Residual binocular interaction in stereoblind humans

Stereodeficient subjects show substantial differences in interocular transfer of two motion adaptation aftereffects

Interocular transfer (IOT) of two motion aftereffects was examined in subjects with normal and deficient stereopsis. Normal subjects showed complete (100%) IOT of motion adaptation on coherent motion thresholds, but only partial IOT of a conventional motion aftereffect, supporting suggestions that the latter aftereffect may be mediated at a lower level in the visual pathway than the extrastriate regions implicated in processing coherent motion. This idea was strengthened by an even greater dissociation between the extent of IOT of the two aftereffects among stereodeficient subjects who exhibited very low IOT of the conventional motion aftereffect, but high (> 87%) IOT of the coherence motion aftereffect.

Dichoptic luminance beat visual evoked potentials in the assessment of binocularity in children

Direct evidence of a distinct cortical binocular pathway has been provided by the production of nonlinear (difference) beats from dichoptic luminance stimulation in stereonormal adults and the absence or diminution of these beats in stereoblind subjects. We have investigated a clinically useful application of this technique in a pediatric population with potentially abnormal binocular vision. We recorded dichoptic luminance beat visual evoked potentials (VEPs) from 20 children (ages 7 months to 8 years) with abnormal binocular ability secondary to strabismus and/or amblyopia and compared this to a control group of 20 children with normal binocularity. Stereoblind children generated significantly lower dichoptic signal-to-noise ratios than stereonormal children (P < .001). Responses to monoptic multifrequency flicker were not significantly different between the two groups (P = .936). This dichoptic VEP can be performed quickly and easily on young children and gives a quantitative assessment of cortical binocularity that may not be determinable by standard clinical methods. This technique may also prove useful for the preoperative gradation of binocular potential and prediction of postoperative binocular fusion.

Visual callosal connections and strabismus

Strabismus is a condition that exists when the visual axes of the two eyes fail to intersect at the fixation point under binocular viewing conditions. When it occurs in mammals during the critical period which corresponds to the period of maximal plasticity early in life, strabismus is known to induce both morphological anomalies and abnormal connections from the retina to the cortex; it further leads to binocular neural changes and to spatial vision deficits, especially at the cortical level. After a brief review of the already known data about the consequences of early strabismus in cats, monkeys and humans on the development of the visual system and of visual perception, new data are presented here concerning interhemispheric connections in the cat. In normally-reared kittens, visual callosal transfer is shown to be almost adult-like as soon as 12 days after birth: it is almost limited to the 17/18 border of the visual cortex when using visual stimulations in spite of the presence of still numerous juvenile exuberant callosal projections. In contrast, callosal transfer of visual information is extended to both areas 17 and 18 after strabismus, leading to the conclusion that at least some juvenile exuberant callosal projections are not only anatomically but also functionally stabilized after such an oculomotor disease. The possibility that similar abnormalities might be present in monkeys and humans is discussed.

Complete interocular transfer of motion aftereffect with flickering test

It has been suggested that motion aftereffect with static test patterns (static MAE) reflects activities at a lower level system that dominantly processes first-order motion, while MAE with a directionally ambiguous test (flicker MAE) reveals a higher level system where second-order motion signals as well as first-order signals are available. To test this hypothesis, we examined interocular transfer of static and flicker MAE. Flicker MAE should transfer more efficiently than static MAE if it occurs at a higher level system. In the first experiment, the adaptation stimulus was a drifting luminance grating (first-order motion), or a drifting grating defined by flicker or texture difference (second-order motion). The test stimulus was a luminance grating, either static or counterphasing. The results indicated that static MAE, which was induced only by first-order motion, transferred partially, as has been reported in previous studies, but the transfer of flicker MAE was nearly perfect with either first- or second-order adaptation stimuli. The second experiment with varied adaptation contrast indicated that this complete transfer was not due to a ceiling effect. These results supported the hypothesis that the underlying mechanism for flicker MAE is located at a level higher than the mechanism for static MAE.

Complete interocular transfer of motion adaptation effects on motion coherence thresholds

The binocularity of visual mechanisms in humans can be investigated by measuring the interocular transfer (IOT) of visual aftereffects. Cells in extrastriate visual areas of macaque, e.g. the middle temporal (MT) area, are uniformly binocular, whereas cells in striate area V1 vary in their degree of binocularity. Therefore, IOT of aftereffects mediated by extrastriate cortex should be nearly complete compared to the partial transfer (about 70%) found for aftereffects thought to be mediated by V1. If MT and other extrastriate areas play a significant role in motion perception, then IOT of motion adaptation aftereffects on the perception of moving stimuli should be nearly complete. After motion adaptation, the perception of global movement direction in partially coherent random dot kinematograms (RDKs) is temporarily impaired if the predominant direction of dots in the test stimulus matches that of the adaptation stimulus. I measured the IOT of this motion incoherence aftereffect in four observers. Post-adaptation motion coherence thresholds were elevated equally for interocular and intraocular adaptation, indicating complete transfer of the aftereffect. Measurement of the classical motion aftereffect using the same stimuli and conditions showed partial or absent transfer. These data support the idea that extrastriate areas play a key role in motion perception and suggest that the motion incoherence aftereffect and the classical motion aftereffect may involve different mechanisms.

Measurement of visual aftereffects and inferences about binocular mechanisms in human vision

There is conflicting evidence concerning the characteristics of binocular channels in the human visual system with respect to the existence of a 'pure' binocular channel that responds only to simultaneous stimulation of both eyes. Four experiments were conducted to resolve these discrepancies and to evaluate the evidence for the existence of such an exclusive binocular channel. In the first three studies, tilt aftereffects were measured after monocular adaptation. The relative sizes of the direct, interocularly transferred, and binocular aftereffects were not influenced by the configuration of the adapting pattern (experiment 1), or by the eye used for adaptation (experiment 2). There were also consistent interobserver differences in the relative sizes of the aftereffect seen after monocular adaptation (experiment 3). Taken together, these data raise questions about the appropriateness of a monocular adaptation paradigm for evaluating the presence of a pure binocular channel in observers with normal binocular vision. In experiment 4, in which the paradigm of alternating monocular adaptation was used, data were obtained that are consistent with the presence of a pure binocular channel.

Binocular combination of contrast signals

We studied the detectability of dichoptically presented vertical grating patterns that varied in the ratio of the contrasts presented to the two eyes. The resulting threshold data fall on a binocular summation contour well described by a power summation equation with an exponent near 2. We studied the effect of adding one-dimensional visual noise, either correlated or uncorrelated between the eyes, to the grating patterns. The addition of uncorrelated noise elevated thresholds uniformly for all interocular ratios, while correlated noise elevated thresholds for stimuli whose ratios were near 1 more than thresholds for other stimuli. We also examined the effects of monocular adaptation to a high-contrast grating on the form of the summation contour. Such adaptation elevates threshold in a manner that varies continuously with the interocular contrast ratio of the test targets, and increases the amount of binocular summation. Each of several current models can explain some of our results, but no one of them seems capable of accounting for all three sets of data. We therefore develop a new multiple-channel model, the distribution model, which postulates a family of linear binocular channels that vary in their sensitivities to the two monocular inputs. This model can account for our data and those of others concerning binocular summation, masking, adaptation and interocular transfer. We conclude that there exists a system of ocular dominance channels in the human visual system.

Binocular beats: psychophysical studies of binocular interaction in normal and stereoblind humans

We describe a psychophysical method for assessing binocular integration using dichoptically-presented uniform fields. By temporally modulating uniform field luminances at different frequencies between the eyes, a rhythmic beat is produced--a visual percept characterized by undulations in luminance at a frequency equal to the arithmetic difference between the two monocular stimulus frequencies. Using a signal detection paradigm, we studied the beat as a function of modulation depth in normal and binocularly deficient subjects. Normal subjects easily detected the beat, even at low modulation depths, while stereoblind subjects (with stereoacuity worse than 2000 sec arc) failed to detect beats at any modulation depth or with any combination of stimulus frequencies tested. The beat provides evidence for the confluence of monocular signals into binocular integrating mechanisms. Our results therefore suggest that the status of functioning binocular mechanisms is related to the detectability of the beat. This uniform-field stimulus does not require accurate accommodation, fixation, vergence or high spatial resolution, thus making this technique particularly attractive for the study of binocular interaction in developing infants and in binocularly deficient adults.

Selective losses in binocular vision in anisometropic amblyopes

Human anisometropic amblyopes typically exhibit reduced contrast sensitivity in the amblyopic eye, especially at higher spatial frequencies. We determined whether this spatial frequency selective loss in contrast sensitivity is accompanied by selective losses in binocular function. Binocular summation (the improvement in one eye's detection performance produced by a subthreshold pattern presented to the fellow eye) was measured at several spatial frequencies. Normal observers exhibited equivalent binouclar summation at all spatial frequencies, whereas all anisometropic amblyopes exhibited normal summation at low spatial frequencies but none at high spatial frequencies. Stereoacuity (minimum resolvable disparity) was also measured as a function of spatial frequency. For normal observers, stereoacuity was best at the highest spatial frequency; for anisometropes stereoacuity was normal at low spatial frequencies, subnormal at intermediate spatial frequencies, and unmeasurable at higher spatial frequencies. Anisometropia may represent a form of selective binocular deprivation that affects neural mechanisms underlying binocular summation and stereopsis.

The binocular contribution to monocular optokinetic nystagmus and after nystagmus asymmetries in humans

Stereoblind subjects show marked monocular asymmetries in both OKN and OKAN, while normal subjects produce more balanced nystagmus. There is a negative relationship between the magnitude of the asymmetries and the magnitude of a binocular contribution to OKN production as measured with dichoptic, strobe illumination. A second experiment with normal subjects corroborated this relationship, revealing a small, but significant asymmetry in these subjects' OKAN. Further, there was no correlation between the monocular nystagmus asymmetries and stereoacuity. The results are explained in terms of subcortical binocular mechanisms.

On the relation of stereoacuity to interocular transfer of the motion and the tilt aftereffects

Interocular transfer of both the motion and the tilt aftereffect were tested in 43 subjects with varying degrees of stereopsis, ranging from normal to stereoblind. Although there was an overall tendency for transfer to diminish with decreasing stereovision, stereoblind subjects always showed some transfer of the tilt aftereffect, and sometimes also of the motion aftereffect, while some subjects with normal stereothresholds had greatly reduced or no interocular transfer. No quantitative correlation between stereothresholds and amount of transfer could be found. The results indicate that there is no simple relationship between interocular transfer, stereopsis and cortical binocularity, as had been suggested previously.

Binocular interactions in normal and anomalous binocular vision: effects of flicker

Temporal modulation thresholds were determined for monocular viewing and for binocular viewing of stimuli presented in phase or in counterphase to each eye of observers with normal binocular vision and those lacking stereopsis. The results showed that in individuals with normal binocular vision sensitivity was much greater for in-phase than for counterphase stimulation at low temporal frequencies, but that this superiority declined at higher temporal frequencies. Averaged across frequencies, binocular sensitivity for in-phase stimulation was 40-50% higher than monocular sensitivity. In contrast, in the observers lacking stereopsis the ratios of binocular in-phase/monocular sensitivity averaged 1.02, and there were no significant differences in sensitivity to in-phase and counterphase stimulation. This failure of binocular integration at threshold does not result from differences in transmission time between the 2 eyes. However, while individuals lacking stereopsis showed an absence of binocular interaction for uniform-field flicker at threshold, they showed suprathreshold dichoptic temporal frequency masking which was similar to that found in normal persons.

A study of interocular transfer of spatial adaptation

The threshold-elevation aftereffect was measured ipsiocularly and interocularly following grating adaptation of one eye. The functions relating aftereffect magnitude to adapting contrast and adaptation time were similar under the two testing conditions, with interocular transfer remaining fairly constant; decay times were similar for ipsiocular and interocular aftereffects of comparable magnitude, and their frequency selectivities were the same. It is concluded that the stimulus-response characteristics of monocular and binocular spatial channels are fundamentally similar.