Dysfunction of early-stage visual processing in schizophrenia.

OBJECTIVE: Schizophrenia is associated with deficits in higher-order processing of visual information. This study evaluated the integrity of early visual processing in order to evaluate the overall pattern of visual dysfunction in schizophrenia. METHOD: Steady-state visual-evoked potential responses were recorded over the occipital cortex in patients with schizophrenia and in age- and sex-matched comparison volunteers. Visual-evoked potentials were obtained for stimuli composed of isolated squares that were modulated sinusoidally in luminance contrast, number of squares, or chromatic contrast in order to emphasize magnocellular or parvocellular visual pathway activity. RESULTS: Responses of patients to magnocellular-biased stimuli were significantly lower than those of comparison volunteers. These lower response levels were observed in conditions using both low luminance contrast and large squares that biased processing toward the magnocellular pathway. In contrast, responses to stimuli that biased processing toward the parvocellular pathway were not significantly different between schizophrenia patients and comparison volunteers. A significant interaction of group and stimulus type was observed in the condition using low luminance contrast. CONCLUSIONS: These findings suggest a dysfunction of lower-level visual pathways, which was more prominent for magnocellular than parvocellular biased stimuli. The magnocellular pathway helps in orienting toward salient stimuli. A magnocellular pathway deficit could contribute to higher-level visual cognitive deficits in schizophrenia.     

On identifying magnocellular and parvocellular responses on the basis of contrast-response functions

It has been proposed that magnocellular and parvocellular sensitivity in schizophrenic individuals can be assessed using steady-state visually evoked potentials (VEPs) to either low-contrast stimuli or stimuli whose contrast is modulated around a high contrast "pedestal" (Green MF, Butler PD, Chen Y, et al. Schizophr Bull. 2009;35:163-181). This suggestion faces 2 difficulties: (1) To use low-contrast stimuli to activate the magnocellular system is inconsistent with lesion studies that have shown that under many conditions, the parvocellular system responds to the lowest contrasts and (2) To rely on contrast-response relationships to identify magnocellular and parvocellular responses is difficult because other neurons exist in the visual system that have contrast-response relationships similar to those of magnocellular and parvocellular cells.     

Magnocellular contributions to impaired motion processing in schizophrenia

Patients with schizophrenia show impairments in motion processing, along with deficits in lower level processing primarily involving the magnocellular visual pathway. The present study investigates potential magnocellular contributions to impaired motion processing in schizophrenia using a combined neurophysiological and behavioral approach. As compared to prior motion studies in schizophrenia, thresholds were determined for both incoherent and coherent visual motion. In this study, velocity discrimination thresholds were measured for schizophrenia patients (n=14) and age-matched normal control subjects (n=16) using a staircase procedure. Early visual processing was evaluated using steady-state visual evoked potentials (ssVEP), with stimuli biased toward activation of either the magnocellular or parvocellular visual pathways through luminance contrast manipulation. Patients with schizophrenia showed poor velocity discrimination for both incoherent and coherent motion, with no significant group x task interaction. Further, when coherent motion performance was measured at individually determined incoherent motion thresholds, accuracy levels for patients were similar to controls, also indicating similarity of deficit for incoherent vs. coherent motion discrimination. Impairments in velocity discrimination correlated significantly with reduced amplitude of ssVEP elicited by magnocellular -- but not parvocellular -- selective stimuli. This study demonstrates that deficits in motion processing in schizophrenia are significantly related to reduced activation of the magnocellular visual system. Further, this study supports and extends prior reports of impaired motion processing in schizophrenia, and indicates significant bottom-up contributions to higher-order cognitive impairments.     

Atypical cortical representation of peripheral visual space in children with an autism spectrum disorder

A key feature of early visual cortical regions is that they contain discretely organized retinotopic maps. Titration of these maps must occur through experience, and the fidelity of their spatial tuning will depend on the consistency and accuracy of the eye movement system. Anomalies in fixation patterns and the ballistics of eye movements are well documented in autism spectrum disorder (ASD), with off‐center fixations a hallmark of the phenotype. We hypothesized that these atypicalities might affect the development of visuo‐spatial maps and specifically that peripheral inputs might receive altered processing in ASD. Using high‐density recordings of visual evoked potentials (VEPs) and a novel system‐identification approach known as VESPA (visual evoked spread spectrum analysis), we assessed sensory responses to centrally and peripherally presented stimuli. Additionally, input luminance was varied to bias responsiveness to the magnocellular system, given previous suggestions of magnocellular‐specific deficits in ASD. Participants were 22 ASD children (7–17 years of age) and 31 age‐ and performance‐IQ‐matched neurotypical controls. Both VEP and VESPA responses to central presentations were indistinguishable between groups. In contrast, peripheral presentations resulted in significantly greater early VEP and VESPA amplitudes in the ASD cohort. We found no evidence that anomalous enhancement was restricted to magnocellular‐biased responses. The extent of peripheral response enhancement was related to the severity of stereotyped behaviors and restricted interests, cardinal symptoms of ASD. The current results point to differential visuo‐spatial cortical mapping in ASD, shedding light on the consequences of peculiarities in gaze and stereotyped visual behaviors often reported by clinicians working with this population.     

Maturation of visual evoked potentials across adolescence

Adolescence represents the period of transition from childhood to adulthood and is characterized by significant changes in brain structure and function. We studied changes in the functional visual processing in the brain across adolescence. Visual evoked potentials (VEPs) to three types of pattern reversal checkerboard stimuli were measured in 90 adolescents (10-18 years) and 10 adults. Across adolescence, the N75 and P100 VEP peaks decreased in size while the N135 peak increased slightly in size. The latency of VEP peaks showed no reliable change across adolescence. The results suggest that even very basic visual sensory function continues to develop throughout adolescence. The results indicate significant changes in visual parvocellular and magnocellular pathways across adolescence.     

The effect of spatial frequency on chromatic and achromatic steady-state visual evoked potentials.

OBJECTIVE: Little is known about the physiological properties of the major components of steady-state visual evoked potentials (VEPs). Based on the hypothesis that isoluminant color and high contrast pattern differentially activate the parvo- and magnocellular pathways, we studied difference in spatial frequency function between chromatic and achromatic VEPs to sinusoidal gratings. METHODS: Steady-state VEPs to isoluminant chromatic (red-green) and high contrast (90%) achromatic (black-white) sinusoidal gratings with nine spatial frequencies (0.5 to 8.0 cycles/degrees (cpd)) at 4 Hz (8 reversals/s) were recorded in 13 normal subjects. VEPs were Fourier analyzed to obtain phase and amplitude of the second (2F) and fourth (4F) harmonic responses. RESULTS: The 2F amplitude of chromatic VEPs decreased above 4.0 cpd in a low-pass function while that of achromatic VEPs showed a band-pass function with a peak at 4.0 cpd. The 4F amplitude of chromatic VEPs was not affected significantly by spatial frequency whereas that of achromatic VEPs exhibited a high-pass function. The phases of 2F and 4F showed a non-monotonic function of spatial frequency in both chromatic and achromatic stimuli with peaks at middle spatial frequencies. CONCLUSION: Chromatic and achromatic visual stimuli differently affected 2F and 4F components, which thus suggests that 2F and 4F components are generated from different neuronal subgroups largely in the parvocellular pathway.     

A subcortical magnocellular pathway is responsible for the fast processing of topological properties of objects: A transcranial magnetic stimulation study

Rapid object recognition has survival significance. The extraction of topological properties (TP) is proposed as the starting point of object perception. Behavioral evidence shows that TP processing takes precedence over other geometric properties and can accelerate object recognition. However, the mechanism of the fast TP processing remains unclear. The magnocellular (M) pathway is well known as a fast route to convey “coarse” information, compared with the slow parvocellular (P) pathway. Here, we hypothesize that the fast processing of TP occurs in a subcortical M pathway. We applied single‐pulse transcranial magnetic stimulation (TMS) over the primary visual cortex to temporarily disrupt cortical processing. Besides, stimuli were designed to preferentially engage M or P pathways (M‐ or P‐biased conditions). We found that, when TMS disrupted cortical function at the early stages of stimulus processing, non‐TP shape discrimination was strongly impaired in both M‐ and P‐biased conditions, whereas TP discrimination was not affected in the M‐biased condition, suggesting that early M processing of TP is independent of the visual cortex, but probably occurs in a subcortical M pathway. Using an unconscious priming paradigm, we further found that early M processing of TP can accelerate object recognition by speeding up the processing of other properties, e.g., orientation. Our findings suggest that the human visual system achieves efficient object recognition by rapidly processing TP in the subcortical M pathway.     

Stimulus Contrast Affects Spatial Integration in the Lateral Geniculate Nucleus of Macaque Monkeys

Gain-control mechanisms adjust neuronal responses to accommodate the wide range of stimulus conditions in the natural environment. Contrast gain control and extraclassical surround suppression are two manifestations of gain control that govern the responses of neurons in the early visual system. Understanding how these two forms of gain control interact has important implications for the detection and discrimination of stimuli across a range of contrast conditions. Here, we report that stimulus contrast affects spatial integration in the lateral geniculate nucleus of alert macaque monkeys (male and female), whereby neurons exhibit a reduction in the strength of extraclassical surround suppression and an expansion in the preferred stimulus size with low-contrast stimuli compared with high-contrast stimuli. Effects were greater for magnocellular neurons than for parvocellular neurons, indicating stream-specific interactions between stimulus contrast and stimulus size. Within the magnocellular pathway, contrast-dependent effects were comparable for ON-center and OFF-center neurons, despite ON neurons having larger receptive fields, less pronounced surround suppression, and more pronounced contrast gain control than OFF neurons. Together, these findings suggest that the parallel streams delivering visual information from retina to primary visual cortex, serve not only to broaden the range of signals delivered to cortex, but also to provide a substrate for differential interactions between stimulus contrast and stimulus size that may serve to improve stimulus detection and stimulus discrimination under pathway-specific lower and higher contrast conditions, respectively.SIGNIFICANCE STATEMENT Stimulus contrast is a salient feature of visual scenes. Here we examine the influence of stimulus contrast on spatial integration in the lateral geniculate nucleus (LGN). Our results demonstrate that increases in contrast generally increase extraclassical suppression and decrease the size of optimal stimuli, indicating a reduction in the extent of visual space from which LGN neurons integrate signals. Differences between magnocellular and parvocellular neurons are noteworthy and further demonstrate that the feedforward parallel pathways to cortex increase the range of information conveyed for downstream cortical processing, a range broadened by diversity in the ON and OFF pathways. These results have important implications for more complex visual processing that underly the detection and discrimination of stimuli under varying natural conditions.     

Impaired parvocellular pathway in dyslexic children

Recent studies report that some children with dyslexia have impaired visual processing, specifically in the fast-processing magnocellular pathway. The objective was to study the effect of varying luminance and temporal and spatial frequency on the latency and amplitude of the visual evoked potentials (VEPs) in normal and dyslexic Egyptian children who speak Arabic (a right-left reading and writing system). VEPs were recorded in 52 dyslexic and 41 normal children in the fourth grade using a black and white checkerboard pattern with different checkerboard sizes and different rates of stimuli at high- and low-contrast media. The peak of the major positive wave component (P100) of each waveform and the trough of the previous major negative wave component were identified, and the peak-to-trough amplitude was measured. The latency and amplitude of VEPs in response to different experimental conditions showed significant shortening of P100 latency under high-contrast media and under low spatial frequency in children with dyslexia compared with normal readers. Furthermore, dyslexia children showed prolonged P100 latency in response to high spatial frequency stimulation compared with the low spatial frequency (P=0.003) and significantly higher N1-P1 amplitude under high-contrast media compared with low-contrast media (P=0.02), whilst no such changes were observed in normal readers. These results are suggestive of deficiency within the parvocellular pathway rather than the magnocellular pathway. As reading apparently places demands primarily on the ability to discriminate fine details, which is to say, on the parvocellular system, we suggested that deficiency in this system, at least in Arabic speaking children, could be a predisposing factor in dyslexia.     

Magnocellular and parvocellular contributions to responses in the middle temporal visual area (MT) of the macaque monkey

Many lines of evidence suggest that the visual signals relayed through the magnocellular and parvocellular subdivisions of the primate dorsal LGN remain largely segregated through several levels of cortical processing. It has been suggested that this segregation persists through to the highest stages of the visual cortex, and that the pronounced differences between the neuronal response properties in the parietal cortex and inferotemporal cortex may be attributed to differential contributions from magnocellular and parvocellular signals. We have examined this hypothesis directly by recording the responses of cortical neurons while selectively blocking responses in the magnocellular or parvocellular layers of the LGN. Responses were recorded from single units or multiunit clusters in the middle temporal visual area (MT), which is part of the pathway leading to parietal cortex and thought to receive primarily magnocellular inputs. Responses in the MT were consistently reduced when the magnocellular subdivision of the LGN was inactivated. The reduction was almost always pronounced and often complete. In contrast, parvocellular block rarely produced striking changes in MT responses and typically had very little effect. Nevertheless, unequivocal parvocellular contributions could be demonstrated for a minority of MT responses. At a few MT sites, responses were recorded while magnocellular and parvocellular blocks were made simultaneously. Responses were essentially eliminated for all these paired blocks. These results provide direct evidence for segregation of magnocellular and parvocellular contributions in the extrastriate visual cortex and support the suggestion that these signals remain largely segregated through the highest levels of cortical processing.     

Differential inhibition of chromatic and achromatic perception by transcranial magnetic stimulation of the human visual cortex.

The magnocellular visual pathway is devoted to low-contrast achromatic and motion perception whereas the parvocellular pathway deals with chromatic and high resolution spatial vision. To specifically separate perception mediated by these pathways we have used low-contrast Gaussian filtered black-white or coloured visual stimuli. By use of transcranial magnetic stimulation (TMS) over the visual cortex inhibition of magnocellular stimuli was achieved distinctly earlier by about 40 ms compared with parvocellular information. A nonspecific inhibition of all stimuli could be seen peaking at 75-90 ms, significantly higher for magnocellular stimuli. The particular vulnerability of magnocellular stimuli to TMS is correlated with distinct physiological properties of this pathway such as faster conduction velocity and non-linear stimulus encoding.     

A new color vep procedure discloses asymptomatic visual impairments in optic neuritis and glaucoma suspects

OBJECTIVE: To evaluate the reliability of visual evoked potentials obtained with a set of multiple chromatic and achromatic patterns (C-VEPs) in differentiating asymptomatic perifoveal retinal impairment from central conduction impairment. METHODS: We propose a set of colored pattern stimuli that allows relatively differential activation of the magnocellular and parvocellular pathways. The system runs on a standard Pentium PC with peripherals that present stimuli and collect, analyze and print data. P1 latencies of C-VEPs obtained with achromatic (black/white) and chromatic (blue/black and red/black isocontrast) checkerboards were evaluated in normal subjects and patients with subclinical retinal impairment (glaucoma suspects) or mild neural conduction impairment (optic neuritis), none of whom had subjective visual defects. RESULTS: The procedure evoked robust cortical signals and statistically distinguished the 3 groups of subjects. The achromatic and chromatic stimuli used distinguished controls from glaucoma suspects and patients with optic neuritis. Glaucoma suspects had greater impairment of C-VEPs to blue/black checkerboards whereas patients with optic neuritis had greater impairment of responses to red/black stimuli. CONCLUSIONS: Our data suggest that chromatic patterns (color/ black, red and blue), that may activate the parvocellular and magnocellular systems differentially but not selectively, can distinguish between mild perifoveal or foveal conduction impairment. They have the additional advantage of evoking large, stable responses across all the subjects.     

Williams syndrome: neuronal size and neuronal-packing density in primary visual cortex

BACKGROUND: Williams syndrome (WMS) is a rare, genetically based syndrome associated with a hemideletion in chromosome 7 (7q11.22-23) and characterized by a unique constellation of somatic, brain, and cognitive features. Individuals with WMS demonstrate an unusual and uneven neuropsychological profile showing cognitive and visual spatial deficits juxtaposed with relative language preservation and excellent facial recognition. OBJECTIVES: A neuroanatomical hypothesis for these behavioral findings suggests predominant involvement of the dorsal portions of the hemispheres relative to the ventral portions, including preferential involvement of peripheral visual field cortical representations over central representation. Predominant involvement of magnocellular visual pathways, as opposed to parvocellular pathways, is also suggested by this hypothesis. SUBJECTS: We examined primary visual cortical area 17 in the right and left hemispheres in 6 age- and sex-matched autopsy specimens from 3 WMS-affected brains (1 male and 2 females; mean [SD] age, 44 [14] years) and 3 control brains (1 male and 2 females; mean age, 43 [11] years). DESIGN: Neurons in layers II, III, IVA, IVB, IVCalpha, IVCbeta, V, and VI were measured using an optical dissector method to determine possible differences between WMS-affected and control brains in cell-packing density, neuronal size, and neuronal size distribution. RESULTS: We found abnormalities in peripheral visual cortex in WMS-affected brains, but not in magnocellular subdivisions. There was a hemisphere by layer IV interaction and a layer IV left hemisphere and diagnosis interaction in cell-packing density. Williams syndrome-affected brains showed increased cell-packing density in left sublayer IVCbeta and an excess of small neurons in left layers IVA, IVCalpha, IVCbeta, V, and VI. CONCLUSIONS: Cell measurements differ in peripheral visual cortical fields of WMS, with significantly smaller, more closely packed cells in some layers on the left side. These cell-packing density and neuronal size differences may be related to visuospatial deficits in this population.     

Variation of visual evoked potential delay to stimulation of central, nasal, and temporal regions of the macula in optic neuritis

OBJECTIVES: To compare the degree of visual evoked potential (VEP) delay to stimulation of central, nasal, and temporal regions of the macula in optic neuritis, to determine whether the differential involvement of parvocellular and magnocellular fibre types suggested by other studies is governed by retinotopic factors. METHODS: VEPs were recorded to reversal of 40' checks in the central (4 degrees radius) and the left and right surrounding regions of the visual field (as far as 10 degrees vertical and 14 degrees horizontal) in 30 patients recently recovered from the acute stage of optic neuritis, and in 17 age matched controls. RESULTS: In the control group, VEP latencies were similar to stimulation of the central and temporal regions of the macula, marginally shorter from the nasal region. In the patients with optic neuritis, VEPs were significantly more delayed from the central region, on average by about twice as much as from the nasal and temporal regions. Delays seen in some of the VEPs from the patients' fellow eyes tended to be more uniformly distributed. CONCLUSIONS: Although the central region of the macula is where the density of parvocellular innervation is greatest, there is no reason to suppose that the VEPs to stimulation of the nasal and temporal regions (almost all P100 activity arising from within the central 10 degrees ) are mediated by fibres of another type. Consequently it is suggested that the central fibres were most affected by demyelination, not on account of their belonging to the parvocellular type but because of their particular situation in the optic nerve. Centrally located fibres may experience greater exposure to factors causing demyelination, or fibres located closer to the edge of the plaque may undergo more effective remyelination in the first few weeks after the acute episode.     

Altered cortical visual processing in individuals with a spreading photoparoxysmal EEG response

Photosensitive individuals respond with epileptiform electroencephalography (EEG) discharges to intermittent photic stimulation. The pathogenetic mechanisms underlying this photoparoxysmal response (PPR) remain to be clarified. We investigated the involvement of magnocellular and parvocellular pathways in the processing of nonprovocative visual stimuli in healthy subjects with different phenotypic expressions of PPR (15 individuals with a local PPR, i.e. occipital discharges only, and 15 with a PPR propagating to anterior brain regions) and in 17 PPR-negative healthy controls using pattern-reversal visual evoked potentials (VEP). Checkerboard stimulation was performed at a low and a high spatial frequency to preferentially activate the magnocellular and parvocellular pathways. VEP habituation was also assessed over 15 blocks (each 100 trials) of recording. PPR-positive individuals with propagating PPR showed an increase in the N75-P100 and P100-N135 VEP components for both spatial frequencies, whereas individuals with a local PPR had normal VEP amplitudes. Individuals with propagating PPR also showed a stronger VEP habituation and reported more aversive sensations during continuous visual stimulation with the high spatial frequency checkerboard. The selective increase in VEP amplitudes in individuals with propagating PPR corroborates the notion that PPR with propagation is pathophysiologically distinct from local PPR. The increase in VEP amplitudes was independent of the spatial frequency of visual stimulation, indicating an increased neuronal excitability in both the parvocellular and magnocellular pathways. The stronger habituation in these individuals may reflect a compensatory mechanism to stabilize excitability in the visual system.     

Spatial attention has different effects on the magno- and parvocellular pathways.

Attention was directed to the left or to the right of the fixation point by the lateral presentation of a target on which the subject had to perform an attention demanding task. A (task-irrelevant) grating displayed in the left visual field was the visual evoked potential (VEP) stimulus. Gratings modulated either in luminance or colour contrast at various temporal frequencies were used in order to maximise the activation of magno- or parvocellular pathways. VEPs recorded in attended and unattended conditions were compared. For luminance stimuli, both latency and amplitude of VEPs were modified by attention. For chromatic stimuli, attention affected the amplitude but not the latency of VEPs. Spatial attention uses different mechanisms when magno- or parvocellular systems are involved.     

Stereologic analysis of the lateral geniculate nucleus of the thalamus in normal and schizophrenic subjects

Reduction of volume and neuronal number has been found in several association nuclei of the thalamus in schizophrenic subjects. Recent evidence suggests that schizophrenic patients exhibit abnormalities in early visual processing and that many of the observed perceptual deficits are consistent with dysfunction of the magnocellular pathway, i.e. the visual relay from peripheral retinal cells to the two ventrally located magnocellular layers of the lateral geniculate nucleus (LGN). The present study was undertaken to determine whether abnormalities in cell number and volume of the LGN are associated with schizophrenia and whether the structural alterations are restricted to either the magnocellular or parvocellular subdivisions of the LGN. Series of Nissl-stained sections spanning the LGN were obtained from 15 schizophrenic and 15 normal control subjects. The optical disector/fractionator sampling method was used to estimate total neuronal number, total glial number and volume of the magnocellular and parvocellular subdivisions of the LGN. Cell number and volume of the LGN in schizophrenic subjects were not abnormal. Volume of both parvocellular and magnocellular layers of the LGN decreased with age. These findings do not support the hypothesis that early visual processing deficits in schizophrenic subjects are due to reduction of neuronal number in the LGN.     

Color and contrast sensitivity in the lateral geniculate body and primary visual cortex of the macaque monkey

We tested color and contrast sensitivity in the magnocellular and parvocellular subdivisions of the lateral geniculate body and in layers 2, 3, 4B, and 4C alpha of visual area 1 to obtain physiological data on the degree of segregation of the 2 pathways and on the fate of the color and contrast information as it is transmitted from the geniculate to the cortex. On average, magnocellular geniculate cells were much less responsive than parvocellular cells to shifts between 2 equiluminant colors. Nevertheless, many magnocellular cells (though not all) continued to give some response at equiluminance. As expected from previous studies, luminance contrast sensitivity differed markedly between magnocellular and parvocellular layers. In V-1, the properties of cells in the magnorecipient layers 4C alpha and 4B faithfully reflected the properties of magnocellular geniculate cells, showing no evidence of any parvocellular input. Like magnocellular geniculate cells, they showed high contrast sensitivity, and with color contrast stimuli they showed large response decrements at equiluminance. In the interblob regions of cortical layers 2 and 3, which anatomically appear to receive most of their inputs from parvorecipient layer 4C beta, contrast sensitivities of some of the cells were compatible with a predominantly parvocellular input. Other interblob cells had sensitivities intermediate between magno- and parvocellular geniculate cells, suggesting a possible contribution from the magnocellular system. Many cells in cortical layers 2 and 3 responded to color-contrast borders equally well at all relative brightnesses of the 2 colors, including equiluminance. We recorded from many direction- and disparity-selective cells in V-1: most of the direction-selective and all of the clearly stereo-selective cells were located in layer 4B.     

Visual evoked potential abnormalities in dyslexic children.

Developmental reading disability (dyslexia) has traditionally been attributed to impaired linguistic skills. Recent psychophysical data suggest that dyslexia may be related to a visual perceptual deficit. A few visual evoked potential (VEP) studies have addressed this hypothesis, but their results are far from consistent. We submitted 9 dyslexic subjects and 9 age- and sex-matched normal controls to checkerboard pattern reversal VEPs. The main experimental variables were: large (0.5 cycles per degree; cpd) and small (2 cpd) checks and two reversal frequencies (2.1 Hz and 8 Hz); mean luminance and contrast (60 cd/m2 and 50%, respectively) were kept constant in all four conditions. Transient VEP (2.1 Hz) parameters did not differ between controls and dyslexics at 2 cpd. At 0.5 cpd, N70 amplitude was significantly smaller and N70 latency significantly shorter in dyslexics. Amplitudes for the fundamental frequency (8 Hz), as well as for the second and third harmonics of the steady-state VEPs were smaller in dyslexics for both stimulus sizes. A discriminant analysis correctly classified each subject. Our data confirm the hypothesis of a perceptual deficit in dyslexic subjects. The abnormalities are related to spatial and temporal stimulus frequencies: they appear when large stimuli are presented, or when the stimulation frequency is high. These data support the hypothesis of selective magnocellular dysfunction in dyslexia.     

Timing and distribution of flash-evoked activity in the lateral geniculate nucleus of the alert monkey

Simultaneous recording of activity from multiple cortical laminae in alert monkeys, using multichannel electrodes, has been used to identify the intracranial generators of surface-recorded, visually evoked potentials (VEP) to stroboscopic flash. Beyond their clinical implications, these results offer an unique view of the timing and sequence of cortical visual processing in the alert monkey, including the somewhat surprising findings of an extremely short-latency response in lamina IVA, a contra- over ipsilateral latency advantage throughout lamina IV, and the lack of a consistent flash-evoked response in the major cortical recipient of the magnocellular system, lamina IVCa. The present study used similar techniques to examine flash-evoked activity in LGN and in optic tract, both to elucidate the role of the subcortical pathways in establishing this pattern, and to provide a parallel, detailed view of the timing of visual activity in LGN and optic tract in the alert monkey. Flash-evoked responses are robust in both parvo- and magnocellular laminae, but these responses differ along several dimensions: (1) parvocellular multiunit activity (MUA) is 1/4 to 1/2 the amplitude of magnocellular MUA; (2) oscillatory activity is higher in frequency and shorter in duration in parvo- than in magnocellular responses; (3) inhibitory processes appear less prominent and diverse in parvo- than in magnocellular activity; (4) mean onset latencies of MUA are longer in parvo- than in magnocellular laminae, but there is extensive overlap in these distributions. Latencies encountered in ipsilateral lamina 3, and at laminar borders dorsal to 3, group more clearly with those of the magnocellular laminae than with those of the other parvocellular laminae. As a result, in the parvocellular division as a whole, the average latency to ipsilateral stimulation is shorter than that to contralateral stimulation. The optic tract exhibits a dorsal-to-ventral progression of onset latency and oscillation frequency consistent with a dorsal/ventral segregation of the inputs to parvo- and magnocellular layers. Comparison of optic tract and LGN data reveals that while many LGN response characteristics are initiated in the retina, significant modification of retinal output occurs at LGN. The techniques used here permit a particularly sensitive and reliable assessment of the timing and distribution of visual responses in the optic tract and LGN of alert monkeys. Our data support the view that in the alert monkey, the surface-VEP to passive, binocular flash primarily reflects activation of parvocellular thalamorecipient laminae of Area 17.(ABSTRACT TRUNCATED AT 400 WORDS)