Contrast sensitivity and vernier acuity in amblyopic monkeys

Responses to visual motion of neurons in the extrastriate visual cortex of macaque monkeys with experimental amblyopia

Amblyopia is a developmental disorder that results from abnormal visual experience in early life. Amblyopia typically reduces visual performance in one eye. We studied the representation of visual motion information in area MT and nearby extrastriate visual areas in two monkeys made amblyopic by creating an artificial strabismus in early life, and in a single age-matched control monkey. Tested monocularly, cortical responses to moving dot patterns, gratings, and plaids were qualitatively normal in awake, fixating amblyopic monkeys, with primarily subtle differences between the eyes. However, the number of binocularly driven neurons was substantially lower than normal; of the neurons driven predominantly by one eye, the great majority responded only to stimuli presented to the fellow eye. The small population driven by the amblyopic eye showed reduced coherence sensitivity and a preference for faster speeds in much the same way as behavioral deficits. We conclude that, while we do find important differences between neurons driven by the two eyes, amblyopia does not lead to a large scale reorganization of visual receptive fields in the dorsal stream when tested through the amblyopic eye, but rather creates a substantial shift in eye preference toward the fellow eye.

Development of radial frequency pattern perception in macaque monkeys

Infant primates see poorly, and most perceptual functions mature steadily beyond early infancy. Behavioral studies on human and macaque infants show that global form perception, as measured by the ability to integrate contour information into a coherent percept, improves dramatically throughout the first several years after birth. However, it is unknown when sensitivity to curvature and shape emerges in early life or how it develops. We studied the development of shape sensitivity in 18 macaques, aged 2 months to 10 years. Using radial frequency stimuli, circular targets whose radii are modulated sinusoidally, we tested monkeys' ability to radial frequency stimuli from circles as a function of the depth and frequency of sinusoidal modulation. We implemented a new four-choice oddity task and compared the resulting data with that from a traditional two-alternative forced choice task. We found that radial frequency pattern perception was measurable at the youngest age tested (2 months). Behavioral performance at all radial frequencies improved with age. Performance was better for higher radial frequencies, suggesting the developing visual system prioritizes processing of fine visual details that are ecologically relevant. By using two complementary methods, we were able to capture a comprehensive developmental trajectory for shape perception.

Interocular suppression in primary visual cortex in strabismus: impact of staggering the presentation of stimuli to the eyes

Diplopia (double vision) in strabismus is prevented by suppression of the image emanating from one eye. In a recent study conducted in two macaques raised with exotropia (an outward ocular deviation) but having normal acuity in each eye, simultaneous display of stimuli to each eye did not induce suppression in V1 neurons. Puzzled by this negative result, we have modified our protocol to display stimuli in a staggered sequence, rather than simultaneously. Additional recordings were made in the same two macaques, following two paradigms. In trial type 1, the receptive field in one eye was stimulated with a sine-wave grating while the other eye was occluded. After 5 s, the occluder was removed and the neuron was stimulated for another 5 s. The effect of uncovering the eye, which potentially exposed the animal to diplopia, was quantified by the peripheral retinal interaction index (PRII). In trial type 2, the receptive field in the fixating eye was stimulated with a grating during binocular viewing. After 5 s, a second grating appeared in the receptive field of the nonfixating eye. The impact of the second grating, which had the potential to generate visual confusion, was quantified by the receptive field interaction index (RFII). For 82 units, the mean PRII was 0.48 ± 0.05 (0.50 = no suppression) and the mean RFII was 0.46 ± 0.08 (0.50 = no suppression). These values suggest mild suppression, but the modest decline in spike rate registered during the second epoch of visual stimulation might have been due to neuronal adaptation, rather than interocular suppression. In a few instances neurons showed unequivocal suppression, but overall, these recordings did not support the contention that staggered stimulus presentation is more effective than simultaneous stimulus presentation at evoking interocular suppression in V1 neurons.NEW & NOTEWORTHY In strabismus, double vision is prevented by interocular suppression. It has been reported that inhibition of neuronal firing in the primary visual cortex occurs only when stimuli are presented sequentially, rather than simultaneously. However, these recordings in alert macaques raised with exotropia showed, with rare exceptions, little evidence to support the concept that staggered stimulus presentation is more effective at inducing interocular suppression of V1 neurons.

Contrast Sensitivity in Early to Intermediate Age-Related Macular Degeneration (AMD)

PURPOSE

Previous studies indicated that advanced age-related macular degeneration (AMD) affects contrast sensitivity (CS) in humans. The CS results for early/intermediate AMD patients are contradictory. The purpose of this study was to determine if CS testing discriminates early/intermediate AMD patients with normal acuity from normal patients.

METHODS

Forty-nine subjects (25 control and 24 early/intermediate AMD patients) were chosen for this project. The age (p = .16) and acuity (p = .34) was not significantly different between the groups. The average simplified AREDS AMD grade for the AMD patients was 2.75 ± 1.03. Three CS functions employing a descending method of limits were measured at the fovea (1. stationary stimulus and, 2. 16 Hz counter-phase stimulus under photopic conditions and 3. the stationary stimulus viewed through a 2 log unit neutral density filter (mesopic condition, background luminance of 1 cd/m²)) and at 4 deg right or left of the fovea with a horizontally oriented sine wave grating (5 deg diameter) viewed on a VPixx monitor (luminance of 100 cd/m²).

RESULTS

The early AMD patients were no different from the control patients for any test condition. The intermediate AMD patients were significantly different from the control patients for the mesopic CS function (p = .05). Post-hoc 2-sample t-tests for the intermediate AMD patients were significantly different from the control patients under the stationary photopic and mesopic conditions for the 1.5 cycle per degree stimulus.

CONCLUSIONS

Group differences in CS were only found in intermediate AMD patients. The loss in CS increased for the intermediate AMD patients under low light levels. Thus, CS may not be the optimal test to discriminate early AMD from control patients so other tests measured under dark adapted conditions should be investigated.

Interocular Suppression in Primary Visual Cortex in Strabismus

People with strabismus acquired during childhood do not experience diplopia (double vision). To investigate how perception of the duplicate image is suppressed, we raised two male monkeys with alternating exotropia by disinserting the medial rectus muscle in each eye at age four weeks. Once the animals were mature, they were brought to the laboratory and trained to fixate a small spot while recordings were made in primary visual cortex (V1). Drifting gratings were presented to the receptive fields of 500 single neurons for eight interleaved conditions: (1) right eye monocular; (2) left eye monocular; (3) right eye's field, right eye fixating; (4) right eye's field, left eye fixating; (5) left eye's field, right eye fixating; (6) left eye's field, left eye fixating; (7) both eyes' fields, right eye fixating; (8) both eyes' fields, left eye fixating. As expected, ocular dominance histograms showed a monocular bias compared with normal animals, but many cells could still be driven via both eyes. Overall, neuronal responses were not affected by switches in ocular fixation. Individual neurons exhibited binocular interactions, but mean population indices indicated no net interocular suppression or facilitation. Even neurons located in cortex with reduced cytochrome oxidase (CO) activity, representing portions of the nasal visual field where perception is suppressed during binocular viewing, showed no net inhibition. These data indicate that V1 neurons do not appear to reflect strabismic suppression and therefore the elimination of diplopia is likely to be mediated at a higher cortical level.SIGNIFICANCE STATEMENT In patients with strabismus, images fall on non-corresponding points in the two retinas. Only one image is perceived, because signals emanating from the other eye that convey the duplicate image are suppressed. The benefit is that diplopia is prevented, but the penalty is that the visual feedback required to adjust eye muscle tone to realign the globes is eliminated. Here, we report the first electrophysiological recordings from the primary visual cortex (V1) in awake monkeys raised with strabismus. The experiments were designed to reveal how perception of double images is avoided.

Repetitive visual cortex transcranial random noise stimulation in adults with amblyopia

We tested the hypothesis that five daily sessions of visual cortex transcranial random noise stimulation would improve contrast sensitivity, crowded and uncrowded visual acuity in adults with amblyopia. Nineteen adults with amblyopia (44.2 ± 14.9 years, 10 female) were randomly allocated to active or sham tRNS of the visual cortex (active, n = 9; sham, n = 10). Sixteen participants completed the study (n = 8 per group). tRNS was delivered for 25 min across five consecutive days. Monocular contrast sensitivity, uncrowded and crowded visual acuity were measured before, during, 5 min and 30 min post stimulation on each day. Active tRNS significantly improved contrast sensitivity and uncrowded visual acuity for both amblyopic and fellow eyes whereas sham stimulation had no effect. An analysis of the day by day effects revealed large within session improvements on day 1 for the active group that waned across subsequent days. No long-lasting (multi-day) improvements were observed for contrast sensitivity, however a long-lasting improvement in amblyopic eye uncrowded visual acuity was observed for the active group. This improvement remained at 28 day follow up. However, between-group differences in baseline uncrowded visual acuity complicate the interpretation of this effect. No effect of tRNS was observed for amblyopic eye crowded visual acuity. In agreement with previous non-invasive brain stimulation studies using different techniques, tRNS induced short-term contrast sensitivity improvements in adult amblyopic eyes, however, repeated sessions of tRNS did not lead to enhanced or long-lasting effects for the majority of outcome measures.

Desenvolvimento e validação de medidas psicofísicas de sensibilidade ao contraste de segunda-ordem

A medida de sensibilidade ao contraste (SC) de primeira ordem é frequentemente utilizada para avaliação da percepção espacial. Nosso objetivo foi desenvolver e validar um teste de SC de segunda ordem para aplicação clínica. Modificações metodológicas foram realizadas na rotina psicofísica para redução do tempo de testagem e no primeiro experimento validamos a nova metodologia. Em um segundo experimento, dezesseis participantes foram testados nas mesmas condições do primeiro experimento. As medidas de consistência interna por alfa de Cronbach foram robustas para a medida de primeira ordem sendo α= 0,788, segunda ordem por ruído branco α= 0,668 e por ruído rosa α= 0,717. O desenvolvimento e validação deste novo experimento para medidas de SC de segunda ordem permitirá avançar nos estudos dos mecanismos básicos da percepção de espaço para estímulos complexos, assim como a aplicação clínica em diversas doenças.

Habitual higher order aberrations affect Landolt but not Vernier acuity

To assess whether the eye's optical imperfections are relevant for hyperacute vision, we measured ocular wave aberrations, visual hyperacuity, and acuity thresholds in 31 eyes of young adults. Although there was a significant positive correlation between the subjects' performance in Vernier- and Landolt-optotype acuity tasks, we found clear differences in how far both acuity measures correlate with the eyes' optics. Landolt acuity thresholds were significantly better in eyes with low higher order aberrations and high visual Strehl ratios (r² = 0.22, p = 0.009), and significantly positively correlated with axial length (r² = 0.15, p = 0.03). A retinal image quality metric, calculated as two-dimensional correlation between perfect and actual retinal image, was also correlated with Landolt acuity thresholds (r² = 0.27, p = 0.003). No such correlations were found with Vernier acuity performance (r² < 0.03, p > 0.3). Based on these results, hyperacuity thresholds are, contrary to resolution acuity, not affected by higher order aberrations of the eye.

A comparison of contrast sensitivity and sweep visual evoked potential (sVEP) acuity estimates in normal humans

PURPOSE

Several previous studies have demonstrated that for normal adult subjects the optotype acuity measured with charts is better than the acuity determined with the sweep visual evoked potential (sVEP) using gratings or checks. However, there is no difference in psychophysical measures of acuity with optotype or grating charts. Thus, it is unclear whether the acuity discrepancy between optotype charts and the sVEP result from the stimulus design or other methodological differences. The purpose of this experiment is to determine the relationship between acuities extrapolated from a contrast sensitivity function (CSF) that uses optotypes and the sVEP.

METHODS

Normal subjects (N = 10) with acuity of 0.00 logMAR or better (ETDRS chart) were recruited for this study. Two commercially available systems were used to measure CSFs [i.e., the Beethoven System (Ryklin Software, NY) and the qCSF system (Adaptive Sensory Tech, CA)]. The stimuli for the Beethoven were sine wave gratings (0.75-18.50 cpd), and thresholds were determined with a 2-alternative forced choice (2-AFC) procedure combined with a staircase. The stimuli for the qCSF system were spatially filtered letters (10 possible letters, 10-AFC) with the letter sizes and contrasts determined by a Bayesian adaptive procedure. Visual acuity was determined by fitting the data with a double exponential equation and extrapolating the fit to a contrast sensitivity of one. The sVEP was obtained with the PowerDiva (Digital Instrumentation for Visual Assessment, version 3.5, CA). The stimuli were sine wave gratings (80% contrast, 3-36 cpd) counterphased at 7.5 Hz. The final acuity was the average of two estimates each derived from the average of 10 sweeps.

RESULTS

The average logMAR chart (acuity converted to cpd), sVEP, Beethoven, and qCSF acuities were 36.6 ± 4.62 cpd (mean ± SD), 31.2 ± 4.59 cpd, 27.3 ± 7.38 cpd, and 27.6 ± 6.36 cpd, respectively. The logMAR chart acuity was significantly different from the other acuity estimates (all p values < 0.05). The sVEP, Beethoven, and qCSF acuities were not different from one another (all p values > 0.05). The Beethoven and the qCSF acuities had a good intraclass correlation coefficient (ICC = 0.85).

CONCLUSIONS

Similar to previous publications, the sVEP acuity estimate was less than the optotype chart acuity. The acuity determined with the sVEP and the CSFs with letter and grating stimuli were not statistically different, suggesting that the difference in acuity with the sVEP and optotype charts does not result from stimulus differences. Other methodological differences must account for the discrepancy in sVEP and optotype chart acuity.

Comparison of contrast sensitivity in macaque monkeys and humans

Contrast sensitivity functions reveal information about a subject's overall visual ability and have been investigated in several species of nonhuman primates (NHPs) with experimentally induced amblyopia and glaucoma. However, there are no published studies comparing contrast sensitivity functions across these species of normal NHPs. The purpose of this investigation was to compare contrast sensitivity across these primates to determine whether they are similar. Ten normal humans and eight normal NHPs (Macaca fascicularis) took part in this project. Previously published data from Macaca mulatta and Macaca nemestrina were also compared. Threshold was operationally defined as two misses in a row for a descending method of limits. A similar paradigm was used for the humans except that the descending method of limits was combined with a spatial, two-alternative forced choice (2-AFC) technique. The contrast sensitivity functions were fit with a double exponential function. The averaged peak contrast sensitivity, peak spatial frequency, acuity, and area under the curve for the humans were 268.9, 3.40 cpd, 27.3 cpd, and 2345.4 and for the Macaca fascicularis were 99.2, 3.93 cpd, 26.1 cpd, and 980.9. A two-sample t-test indicated that the peak contrast sensitivities (P = 0.001) and areas under the curve (P = 0.010) were significantly different. The peak spatial frequencies (P = 0.150) and the extrapolated visual acuities (P = 0.763) were not different. The contrast sensitivities for the Macaca fascicularis, Macaca mulatta, and Macaca nemestrina were qualitatively and quantitatively similar. The contrast sensitivity functions for the NHPs had lower peak contrast sensitivities and areas under the curve than the humans. Even though different methods have been used to measure contrast sensitivity in different species of NHP, the functions are similar. The contrast sensitivity differences and similarities between humans and NHPs need to be considered when using NHPs to study human disease.

Beyond Rehabilitation of Acuity, Ocular Alignment, and Binocularity in Infantile Strabismus

Infantile strabismus impairs the perception of all attributes of the visual scene. High spatial frequency components are no longer visible, leading to amblyopia. Binocularity is altered, leading to the loss of stereopsis. Spatial perception is impaired as well as detection of vertical orientation, the fastest movements, directions of movement, the highest contrasts and colors. Infantile strabismus also affects other vision-dependent processes such as control of postural stability. But presently, rehabilitative therapies for infantile strabismus by ophthalmologists, orthoptists and optometrists are restricted to preventing or curing amblyopia of the deviated eye, aligning the eyes and, whenever possible, preserving or restoring binocular vision during the critical period of development, i.e., before ~10 years of age. All the other impairments are thus ignored; whether they may recover after strabismus treatment even remains unknown. We argue here that medical and paramedical professionals may extend their present treatments of the perceptual losses associated with infantile strabismus. This hypothesis is based on findings from fundamental research on visual system organization of higher mammals in particular at the cortical level. In strabismic subjects (as in normal-seeing ones), information about all of the visual attributes converge, interact and are thus inter-dependent at multiple levels of encoding ranging from the single neuron to neuronal assemblies in visual cortex. Thus if the perception of one attribute is restored this may help to rehabilitate the perception of other attributes. Concomitantly, vision-dependent processes may also improve. This could occur spontaneously, but still should be assessed and validated. If not, medical and paramedical staff, in collaboration with neuroscientists, will have to break new ground in the field of therapies to help reorganize brain circuitry and promote more comprehensive functional recovery. Findings from fundamental research studies in both young and adult patients already support our hypothesis and are reviewed here. For example, presenting different contrasts to each eye of a strabismic patient during training sessions facilitates recovery of acuity in the amblyopic eye as well as of 3D perception. Recent data also demonstrate that visual recoveries in strabismic subjects improve postural stability. These findings form the basis for a roadmap for future research and clinical development to extend presently applied rehabilitative therapies for infantile strabismus.

Endogenous attention improves perception in amblyopic macaques

Amblyopia, a developmental disorder of vision, affects many aspects of spatial vision as well as motion perception and some cognitive skills. Current models of amblyopic vision based on known neurophysiological deficiencies have yet to provide an understanding of the wide range of amblyopic perceptual losses. Visual spatial attention is known to enhance performance in a variety of detection and discrimination tasks in visually typical humans and nonhuman primates. We investigated whether and how voluntary spatial attention affected psychophysical performance in amblyopic macaques. Full-contrast response functions for motion direction discrimination were measured for each eye of six monkeys: five amblyopic and one control. We assessed whether the effect of a valid spatial cue on performance corresponded to a change in contrast gain, a leftward shift of the function, or response gain, an upward scaling of the function. Our results showed that macaque amblyopes benefit from a valid spatial cue. Performance with amblyopic eyes viewing showed enhancement of both contrast and response gain whereas fellow and control eyes' performance showed only contrast gain. Reaction time analysis showed no speed accuracy trade-off in any case. The valid spatial cue improved contrast sensitivity for the amblyopic eye, effectively eliminating the amblyopic contrast sensitivity deficit. These results suggest that engaging endogenous spatial attention may confer substantial benefit to amblyopic vision.

Altered Balance of Receptive Field Excitation and Suppression in Visual Cortex of Amblyopic Macaque Monkeys

In amblyopia, a visual disorder caused by abnormal visual experience during development, the amblyopic eye (AE) loses visual sensitivity whereas the fellow eye (FE) is largely unaffected. Binocular vision in amblyopes is often disrupted by interocular suppression. We used 96-electrode arrays to record neurons and neuronal groups in areas V1 and V2 of six female macaque monkeys (Macaca nemestrina) made amblyopic by artificial strabismus or anisometropia in early life, as well as two visually normal female controls. To measure suppressive binocular interactions directly, we recorded neuronal responses to dichoptic stimulation. We stimulated both eyes simultaneously with large sinusoidal gratings, controlling their contrast independently with raised-cosine modulators of different orientations and spatial frequencies. We modeled each eye's receptive field at each cortical site using a difference of Gaussian envelopes and derived estimates of the strength of central excitation and surround suppression. We used these estimates to calculate ocular dominance separately for excitation and suppression. Excitatory drive from the FE dominated amblyopic visual cortex, especially in more severe amblyopes, but suppression from both the FE and AEs was prevalent in all animals. This imbalance created strong interocular suppression in deep amblyopes: increasing contrast in the AE decreased responses at binocular cortical sites. These response patterns reveal mechanisms that likely contribute to the interocular suppression that disrupts vision in amblyopes.SIGNIFICANCE STATEMENT Amblyopia is a developmental visual disorder that alters both monocular vision and binocular interaction. Using microelectrode arrays, we examined binocular interaction in primary visual cortex and V2 of six amblyopic macaque monkeys (Macaca nemestrina) and two visually normal controls. By stimulating the eyes dichoptically, we showed that, in amblyopic cortex, the binocular combination of signals is altered. The excitatory influence of the two eyes is imbalanced to a degree that can be predicted from the severity of amblyopia, whereas suppression from both eyes is prevalent in all animals. This altered balance of excitation and suppression reflects mechanisms that may contribute to the interocular perceptual suppression that disrupts vision in amblyopes.

Observations on the relationship between anisometropia, amblyopia and strabismus

We investigated the potential causal relationships between anisometropia, amblyopia and strabismus, specifically to determine whether either amblyopia or strabismus interfered with emmetropization. We analyzed data from non-human primates that were relevant to the co-existence of anisometropia, amblyopia and strabismus in children. We relied on interocular comparisons of spatial vision and refractive development in animals reared with 1) monocular form deprivation; 2) anisometropia optically imposed by either contact lenses or spectacle lenses; 3) organic amblyopia produced by laser ablation of the fovea; and 4) strabismus that was either optically imposed with prisms or produced by either surgical or pharmacological manipulation of the extraocular muscles. Hyperopic anisometropia imposed early in life produced amblyopia in a dose-dependent manner. However, when potential methodological confounds were taken into account, there was no support for the hypothesis that the presence of amblyopia interferes with emmetropization or promotes hyperopia or that the degree of image degradation determines the direction of eye growth. To the contrary, there was strong evidence that amblyopic eyes were able to detect the presence of a refractive error and alter ocular growth to eliminate the ametropia. On the other hand, early onset strabismus, both optically and surgically imposed, disrupted the emmetropization process producing anisometropia. In surgical strabismus, the deviating eyes were typically more hyperopic than their fellow fixating eyes. The results show that early hyperopic anisometropia is a significant risk factor for amblyopia. Early esotropia can trigger the onset of both anisometropia and amblyopia. However, amblyopia, in isolation, does not pose a significant risk for the development of hyperopia or anisometropia.

"Global" visual training and extent of transfer in amblyopic macaque monkeys

Perceptual learning is gaining acceptance as a potential treatment for amblyopia in adults and children beyond the critical period. Many perceptual learning paradigms result in very specific improvement that does not generalize beyond the training stimulus, closely related stimuli, or visual field location. To be of use in amblyopia, a less specific effect is needed. To address this problem, we designed a more general training paradigm intended to effect improvement in visual sensitivity across tasks and domains. We used a "global" visual stimulus, random dot motion direction discrimination with 6 training conditions, and tested for posttraining improvement on a motion detection task and 3 spatial domain tasks (contrast sensitivity, Vernier acuity, Glass pattern detection). Four amblyopic macaques practiced the motion discrimination with their amblyopic eye for at least 20,000 trials. All showed improvement, defined as a change of at least a factor of 2, on the trained task. In addition, all animals showed improvements in sensitivity on at least some of the transfer test conditions, mainly the motion detection task; transfer to the spatial domain was inconsistent but best at fine spatial scales. However, the improvement on the transfer tasks was largely not retained at long-term follow-up. Our generalized training approach is promising for amblyopia treatment, but sustaining improved performance may require additional intervention.

Early monocular defocus disrupts the normal development of receptive-field structure in V2 neurons of macaque monkeys

Experiencing different quality images in the two eyes soon after birth can cause amblyopia, a developmental vision disorder. Amblyopic humans show the reduced capacity for judging the relative position of a visual target in reference to nearby stimulus elements (position uncertainty) and often experience visual image distortion. Although abnormal pooling of local stimulus information by neurons beyond striate cortex (V1) is often suggested as a neural basis of these deficits, extrastriate neurons in the amblyopic brain have rarely been studied using microelectrode recording methods. The receptive field (RF) of neurons in visual area V2 in normal monkeys is made up of multiple subfields that are thought to reflect V1 inputs and are capable of encoding the spatial relationship between local stimulus features. We created primate models of anisometropic amblyopia and analyzed the RF subfield maps for multiple nearby V2 neurons of anesthetized monkeys by using dynamic two-dimensional noise stimuli and reverse correlation methods. Unlike in normal monkeys, the subfield maps of V2 neurons in amblyopic monkeys were severely disorganized: subfield maps showed higher heterogeneity within each neuron as well as across nearby neurons. Amblyopic V2 neurons exhibited robust binocular suppression and the strength of the suppression was positively correlated with the degree of hereogeneity and the severity of amblyopia in individual monkeys. Our results suggest that the disorganized subfield maps and robust binocular suppression of amblyopic V2 neurons are likely to adversely affect the higher stages of cortical processing resulting in position uncertainty and image distortion.

Population representation of visual information in areas V1 and V2 of amblyopic macaques

Amblyopia is a developmental disorder resulting in poor vision in one eye. The mechanism by which input to the affected eye is prevented from reaching the level of awareness remains poorly understood. We recorded simultaneously from large populations of neurons in the supragranular layers of areas V1 and V2 in 6 macaques that were made amblyopic by rearing with artificial strabismus or anisometropia, and 1 normally reared control. In agreement with previous reports, we found that cortical neuronal signals driven through the amblyopic eyes were reduced, and that cortical neurons were on average more strongly driven by the non-amblyopic than by the amblyopic eyes. We analyzed multiunit recordings using standard population decoding methods, and found that visual signals from the amblyopic eye, while weakened, were not degraded enough to explain the behavioral deficits. Thus additional losses must arise in downstream processing. We tested the idea that under monocular viewing conditions, only signals from neurons dominated by - rather than driven by - the open eye might be used. This reduces the proportion of neuronal signals available from the amblyopic eye, and amplifies the interocular difference observed at the level of single neurons. We conclude that amblyopia might arise in part from degradation in the neuronal signals from the amblyopic eye, and in part from a reduction in the number of signals processed by downstream areas.

The relationship between anisometropia and amblyopia

This review aims to disentangle cause and effect in the relationship between anisometropia and amblyopia. Specifically, we examine the literature for evidence to support different possible developmental sequences that could ultimately lead to the presentation of both conditions. The prevalence of anisometropia is around 20% for an inter-ocular difference of 0.5D or greater in spherical equivalent refraction, falling to 2-3%, for an inter-ocular difference of 3D or above. Anisometropia prevalence is relatively high in the weeks following birth, in the teenage years coinciding with the onset of myopia and, most notably, in older adults starting after the onset of presbyopia. It has about one-third the prevalence of bilateral refractive errors of the same magnitude. Importantly, the prevalence of anisometropia is higher in highly ametropic groups, suggesting that emmetropization failures underlying ametropia and anisometropia may be similar. Amblyopia is present in 1-3% of humans and around one-half to two-thirds of amblyopes have anisometropia either alone or in combination with strabismus. The frequent co-existence of amblyopia and anisometropia at a child's first clinical examination promotes the belief that the anisometropia has caused the amblyopia, as has been demonstrated in animal models of the condition. In reviewing the human and monkey literature however it is clear that there are additional paths beyond this classic hypothesis to the co-occurrence of anisometropia and amblyopia. For example, after the emergence of amblyopia secondary to either deprivation or strabismus, anisometropia often follows. In cases of anisometropia with no apparent deprivation or strabismus, questions remain about the failure of the emmetropization mechanism that routinely eliminates infantile anisometropia. Also, the chronology of amblyopia development is poorly documented in cases of 'pure' anisometropic amblyopia. Although indirect, the therapeutic impact of refractive correction on anisometropic amblyopia provides strong support for the hypothesis that the anisometropia caused the amblyopia. Direct evidence for the aetiology of anisometropic amblyopia will require longitudinal tracking of at-risk infants, which poses numerous methodological and ethical challenges. However, if we are to prevent this condition, we must understand the factors that cause it to develop.

Visual motion processing by neurons in area MT of macaque monkeys with experimental amblyopia

Early experience affects the development of the visual system. Ocular misalignment or unilateral blur often causes amblyopia, a disorder that has become a standard for understanding developmental plasticity. Neurophysiological studies of amblyopia have focused almost entirely on the first stage of cortical processing in striate cortex. Here we provide the first extensive study of how amblyopia affects extrastriate cortex in nonhuman primates. We studied macaque monkeys (Macaca nemestrina) for which we have detailed psychophysical data, directly comparing physiological findings to perceptual capabilities. Because these subjects showed deficits in motion discrimination, we focused on area MT/V5, which plays a central role in motion processing. Most neurons in normal MT respond equally to visual stimuli presented through either eye; most recorded in amblyopes strongly preferred stimulation of the nonamblyopic (fellow) eye. The pooled responses of neurons driven by the amblyopic eye showed reduced sensitivity to coherent motion and preferred higher speeds, in agreement with behavioral measurements. MT neurons were more limited in their capacity to integrate motion information over time than expected from behavioral performance; neurons driven by the amblyopic eye had even shorter integration times than those driven by the fellow eye. We conclude that some, but not all, of the motion sensitivity deficits associated with amblyopia can be explained by abnormal development of MT.

Development of sensitivity to visual texture modulation in macaque monkeys

In human and non-human primates, higher form vision matures substantially later than spatial acuity and contrast sensitivity, as revealed by performance on such tasks as figure-ground segregation and contour integration. Our goal was to understand whether delayed maturation on these tasks was intrinsically form-dependent or, rather, related to the nature of spatial integration necessary for extracting task-relevant cues. We used an intermediate-level form task that did not call for extensive spatial integration. We trained monkeys (6-201 weeks) to discriminate the orientation of pattern modulation in a two-alternative forced choice paradigm. We presented two families of form patterns, defined by texture or contrast variations, and luminance-defined patterns for comparison. Infant monkeys could discriminate texture- and contrast-defined form as early as 6 weeks; sensitivity improved up to 40 weeks. Surprisingly, sensitivity for texture- and contrast-defined form matured earlier than for luminance-defined form. These results suggest that intermediate-level form vision develops in concert with basic spatial vision rather than following sequentially. Comparison with earlier results reveals that different aspects of form vision develop over different time courses, with processes that depend on comparing local image content maturing earlier than those requiring "global" linking of multiple visual elements across a larger spatial extent.

Neural Correlate of Vernier Acuity Tasks Assessed by Functional MRI (fMRI)

Vernier acuity refers to the ability to discern a small offset within a line. However, while Vernier acuity has been extensively studied psychophysically, its neural correlates are uncertain. Based upon previous psychophysical and electrophysiologic data, we hypothesized that extrastriate areas of the brain would be involved in Vernier acuity tasks, so we designed event-related functional MRI (fMRI) paradigms to identify cortical regions of the brain involved in this behavior. Normal subjects identified suprathreshold and subthreshold Vernier offsets. The results suggest a cortical network including frontal, parietal, occipital, and cerebellar regions subserves the observation, processing, interpretation, and acknowledgment of briefly presented Vernier offsets.

Behavioral measurement of temporal contrast sensitivity development in macaque monkeys (Macaca nemestrina)

We measured the developmental time course for temporal contrast sensitivity in macaque monkeys. The animals, aged 5 weeks to 4 years, detected an unpatterned field of light sinusoidally modulated over time at frequencies ranging from 1 to 40 Hz. Young infants showed reduced sensitivity for all frequencies, and a reduced range of detectable frequencies. Sensitivity to high and low frequencies developed at different rates, but the shape of the temporal contrast sensitivity function did not change significantly with age. Temporal contrast sensitivity matures earlier than spatial contrast sensitivity. The development of high, but not low, frequency sensitivity may be limited by maturation of the magnocellular pathway.

Rotated prism-wear disrupts emmetropization but does not reliably induce hyperopia in the New World monkey

To determine whether a disruption of binocular vision that has been previously shown to be amblyogenic disturbs visually guided growth, and in particular to follow-up the observation by Kiorpes and Wallman [Kiorpes, L., & Wallman, J. (1995). Does experimentally-induced amblyopia cause hyperopia in monkeys? Vision Research, 35(9), 1289-1297] that monkeys in whom strabismus had been induced some years earlier were hyperopic in eyes that had become amblyopic, we induced unilateral fixation in five infant New World monkeys (marmosets) through the wearing of a Fresnel prism (of 15 or 30 prism dioptres power) in front of one eye for four weeks. The prism was rotated every three hours during the prism-wear period to encourage a preference for fixating with the contralateral eye. Refractive error and intraocular axial dimensions were measured before, and at intervals after the prism-wearing period. Fixation preference was measured behaviourally, during and after the prism-wear period. Cortical visual function was subsequently assessed through recording of pattern-reversal VEPs in each marmoset between 11 and 14 months of age to assess whether amblyopia had developed in the non-fixing eye. All marmosets used the untreated eye almost exclusively for a monocular visual task by the end of the prism-rearing period. This preference was still present up to at least 7 months after prism-wear had ceased. VEP measures showed a loss of sensitivity at low spatial frequencies (the only ones we were able to test), compatible with amblyopia having developed in the non-fixating eyes of the prism-reared marmosets. Eyes that wore prisms were not significantly different from their fellow eyes in mean refractive error or mean vitreous chamber depth (repeated measures ANOVA; P>0.05) before or at any time after prism-wear had ceased. Two marmosets developed 2-3D of anisometropia (one hyperopic and one myopic) at the end of prism-wear, that was attributable to interocular differences in vitreous chamber depth, and which decreased towards isometropia in the period following prism-wear removal. Disruption of binocular vision with rotating prisms can influence emmetropization and ocular growth, although it does not appear to do so in a consistent way.

Vernier acuity in barn owls

Vernier acuity thresholds were obtained psychophysically in three adult barn owls with vertical bars and sinusoidal gratings. A minimal displacement threshold of 0.58 arcmin was observed with the bar stimulus under binocular viewing conditions. The mean binocular bar threshold was 2.51 arcmin. Bar thresholds were lower than grating thresholds. Monocular thresholds, obtained in one bird only, were typically higher than binocular thresholds. With grating acuity being about 3.75 arcmin in this species, we conclude that the findings reported here indicate that vernier acuity is hyperacute in the barn owl. The data presented here are the first demonstration of vernier acuity thresholds in birds.

Temporal response properties of the primary and secondary rod-signaling pathways in normal and Gnat2 mutant mice

Multiple signaling pathways have been proposed for rod vision in the mammalian retina. The primary and secondary rod pathways have been characterized in humans with the scotopic 15-Hz flicker electroretinogram (ERG). The purpose of this study was to determine whether the response properties of these pathways in the mouse are similar to those of humans. C57BL/6J and Gnat2(cpfl3) mutant mice lacking functional cones were used in these experiments. Standard ERG recording techniques were employed. Response functions were obtained for a range of flash intensities (-4.7logcd-s/m(2) to -0.2logcd-s/m(2)) and temporal modulation frequencies (1-30Hz). The mouse intensity-response functions to 15-Hz flickering stimuli possessed the same features as that of humans - a local amplitude minimum and a rapid phase change in the intensity region where the primary and secondary pathways are mutually inhibitory. However, the secondary pathway in the mouse did not achieve the same level of sensitivity as previously shown for humans, suggesting inter-species differences in post-receptoral signal processing. In Gnat2(cpfl3) mutant mice, the secondary pathway was completely abolished. Measurements of temporal acuity indicated that the primary and secondary rod pathways could mediate temporal frequencies as high as 30 and 50Hz, respectively. The response functions for mice are similar to those of humans, although the evidence suggests that the primary rod pathway dominates all rod-mediated signal processing in the mouse. Nevertheless, these results demonstrate the feasibility of measuring non-invasively the performance characteristics of the primary and secondary rod retinal pathways in the mouse and provide a mechanism for testing hypotheses about the action of disease where post-receptoral cells are differentially affected.

Interocular interactions during acuity measurement in children and adults, and in adults with amblyopia

The binocular interactions that occur during dichoptic and binocular viewing were investigated using a letter acuity task in normally sighted children (age range 6-14 years) and adults, and in adults with anisometropic amblyopia. Our aims were to investigate the nature of binocular interactions that occur in each group, and the extent to which the characteristics of binocular interactions differ across the groups. The non-tested eye was occluded during monocular (baseline) viewing, and was allowed to view a uniform stimulus with fusion lock in dichoptic viewing. In adults and children with normal vision, acuity under dichoptic viewing was unchanged relative to monocular baseline in the dominant eyes, while acuity of the non-dominant eye improved under dichoptic viewing relative to baseline. The magnitude of dichoptic change in the non-dominant eyes was similar in the two normally sighted groups, but the dichoptic advantage was found to decrease with increasing age within the children tested. Binocular acuity was better than monocular acuity in normal subjects, and a decrease in binocular summation with age was noted within the age range of the children tested. In contrast, the amblyopic observers showed no change in acuity with viewing conditions. The results demonstrate development of interocular interactions during childhood, and wide inter-individual variation in pattern of interocular interactions among anisometropic amblyopic adults.

Sensitivity to visual motion in amblyopic macaque monkeys

Amblyopia is usually considered to be a deficit in spatial vision. But there is evidence that amblyopes may also suffer specific deficits in motion sensitivity as opposed to losses that can be explained by the known deficits in spatial vision. We measured sensitivity to visual motion in random dot displays for strabismic and anisometropic amblyopic monkeys. We used a wide range of spatial and temporal offsets and compared the performance of the fellow and amblyopic eye for each monkey. The amblyopes were severely impaired at detecting motion at fine spatial and long temporal offsets, corresponding to fine spatial scale and slow speeds. This impairment was also evident for the untreated fellow eyes of strabismic but not anisometropic amblyopes. Motion sensitivity functions for amblyopic eyes were shifted toward large spatial scales for amblyopic compared to fellow eyes, to a degree that was correlated with the shift in scale of the spatial contrast sensitivity function. Amblyopic losses in motion sensitivity, however, were not correlated with losses in spatial contrast sensitivity. This, combined with the specific impairment for detecting long temporal offsets, reveals a deficit in spatiotemporal integration in amblyopia which cannot be explained by the lower spatial resolution of amblyopic vision.

The visual evoked potential in the mouse--origins and response characteristics

The visual evoked potential (VEP) in the mouse is characterized and compared to responses obtained with the electroretinogram (ERG). The results indicate that: 1, the VEP originates in the visual cortex; 2, the rod and cone pathways contribute separately to the VEP; 3, temporal tuning functions for rod and cone ERGs are low pass and band pass, respectively; VEP tuning functions are both band pass; and 4, VEP acuity is 0.62+/-0.156 cycles/degree. The differences in the spatial and temporal tuning functions obtained from the retina and visual cortex provides a tool to investigate signal processing through the visual system.

Effect of Artificial Tears on Visual Performance in Subjects With Dry Eye

PURPOSE

Disruption of the anterior refracting surface of the eye (i.e., the tear layer) reduces visual performance. Tear layer breakup occurs soon after a blink in contact lens wearers and patients with dry eye. This study determined whether artificial tears stabilize the tear film and improve visual performance in contact lens wearers who also exhibit a dry eye.

METHODS

Five subjects with mild to moderate dry eye (probably as a result of an evaporative dry eye) during spectacle and contact lens wear were fitted with a Focus Night & Day hydrogel lens for this study. A temporal, two-alternative, forced-choice paradigm was used to measure contrast sensitivity. The stimuli were vertically oriented sine wave gratings (between 0.5 and 14 cpd) presented for 16.67 msec. The stimuli were presented at two different times after blink detection: 2 sec after blink detection (i.e., before tear layer breakup) or 4 sec after tear film breakup. Four conditions were investigated at 4 sec after tear layer breakup: 1) without artificial tears added, 2) with Clerz2 (Alcon, Fort Worth, TX) instilled, 3) with Sensitive Eyes (Bausch & Lomb, Rochester, NY), and 4) with GenTeal (Novartis, Basel, Switzerland) applied. The artificial tears were instilled at 10-min intervals during the data collection. The short-term visual effects of drop instillation were also investigated by continually monitoring contrast sensitivity for a 14-cpd grating after a single-drop administration.

RESULTS

High spatial frequency contrast sensitivity and visual acuity were found to be reduced after tear film breakup in the absence of supplementation with artificial tears. For the group data (and four of five subjects), the instillation of Sensitive Eyes improved the contrast sensitivity and visual acuity to the level attained before tear breakup, thus prolonging visual performance. Clerz2 and GenTeal did not produce any enhancement in visual performance. A short-term decrease in contrast sensitivity was also observed with a single administration of Clerz2 and GenTeal.

CONCLUSIONS

This study indicates that there was a benefit of Bausch & Lomb Sensitive Eyes tear supplementation on visual performance in subjects with an evaporative dry eye. This may be the result of 1) aqueous supplementation in these subjects and/or 2) the minimal tear layer disruption found with Sensitive Eyes drop administration. The results suggest that practitioners need to identify those patients who can benefit from the use of appropriate artificial tear supplements.

Development of sensitivity to visual motion in macaque monkeys

The development of spatial vision is relatively well documented in human and nonhuman primates. However, little is known about the development of sensitivity to motion. We measured the development of sensitivity to direction of motion, and the relationship between motion and contrast sensitivity in macaque monkeys as a function of age. Monkeys (Macaca nemestrina, aged between 10 days and 3 years) discriminated direction of motion in random-dot kinematograms. The youngest monkeys showed directionally selective orienting and the ability to integrate motion signals at large dot displacements and fast speeds. With age, coherence sensitivity improved for all spatial and temporal dot displacements tested. The temporal interval between the dots was far less important than the spatial offset in determining the animals' performance at all but the youngest ages. Motion sensitivity improved well beyond the end of the first postnatal year, when mid-spatial-frequency contrast sensitivity reached asymptote, and continued for at least 3 years. Sensitivity to contrast at high spatial frequencies also continued to develop beyond the end of the first year. We conclude that the development of motion sensitivity depends on mechanisms beyond the low-level filters presumed to limit acuity and contrast sensitivity, and most likely reflects the function of extrastriate visual areas.

The spatial localization deficit in visually deprived kittens

We measured the spatial localization abilities (alignment accuracy) of visually deprived kittens by use of similar spatially bandpass stimuli (Gaussian blobs) to those employed for the assessment of human amblyopes. The tests of vision were conducted on kittens reared with either strabismus or following different periods of monocular deprivation. As with amblyopic humans, the deficits in alignment accuracy were scaled in proportion to blob size and were not only considerably larger than those of grating acuity but also were not correlated with either the acuity or contrast sensitivity losses. Tests with stimuli of various contrast revealed that the deficits could not be explained in terms of the contrast sensitivity loss in this eye. The positional deficits that arise from anomalous visual development are independent of the contrast sensitivity loss and profound.

Single-neuron responses and neuronal decisions in a vernier task

Vernier acuity is a measure of the smallest horizontal offset between two vertical lines that can be behaviorally discriminated. To examine the link between the neuronal responses in a retinotopic mosaic and vernier acuity, we recorded the responses of single cells in cat lateral geniculate nucleus to a vertical bar stimulus that was stepped in small increments through the receptive fields of cells. Based on the single-trial responses evoked by stimuli at different positions, we calculated the spatial resolution that could be achieved. If the stimulus could fall anywhere in their receptive fields, single neurons had spatial resolutions two times worse than previously reported vernier thresholds. Given the known coverage factor in a cat retina, we developed a two-stage decision model to examine how the responses of neurons in a retinotopic mosaic could be processed to achieve vernier acuity. In order for psychophysical thresholds to be accounted for by the responses of a single cell, the stimulus must fall in the quarter of the receptive field that provides the most information about stimulus position. Alternatively, both the absolute psychophysical threshold for vernier acuity and its dependence on stimulus length can be realized by pooling the responses of a few neurons, all located on one side of the bar stimulus.

Development of contour integration in macaque monkeys

Studies of visual development show that basic metrics of visual development such as spatial resolution develop over the first 6-9 months in monkeys and over the first 6 or so years in humans. However, more complex visual functions may develop over different, or more protracted, time courses. To address the question of whether global perceptual processing is linked to or otherwise dependent on the development of basic spatial vision, we studied the development of contour integration, a global perceptual task, in comparison to that of grating acuity in macaque monkeys. We find that contour integration develops substantially later than acuity. Contour integration begins to develop at 5-6 months, near the time that acuity development is complete and continues to mature well into the second postnatal year. We discuss this later development in terms poor central efficiency and consider the relevant anatomy and physiology of the developing visual system. We conclude that contour integration is not likely to be limited by the same mechanisms that are permissive to acuity development, and may instead reflect the emergence of function central to V1.

Contour integration in amblyopic monkeys

Amblyopia is characterized by losses in a variety of aspects of spatial vision, such as acuity and contrast sensitivity. Our goal was to learn whether those basic spatial deficits lead to impaired global perceptual processing in strabismic and anisometropic amblyopia. This question is unresolved by the current human psychophysical literature. We studied contour integration and contrast sensitivity in amblyopic monkeys. We found deficient contour integration in anisometropic as well as strabismic amblyopic monkeys. Some animals showed poor contour integration in the fellow eye as well as in the amblyopic eye. Orientation jitter of the elements in the contour systematically decreased contour-detection ability for control and fellow eyes, but had less effect on amblyopic eyes. The deficits were not clearly related to basic losses in contrast sensitivity and acuity for either type of amblyopia. We conclude that abnormal contour integration in amblyopes reflects disruption of mechanisms that are different from those that determine acuity and contrast sensitivity, and are likely to be central to V1.

Effects of contrast and length on vernier acuity explained with noisy templates

Vernier acuity depends on the integration of information from multiple photoreceptors. For this reason, vernier acuity thresholds ought to exhibit effects of stimulus size and contrast analogous to those that occur in area summation experiments. In this paper, we consider some area and contrast effects found in vernier acuity experiments, and explain them with a model of detection and discrimination which we call the Noisy Template model. The Noisy Template model assumes that psychophysical tasks are performed (or can be approximated) by cross-correlation of the stimulus with a decision template which is optimal for the task at hand. The Noisy Template model crucially adds the assumption that the template contains noise. This yields inefficiency in the decision process which increases with stimulus size and contrast. Predictions of the Noisy Template model are derived for the case of vernier acuity, and compared with existing experiments.

Is second-order spatial loss in amblyopia explained by the loss of first-order spatial input?

The purpose of the study was to determine whether amblyopes show detection loss for second-order spatial information, and if present, whether the loss is explained by the loss of first-order spatial input. We psychophysically determined detection thresholds for the amblyopic and non-amblyopic eyes of five adult amblyopes and the dominant eyes of three control observers. We found that four amblyopic eyes and two non-amblyopic eyes showed second-order loss relative to the control eyes. The second-order loss was greater than the first-order loss at the carrier spatial frequency (first-order input). The extra second-order loss indicates an early amplification of cortical neural loss that we speculate is due to deficient binocular input to second-order neurons.

Conditions of perceptual selection and suppression during interocular rivalry in strabismic and normal cats

Presenting the two eyes with incongruent stimuli leads to the phenomenon of interocular rivalry. At any given time, one of the stimuli is perceptually suppressed in order to avoid double vision. In squinting subjects, rivalry occurs permanently also for congruent stimuli because of developmental rearrangement of cortical circuitry. In this study, we have investigated the dynamics and stimulus dependence of rivalry in six esotropic, four exotropic and three non-strabismic cats. As an indicator for perception, we used optokinetic nystagmus that was induced by moving gratings. The esotropic cats were tested for their visual acuity by means of a jumping stand procedure. The results show that one eye can dominate perception even if both eyes have equal visual acuity and are presented with stimuli of equal contrast. Strong eye dominance asymmetry was found in all but one of the tested cats. Notably, all three of the normal cats showed a clear asymmetry in perceptual selection. Measurements with varying contrast and velocity of the stimuli revealed that the influence of these parameters on perceptual selection was independent of the presence of strabismus. In all cats, the time during which a given eye dominated perception increased with the contrast and decreases with the velocity of the stimulus presented to this eye.

Factors limiting contrast sensitivity in experimentally amblyopic macaque monkeys

Contrast detection is impaired in amblyopes. To understand the contrast processing deficit in amblyopia, we studied the effects of masking noise on contrast threshold in amblyopic macaque monkeys. Amblyopia developed as a result of either experimentally induced strabismus or anisometropia. We used random spatiotemporal broadband noise of varying contrast power to mask the detection of sinusoidal grating patches. We compared masking in the amblyopic and non-amblyopic eyes. From the masking functions, we calculated equivalent noise contrast (the noise power at which detection threshold was elevated by square root of 2) and signal-to-noise ratio (the ratio of threshold contrast to noise contrast at high noise power). The relation between contrast threshold and masking noise level was similar for amblyopic and non-amblyopic eyes. Although in most cases there was some elevation in equivalent noise for amblyopic compared to fellow eyes, signal-to-noise ratio showed greater variation with the extent of amblyopia. These results support the idea that the contrast detection deficit in amblyopia is a cortical deficit.

Hyperacuity deficits in anisometropic and strabismic amblyopes with known ages of onset

In order to evaluate the influence of etiology of amblyopia and of age at onset of amblyopia on the resulting constellation of spatial vision deficits, resolution/vernier and recognition/resolution acuity ratios were measured in groups of children with either strabismic amblyopia or anisometropic amblyopia with known ages of onset. Strabismic amblyopia with infantile onset (<9 months) and strabismic amblyopia with late onset (18-30 months) were both associated with abnormally low resolution/vernier and abnormally high recognition/resolution acuity ratios. Among amblyopes with infantile onset (<9 months), moderate amblyopia was associated with different resolution/vernier and recognition/resolution acuity ratios in anisometropic and strabismic groups. Infantile amblyopes with poor acuity outcomes included children who initially presented with anisometropia but later developed strabismus and children who initially presented with esotropia but later developed anisometropia; both subgroups with mixed amblyopia had poor resolution/vernier acuity ratios. Data from moderate amblyopes support the hypothesis that anisometropia and strabismus disrupt visual maturation in fundamentally different ways rather than simply at different stages in visual development.

Discrimination of line orientation in humans and monkeys

Orientation discrimination, the capacity to recognize an orientation difference between two lines presented at different times, probably involves cortical processes such as stimuli encoding, holding them in memory, comparing them, and then deciding. To correlate discrimination with neural activity in combined psychophysical and electrophysiological experiments, precise knowledge of the strategies followed in the completion of the behavioral task is necessary. To address this issue, we measured human and nonhuman primates' capacities to discriminate the orientation of lines in a fixed and in a continuous variable task. Subjects have to indicate whether a line (test) was oriented to one side or to the other of a previously presented line (reference). When the orientation of the reference line did not change across trials (fixed discrimination task), subjects can complete the task either by categorizing the test line, thus ignoring the reference, or by discriminating between them. This ambiguity was avoided when the reference stimulus was changed randomly from trial to trial (continuous discrimination task), forcing humans and monkeys to discriminate by paying continuous attention to the reference and test stimuli. Both humans and monkeys discriminated accurately with stimulus duration as short as 150 ms. Effective interstimulus intervals were of 2.5 s for monkeys but much longer (>6 s) in humans. These results indicated that the fixed and continuous discrimination tasks are different, and accordingly humans and monkeys do use different behavioral strategies to complete each task. Because both tasks might involve different neural processes, these findings have important implications for studying the neural mechanisms underlying visual discrimination.

Neural mechanisms underlying amblyopia

The nature of the neural basis of amblyopia is a matter of some debate. Recent neurophysiological data show correlates of amblyopia in the spatial properties of neurons in primary visual cortex. These neuronal deficits are probably the initial manifestation of the visual loss, but there are almost certainly additional deficits at higher levels of the visual pathways.

Neuronal correlates of amblyopia in the visual cortex of macaque monkeys with experimental strabismus and anisometropia

Amblyopia is a developmental disorder of pattern vision. After surgical creation of esotropic strabismus in the first weeks of life or after wearing -10 diopter contact lenses in one eye to simulate anisometropia during the first months of life, macaques often develop amblyopia. We studied the response properties of visual cortex neurons in six amblyopic macaques; three monkeys were anisometropic, and three were strabismic. In all monkeys, cortical binocularity was reduced. In anisometropes, the amblyopic eye influenced a relatively small proportion of cortical neurons; in strabismics, the influence of the two eyes was more nearly equal. The severity of amblyopia was related to the relative strength of the input of the amblyopic eye to the cortex only for the more seriously affected amblyopes. Measurements of the spatial frequency tuning and contrast sensitivity of cortical neurons showed few differences between the eyes for the three less severe amblyopes (two strabismic and one anisometropic). In the three more severely affected animals (one strabismic and two anisometropic), the optimal spatial frequency and spatial resolution of cortical neurons driven by the amblyopic eye were substantially and significantly lower than for neurons driven by the nonamblyopic eye. There were no reliable differences in neuronal contrast sensitivity between the eyes. A sample of neurons recorded from cortex representing the peripheral visual field showed no interocular differences, suggesting that the effects of amblyopia were more pronounced in portions of the cortex subserving foveal vision. Qualitatively, abnormalities in both the eye dominance and spatial properties of visual cortex neurons were related on a case-by-case basis to the depth of amblyopia. Quantitative analysis suggests, however, that these abnormalities alone do not explain the full range of visual deficits in amblyopia. Studies of extrastriate cortical areas may uncover further abnormalities that explain these deficits.

Peripheral and central factors limiting the development of contrast sensitivity in macaque monkeys

The aim of this study was to evaluate the contribution of peripheral and central factors to the development of visual sensitivity. We used the approach of (Pelli, 1981, 1990) to evaluate the hypothesis that intrinsic noise is high in infants compared with adults, and therefore sets an important limit on contrast sensitivity in infants. We measured contrast thresholds in the presence of various levels of dynamic spatiotemporal broadband noise in infant monkeys, and evaluated the developmental changes in contrast threshold and intrinsic noise. Our data show that intrinsic noise is high in infants and falls with contrast threshold during development. However, contrast thresholds in high-contrast noise also fall during development, although by a smaller amount. Therefore, while changes in intrinsic noise set an important limit on the development of contrast sensitivity across spatial frequencies, changes in non-additive sources of noise also contribute, particularly at high spatial frequencies. We interpret these results in terms of Pelli's hypothesis about the sources of additive and non-additive noise affecting visual detection. In these terms, additive noise reflects peripheral factors and non-additive noise reflects central ones. Our results suggest that changes in peripheral sources of noise represent an important limit for the development of visual sensitivity.

Pattern of ocular dominance columns and cytochrome oxidase activity in a macaque monkey with naturally occurring anisometropic amblyopia

Unilateral eyelid suture, a model for amblyopia induced by congenital cataract, produces shrinkage of the deprived eye's ocular dominance columns in the striate cortex. Loss of geniculocortical projections are thought to account for the poor vision in the amblyopic eye. It is uncertain whether ocular dominance columns become shrunken in other forms of amblyopia. We examined the striate cortex in a pigtailed macaque with natural anisometropia discovered at age 5 months. Amblyopia in the left eye was documented at 1 year by behavioral testing. At age 6 years, the left eye was injected with [3H]proline and the striate cortex was processed for autoradiography and cytochrome oxidase (CO). The ocular dominance columns in layer IVc labelled with [3H]proline were normal. CO staining showed a novel pattern of thin dark bands in layer IV. These bands occupied the core zones at the center of the ocular dominance columns. Their appearance resulted from relative loss of CO activity along the borders of the ocular dominance columns, regions specialized for binocular processing. These findings indicate that not all forms of amblyopia are accompanied by shrinkage of ocular dominance columns. The unusual pattern of CO staining in layer IVc reflected a subtle alteration in metabolic activity which may have resulted from impairment of binocular function in anisometropic amblyopia.

Effects of early-onset artificial strabismus on pursuit eye movements and on neuronal responses in area MT of macaque monkeys

In humans, esotropia of early onset is associated with a profound asymmetry in smooth pursuit eye movements. When viewing is monocular, targets are tracked well only when they are moving nasally with respect to the viewing eye. To determine whether this pursuit abnormality reflects an anomaly in cortical visual motion processing, we recorded eye movements and cortical neural responses in nonamblyopic monkeys made strabismic by surgery at the age of 10-60 d. Eye movement recordings revealed the same asymmetry in the monkeys' pursuit eye movements as in humans with early-onset esotropia. With monocular viewing, pursuit was much stronger for nasalward motion than for temporalward motion, especially for targets presented in the nasal visual field. However, for targets presented during ongoing pursuit, temporalward and nasalward image motion was equally effective in modulating eye movement. Single-unit recordings made from the same monkeys, under anesthesia, revealed that MT neurons were rarely driven binocularly, but otherwise had normal response properties. Most were directionally selective, and their direction preferences were uniformly distributed. Our neurophysiological and oculomotor measurements both suggest that the pursuit defect in these monkeys is not due to altered cortical visual motion processing. Rather, the asymmetry in pursuit may be a consequence of imbalances in the two eyes' inputs to the "downstream" areas responsible for the initiation of pursuit.

Vernier acuity in amblyopic and nonamblyopic children

We measured vernier acuity in normal and amblyopic children using a procedure that resembles a video game and is suitable for testing most children older than 4 years old. In this procedure, subjects align bars using two keys of a computer keypad. Following binocular practice trials, monocular vernier acuity was measured in 38 control children, 5 to 15 years old, and in 18 children with histories of amblyopia. Vernier acuity was defined as the standard deviation of adjusted position across a block of six trials. Vernier acuity improved as a function of age in the control subjects, indicating developmental improvement. Amblyopic subjects with strabismus (n = 5) and with both anisometropia and strabismus (n = 8) showed markedly impaired vernier acuity in their amblyopic eyes, with vernier acuitys four to five times larger than those of age-matched controls. However, the amblyopic subjects who were anisometropic (n = 5), without any history of strabismus, were not significantly different from control subjects in either eye. Testing with bars and gratings gave similar results. To our knowledge, this is the first study showing that vernier acuity deficits seen in adult amblyopes are also seen in child amblyopes.

Spatial frequency channels in experimentally strabismic monkeys revealed by oblique masking

Although the spatial vision deficits of human strabismic amblyopes have been well documented, surprisingly little is known about the mechanisms underlying their visual performance. In an effort to reveal the structure underlying the spatial vision deficits associated with strabismic amblyopia, we measured the performance of monkeys (Macaca nemestrina) with experimental strabismus in a contrast detection task with oblique masks. The masks were two adjacent identical oblique sine-wave gratings modulated in space by a Gaussian envelope. The target stimulus was a vertically oriented Gabor patch that appeared superimposed on the center of either the left or the right mask. The animals were trained by operant methods to indicate the location of the target. We measured detection thresholds in each eye independently for a large number of test and mask spatial frequencies. For each test spatial frequency, detection thresholds were elevated in the presence of the mask. The threshold evaluations showed a peak for a particular spatial frequency that was typically similar to the test spatial frequency. This pattern of results is consistent with the idea that the tests are detected by a discrete number of channels tuned to a narrow range of spatial frequencies. The data from the deviated eyes did not appear qualitatively different from those of the fellow eyes, and could be accounted by the same number of channels in both eyes. Quantitative estimates of the channels' characteristics revealed that the channels derived from the deviated eyes' data were similar to those yielded by the fellow eyes, but showed a reduction in their sensitivity to contrast.

Does experimentally-induced amblyopia cause hyperopia in monkeys?

We assessed refractive errors in 19 monkeys (Macaca nemestrina) raised with experimentally produced strabismus or unilateral defocus. These procedures resulted in hyperopic anisometropia in 10 monkeys. All 10 of the hyperopic animals were amblyopic; the amblyopic eye was always the more hyperopic eye. The degree of anisometropia was correlated with the degree of amblyopia. Hyperopic anisometropia did not develop in non-amblyopic animals. There was an association between early onset of visual abnormality and later development of hyperopic anisometropia. Since the refractive changes were correlated with changes in axial length and vitreous chamber depth, we suggest that amblyopia may cause alterations in eye growth and late-onset hyperopia.

Discrimination of position and contrast in amblyopic and peripheral vision

Many computational models of normal vernier acuity make predictions based on the just-noticeable contrast difference. Recently, Hu, Klein and Carney [(1993) Vision Research, 33, 1241-1258] compared vernier acuity and contrast discrimination (jnd) in normal foveal viewing using cosine gratings. In the jnd stimulus the test grating was added in-phase to the (sinusoidal) pedestal, whereas in the vernier stimulus the same test grating was added with an approx. 90 deg phase shift to the pedestal. In the present experiments, we measured thresholds for discriminating changes in relative position and changes in relative contrast for abutting, horizontal cosine gratings in a group of amblyopes using the Hu et al., test-pedestal approach. The approach here is to ask whether the reduced vernier acuity of amblyopes can be understood on the basis of reduced contrast sensitivity or contrast discrimination. Our results show that (i) abutting cosine vernier acuity is strongly dependent on stimulus contrast. (ii) In both anisometropic and strabismic amblyopes, abutting cosine vernier discrimination thresholds are elevated at all contrast levels, even after accounting for reduced target visibility, or contrast discrimination. (iii) For both strabismic and anisometropic amblyopes, the vernier Weber fraction is markedly degraded, while the contrast Weber fraction is normal or nearly so. (iv) In anisometropic amblyopes the elevated vernier thresholds are consistent with the observers' reduced cutoff spatial frequency, i.e. the loss can be accounted for on the basis of a shift in spatial scale. (v) In strabismic amblyopes and in the normal periphery, there appears to be an extra loss, which can not be accounted for by either reduced contrast sensitivity and contrast discrimination or by a shift in spatial scale. (vi) This extra loss cannot be quantitatively mimicked by "undersampling" the stimulus. (vii) Surprisingly, in some strabismics, and in the periphery, at relatively high spatial frequencies, vernier thresholds appear to lose their contrast dependence, suggesting the possibility that there may be qualitative differences between the normal fovea and these degraded visual systems. (viii) This contrast saturation can be mimicked by "undersampling" the target, or by introducing strips of mean luminance between the two vernier gratings, thus mimicking a "scotoma". Taken together with the preceding paper, our results suggest that the extra loss in position acuity of strabismic amblyopes and the normal periphery may be a consequence of noise at a second stage of processing, which selectively degrades position but not contrast discrimination.

Spatial scale shifts in amblyopia

We used a masking paradigm to uncover the properties of the mechanisms engaged by the amblyopic visual system for vernier acuity and line detection. Line vernier and line detection thresholds were measured in the presence of one-dimensional noise masks varying in orientation, spatial frequency content or contrast. Our results reveal that in both normal and amblyopic eyes, there is a bimodal orientation tuning function for vernier acuity, i.e. vernier acuity is most strongly masked by mask orientations approx. +/- 10 deg on either side of the target lines. In contrast, in both normal and amblyopic eyes, line detection is most strongly masked when the mask and line target have the same orientation. In the normal fovea, the spatial frequency tuning is bandpass, with a peak spatial frequency of about 10 c/deg. In the amblyopic eyes, the spatial tuning is similar in specificity; however the peak is shifted to lower spatial frequencies, suggesting a shift in the scale of spatial processing of line stimuli. For all of the amblyopic eyes, the increased line detection thresholds are approximately proportional to the shift in spatial scale. In anisometropic amblyopes, the (unmasked) vernier threshold is elevated in proportion to the shift in spatial scale; however in some amblyopes with constant strabismus the shift in spatial scale is not sufficient to account for the degraded vernier acuity. The "extra" increase in vernier thresholds associated with strabismus may be a consequence of a high degree of positional uncertainty which adds noise at a stage following the combination of filter responses.