High-contrast Stimuli Research Articles

Retrospective Attention Interacts with Stimulus Strength to Shape Working Memory Performance.

Orienting attention retrospectively to selective contents in working memory (WM) influences performance. A separate line of research has shown that stimulus strength shapes perceptual representations. There is little research on how stimulus strength during encoding shapes WM performance, and how effects of retrospective orienting might vary with changes in stimulus strength. We explore these questions in three experiments using a continuous-recall WM task. In Experiment 1 we show that benefits of cueing spatial attention retrospectively during WM maintenance (retrocueing) varies according to stimulus contrast during encoding. Retrocueing effects emerge for supraliminal but not sub-threshold stimuli. However, once stimuli are supraliminal, performance is no longer influenced by stimulus contrast. In Experiments 2 and 3 we used a mixture-model approach to examine how different sources of error in WM are affected by contrast and retrocueing. For high-contrast stimuli (Experiment 2), retrocues increased the precision of successfully remembered items. For low-contrast stimuli (Experiment 3), retrocues decreased the probability of mistaking a target with distracters. These results suggest that the processes by which retrospective attentional orienting shape WM performance are dependent on the quality of WM representations, which in turn depends on stimulus strength during encoding.

Spontaneous recovery of effects of contrast adaptation without awareness

Prolonged exposure to a high contrast stimulus reduces the neural sensitivity to subsequent similar patterns. Recent work has disclosed that contrast adaptation is controlled by multiple mechanisms operating over differing timescales (Bao and Engel, 2012; Bao, Fast, Mesik, & Engel, 2013). Adaptation to high contrast for a relatively longer period can be rapidly eliminated by adaptation to a lower contrast (or meanfield in the present study). Such rapid deadaptation presumably causes a short-term mechanism to signal for a sensitivity increase, canceling ongoing signals from long-term mechanisms. Once deadaptation ends, the short-term mechanism rapidly returns to baseline, and the slowly decaying effects in the long-term mechanisms reemerge, allowing the perceptual aftereffects to recover during continued testing. Although this spontaneous recovery effect is considered strong evidence supporting the multiple mechanisms theory, it remains controversial whether the effect is mainly driven by visual memory established during the initial longer-term adaptation period. In Experiment 1, adapting stimuli were either visible or rendered invisible with a modified Continuous Flash Suppression (CFS) paradigm. We used a contrast matching task to track the dynamic of effects of adaptation. We found reliable spontaneous recovery emerging rapidly in the initial phase of the post-test for both visible and invisible adapters. In Experiment 2, the adapting grating was either displayed alone or flanked by other similar gratings (i.e. a 'crowding' paradigm introduced by He, Cavanagh, & Intriligator, 1996). The time course of contrast detection thresholds were tracked on the same purpose. Spontaneous recovery was found not only in the adapter-alone condition but also in the crowded condition (orientation indiscernible). These results exclude the possibility that spontaneous recovery found in the previous work is merely the consequence of explicit visual memory. Our findings also demonstrate that contrast adaptation, even at the unconscious processing levels, is controlled by multiple mechanisms. Meeting abstract presented at VSS 2016

Multiple asynchronous stimulus- and task-dependent hierarchies (STDH) within the visual brain's parallel processing systems.

Results from a variety of sources, some many years old, lead ineluctably to a re-appraisal of the twin strategies of hierarchical and parallel processing used by the brain to construct an image of the visual world. Contrary to common supposition, there are at least three 'feed-forward' anatomical hierarchies that reach the primary visual cortex (V1) and the specialized visual areas outside it, in parallel. These anatomical hierarchies do not conform to the temporal order with which visual signals reach the specialized visual areas through V1. Furthermore, neither the anatomical hierarchies nor the temporal order of activation through V1 predict the perceptual hierarchies. The latter shows that we see (and become aware of) different visual attributes at different times, with colour leading form (orientation) and directional visual motion, even though signals from fast-moving, high-contrast stimuli are among the earliest to reach the visual cortex (of area V5). Parallel processing, on the other hand, is much more ubiquitous than commonly supposed but is subject to a barely noticed but fundamental aspect of brain operations, namely that different parallel systems operate asynchronously with respect to each other and reach perceptual endpoints at different times. This re-assessment leads to the conclusion that the visual brain is constituted of multiple, parallel and asynchronously operating task- and stimulus-dependent hierarchies (STDH); which of these parallel anatomical hierarchies have temporal and perceptual precedence at any given moment is stimulus and task related, and dependent on the visual brain's ability to undertake multiple operations asynchronously.

Visual Rehabilitation in Chronic Cerebral Blindness: A Randomized Controlled Crossover Study.

The treatment of patients suffering from cerebral blindness following stroke is a topic of much recent interest. Several types of treatment are under investigation, such as substitution with prisms and compensation training of saccades. A third approach, aimed at vision restitution is controversial, as a proper controlled study design is missing. In the current study, 27 chronic stroke patients with homonymous visual field defects were trained at home with a visual training device. We used a discrimination task for two types of stimuli: a static point stimulus and a new optic flow-discontinuity stimulus. Using a randomized controlled crossover design, each patient received two successive training rounds, one with high contrast stimuli in their affected hemifield (test) and one round with low-contrast stimuli in their intact hemifield (control). Goldmann and Humphrey perimetry were performed at the start of the study and following each training round. In addition, reading performance was measured. Goldmann perimetry revealed a statistically significant reduction of the visual field defect after the test training, but not after the control training or after no intervention. For both training rounds combined, Humphrey perimetry revealed that the effect of a directed training (sensitivity change in trained hemifield) exceeded that of an undirected training (sensitivity change in untrained hemifield). The interaction between trained and tested hemifield was just above the threshold of significance (p = 0.058). Interestingly, reduction of the field defect assessed by Goldmann perimetry increases with the difference between defect size as measured by Humphrey and Goldmann perimetry prior to training. Moreover, improvement of visual sensitivity measured by Humphrey perimetry increases with the fraction of non-responsive elements (i.e., more relative field loss) in Humphrey perimetry prior to training. Reading speed revealed a significant improvement after training. Our findings demonstrate that our training can result in reduction of the visual field. Improved reading performance after defect training further supports the significance of our training for improvement in daily life activities.

Attention Reorients Periodically

Reorienting of voluntary attention enables the processing of stimuli at previously unattended locations. Although studies have identified a ventral fronto-parietalnetwork underlying attention [1, 2], little is known about whether and how early visual areas are involved in involuntary [3, 4] and even less in voluntary [5] reorienting, and their temporal dynamics are unknown. We used transcranial magnetic stimulation (TMS) over the occipital cortex to interfere with attentional reorienting and study its role and temporal dynamics in this process. Human observers performed an orientation discrimination task, with either valid or invalid attention cueing, across a range of stimulus contrasts. Valid cueing induced a behavioral response gain increase, higher asymptotic performance for attended than unattended locations. During subsequent TMS sessions, observers performed the same task, with high stimulus contrast. Based on phosphene mapping, TMS double pulses were applied at one of various delays to a consistent brain location in retinotopic areas (V1/V2), corresponding to the evoked signal of thetarget or distractor, in a valid or invalid trial. Thus, the stimulation was identical for the four experimental conditions (valid/invalid cuecondition× target/distractor-stimulated). TMS modulation of the target and distractor were both periodic (5Hz, theta) and out of phase with respect to each other in invalid trials only, when attention had to be disengaged from the distractor and reoriented to the target location. Reorientation of voluntary attention periodically involves V1/V2 at the theta frequency. These results suggest that TMS probes theta phase-reset by attentional reorienting and help link periodic sampling in time and attention reorienting in space.

Use of a steady-state baseline to address evoked vs. oscillation models of visual evoked potential origin

There has been a long debate about the neural mechanism of event-related potentials (ERPs). Previously, no evidence or method was apparent to validate the two competing models, the evoked model and the oscillation model. One argument is whether the pre-stimulus brain oscillation could influence the following ERP. This study carried out an innovative visual oddball task experiment to investigate the dynamic process of visual evoked potentials. A period of stable oscillations of specified dominant frequencies and initial phases, i.e. the steady-state baseline, would be induced before responses to transient stimuli of different contrasts, which could overcome the artifact problem caused by the ‘sorting’ method. The result first revealed a ‘three-period-transition’ for the generation of visual evoked potentials by an objective decomposition. The ERP almost retained the preceding oscillation during the first period, provided an unstable negative potential in the second period, and generated the N1 component in the third period. The cross term analysis showed that the evoked model couldn't be the whole explanation for the ERP generation. Furthermore, the component analysis revealed that the N1 latency was sensitive to the initial phase under the low stimulus contrast (supporting the oscillation model) but not under the high stimulus contrast (supporting the evoked model). It demonstrated that the external stimulus contrast is a significant factor deciding the explicit model for ERPs. Our method and preliminary results may help reconcile the previous, seemly contradictory findings on the ERP mechanism.

Observers' Judgments of the Effects of Glare on Their Visual Acuity for High and Low Contrast Stimuli.

Disability glare refers to a reduction in the ability to discern a stimulus that is positioned near another stimulus that has a much higher luminance. While it is common for drivers to report that they have been "blinded" by oncoming headlights, it is unclear whether observers can accurately judge when they are visually disabled by glare. This experiment sought to quantify the accuracy with which observers can judge when a glare source reduces their visual acuity. Seventeen observers estimated their disability glare threshold (DGT)-the luminance of a glare source that would be just sufficient to impair their ability to discern the orientation of a Landolt C that was surrounded by the glare source. These estimated DGTs were compared to the participant's actual DGTs. Participants consistently underestimated the intensity of glare that was required to impair their acuity. On average, estimates of glare threshold were 88% lower than actual glare threshold intensities. Participants' judgments were affected by stimulus size but not stimulus contrast. These results suggest that observers can exaggerate the debilitating effects of glare and that they can fail to appreciate that high contrast stimuli are more robust to glare. A driver who believes that even the lowest intensities of headlight glare can visually impair an oncoming driver may be reluctant to use high beam headlamps, despite their significant visibility advantages.

Differential effects of forward and backward masks on the relationship between perception and action.

A recent series of experiments has shown that the effects of near-threshold stimuli on perceptual and motor responses are highly dependent on experimental conditions. In particular, motor influences of near-threshold distractors were observed when using low-contrast unmasked stimuli and high-contrast masked stimuli although only the latter affected motor responses in the absence of stimulus awareness. These results are compatible with neurophysiological models of visual masking, suggesting that early neural responses to a visual stimulus can be decomposed in feedforward activations to-and feedback activations from-higher visual areas, correlating respectively with the actual presence of the stimulus and its conscious perception. We tested the compatibility between these neurophysiological models and the behavioural data obtained in near-threshold experiments. We recorded fast reaching movements directed to a highly visible target followed by a report of the presence of a near-threshold distractor presented either at low contrast without mask or at high contrast with a backward or forward mask. Analysis of hand trajectories revealed that deviations toward the distractor were observed in the no-mask condition when the distractor was present and reported, and when it was present but not reported in the backward and forward mask conditions, although the effect was weaker in the latter condition. These data reveal that the presence or absence of perception-action dissociations in behavioural studies are well accounted for by neurophysiological models of visual masking and that behavioural effects of near-threshold distractors cannot result merely from on a dichotomic visual system for perception and action.

Cue-dependent circuits for illusory contours in humans

Objects' borders are readily perceived despite absent contrast gradients, e.g. due to poor lighting or occlusion. In humans, a visual evoked potential (VEP) correlate of illusory contour (IC) sensitivity, the “IC effect”, has been identified with an onset at ~90ms and generators within bilateral lateral occipital cortices (LOC). The IC effect is observed across a wide range of stimulus parameters, though until now it always involved high-contrast achromatic stimuli. Whether IC perception and its brain mechanisms differ as a function of the type of stimulus cue remains unknown. Resolving such will provide insights on whether there is a unique or multiple solutions to how the brain binds together spatially fractionated information into a cohesive perception. Here, participants discriminated IC from no-contour (NC) control stimuli that were either comprised of low-contrast achromatic stimuli or instead isoluminant chromatic contrast stimuli (presumably biasing processing to the magnocellular and parvocellular pathways, respectively) on separate blocks of trials. Behavioural analyses revealed that ICs were readily perceived independently of the stimulus cue—i.e. when defined by either chromatic or luminance contrast. VEPs were analysed within an electrical neuroimaging framework and revealed a generally similar timing of IC effects across both stimulus contrasts (i.e. at ~90ms). Additionally, an overall phase shift of the VEP on the order of ~30ms was consistently observed in response to chromatic vs. luminance contrast independently of the presence/absence of ICs. Critically, topographic differences in the IC effect were observed over the ~110–160ms period; different configurations of intracranial sources contributed to IC sensitivity as a function of stimulus contrast. Distributed source estimations localized these differences to LOC as well as V1/V2. The present data expand current models by demonstrating the existence of multiple, cue-dependent circuits in the brain for generating perceptions of illusory contours.

On-off selectivity and asymmetry in apparent contrast: An adaptation study.

Using textures composed of sparse bright/dark elements that can activate either on or off sensors selectively, Sato, Motoyoshi, and Sato (2012) reported simultaneous contrast-contrast effects tuned for contrast polarity. As with contrast-contrast effects, prolonged viewing of high-contrast stimuli reduces the perceived contrast of a subsequently presented stimulus. The present study examined whether contrast aftereffects are also selective for luminance polarity using texture patterns composed of sparse bright/dark elements. Results revealed that contrast aftereffects are selective for luminance polarity (polarity selectivity) but that adaptation aftereffects occur asymmetrically depending on the polarity of the adapter (polarity asymmetry). Polarity selectivity and asymmetry in adaptation aftereffects are reduced but not completely diminished if adapter and test stimuli are presented to separate eyes (dichoptically). Our results support the idea that suprathreshold contrast perception and its adaptational shifts are jointly determined by responses between monocular and binocular units.

Monitoring occlusion therapy in amblyopic children using pattern visual evoked potential

Background Amblyopia is a unilateral or bilateral reduction of best-corrected vision that cannot be attributed only to a structural abnormality of the eye. It is a common childhood problem, and can be reversed if treated while the visual system is still maturing. Objective The aim of the present study was to investigate the role of pattern visual evoked potential (PVEP) in the assessment of visual function after occlusion therapy for children with unilateral strabismic amblyopia. Patients and methods Visual function was assessed clinically and using monocular PVEP, elicited by high-contrast checkerboard-patterned stimuli, before and after occlusion of the healthy eye for 1 week for every year of life in 20 children presenting with unilateral strabismic amblyopia. Results PVEP responses were significantly prolonged in latency in amblyopic eyes after the first assessment compared with nonamblyopic eyes. There was no significant difference in the interside amplitude. Assessment after occlusion showed a statistically significant reduction in the latency of the amblyopic eye and statistically significant improvement in the visual acuity. Conclusion PVEP can be used as a method of assessment of visual function after the occlusion therapy for children with unilateral strabismic amblyopia.

Enhancing visual performance in individuals with cortical visual impairment (homonymous hemianopsia): Tapping into blindsight

Abstract Homonymous hemianopsia is a type of cortical blindness in which vision is lost completely or partially in the left half or the right half of the field of vision. It is prevalent in approximately 12% of traumatic brain injury and 35% of strokes. Patients often experience difficulty with activities such as ambulating, eating, reading, and driving. Due to the high prevalence of homonymous hemianopsia and its associated difficulties, it is imperative to find methods for visual rehabilitation in this condition. Traditional methods such as prism glasses can cause visual confusion and result in patient noncompliance. There is a large unmet medical need for improving this condition. In this article, we propose that modifying visual stimuli to activate non-cortical areas of visual processing, such as lateral geniculate nucleus and superior colliculus, may result in increased visual awareness. Presenting high contrast and low spatial frequency visual stimuli can increase visual detection in patients with cortical blindness, a phenomenon known as blindsight. Augmented virtual reality goggles have the potential to alter real-time visual input to high contrast and low spatial frequency images, possibly improving visual detection in the blind hemifield and providing an alternative therapy for homonymous hemianopsia.

Greater sensitivity of the cortical face processing system to perceptually-equated face detection

Face detection, the perceptual capacity to identify a visual stimulus as a face before probing deeper into specific attributes (such as its identity or emotion), is essential for social functioning. Despite the importance of this functional capacity, face detection and its underlying brain mechanisms are not well understood. This study evaluated the roles that the cortical face processing system, which is identified largely through studying other aspects of face perception, play in face detection. Specifically, we used functional magnetic resonance imaging (fMRI) to examine the activations of the fusifom face area (FFA), occipital face area (OFA) and superior temporal sulcus (STS) when face detection was isolated from other aspects of face perception and when face detection was perceptually-equated across individual human participants (n=20). During face detection, FFA and OFA were significantly activated, even for stimuli presented at perceptual-threshold levels, whereas STS was not. During tree detection, however, FFA and OFA were responsive only for highly salient (i.e., high contrast) stimuli. Moreover, activation of FFA during face detection predicted a significant portion of the perceptual performance levels that were determined psychophysically for each participant. This pattern of result indicates that FFA and OFA have a greater sensitivity to face detection signals and selectively support the initial process of face vs. non-face object perception.

Spontaneous recovery of effects of contrast adaptation without awareness.

Prolonged exposure to a high contrast stimulus reduces the neural sensitivity to subsequent similar patterns. Recent work has disclosed that contrast adaptation is controlled by multiple mechanisms operating over differing timescales. Adaptation to high contrast for a relatively longer period can be rapidly eliminated by adaptation to a lower contrast (or meanfield in the present study). Such rapid deadaptation presumably causes a short-term mechanism to signal for a sensitivity increase, canceling ongoing signals from long-term mechanisms. Once deadaptation ends, the short-term mechanism rapidly returns to baseline, and the slowly decaying effects in the long-term mechanisms reemerge, allowing the perceptual aftereffects to recover during continued testing. Although this spontaneous recovery effect is considered strong evidence supporting the multiple mechanisms theory, it remains controversial whether the effect is mainly driven by visual memory established during the initial longer-term adaptation period. To resolve this debate, we used a modified Continuous Flash Suppression (CFS) and visual crowding paradigms to render the adapting stimuli invisible, but still observed the spontaneous recovery phenomenon. These results exclude the possibility that spontaneous recovery found in the previous work was merely the consequence of explicit visual memory. Our findings also demonstrate that contrast adaptation, even at the unconscious processing levels, is controlled by multiple mechanisms.

Interaction effects of visual stimulus speed and contrast on postural sway.

Manipulating the characteristics of visual stimuli that simulate self-motion through the environment can affect the resulting postural sway magnitude. In the present study, we address the question whether varying the contrast and speed of a linear translating dot pattern influences medial–lateral postural sway. In a first experiment, we investigated whether the postural sway magnitude increases with increasing dot speed, as was previously demonstrated for expanding and contracting stimuli. In a second experiment, we also manipulated the contrast of the stimuli. For reasons that high-contrast stimuli can be considered ‘perceptually’ stronger, we expect that higher-contrast stimuli induce more sway than lower-contrast stimuli. The results of the first experiment show that dot speed indeed influences postural sway, although in an unexpected way. For higher speeds, the sway is in the direction of the stimulus motion, yet for lower speeds the sway is in a direction opposite to the stimulus motion. The results of the second experiment show that dot contrast does affect postural sway, but that this depends on the speed of the moving dots. Interestingly, the direction of postural sway induced by a relatively low dot speed (4°/s) depends on dot contrast. Taken together, our results suggest that interactions between the visual, vestibular and proprioceptive system appear to be influenced by an internal representation of the visual stimulus, rather than being influenced by the external visual stimulus characteristics only.

Revealing hidden states in visual working memory using electroencephalography.

It is often assumed that information in visual working memory (vWM) is maintained via persistent activity. However, recent evidence indicates that information in vWM could be maintained in an effectively “activity-silent” neural state. Silent vWM is consistent with recent cognitive and neural models, but poses an important experimental problem: how can we study these silent states using conventional measures of brain activity? We propose a novel approach that is analogous to echolocation: using a high-contrast visual stimulus, it may be possible to drive brain activity during vWM maintenance and measure the vWM-dependent impulse response. We recorded electroencephalography (EEG) while participants performed a vWM task in which a randomly oriented grating was remembered. Crucially, a high-contrast, task-irrelevant stimulus was shown in the maintenance period in half of the trials. The electrophysiological response from posterior channels was used to decode the orientations of the gratings. While orientations could be decoded during and shortly after stimulus presentation, decoding accuracy dropped back close to baseline in the delay. However, the visual evoked response from the task-irrelevant stimulus resulted in a clear re-emergence in decodability. This result provides important proof-of-concept for a promising and relatively simple approach to decode “activity-silent” vWM content using non-invasive EEG.

Global motion perception is independent from contrast sensitivity for coherent motion direction discrimination and visual acuity in 4.5-year-old children

Global motion processing depends on a network of brain regions that includes extrastriate area V5 in the dorsal visual stream. For this reason, psychophysical measures of global motion perception have been used to provide a behavioral measure of dorsal stream function. This approach assumes that global motion is relatively independent of visual functions that arise earlier in the visual processing hierarchy such as contrast sensitivity and visual acuity. We tested this assumption by assessing the relationships between global motion perception, contrast sensitivity for coherent motion direction discrimination (henceforth referred to as contrast sensitivity) and habitual visual acuity in a large group of 4.5-year-old children (n=117). The children were born at risk of abnormal neurodevelopment because of prenatal drug exposure or risk factors for neonatal hypoglycemia. Motion coherence thresholds, a measure of global motion perception, were assessed using random dot kinematograms. The contrast of the stimuli was fixed at 100% and coherence was varied. Contrast sensitivity was measured using the same stimuli by fixing motion coherence at 100% and varying dot contrast. Stereoacuity was also measured. Motion coherence thresholds were not correlated with contrast sensitivity or visual acuity. However, lower (better) motion coherence thresholds were correlated with finer stereoacuity (ρ=0.38, p=0.004). Contrast sensitivity and visual acuity were also correlated (ρ=−0.26, p=0.004) with each other. These results indicate that global motion perception for high contrast stimuli is independent of contrast sensitivity and visual acuity and can be used to assess motion integration mechanisms in children.

Temporal evolution of motion direction judgments

Existing models of perceptual decision-making for moving stimuli assume a continuous accumulation of sensory signals for a given direction of motion (Smith & Ratcliff, 2004). This predicts concurrent improvements in estimating motion axis (orientation of motion direction) and motion direction over time. Here, we present a novel finding that the visual system can extract motion axis orientation before detecting motion direction. Methods and results: To estimate the temporal evolution of motion direction judgments, we asked participants to adjust an on-screen arrow to indicate perceived direction of moving stimuli varying in duration (rigid texture motion; randomly chosen direction; 99% contrast, 11° radius, 5°/s speed; stimuli presented at 360Hz). As expected, participants’ responses were completely random for very brief stimulus motions (5ms) and distributed around the actual motion direction for long durations (>90ms). Interestingly, for intermediate durations (30-60ms), participants’ responses exhibited a clear bimodal distribution with equal peaks in the actual motion direction and in the opposite direction. These results indicate that participants extract motion axis information before having a sense of the actual motion direction. In a related experiment, we found that for certain brief stimulus durations, subjects were 100% correct at estimating motion axis while being at chance at perceiving motion direction. Next, we ran several experiments to rule out possible contributions of spatial signals (i.e., the well-known motion streaks). Finally, we explored the contrast and size dependency of this result (5-99% contrast; 1°-11° radius). The results reveled that rapid and early coding of motion axis was mainly associated with large, high-contrast moving stimuli, stimuli associated with center-surround suppression of motion signals (Tadin et al., 2003). Conclusion: We show the visual system can extract motion axis information considerably faster than motion direction information. The specificity to large, high-contrast motion stimuli offers new clues how center-surround suppression affects motion perception. Meeting abstract presented at VSS 2015

Mathematical impairment associated with high-contrast abnormalities in change detection and magnocellular visual evoked response.

The cause of developmental dyscalculia, a specific deficit in acquisition of arithmetic skills, particularly of enumeration, has never been investigated with respect to the patency of the visual magnocellular system. Here, the question of dysfunction of the afferent magnocellular cortical input and its dorsal stream projections was tested directly using nonlinear analysis of the visual evoked potential (VEP) and through the psychophysical ability to rapidly detect visual change. A group of young adults with self-reported deficiencies of arithmetical ability, showed marked impairment in magnitude estimation and enumeration performance-though not in lexical decision reaction times when compared with an arithmetically capable group controlled for age and handedness. Multifocal nonlinear VEPs were recorded at low (24 %) and high (96 %) contrast. First- and second-order VEP kernels were comparable between groups at low contrast, but not at high contrast. The mathematically impaired group showed an abnormal lack of contrast saturation in the shortest latency first-order peak (N60) and a delayed P100 positivity in the first slice of the second-order kernel. Both features have previously been argued to be physiological markers of magnocellular function. Mathematically impaired participants also performed worse on a gap paradigm change detection for digit task showing increased reaction times for high-contrast stimuli but not for low-contrast stimuli compared with controls. The VEP results give direct evidence of abnormality in the occipital processing of magnocellular information in those with mathematical impairment. The anomalous high visual contrast physiological and psychophysical performance suggests an abnormality in the inhibitory processes that normally result in saturation of contrast gain in the magnocellular system.

Abnormal Contrast Responses in the Extrastriate Cortex of Blindsight Patients

When the human primary visual cortex (V1) is damaged, the dominant geniculo-striate pathway can no longer convey visual information to the occipital cortex. However, many patients with such damage retain some residual visual function that must rely on an alternative pathway directly to extrastriate occipital regions. This residual vision is most robust for moving stimuli, suggesting a role for motion area hMT+. However, residual vision also requires high-contrast stimuli, which is inconsistent with hMT+ sensitivity to contrast in which even low-contrast levels elicit near-maximal neural activation. We sought to investigate this discrepancy by measuring behavioral and neural responses to increasing contrast in patients with V1 damage. Eight patients underwent behavioral testing and functional magnetic resonance imaging to record contrast sensitivity in hMT+ of their damaged hemisphere, using Gabor stimuli with a spatial frequency of 1 cycle/°. The responses from hMT+ of the blind hemisphere were compared with hMT+ and V1 responses in the sighted hemisphere of patients and a group of age-matched controls. Unlike hMT+, neural responses in V1 tend to increase linearly with increasing contrast, likely reflecting a dominant parvocellular channel input. Across all patients, the responses in hMT+ of the blind hemisphere no longer showed early saturation but increased linearly with contrast. Given the spatiotemporal parameters used in this study and the known direct subcortical projections from the koniocellular layers of the lateral geniculate nucleus to hMT+, we propose that this altered contrast sensitivity in hMT+ could be consistent with input from the koniocellular pathway.