The contrast sensitivity of retinal ganglion cells of the cat

1. Spatial summation within cat retinal receptive fields was studied by recording from optic-tract fibres the responses of ganglion cells to grating patterns whose luminance perpendicular to the bars varied sinusoidally about the mean level. 2. Summation over the receptive fields of some cells (X-cells) was found to be approximately linear, while for other cells (Y-cells) summation was very non-linear. 3. The mean discharge frequency of Y-cells (unlike that of X-cells) was greatly increased when grating patterns drifted across their receptive fields. 4. In twenty-one X-cells the relation between the contrast and spatial frequency of drifting sinusoidal gratings which evoked the same small response was measured. In every case it was found that the reciprocal of this relation, the contrast sensitivity function, could be satisfactorily described by the difference of two Gaussian functions. 5. This finding supports the hypothesis that the sensitivities of the antagonistic centre and surround summating regions of ganglion cell receptive fields fall off as Gaussian functions of the distance from the field centre. 6. The way in which the sensitivity of an X-cell for a contrast-edge pattern varied with the distance of the edge from the receptive field centre was determined and found to be consistent with the cell's measured contrast sensitivity function. 7. Reducing the retinal illumination produced changes in the contrast sensitivity function of an X-cell which suggested that the diameters of the summating regions of the receptive field increased while the surround region became relatively ineffective.

Normalization of cell responses in cat striate cortex

Simple cells in the striate cortex have been depicted as half-wave-rectified linear operators. Complex cells have been depicted as energy mechanisms, constructed from the squared sum of the outputs of quadrature pairs of linear operators. However, the linear/energy model falls short of a complete explanation of striate cell responses. In this paper, a modified version of the linear/energy model is presented in which striate cells mutually inhibit one another, effectively normalizing their responses with respect to stimulus contrast. This paper reviews experimental measurements of striate cell responses, and shows that the new model explains a significantly larger body of physiological data.

The normalization model of attention

Attention has been found to have a wide variety of effects on the responses of neurons in visual cortex. We describe a model of attention that exhibits each of these different forms of attentional modulation, depending on the stimulus conditions and the spread (or selectivity) of the attention field in the model. The model helps reconcile proposals that have been taken to represent alternative theories of attention. We argue that the variety and complexity of the results reported in the literature emerge from the variety of empirical protocols that were used, such that the results observed in any one experiment depended on the stimulus conditions and the subject's attentional strategy, a notion that we define precisely in terms of the attention field in the model, but that has not typically been completely under experimental control.

Attentional modulation of visual processing

Single-unit recording studies in the macaque have carefully documented the modulatory effects of attention on the response properties of visual cortical neurons. Attention produces qualitatively different effects on firing rate, depending on whether a stimulus appears alone or accompanied by distracters. Studies of contrast gain control in anesthetized mammals have found parallel patterns of results when the luminance contrast of a stimulus increases. This finding suggests that attention has co-opted the circuits that mediate contrast gain control and that it operates by increasing the effective contrast of the attended stimulus. Consistent with this idea, microstimulation of the frontal eye fields, one of several areas that control the allocation of spatial attention, induces spatially local increases in sensitivity both at the behavioral level and among neurons in area V4, where endogenously generated attention increases contrast sensitivity. Studies in the slice have begun to explain how modulatory signals might cause such increases in sensitivity.

Supernormal vision and high-resolution retinal imaging through adaptive optics

Even when corrected with the best spectacles or contact lenses, normal human eyes still suffer from monochromatic aberrations that blur vision when the pupil is large. We have successfully corrected these aberrations using adaptive optics, providing normal eyes with supernormal optical quality. Contrast sensitivity to fine spatial patterns was increased when observers viewed stimuli through adaptive optics. The eye's aberrations also limit the resolution of images of the retina, a limit that has existed since the invention of the ophthalmoscope. We have constructed a fundus camera equipped with adaptive optics that provides unprecedented resolution, allowing the imaging of microscopic structures the size of single cells in the living human retina.

The "independent components" of natural scenes are edge filters

It has previously been suggested that neurons with line and edge selectivities found in primary visual cortex of cats and monkeys form a sparse, distributed representation of natural scenes, and it has been reasoned that such responses should emerge from an unsupervised learning algorithm that attempts to find a factorial code of independent visual features. We show here that a new unsupervised learning algorithm based on information maximization, a nonlinear "infomax" network, when applied to an ensemble of natural scenes produces sets of visual filters that are localized and oriented. Some of these filters are Gabor-like and resemble those produced by the sparseness-maximization network. In addition, the outputs of these filters are as independent as possible, since this infomax network performs Independent Components Analysis or ICA, for sparse (super-gaussian) component distributions. We compare the resulting ICA filters and their associated basis functions, with other decorrelating filters produced by Principal Components Analysis (PCA) and zero-phase whitening filters (ZCA). The ICA filters have more sparsely distributed (kurtotic) outputs on natural scenes. They also resemble the receptive fields of simple cells in visual cortex, which suggests that these neurons form a natural, information-theoretic coordinate system for natural images.

Improvement in visual sensitivity by changes in local context: parallel studies in human observers and in V1 of alert monkeys

To explore the role of primary visual cortex in contour integration, we measured the contextual sensitivity of human contrast thresholds and of superficial layer complex cells in monkey V1. An observer's contrast detection was 40% improved by a second suprathreshold bar; the effect was decreased as the two bars were separated along their axis of orientation, were displaced from colinearity, and had their relative orientation changed. Recordings from V1 showed that 42% of complex cells demonstrated facilitation for a second bar outside their classical receptive fields with a similar dependency on relative location and orientation. Both effects were eliminated by an orthogonal line between the two iso-oriented lines. Multiple randomly placed and oriented lines in the receptive field surround often caused a reduction in a cell's response to an optimally oriented stimulus, but this inhibition could be eliminated by changing the orientation of a few of these elements to colinearity with the centrally located target.

Motion illusions as optimal percepts

The pattern of local image velocities on the retina encodes important environmental information. Although humans are generally able to extract this information, they can easily be deceived into seeing incorrect velocities. We show that these 'illusions' arise naturally in a system that attempts to estimate local image velocity. We formulated a model of visual motion perception using standard estimation theory, under the assumptions that (i) there is noise in the initial measurements and (ii) slower motions are more likely to occur than faster ones. We found that specific instantiation of such a velocity estimator can account for a wide variety of psychophysical phenomena.

Attention alters appearance

Does attention alter appearance? This critical issue, debated for over a century, remains unsettled. From psychophysical evidence that covert attention affects early vision-it enhances contrast sensitivity and spatial resolution-and from neurophysiological evidence that attention increases the neuronal contrast sensitivity (contrast gain), one could infer that attention changes stimulus appearance. Surprisingly, few studies have directly investigated this issue. Here we developed a psychophysical method to directly assess the phenomenological correlates of attention in humans. We show that attention alters appearance; it boosts the apparent stimulus contrast. These behavioral results are consistent with neurophysiological findings suggesting that attention changes the strength of a stimulus by increasing its 'effective contrast' or salience.

Lateral interactions between spatial channels: suppression and facilitation revealed by lateral masking experiments

We measured contrast detection thresholds for a foveal Gabor signal flanked by two high contrast Gabor signals. The spatially localized target and masks enabled investigation of space dependent lateral interactions between foveal and neighboring spatial channels. Our data show a suppressive region extending to a radius of two wavelengths, in which the presence of the masking signals have the effect of increasing target threshold. Beyond this range a much larger facilitatory region (up to a distance of ten wavelengths) is indicated, in which contrast thresholds were found to decrease by up to a factor of two. The interactions between the foveal target and the flanking Gabor signals are spatial-frequency and orientation specific in both regions, but less specific in the suppression region.

Neuronal selectivities to complex object features in the ventral visual pathway of the macaque cerebral cortex

1. To infer relative roles of cortical areas at different stages of the ventral visual pathway, we quantitatively examined visual responses of cells in V2, V4, the posterior part of the inferotemporal cortex (posterior IT), and the anterior part of the inferotemporal cortex (anterior IT), using anesthetized macaque monkeys. 2. The critical feature for the activation was first determined for each recorded cell by using a reduction method. We started from images of three-dimensional complex objects and simplified the image of effective stimuli step by step by eliminating a part of the features present in the image. The simplest feature that maximally activated the cell was determined as the critical feature. The response to the critical feature was then compared with responses of the same cell to a routine set of 32 simple stimuli, which included white and black bars of four different orientations and squares or spots of four different colors. 3. Cells that responded maximally to particular complex object features were found in posterior IT and V4 as well as in anterior IT. The cells in posterior IT and V4 were, however, different from the cells in anterior IT in that many of them responded to some extent to some simple features, that the size of the receptive field was small, and that they intermingled in single penetrations with cells that responded maximally to some simple features. The complex critical features in posterior IT and V4 varied; they consisted of complex shapes, combinations of a shape and texture, and combinations of a shape and color. 4. We suggest that local neuronal networks in V4 and posterior IT play an essential role in the formation of selective responses to complex object features.

Coding of image contrast in central visual pathways of the macaque monkey

Measurements of contrast sensitivity were obtained from isolated neurons in the lateral geniculate nucleus, striate cortex, and middle temporal visual area of macaque monkeys. Between the lateral geniculate nucleus and the middle temporal area contrast sensitivity functions become progressively steeper. Furthermore, many neurons in the middle temporal area are more sensitive than any cell encountered in early stages. Measurements made with stimuli of different sizes show that this high sensitivity depends on areal summation across the receptive field.

Distributed and Overlapping Cerebral Representations of Number, Size, and Luminance during Comparative Judgments

How are comparative judgments performed in the human brain? We scanned subjects with fMRI while they compared stimuli for size, luminance, or number. Regions involved in comparative judgments were identified using three criteria: task-related activation, presence of a distance effect, and interference of one dimension onto the other. We observed considerable overlap in the neural substrates of the three comparison tasks. Interestingly, the amount of overlap predicted the amount of cross-dimensional interference: in both behavior and fMRI, number interfered with size, and size with luminance, but number did not interfere with luminance. The results suggest that during comparative judgments, the relevant continuous quantities are represented in distributed and overlapping neural populations, with number and size engaging a common parietal spatial code, while size and luminance engage shared occipito-temporal perceptual representations.

Relation of visual function to retinal nerve fiber layer thickness in multiple sclerosis

PURPOSE

To examine the relation of visual function to retinal nerve fiber layer (RNFL) thickness as a structural biomarker for axonal loss in multiple sclerosis (MS), and to compare RNFL thickness among MS eyes with a history of acute optic neuritis (MS ON eyes), MS eyes without an optic neuritis history (MS non-ON eyes), and disease-free control eyes.

DESIGN

Cross-sectional study.

PARTICIPANTS

Patients with MS (n = 90; 180 eyes) and disease-free controls (n = 36; 72 eyes).

METHODS

Retinal never fiber layer thickness was measured using optical coherence tomography (OCT; fast RNFL thickness software protocol). Vision testing was performed for each eye and binocularly before OCT scanning using measures previously shown to capture dysfunction in MS patients: (1) low-contrast letter acuity (Sloan charts, 2.5% and 1.25% contrast levels at 2 m) and (2) contrast sensitivity (Pelli-Robson chart at 1 m). Visual acuity (retroilluminated Early Treatment Diabetic Retinopathy charts at 3.2 m) was also measured, and protocol refractions were performed.

MAIN OUTCOME MEASURES

Retinal nerve fiber layer thickness measured by OCT, and visual function test results.

RESULTS

Although median Snellen acuity equivalents were better than 20/20 in both groups, RNFL thickness was reduced significantly among eyes of MS patients (92 mum) versus controls (105 mum) (P<0.001) and particularly was reduced in MS ON eyes (85 mum; P<0.001; accounting for age and adjusting for within-patient intereye correlations). Lower visual function scores were associated with reduced average overall RNFL thickness in MS eyes; for every 1-line decrease in low-contrast letter acuity or contrast sensitivity score, the mean RNFL thickness decreased by 4 mum.

CONCLUSIONS

Scores for low-contrast letter acuity and contrast sensitivity correlate well with RNFL thickness as a structural biomarker, supporting validity for these visual function tests as secondary clinical outcome measures for MS trials. These results also suggest a role for ocular imaging techniques such as OCT in trials that examine neuroprotective and other disease-modifying therapies. Although eyes with a history of acute optic neuritis demonstrate the greatest reductions in RNFL thickness, MS non-ON eyes have less RNFL thickness than controls, suggesting the occurrence of chronic axonal loss separate from acute attacks in MS patients.

Sustained negative BOLD, blood flow and oxygen consumption response and its coupling to the positive response in the human brain

Most fMRI studies are based on the detection of a positive BOLD response (PBR). Here, we demonstrate and characterize a robust sustained negative BOLD response (NBR) in the human occipital cortex, triggered by stimulating part of the visual field. The NBR was spatially adjacent to but segregated from the PBR. It depended on the stimulus and thus on the pattern of neuronal activity. The time courses of the NBR and PBR were similar, and their amplitudes covaried both with increasing stimulus duration and increasing stimulus contrast. The NBR was associated with reductions in blood flow and with decreases in oxygen consumption. Our findings support the contribution to the NBR of (1) a significant component of reduction in neuronal activity and (2) possibly a component of hemodynamic changes independent of the local changes in neuronal activity.

Crowding is unlike ordinary masking: distinguishing feature integration from detection

A letter in the peripheral visual field is much harder to identify in the presence of nearby letters. This is "crowding." Both crowding and ordinary masking are special cases of "masking," which, in general, refers to any effect of a "mask" pattern on the discriminability of a signal. Here we characterize crowding, and propose a diagnostic test to distinguish it from ordinary masking. In ordinary masking, the signal disappears. In crowding, it remains visible, but is ambiguous, jumbled with its neighbors. Masks are usually effective only if they overlap the signal, but the crowding effect extends over a large region. The width of that region is proportional to signal eccentricity from the fovea and independent of signal size, mask size, mask contrast, signal and mask font, and number of masks. At 4 deg eccentricity, the threshold contrast for identification of a 0.32 deg signal letter is elevated (up to six-fold) by mask letters anywhere in a 2.3 deg region, 7 times wider than the signal. In ordinary masking, threshold contrast rises as a power function of mask contrast, with a shallow log-log slope of 0.5 to 1, whereas, in crowding, threshold is a sigmoidal function of mask contrast, with a steep log-log slope of 2 at close spacing. Most remarkably, although the threshold elevation decreases exponentially with spacing, the threshold and saturation contrasts of crowding are independent of spacing. Finally, ordinary masking is similar for detection and identification, but crowding occurs only for identification, not detection. More precisely, crowding occurs only in tasks that cannot be done based on a single detection by coarsely coded feature detectors. These results (and observers' introspections) suggest that ordinary masking blocks feature detection, so the signal disappears, while crowding (like "illusory conjunction") is excessive feature integration - detected features are integrated over an inappropriately large area because there are no smaller integration fields - so the integrated signal is ambiguous, jumbled with the mask. In illusory conjunction, observers see an object that is not there made up of features that are. A survey of the illusory conjunction literature finds that most of the illusory conjunction results are consistent with the spatial crowding described here, which depends on spatial proximity, independent of time pressure. The rest seem to arise through a distinct phenomenon that one might call "temporal crowding," which depends on time pressure ("overloading attention"), independent of spatial proximity.

Adaptation of retinal processing to image contrast and spatial scale

Owing to the limited dynamic range of a neuron's output, neural circuits are faced with a trade-off between encoding the full range of their inputs and resolving gradations among those inputs. For example, the ambient light level varies daily over more than nine orders of magnitude, whereas the firing rate of optic nerve fibres spans less than two. This discrepancy is alleviated by light adaptation: as the mean intensity increases, the retina becomes proportionately less sensitive. However, image statistics other than the mean intensity also vary drastically during routine visual processing. Theory predicts that an efficient visual encoder should adapt its strategy not only to the mean, but to the full shape of the intensity distribution. Here we report that retinal ganglion cells, the output neurons of the retina, adapt to both image contrast-the range of light intensities-and to spatial correlations within the scene, even at constant mean intensity. The adaptation occurs on a scale of seconds, one hundred times more slowly than the immediate light response, and involves 2-5-fold changes in the firing rate. It is mediated within the retinal network: two independent sites of modulation after the photoreceptor cells appear to be involved. Our results demonstrate a remarkable plasticity in retinal processing that may contribute to the contrast adaptation of human vision.

Changes in visual perception at the time of saccades

We frequently reposition our gaze by making rapid ballistic eye movements that are called saccades. Saccades pose problems for the visual system, because they generate rapid, large-field motion on the retina and change the relationship between the object position in external space and the image position on the retina. The brain must ignore the one and compensate for the other. Much progress has been made in recent years in understanding the effects of saccades on visual function and elucidating the mechanisms responsible for them. Evidence suggests that saccades trigger two distinct neural processes: (1) a suppression of visual sensitivity, specific to the magnocellular pathway, that dampens the sensation of motion and (2) a gross perceptual distortion of visual space in anticipation of the repositioning of gaze. Neurophysiological findings from several laboratories are beginning to identify the neural substrates involved in these effects.

Stimulus dependence of neuronal correlation in primary visual cortex of the macaque

Nearby cortical neurons often have correlated trial-to-trial response variability, and a significant fraction of their spikes occur synchronously. These two forms of correlation are both believed to arise from common synaptic input, but the origin of this input is unclear. We investigated the source of correlated responsivity by recording from pairs of single neurons in primary visual cortex of anesthetized macaque monkeys and comparing correlated variability and synchrony for spontaneous activity and activity evoked by stimuli of different orientations and contrasts. These two stimulus manipulations would be expected to have different effects on the cortical pool providing input to the recorded pair: changing stimulus orientation should recruit different populations of cells, whereas changing stimulus contrast affects primarily the relative strength of sensory drive and ongoing cortical activity. Consistent with this predicted difference, we found that correlation was affected by these stimulus manipulations in different ways. Synchrony was significantly stronger for orientations that drove both neurons well than for those that did not, but correlation on longer time scales was orientation independent. Reducing stimulus contrast resulted in a decrease in the temporal precision of synchronous firing and an enhancement of correlated response variability on longer time scales. Our results thus suggest that correlated responsivity arises from mechanisms operating at two distinct timescales: one that is orientation tuned and that determines the strength of temporally precise synchrony, and a second that is contrast sensitive, of low temporal frequency, and present in ongoing cortical activity.

Nature and precision of temporal coding in visual cortex: a metric-space analysis

1. We recorded single-unit and multi-unit activity in response to transient presentation of texture and grating patterns at 25 sites within the parafoveal representation of V1, V2, and V3 of two awake monkeys trained to perform a fixation task. In grating experiments, stimuli varied in orientation, spatial frequency, or both. In texture experiments, stimuli varied in contrast, check size, texture type, or pairs of these attributes. 2. To examine the nature and precision of temporal coding, we compared individual responses elicited by each set of stimuli in terms of two families of metrics. One family of metrics, D(spike), was sensitive to the absolute spike time (following stimulus onset). The second family of metrics, D(interval), was sensitive to the pattern of interspike intervals. In each family, the metrics depend on a parameter q, which expresses the precision of temporal coding. For q = 0, both metrics collapse into the "spike count" metric D(Count), which is sensitive to the number of impulses but insensitive to their position in time. 3. Each of these metrics, with values of q ranging from 0 to 512/s, was used to calculate the distance between all pairs of spike trains within each dataset. The extent of stimulus-specific clustering manifest in these pairwise distances was quantified by an information measure. Chance clustering was estimated by applying the same procedure to synthetic data sets in which responses were assigned randomly to the input stimuli. 4. Of the 352 data sets, 170 showed evidence of tuning via the spike count (q = 0) metric, 294 showed evidence of tuning via the spike time metric, 272 showed evidence of tuning via the spike interval metric to the stimulus attribute (contrast, check size, orientation, spatial frequency, or texture type) under study. Across the entire dataset, the information not attributable to chance clustering averaged 0.042 bits for the spike count metric, 0.171 bits for the optimal spike time metric, and 0.107 bits for the optimal spike interval metric. 5. The reciprocal of the optimal cost q serves as a measure of the temporal precision of temporal coding. In V1 and V2, with both metrics, temporal precision was highest for contrast (ca. 10-30 ms) and lowest for texture type (ca. 100 ms). This systematic dependence of q on stimulus attribute provides a possible mechanism for the simultaneous representation of multiple stimulus attributes in one spike train. 6. Our findings are inconsistent with Poisson models of spike trains. Synthetic data sets in which firing rate was governed by a time-dependent Poisson process matched to the observed poststimulus time histogram (PSTH) overestimated clustering induced by D(count) and, for low values of q, D(spike)[q] and D(intervals)[q]. Synthetic data sets constructed from a modified Poisson process, which preserved not only the PSTH but also spike count statistics accounted for the clustering induced by D(count) but underestimated the clustering induced by D(spike)[q] and D(interval)[q].

Aging and vision

Given the increasing size of the older adult population in many countries, there is a pressing need to identify the nature of aging-related vision impairments, their underlying mechanisms, and how they impact older adults' performance of everyday visual tasks. The results of this research can then be used to develop and evaluate interventions to slow or reverse aging-related declines in vision, thereby improving quality of life. Here we summarize salient developments in research on aging and vision over the past 25 years, focusing on spatial contrast sensitivity, vision under low luminance, temporal sensitivity and motion perception, and visual processing speed.

Perceptual organization and the judgment of brightness

The perceived brightness of a gray patch depends on the surrounding context. For example, a medium-gray patch appears darker when placed on a bright background and brighter when placed on a dark background. Models to explain these effects are usually based on simple low-level mechanisms. A new set of brightness illusions cannot be explained by such models. In these illusions, the brightness percept is strongly influenced by the perceptual organization of the stimuli. Simple modifications of the stimuli that should have little effect on low-level mechanisms greatly alter the strength of the illusion. These effects may be ascribed to more complex mechanisms occurring later in the visual system.

Receptive fields of P and M ganglion cells across the primate retina

We studied the receptive field organization and contrast sensitivity of ganglion cells located within the central 80 (radius of 40) deg of the macaque retina. Ganglion cell activity was monitored as synaptic (S) potentials recorded extracellularly in the lateral geniculate nuclei of anesthetized and paralyzed monkeys. Receptive field center and surround regions of magnocellularly-projecting (M) and parvocellularly-projecting (P) cells increase in area with distance from the fovea, with the center radii of M cells being about twice those of neighboring P cells. Peak sensitivities of center and surround regions are inversely proportional to the regions' areas, so that integrated contrast sensitivities (contrast gains) are constant across the visual field, with the gain of M cells being, on average, six times that of P cells. For both M and P cells, the average ratio of surround/center gain is 0.55. Constant gain of P cells across the visual field is achieved by increasing sensitivity to stimuli falling on the peripheral retina to an extent that counteracts the aberrations introduced by the eye's optics.

Contrast dependence of contextual effects in primate visual cortex

The responses of neurons in the visual cortex to stimuli presented within their receptive fields can be markedly modulated by stimuli presented in surrounding regions that do not themselves evoke responses. This modulation depends on the relative orientation and direction of motion of the centre and surround stimuli, and it has been suggested that local cortical circuits linking cells with similar stimulus selectivities underlie these phenomena. However, the functional relevance and nature of these integrative processes remain unclear. Here we investigate how such integration depends on the relative activity levels of neurons at different points across the cortex by varying the relative contrast of stimuli over the receptive field and surrounding regions. We show that simply altering the balance of the excitation driving centre and surround regions can dramatically change the sign and stimulus selectivity of these contextual effects. Thus, the way that single neurons integrate information across the visual field depends not only on the precise form of stimuli at different locations, but also crucially on their relative contrasts. We suggest that these effects reflect a complex gain-control mechanism that regulates cortical neuron responsiveness, which permits dynamic modification of response properties of cortical neurons.

Enhanced and diminished visuo-spatial information processing in autism depends on stimulus complexity

Visuo-perceptual processing in autism is characterized by intact or enhanced performance on static spatial tasks and inferior performance on dynamic tasks, suggesting a deficit of dorsal visual stream processing in autism. However, previous findings by Bertone et al. indicate that neuro-integrative mechanisms used to detect complex motion, rather than motion perception per se, may be impaired in autism. We present here the first demonstration of concurrent enhanced and decreased performance in autism on the same visuo-spatial static task, wherein the only factor dichotomizing performance was the neural complexity required to discriminate grating orientation. The ability of persons with autism was found to be superior for identifying the orientation of simple, luminance-defined (or first-order) gratings but inferior for complex, texture-defined (or second-order) gratings. Using a flicker contrast sensitivity task, we demonstrated that this finding is probably not due to abnormal information processing at a sub-cortical level (magnocellular and parvocellular functioning). Together, these findings are interpreted as a clear indication of altered low-level perceptual information processing in autism, and confirm that the deficits and assets observed in autistic visual perception are contingent on the complexity of the neural network required to process a given type of visual stimulus. We suggest that atypical neural connectivity, resulting in enhanced lateral inhibition, may account for both enhanced and decreased low-level information processing in autism.