Different temporal structure for form versus surface cortical color systems--evidence from chromatic non-linear VEP

Physiological studies of color processing have typically measured responses to spatially varying chromatic stimuli such as gratings, while psychophysical studies of color include color naming, color and light, as well as spatial and temporal chromatic sensitivities. This raises the question of whether we have one or several cortical color processing systems. Here we show from non-linear analysis of human visual evoked potentials (VEP) the presence of distinct and independent temporal signatures for form and surface color processing. Surface color stimuli produced most power in the second order Wiener kernel, indicative of a slowly recovering neural system, while chromatic form stimulation produced most power in the first order kernel (showing rapid recovery). We find end-spectral saturation-dependent signals, easily separable from achromatic signals for surface color stimuli. However physiological responses to form color stimuli, though varying somewhat with saturation, showed similar waveform components. Lastly, the spectral dependence of surface and form color VEP was different, with the surface color responses almost vanishing with yellow-grey isoluminant stimulation whereas the form color VEP shows robust recordable signals across all hues. Thus, surface and form colored stimuli engage different neural systems within cortex, pointing to the need to establish their relative contributions under the diverse chromatic stimulus conditions used in the literature.

Spatial and temporal dissociation of AQP4 and Kir4.1 expression during induction of refractive errors

PURPOSE

Spatial co-localization of aquaporin water channels (AQP4) and inwardly rectifying potassium ion channels (Kir4.1) on the endfeet regions of glial cells has been suggested as the basis of functionally interrelated mechanisms of osmoregulation in brain edema. The aim of this study was to investigate the spatial and temporal changes in the expression of AQP4 and Kir4.1 channels in an avascular retina during the first week of the optical induction of refractive errors.

METHODS

Three-day-old hatchling chicks were randomly assigned to three groups and either did not wear lenses or were monocularly goggled with +/-10D lenses for varying times up to 7 days before biometric assessment. Retinal tissue was prepared either for western blot analysis to show the presence of the AQP4 and Kir4.1 protein in the chick retina or for immunolocalization using AQP4 and Kir4.1 antibodies to determine the regional distribution and intensity of labeling during the induction of refractive errors.

RESULTS

As expected, ultrasonography demonstrated that all eyes showed rapid elongation post hatching. Negative lens-wearing eyes elongated faster than fellow eyes or normal non goggled eyes and became progressively more myopic with time post lensing. Positive lens-wearing eyes showed reduced ocular growth compared to normal controls and developed a hyperopic refraction. Quantitative immunohistochemistry revealed the upregulation of AQP4 channel expression on Müller cells in the retinal nerve fiber layer during the first 2 days of negative lens wear. Kir4.1 channel upregulation in the inner plexiform layer was only found on day 4 of positive lens wear during the development of refractive hyperopia.

CONCLUSIONS

These results indicate that the expression of AQP4 and Kir4.1 channels on Müller cells is associated with the changes in ocular volume seen during the induction of refractive errors. However, the sites of greatest expression and the temporal pattern of the upregulation of AQP4 and Kir4.1 were dissimilar, indicating a dissociation of AQP4 and Kir4.1 function during refractive error development. Increased AQP4 expression in the nerve fiber layer is suggested to contribute to the rapid axial elongation and movement of fluid into the vitreous cavity in the presence of minus lenses; whereas, upregulation of Kir4.1 channels appears to play a role in limiting axial elongation in the presence of plus lenses.

Magnocellular visual evoked potential delay with high autism spectrum quotient yields a neural mechanism for altered perception

Everyone has autistic characteristics to a greater or lesser degree, encapsulated in the Autism Spectrum Quotient, a scale that measures the degree to which an adult of normal intelligence displays traits associated with autism spectrum disorders. Recent psychophysical analyses of autism spectrum disorders point to superior local processing, and impaired or ignored global and contextual processing. The aim of this study was to test whether low- and high-scoring individuals on the Autism Spectrum Quotient differ on a measure of local and global processing, motion processing and visual pathway integrity. Fifteen low-scoring individuals and 14 high-scoring individuals derived from a normal population participated in the study. The results indicate that the initial cortical response to the magnocellular afferents is weaker at low contrast in the high autistic tendency group and that a second-order response, reflecting magnocellular activity, demonstrated a delay for high versus low scorers when the parvocellular pathway was also active in response to a high contrast stimulus. High-scoring individuals also demonstrated difficulty in identifying the global components of locally salient hierarchical Navon figures. Furthermore, cross-validated discriminant analysis, using four physiologically and three psychophysically derived parameters, correctly classified 83% of individuals who scored either high or low on the Autism Spectrum Quotient. These findings in the group scoring high on the Autism Spectrum Quotient indicate that a delay in primary visual/prestriate cortical processing of magnocellular input diminishes the advantage of its early arrival to primary visual cortex. This appears to be associated with impaired global visual perception, predicting with high accuracy behavioural tendencies associated with autism spectrum disorders. It has been proposed that perceptual impairment in autism may be attributed to a dysfunction of horizontal connections within early visual areas, presumably parvocellular in nature. However, the timing of such form processing aberrations is much later than the timing of abnormal magnocellular visual processing measured directly here. Thus it is proposed that a magnocellular processing delay decreases the ability of autistic individuals to benefit perceptually from feedback normally associated with the magnocellular advantage.

The race that precedes coactivation: development of multisensory facilitation in children

RATIONALE

The facilitating effect of multisensory integration on motor responses in adults is much larger than predicted by race-models and is in accordance with the idea of coactivation. However, the development of multisensory facilitation of endogenously driven motor processes and its relationship to the development of complex cognitive skills in school-age children is largely unexplored.

METHOD

Twenty adults and 95 children where allocated into six age groups: 6, 7, 8, 9, 10-11 and adults. Participants' motor reaction times (MRTs) and accuracy in response to the detection of auditory, visual and audiovisual stimuli were recorded. Children's reading accuracy and nonverbal IQ were also assessed.

RESULTS

In general, MRTs of children were significantly slower with greater variability than those of adults. Although the average level of multisensory facilitation was similar for all age groups, mean cumulative density functions (CDFs) showed that multisensory facilitation in 6 and 10-11-year-olds is within the predictive limits of race-models. Where coactivation was seen in the CDF of individual children it was not as strong or as consistent as that in adults. The degree of multisensory facilitation did not correlate with age, reading accuracy or IQ.

CONCLUSION

The average level of multisensory facilitation to endogenously driven motor responses does not change gradually with age nor is it related to intelligence or reading accuracy. In general, multisensory integration remains immature until 10-11 years of age and lies within the predicted confines of race-models.

A puzzle form of a non-verbal intelligence test gives significantly higher performance measures in children with severe intellectual disability

BACKGROUND

Assessment of 'potential intellectual ability' of children with severe intellectual disability (ID) is limited, as current tests designed for normal children do not maintain their interest. Thus a manual puzzle version of the Raven's Coloured Progressive Matrices (RCPM) was devised to appeal to the attentional and sensory preferences and language limitations of children with ID. It was hypothesized that performance on the book and manual puzzle forms would not differ for typically developing children but that children with ID would perform better on the puzzle form.

METHODS

The first study assessed the validity of this puzzle form of the RCPM for 76 typically developing children in a test-retest crossover design, with a 3 week interval between tests. A second study tested performance and completion rate for the puzzle form compared to the book form in a sample of 164 children with ID.

RESULTS

In the first study, no significant difference was found between performance on the puzzle and book forms in typically developing children, irrespective of the order of completion. The second study demonstrated a significantly higher performance and completion rate for the puzzle form compared to the book form in the ID population.

CONCLUSION

Similar performance on book and puzzle forms of the RCPM by typically developing children suggests that both forms measure the same construct. These findings suggest that the puzzle form does not require greater cognitive ability but demands sensory-motor attention and limits distraction in children with severe ID. Thus, we suggest the puzzle form of the RCPM is a more reliable measure of the non-verbal mentation of children with severe ID than the book form.

Potassium channel and NKCC cotransporter involvement in ocular refractive control mechanisms

Myopia affects well over 30% of adult humans globally. However, the underlying physiological mechanism is little understood. This study tested the hypothesis that ocular growth and refractive compensation to optical defocus can be controlled by manipulation of potassium and chloride ion-driven transretinal fluid movements to the choroid. Chicks were raised with +/-10D or zero power optical defocus rendering the focal plane of the eye in front of, behind, or at the level of the retinal photoreceptors respectively. Intravitreal injections of barium chloride, a non-specific inhibitor of potassium channels in the retina and RPE or bumetanide, a selective inhibitor of the sodium-potassium-chloride cotransporter were made, targeting fluid control mechanisms. Comparison of refractive compensation to 5 mM Ba(2+) and 10(-5) M bumetanide compared with control saline injected eyes shows significant change for both positive and negative lens defocus for Ba(2+) but significant change only for negative lens defocus with bumetanide (Rx(SAL)(-10D) = -8.6 +/- .9 D; Rx(Ba2+)(-10D) = -2.9 +/- .9 D; Rx(Bum)(-10D) = -2.9 +/- .9 D; Rx(SAL)(+10D) = +8.2 +/- .9 D; Rx(Ba2+)(+10D) = +2.8 +/- 1.3 D; Rx(Bum)(+10D) = +8.0 +/- .7 D). Vitreous chamber depths showed a main effect for drug conditions with less depth change in response to defocus shown for Ba(2+) relative to Saline, while bumetanide injected eyes showed a trend to increased depth without a significant interaction with applied defocus. The results indicate that both K channels and the NKCC cotransporter play a role in refractive compensation with NKCC blockade showing far more specificity for negative, compared with positive, lens defocus. Probable sites of action relevant to refractive control include the apical retinal pigment epithelium membrane and the photoreceptor/ON bipolar synapse. The similarities between the biometric effects of NKCC inhibition and biometric reports of the blockade of the retinal ON response, suggest a possible common mechanism. The selective inhibition of refractive compensation to negative lens in chick by loop diuretics such as bumetanide suggests that these drugs may be effective in the therapeutic management of human myopia.

A role for aquaporin-4 during induction of form deprivation myopia in chick

PURPOSE

Aquaporins (AQP) form a family of specialized water channels known to transport water across cell membranes and reduce osmotic gradients. The isoform AQP4 is highly expressed in the astroglia of the brain and Müller cells in the retina. In the brain, AQP4 play a role in the control of cerebral edema by shunting excess fluid into blood vessels and by upregulating during conditions of hyperosmolarity. Thus, on the basis of the hyperosmolarity seen across the retina and choroid of hatchling chickens made myopic by form deprivation (FD), we predicted an upregulation of retinal AQP4 expression during induction of myopia.

METHODS

Two-day-old hatchling chicks were monocularly form-deprived for 48, 72, or 96 h, and then after biometric assessment, the eyes of these animals and the normal controls of the same age were enucleated. Retinal tissue was prepared either for western blot analysis to show the presence of the AQP4 protein in the chick retina or for immunolocalization using polyclonal AQP4 antibodies to determine regional distribution and intensity of labeling during the induction of form deprivation myopia (FDM).

RESULTS

As expected, ultrasonography demonstrated that all post hatchling eyes showed rapid elongation with occluded eyes elongating faster than fellow eyes or normal controls and becoming progressively more myopic with the duration of visual deprivation. Western blot analyses revealed an approximately 30 kDa band immunoreactive for AQP4 protein and confirmed the presence of AQP4 in chicks. Immunohistochemical staining showed the greatest positive immunoreactivity for antibodies to AQP4 in the inner retina along the vitreoretinal interface, nerve fiber layer, ganglion cell layer, and inner plexiform layer in all animals. The control eyes showed relatively constant levels of AQP4 expression until day 5 after which the level appeared to decrease. By comparison, positive AQP4 immunoreactivity in the nerve fiber layer increased significantly over the first 48 h in form-deprived eyes and in fellow eyes and then decreased over the next 48 h but not to the level of expression in the normal untreated eyes.

CONCLUSIONS

This is the first study to demonstrate the presence of AQP4 protein in the chick retina and to associate AQP4 expression in the inner retina with the initiation of form deprivation and the period of fastest axial elongation. This increased expression of AQP4 channels near the vitread border during the time of rapid growth suggests a role for AQP4 as a conduit for movement of retinal fluid into the vitreous in form-deprived chicks.

Evidence for Fast Signals and Later Processing in Human V1/V2 and V5/MT+: A TMS Study of Motion Perception

Evidence from human and primate studies suggests that fast visual processing may utilize signals projecting from primary visual cortex (V1) through the dorsal stream, to area V5/MT+ or beyond and subsequently back into V1. This coincides with the arrival of parvocellular signals en route to the ventral pathway and infero-temporal cortex. Such evidence suggests that the dorsal stream region V5/MT+ is activated rapidly through the traditional hierarchical pathway and also via a less-well-established direct signal to V5/MT+ bypassing V1. To test this, 16 healthy humans underwent transcranial magnetic stimulation (TMS) of V1/V2 and V5/MT+ while performing a motion-direction detection task. A three-alternate forced-choice design (left/right motion, stationary) allowed analysis of the quality of errors made, in addition to the more usual performance measures. Transient disruption of V1/V2 and V5/MT+ significantly reduced accuracy when TMS was applied at or near motion onset. Most participants also showed disrupted performance with TMS application over V1/V2 ∼125 ms post motion onset, and significantly reduced accuracy at 158 ms with V5/MT+ stimulation. The two periods of disruption with V1/V2 TMS are suggestive of feedforward/feedback models, although the earlier period of disruption has not been reported in previous TMS studies. Very early activation of V5/MT+, evidenced by diminished accuracy and reduced perception of motion after TMS may be indicative of a thalamic-extrastriate pathway in addition to the traditionally expected later period of processing. A profound disruption of performance prestimulus onset is more likely to reflect disruption of top-down expectancy than disruption of visual processing.

The visual attentional blink reflects constraints on temporal visual processing, not just a lapse of visual memory

BACKGROUND

Most theories related to the attentional blink (AB), defined as the refractory period of 300 to 500 msecs following a conscious visual identification, during which a second detection task is impaired, argue that limitations of 'visual working memory' (VWM) underlie the phenomenon rather than suppression of attention. The current study examined the relationship between performance on an AB task and classical measures of visual and auditory memory and executive function (EF).

METHODS

Fifty-one adult volunteers completed the AB task but only 27 of these also completed an extensive battery of traditional measures of short-term and working memory, processing speed and EF. Correlation and regression methods were used to analyse the data.

RESULTS

Bivariate correlation showed a significant relationship between AB duration and non-verbal intelligence and processing speed. Multiple regression analysis demonstrated that four factors (non-verbal intelligence, letter-number sequencing, visual search and visuo-spatial recognition memory) explained 40 per cent of the variance, with better reasoning skills and letter-number sequencing associated with a shorter AB, and better visuo-spatial recognition memory associated with a longer AB.

CONCLUSION

The positive beta weight in the model for visuo-spatial recognition memory may reflect that the ability to later recall a target depends on its depth of encoding. On the other hand, the negative betas in the regression model for verbal intelligence and processing speed indicate that AB duration may be a potentially useful measure of temporal rate of visual processing. Finally, the lack of any relationship between the AB and executive function performance measures used suggests that the mechanism of the AB is primarily confined to early sensory processing and lower level working memory.

A role for the 'magnocellular advantage' in visual impairments in neurodevelopmental and psychiatric disorders

Evidence exists implicating abnormal visual information processing and visually driven attention in a number of neurodevelopmental and psychiatric disorders, suggesting that research into such disorders may benefit from a better understanding of more recent advances in visual system processing. A new integrated model of visual processing based on primate single cell and human electrophysiology may provide a framework, to understand how the visual system is involved, by implicating the magnocellular pathway's role in driving attentional mechanisms in higher-order cortical regions, what we term the 'magnocellular advantage'. Evidence is also presented demonstrating visual processing occurs considerably faster than previously assumed, and emphasising the importance of top-down feedback signals into primary visual cortex, as well as considering the possibility of lateral connections from dorsal to ventral visual areas. Such organisation is argued to be important for future research highlighting visual aspects of impairment in disorders as diverse as schizophrenia and autism.

Wots that werd? Pseudowords (non-words) may be a misleading measure of phonological skills in young learner readers

Pseudoword (non-word) reading tasks are a commonly used measure of phonological processing across diverse fields of reading research. However, whether pseudoword reading gives any more information about phonological processing in young learner readers than does the reading of real words has seldom been considered. Here we show that pseudoword and real word reading are so strongly correlated (r=0.94) in the first 4 years of school as to be representative of the same construct. Two of the subskills of phonological processing, phonological awareness and rapid automatic naming also predict almost identical amounts of variance in pseudoword and real word reading. A divergence in the correlations between word and pseudoword reading and phonological awareness and rapid naming only emerges in the fourth year, while a significant correlation between phonological awareness and rapid automatic naming is evident only in the first year of schooling. Thus these results suggest that, at least in young children learning to read, care should be taken when using pseudoword reading to measure either phonological processing ability or phonological awareness as this may misinform the choice of therapy for a child showing symptoms of dyslexia.

Ionic control of ocular growth and refractive change

The physiological mechanisms underlying the abnormal vitreal and ocular growth and myopic refractive errors induced under conditions of visual form deprivation in many animal species, including humans, are unknown. This study demonstrates, using energy dispersive x-ray microanalysis, a systematic pattern of changes in the elemental distribution of K, Na, and Cl across the entire retina in experimental form deprivation myopia and in the 5 days required for refractive normalization after occluder removal. In our report we link the ionic environment associated with physiological activity of the retina under a translucent occluder to refractive change and describe large but reversible environmentally driven increases in potassium, sodium, and chloride abundances in the neural retina. Our results are consistent with the notion of ionically driven fluid movements as the vector underlying the myopic increase in ocular size. New treatments for myopia, which currently affects nearly half of the human population, may result.

Pre- and post-critical period induced reduction of Cat-301 immunoreactivity in the lateral geniculate nucleus and visual cortex of cats Y-blocked as adults or made strabismic as kittens

PURPOSE

To test the post-critical period stability of perineuronal nets by comparing the expression of antigens on aggrecan (a chondroitin sulfate proteoglycan (CSPG) recognized by the monoclonal antibody Cat-301) in the lateral geniculate nucleus (LGN) and striate cortex (A17) of adult Y-blocked cats and cats made strabismic and amblyopic as kittens. The comparison tested the idea that pre- and post-critical period loss of synchronous activity would differentially affect the perineuronal net of Y-type neurons in LGN and A17.

METHODS

Seven adult cats, two normal, three convergent strabismic amblyopic, and two monocularly Y-blocked cats, were used in this study. The strabismic amblyopic cats had been made monocularly esotropic (by tenotomy) at 14 days of age. The Y-block was created acutely by a pressure cuff placed on the optic nerve behind the left eye in adult cats. The efficacy of both procedures was tested electrophysiologically. Frontal frozen sections were incubated with the Cat-301 antibody and the labeling visualized using a DAB kit. The sections were counterstained with cresyl violet. In each section, Cat-301-stained cells with well-defined nucleoli were counted under a 20x objective with a computer-based quantitative microscope image analysis system.

RESULTS

The percentage of positively labeled cells was reduced in LGN laminae that received input from the deviated eye in amblyopic cats and from the pressure-blocked eye in Y-blocked cats compared with normal cats. Surprisingly, the non-blocked laminae of the Y-blocked cats also showed a significant reduction in positively labeled neurons when compared to normals or to strabismic cats. In the visual cortex of both hemispheres of strabismic and Y-blocked cats, the density of immunopositive neurons was significantly reduced compared with normal. The effect was most pronounced in layers IV-VI for Y-blocked cats and in layer IV for strabismic amblyopic cats.

CONCLUSIONS

The results demonstrate that surface expression of aggrecan in adult cat LGN and A17 of adult cat is reduced both by chronic developmental loss of synchronous input from the two eyes and by acute changes in synchronous input in adulthood. Thus both pre- and post-critical plasticity in the expression of epitopes of aggrecan can be demonstrated. The uniform distribution of Cat-301 labeling tangentially within cortical layers of strabismic amblyopic cats indicates that the reduction in immunoreactivity observed with strabismus induced early in life is not simply eye-specific. Indeed, comparison of the immunopositivity of Y-blocked and strabismic animals, both in LGN and cortex, suggests that even after the critical period is ended, the physical removal of monocular Y-type afferent activity and weakening of binocular feedback connections between cortex and thalamus can alter the stability of the perineuronal nets surrounding the affected neurons.

Parietal function in good and poor readers

BACKGROUND

While there are many psychophysical reports of impaired magnocellular pathway function in developmental dyslexia (DD), few have investigated parietal function, the major projection of this pathway, in good and poor readers closely matched for nonverbal intelligence. In view of new feedforward-feedback theories of visual processing, impaired magnocellular function raises the question of whether all visually-driven functions or only those associated with parietal cortex functions are equally impaired and if so, whether parietal performance is more closely related to general ability levels than reading ability.

METHODS

Reading accuracy and performance on psychophysical tasks purported to selectively activate parietal cortex such as motion sensitivity, attentional tracking, and spatial localization was compared in 17 children with DD, 16 younger reading-age matched (RA) control children, and 46 good readers of similar chronological-age (CA) divided into CA-HighIQ and a CA-LowIQ matched to DD group nonverbal IQ.

RESULTS

In the age-matched groups no significant differences were found between DD and CA controls on any of the tasks relating to parietal function, although performance of the DD group and their nonverbal IQ scores was always lower. As expected, CA and RA group comparisons indicated purported parietal functioning improves with age. No difference in performance was seen on any of the parietally driven tasks between the DD and age-nonverbal IQ matched groups, whereas performance differentiated the DD group from the age-matched, higher nonverbal IQ group on several such tasks. An unexpected statistical difference in performance between lower reading age (DD and RA children) and all higher reading age (CA) children was seen on a test of chromatic sensitivity, whereas when high and low nonverbal IQ normal readers were compared performance was not different

CONCLUSION

The results indicate that performance on purported parietal functions improves with age and may be more associated with nonverbal mentation than reading accuracy. Performance on a cognitively demanding task, traditionally considered to rely on ventral stream functions, was more related to reading accuracy.

Low frequency temporal modulation of light promotes a myopic shift in refractive compensation to all spectacle lenses

Emmetropization, the process by which ocular growth of young animals adapts to ensure focussed retinal images, can be disrupted by high frequency flicker, causing a hypermetropic shift. Emmetropization can also be disrupted differentially, in a sign dependent manner, by pharmacological alteration of the balance of activation of the ON and OFF retinal sub-systems in normal light or by rearing in an environment with a moving spatiotemporally varied diamond pattern (yielding local sawtooth illumination on the retina). Thus the aim of this experiment was to determine whether low frequency temporal modulation alone was sufficient to cause defocus sign-dependent interference with compensation. Chicks were reared for 6 or 7 days with monocular +/-10 D, 0 D, or No Lenses in a 12h light/dark cycle. Luminance of the environment was temporally modulated during the light cycle with a non-square wave profile pulse of 250 msec duration, with the illumination fluctuating between 1.5 and 180 lux at 1 Hz, 2 Hz, 4 Hz or with no flicker (0 Hz-180 lux). Final refractive state and ocular dimensions, measured using retinoscopy and A-scan ultrasonography, demonstrated that in the absence of temporal luminance modulation (0 Hz), chicks compensated to induced defocus in the expected sign-dependent manner. However, under 1, 2 and 4 Hz flickering light conditions, there was an overall myopic offset of approximately 6D across lens groups with refractive compensation to positive lenses more strongly inhibited. This myopic offset was reflected by increases in the depth of both vitreous and anterior chambers. However, luminance modulation had no effect on refraction or ocular parameters in the No Lens conditions. This is a hitherto unreported strong interaction between lens wear and low frequency temporally modulated light, with the refractive compensation mechanism being overridden by a generalized myopic shift.

Measurement error: Implications for diagnosis and discrepancy models of developmental dyslexia

The diagnosis of developmental dyslexia (DD) is reliant on a discrepancy between intellectual functioning and reading achievement. Discrepancy-based formulae have frequently been employed to establish the significance of the difference between 'intelligence' and 'actual' reading achievement. These formulae, however, often fail to take into consideration test reliability and the error associated with a single test score. This paper provides an illustration of the potential effects that test reliability and measurement error can have on the diagnosis of dyslexia, with particular reference to discrepancy models. The roles of reliability and standard error of measurement (SEM) in classic test theory are also briefly reviewed. This is followed by illustrations of how SEM and test reliability can aid with the interpretation of a simple discrepancy-based formula of DD. It is proposed that a lack of consideration of test theory in the use of discrepancy-based models of DD can lead to misdiagnosis (both false positives and false negatives). Further, misdiagnosis in research samples affects reproducibility and generalizability of findings. This in turn, may explain current inconsistencies in research on the perceptual, sensory, and motor correlates of dyslexia.

Changes in ocular accommodation when shifting between global and local attention

BACKGROUND

The link between cognitive changes in attentional focus and the physiological parameters of the eye is not well understood. The aim of the current work was to examine the role of ocular accommodation, that is, the process of changing the shape of the crystalline lens in order to focus an image onto the retina, in active shifts of visual attention between global and local information.

METHOD

Ten adult participants (aged 19 to 27) viewed sequences of complex global/local figures presented at the same location. They were asked to identify either a global or local red target letter and to look for either a global or local letter x (probe) in the sequence following. Target and probe items were separated by a temporal gap of approximately one second. Refraction was measured using a Canon Autorefractor R1 at the time of target appearance and again at the time of probe presentation. Particular interest was paid to trials where participants were required to shift attention either from a local to global level or in the reverse direction. The difference between the two measurements of refraction gave a 'change score' which provided an indication of the change of accommodation.

RESULTS

No significant change in refraction was observed when shifting from the local to the global condition. A mean change in refraction of -0.128 dioptres was observed for the global to local condition. The 95 per cent confidence interval for this difference did not overlap zero, indicating a significant change in refraction, which was attributed to an increase in accommodation.

DISCUSSION

The results suggest that while a change in accommodation occurs in shifting attention from the global to the local aspect of the complex figures, a similar change is not observed in the reverse direction. These results are consistent with the hypothesis that it is more difficult to restrict attentional focus than it is to widen it.

Structural and Elemental Evidence for Edema in the Retina, Retinal Pigment Epithelium, and Choroid during Recovery from Experimentally Induced Myopia

PURPOSE

The purpose of this study was to monitor temporal changes in the retina, retinal pigment epithelium (RPE), and choroid of chick eyes using biometric, ultrastructural, and elemental microanalysis techniques as a means of visualizing more detailed signs of the physiological processes underlying choroidal expansion and refractive normalization during recovery from form deprivation.

METHODS

Axial dimensions and refractions were measured on form-deprived and fellow eyes of 117 experimental chickens reared with monocular translucent occlusion from days 1 to 15 and given different lengths of visual experience (T = 0-144 hours) before death. Tissue was analyzed ultrastructurally by electron microscopy and relative sodium (Na) and chloride (Cl) ion abundances, by using x-ray microanalysis to determine changes in the presence of these indicators of tissue hydration.

RESULTS

Refractive error decreased from more than 20 D of myopia almost linearly over the first 144 hours after occlusion. Concurrent changes in thickness in the retina, RPE, and choroid were seen as a series of thickness increases and edema, which returned to normal thickness, first in the retina, and did not reach maximum until 3 days after occluder removal in the choroid. In freeze-dried tissue, Na and Cl ion concentrations were greatest in the RPE photoreceptor outer segments and extravascular choroid at T = 0, decreasing toward fellow eye levels by T = 48 in the RPE and choroid. Na and Cl ion abundances in the frozen lymph of choroidal lymphatics were nearly at control levels (T = 0) and increased later as the vessels became more distended after the extravascular edema became significant.

CONCLUSIONS

The results suggest that occluder removal induces edema across the retina and choroid and that this fluid may be the vector eliciting choroidal expansion during recovery from form deprivation possibly driven by the hyperosmolarity in the choroid, RPE, and photoreceptor outer segments that accompanies deprivation.

Clinical application of the multifocal visual evoked potential

BACKGROUND

Measures of visual function thresholds such as visual acuity and visual fields are generally dependent on subjective responses and assume maintenance of fixation, attention and motivation. In the young, elderly, cognitively impaired or malingering populations, such measures may be inaccurate or difficult to obtain. The Visual Evoked Response Imaging System (VERIS) has been claimed to give more objective topographic recordings of retinal and cortical function. This paper aims to explore the adequacy of this technique in four unusual, unrelated, clinically difficult cases.

METHODS

Multifocal visual evoked potentials (mfVEPs) recorded on the VERIS System 3.01 are used to assess visual function in four cases with contradictory clinical findings or unreliable subjective responses.

RESULTS

Patient 1 had sustained a head injury and had normal ocular and pupil examination but light perception in the right eye and 6/5 acuity in the left. Multifocal VEPs showed a marked depression of the right visual field with little macular response. Patient 2 had sustained a head injury, had a left field hemianopia, possible macular sparing and loss of much of the right field, reduced but variable visual acuities, good near vision and normal ocular fundi. Multifocal VEPs showed a severe depression in both visual fields (L more than R) with little macular response. Patient 3 had a left optic nerve meningioma and experienced great difficulty with visual field assessment. mfVEPs showed a bilateral depression in the superior field particularly the left field, with a larger deficit in the left eye. Patient 4 had unexplained visual acuity and peripheral field deficits. mfVEP results were inconclusive in this case.

DISCUSSION

Where there is difficulty performing traditional techniques or conflicting clinical findings, mfVEPs may provide additional objective information to aid in the assessment of patients.