Coherence Thresholds Research Articles

Improved Visual Cognition through Stroboscopic Training

Humans have a remarkable capacity to learn and adapt, but surprisingly little research has demonstrated generalized learning in which new skills and strategies can be used flexibly across a range of tasks and contexts. In the present work we examined whether generalized learning could result from visual–motor training under stroboscopic visual conditions. Individuals were assigned to either an experimental condition that trained with stroboscopic eyewear or to a control condition that underwent identical training with non-stroboscopic eyewear. The training consisted of multiple sessions of athletic activities during which participants performed simple drills such as throwing and catching. To determine if training led to generalized benefits, we used computerized measures to assess perceptual and cognitive abilities on a variety of tasks before and after training. Computer-based assessments included measures of visual sensitivity (central and peripheral motion coherence thresholds), transient spatial attention (a useful field of view – dual task paradigm), and sustained attention (multiple-object tracking). Results revealed that stroboscopic training led to significantly greater re-test improvement in central visual field motion sensitivity and transient attention abilities. No training benefits were observed for peripheral motion sensitivity or peripheral transient attention abilities, nor were benefits seen for sustained attention during multiple-object tracking. These findings suggest that stroboscopic training can effectively improve some, but not all aspects of visual perception and attention.

Scale-dependent loss of global form perception in strabismic amblyopia

Amblyopic humans are known to have a range of spatial vision abnormalities. Prior studies have documented amblyopic deficits in global form perception but have typically used only one set of stimulus parameters. Our aim in this study was to examine the extent and nature of global form perception deficits in strabismic amblyopia using a range of spatial scales and pattern types. Glass patterns are random dot stimuli in which the local orientations of paired dots must be integrated over space to yield a global form percept. We measured coherence thresholds for discrimination of pattern structure in translational (linear) and concentric Glass patterns at three spatial scales in two control and six amblyopic observers. We found that sensitivity to Glass patterns depended on both spatial scale and pattern type in all observers. Participants with a history of abnormal early visual experience showed greater interocular threshold difference when the discrimination was based on translational patterns than when it was based on concentric patterns, and the degree of amblyopic loss was greatest at fine spatial scale. Our results show that the nature and extent of global form vision deficits vary substantially with stimulus parameters and are greatest at fine spatial scales.

The role of contrast sensitivity in global motion processing deficits in the elderly

This study compared the effects of age on the perception of translational, radial, and rotational global motion patterns. Motion coherence thresholds were measured for judging the direction of each motion type as a function of contrast (visibility) and temporal sampling rate in young and elderly participants. Coherence thresholds decreased as dot contrast increased asymptoting at high dot contrasts but were higher in elderly compared to young participants. This equated to global motion impairment in the elderly of a factor of around 2, characterized by a shift of the threshold vs. contrast function along the horizontal axes (dot contrast). The effect of contrast interacted with the temporal sampling rate. Old participants were deleteriously affected by reduced temporal sampling particularly at low contrasts. The findings suggest that age-related changes in global motion perception may be driven principally by deficits in contrast encoding, rather than by deficits in motion integration and suggest a role for increased internal noise in the older visual system.

Quantifying Sensory Eye Dominance in the Normal Visual System: A New Technique and Insights into Variation across Traditional Tests

Although eye dominance assessment is used to assist clinical decision-making, current understanding is limited by inconsistencies across the range of available tests. A new psychophysical test of sensory eye dominance has been developed that objectively measures the relative contribution of each eye to a fused suprathreshold binocular percept. Six standard tests and the newly developed test were used to measure motor and sensory dominance in a group of 44 binocularly normal individuals (mean age, 29.5 ± 9.10 years). The new test required observers to perform a motion coherence task under dichoptic viewing conditions, wherein a population of moving, luminance-defined signal (coherently moving) and noise (randomly moving) dots were presented separately to each eye. The observers judged the motion direction of the signal dots. Motion coherence thresholds were measured by varying the ratio of signal-to-noise dots, in a staircase procedure. The new dichoptic motion coherence threshold test revealed a clear bimodal distribution of sensory eye dominance strength, wherein the majority of the participants (61%) showed weak dominance, but a significant minority (39%) showed strong dominance. Subsequent analysis revealed that the strong-dominance group showed greater consistency across the range of traditional eye dominance tests used. This new quantitative dichoptic motion coherence threshold technique suggests that there are two separate sensory eye dominance strength distributions among observers with normal binocular vision: weak and strong eye dominance. This finding may provide a basis for clinical decision-making by indicating whether eye dominance is likely to be an important consideration in a particular patient.

The effect of speed on the typical and atypical development of motion-defined form perception

Our previous work on motion-defined form perception showed: 1) slow maturation to adult levels by 7 years of age, and 2) large fellow-eye deficits in children with amblyopia. These developmental changes could reflect the functioning of the M/dorsal or the P/ventral pathway. The current study used motion speed to bias the relative contribution of the M/dorsal (fast motion) and P/ventral (slow motion) pathways to the processing of motion-defined form. Motion-defined rectangles (vertical or horizontal) were created by moving a proportion of dots coherently (up or down) while the remaining dots moved in random directions at the same speed within the rectangle. Outside the rectangle, dots moved in the opposite direction with the same speed and coherence. Monocular motion coherence thresholds, for discriminating rectangle orientation, were determined at slow (0.85 deg/s) and at fast speeds (5.1 deg/s) using a staircase procedure. First we examined normal development in children (4–6 years), adolescents (11–12 years) and young adults. Consistent with our previous results, coherence thresholds were higher in children than in adolescents and adults, with no difference between the two older groups. In addition, coherence thresholds were lower at the slow than at the fast speed for all three groups. Next we examined patients aged 11 years and older with anisometropic and/or strabismic amblyopia. A few individuals showed a deficit in either eye at both speeds, and thresholds were lower in those with compromised binocularity. As a group, however, performance was similar to that of the controls. This is contrary to our previous results, possibly due to the older age of the observers or to the use of coherence rather than minimum-speed thresholds. Our results provide no evidence for a developmental difference in M/dorsal versus P/ventral pathways. The speed effect may reflect the stronger contribution of the slow-motion system to motion-defined form perception.

Brain responses to global perceptual coherence

Coherence thresholds for concentric patterns of contour segments or dot trajectories have revealed differential development and vulnerability of extrastriate form and motion pathways respectively (Braddick et al, Neuropsychologia, 2003). Here we explore the properties of visual evoked potentials (VEPs) specific to form and motion coherence. Coherent contour or motion arrays alternate with incoherent arrays at 0.5-4 Hz. A 2nd harmonic VEP signal (F2) may arise from local changes occuring at each transition, but a first harmonic (F1) must reflect differential responses to coherence and incoherence, indicating global processing. F1 amplitude is approximately linear with coherence for both form and motion, confirming that F1 measures global processing. F2 is independent of coherence for form, and nearly so for motion, implying that it reflects local changes. Occipital F1 amplitudes for motion: form coherence are approximately 2:1. However, this does not reflect underlying sensitivity: psychophysical form and motion coherence thresholds are similar, and extrapolation of the linear region of F1 amplitude vs coherence reaches zero close to threshold coherence in each case. F1 amplitude for both displays increases gradually from 0.5-4 Hz. However, this is opposite to psychophysical coherence sensitivity, which falls over the same range. We conclude: (a) coherence-dependent VEPs provide measures of global form and motion processing, practical for infants and non-verbal (e.g. autistic) children; (b) this response is elicited efficiently at relatively high presentation rates; (c) the level tapped by the VEP does not provide the limiting factor on temporal integration in global perceptual processing.

Motion coherence thresholds can be elevated by flicker adaptation or red background

Purpose: Previous studies have shown that motion coherence thresholds are elevated in several disorders including dyslexia (Cornelissen et al. 1995), glaucoma (Silverman et al. 1990), Alzheimer's disease (Gilmore et al. 1994) and Parkinson's disease (Trick et al. 1994). This threshold elevation is consistent with a deficit in the functioning of the magnocellular (M) pathway. We compared two psychophysical methods of temporarily disrupting normal M-pathway functioning as indexed by elevated motion coherence thresholds. Methods: 30 normal adults participated in this study. We measured motion coherence thresholds for limited lifetime dots moving left or right at a speed of 0.935 deg/s. Dot density was 1 dot/deg2. Coherence thresholds were determined in 4 conditions: after 2 minutes of adaptation to (i) a uniform field flickering at 9 Hz (experimental 1) (ii) a uniform gray field (control 1) and with dots presented on (iii) a uniform red background (experimental 2) and (iv) a gray background (control 2). Results: Motion coherence thresholds were significantly elevated in both experimental conditions (experimental 1 M = 0.2771, control 1 M = 0.1981, experimental 2 M = 0.2198, control 2 M = 0.1873). In addition, the threshold elevation was significantly greater after flicker adaptation than in the presence of a uniform red background. Conclusions: Either flicker adaptation or a red background can be used to disrupt direction selective global motion perception. Physiological studies have shown that magnocellular layers of the LGN are inhibited by a red background (Wiesel & Hubel, 1966) and are essential for flicker perception in monkeys (Merigan, Byrne & Maunsell, 1991). Therefore our results suggest that interference with processing at this level in the M pathway disrupts global motion perception. These techniques are currently being used to study the relationship between M-pathway functioning and reading in people with dyslexia.

The effect of dot lifetime, dot size, & percent area covered by dots on motion coherence thresholds: Implications for diagnosing reading difficulties

The effect of dot lifetime, dot size, & percent area covered by dots on motion coherence thresholds: Implications for diagnosing reading difficulties

Age-related deficits in motion coherence thresholds

Age-related deficits in motion coherence thresholds

Sensitivity of the Goldfish Motion Detection System Revealed by Incoherent Random Dot Stimuli: Comparison of Behavioural and Neuronal Data

BackgroundGlobal motion detection is one of the most important abilities in the animal kingdom to navigate through a 3-dimensional environment. In the visual system of teleost fish direction-selective neurons in the pretectal area (APT) are most important for global motion detection. As in all other vertebrates these neurons are involved in the control of slow phase eye movements during gaze stabilization. In contrast to mammals cortical pathways that might influence motion detection abilities of the optokinetic system are missing in teleost fish.ResultsTo test global motion detection in goldfish we first measured the coherence threshold of random dot patterns to elicit horizontal slow phase eye movements. In addition, the coherence threshold of the optomotor response was determined by the same random dot patterns. In a second approach the coherence threshold to elicit a direction selective response in neurons of the APT was assessed from a neurometric function. Behavioural thresholds and neuronal thresholds to elicit slow phase eye movements were very similar, and ranged between 10% and 20% coherence. In contrast to these low thresholds for the optokinetic reaction and APT neurons the optomotor response could only be elicited by random dot patterns with coherences above 40%.ConclusionOur findings suggest a high sensitivity for global motion in the goldfish optokinetic system. Comparison of neuronal and behavioural thresholds implies a nearly one-to-one transformation of visual neuron performance to the visuo-motor output. In addition, we assume that the optomotor response is not mediated by the optokinetic system, but instead by other motion detection systems with higher coherence thresholds.

Reorganization of Global Form and Motion Processing during Human Visual Development

The functional selectivity of human primary visual cortex (V1) for orientation and motion direction is established by around 3 months of age [1-3], but there have been few studies of the development of extrastriate visual areas that integrate outputs from V1 [4-8]. We investigated sensitivity and topographical organization for global form and motion with high-density visual event-related potentials (VERPs) in 4- to 5-month-old infants and adults. Responses were measured to transitions between concentrically organized elements (short arc segments for form, dot trajectories for motion) and random arrangements. Adults showed topographically separate responses, with midline motion and more lateral form responses. Of 26 infants, 25 showed significant motion responses but only 13 showed form responses, suggesting more advanced development for extrastriate motion areas than form. Infants' form and motion responses were topographically distinct but contrasted with the corresponding adult topographies, with infants' motion responses more lateral than form responses. These results imply distinct neural sources at both ages and raise the possibility of substantial reorganization of extrastriate networks between infancy and adulthood. We speculate that global motion responses arise from area V5 in infants but are dominated by more medial areas such as V3/V3A and V6 in adults.

Development of motion processing in children with autism

Recent findings suggest that children with autism may be impaired in the perception of biological motion from moving point-light displays. Some children with autism also have abnormally high motion coherence thresholds. In the current study we tested a group of children with autism and a group of typically developing children aged 5 to 12 years of age on several motion perception tasks, in order to establish the specificity of the biological motion deficit in relation to other visual discrimination skills. The first task required the recognition of biological from scrambled motion. Three quasi-psychophysical tasks then established individual thresholds for the detection of biological motion in dynamic noise, of motion coherence and of form-from-motion. Lastly, individual thresholds for a task of static perception--contour integration (Gabor displays)--were also obtained. Compared to controls, children with autism were particularly impaired in processing biological motion in relation to any developmental measure (chronological or mental age). In contrast, there was some developmental overlap in ability to process other types of visual motion between typically developing children and the children with autism, and evidence of developmental change in both groups. Finally, Gabor display thresholds appeared to develop typically in children with autism.

Temporal characteristics of global motion processing revealed by transcranial magnetic stimulation

The ability to detect the motion of objects is critical to survival, and understanding the cortical mechanisms involved in this process remains a key challenge in sensory neuroscience. A relatively new approach to this problem is to temporarily disrupt processing at specific cortical sites and measure the behavioural consequences. Several previous studies have shown that transcranial magnetic stimulation (TMS) of human visual area V5/MT disrupts global motion perception, but reports vary widely in the timescale of this effect. To resolve this issue we employed psychophysical techniques to investigate how discrimination of translational, rotational and radial global motion is affected by TMS. Prior to applying TMS we established baseline coherence thresholds for global motion perception. Adopting each observer's coherence level at threshold we examined how TMS delivered to V5/MT modulated performance. Importantly, we measured the influence of single-pulse TMS over a broad temporal range to reveal the fine temporal structure of the disruption profile for global motion perception. Results show that the disruption profile consisted of two distinct epochs during which global direction judgments were reliably impaired, separated by an interval in which performance was unaffected. The bimodal nature of the distribution profiles is consistent with feedforward and feedback processing between visual areas mediating global motion processing. We present a novel quantitative model that characterizes the contribution of each process to visual motion perception.

Impaired recognition of emotions from body movements is associated with elevated motion coherence thresholds in autism spectrum disorders

Recent research has confirmed that individuals with autism spectrum disorder (ASD) have difficulties in recognizing emotions from body movements. Difficulties in perceiving coherent motion are also common in ASD. Yet it is unknown whether these two impairments are related. Thirteen adults with ASD and 16 age- and IQ-matched typically developing (TD) adults classified basic emotions from point-light and full-light displays of body movements and discriminated the direction of coherent motion in random-dot kinematograms. The ASD group was reliably less accurate in classifying emotions regardless of stimulus display type, and in perceiving coherent motion. As predicted, ASD individuals with higher motion coherence thresholds were less accurate in classifying emotions from body movements, especially in the point-light displays; this relationship was not evident for the TD group. The results are discussed in relation to recent models of biological motion processing and known abnormalities in the neural substrates of motion and social perception in ASD.

Deficits in local and global motion perceptionarising from abnormal eye movements

Albino mammals exhibit a range of visual deficits including disrupted hemispheric pathways, an underdeveloped central retina, and nystagmus. Recently, it has been reported that albino animals also show deficits in the processing of visual motion, exhibiting higher motion coherence thresholds (MCTs; the proportion of coherently moving elements within a field of randomly moving distracters required to reliably report direction). Here we compare MCTs-collected from human observers with albinism-with an equivalent noise analysis of their fine-direction discrimination and report that their loss in motion sensitivity operates at both the level of local motion processing (of small objects) and at the later stage of global motion pooling. We also compare results from observers with aniridia (characterized by underdeveloped central retina and nystagmus but normal hemispheric visual pathways) and a rare group of observers with albinism who show no nystagmus. For the observers tested, nystagmus proved to be a common feature of individuals showing elevated MCTs. Since it is likely that motion perception is influenced by environmental factors early in development we postulate that the effect of congenital nystagmus on the temporal structure of the natural visual diet disrupts the ability of motion pathways to form normally.

Assessing Attention in Children with Williams Syndrome and Down's Syndrome Using a New Comprehensive Attention Battery: Do These Problems Relate to Visual Dorsal Cortical Stream Deficits?

Event Abstract Back to Event Assessing Attention in Children with Williams Syndrome and Down's Syndrome Using a New Comprehensive Attention Battery: Do These Problems Relate to Visual Dorsal Cortical Stream Deficits? K. Breckenridge1* and J. Atkinson1 1 University College London, Visual Development Unit,, United Kingdom Purpose: There are many anecdotal reports of attention problems in both Williams syndrome (WS) and Down’s syndrome (DS), with some recent studies suggesting difficulties on frontal lobe executive function tasks (e.g. Atkinson et al, 2003). However, for many children with WS and DS who have a mental age below 6 years, there has been no standardized single battery to assess the different components of attention (selective attention, sustained attention and attentional / executive control) which are thought to be underpinned by different neural networks of brain areas in the adult brain and in typically developing older children (e.g. Posner & Petersen, 1990; Manly et al, 2001). In the Visual Development Unit, we have recently devised and normalized a new battery designed to assess different attention components in children of 3-6 years mental age (MA), including children with developmental disorders. We report the results of a study using this battery to map profiles of attention in children with WS and DS and consider the relationship between attentional deficits and dorsal stream deficits, as manifested in the comparison of motion and form coherence thresholds (e.g. Atkinson et al, 1997; Braddick et al, 2003). Methods: There were 8 short subtests in the battery, each test lasting between 2 and 10 minutes. These subtests included a visual search and a flanker task, intended to tap selective attention; vigilance-type sustained attention tasks in visual and auditory modalities; and tests of verbal inhibition, motor inhibition, and sorting with rule shifts to assess aspects of attentional / executive control. Children in the current study completed these new attention tasks, plus measures of verbal and non-verbal intelligence, dorsal and ventral stream processing (motion and form coherence computer game), and global vs. local level visual processing with Navon figures. Results and discussion: Results to date confirm earlier preliminary data showing marked delays for both groups compared to chronological age and MA, and suggest that sustained attention is less affected than selective or executive attention in both syndrome groups. We will discuss how profiles of attention in these groups relate to (i) the changes in attention seen over the preschool years in typical development, (ii) vulnerability of the dorsal visual stream in developmental disorders, and (iii) known structural and functional brain abnormalities in these groups. Conference: 12th International Professional Conference on Williams Syndrome, Garden Grove,CA, United States, 13 Jul - 14 Jul, 2008. Presentation Type: Oral Presentation Topic: SESSION 6: Updates on Cognition in Williams Syndrome Citation: Breckenridge K and Atkinson J (2009). Assessing Attention in Children with Williams Syndrome and Down's Syndrome Using a New Comprehensive Attention Battery: Do These Problems Relate to Visual Dorsal Cortical Stream Deficits?. Conference Abstract: 12th International Professional Conference on Williams Syndrome. doi: 10.3389/conf.neuro.09.2009.07.020 Copyright: The abstracts in this collection have not been subject to any Frontiers peer review or checks, and are not endorsed by Frontiers. They are made available through the Frontiers publishing platform as a service to conference organizers and presenters. The copyright in the individual abstracts is owned by the author of each abstract or his/her employer unless otherwise stated. Each abstract, as well as the collection of abstracts, are published under a Creative Commons CC-BY 4.0 (attribution) licence (https://creativecommons.org/licenses/by/4.0/) and may thus be reproduced, translated, adapted and be the subject of derivative works provided the authors and Frontiers are attributed. For Frontiers’ terms and conditions please see https://www.frontiersin.org/legal/terms-and-conditions. Received: 30 Apr 2009; Published Online: 30 Apr 2009. * Correspondence: K. Breckenridge, University College London, Visual Development Unit,, London, United Kingdom, k.breckenridge@ucl.ac.uk Login Required This action requires you to be registered with Frontiers and logged in. To register or login click here. Abstract Info Abstract The Authors in Frontiers K. Breckenridge J. Atkinson Google K. Breckenridge J. Atkinson Google Scholar K. Breckenridge J. Atkinson PubMed K. Breckenridge J. Atkinson Related Article in Frontiers Google Scholar PubMed Abstract Close Back to top Javascript is disabled. Please enable Javascript in your browser settings in order to see all the content on this page.

Ongoing Activity Fluctuations in hMT+ Bias the Perception of Coherent Visual Motion

We have recently shown that intrinsic fluctuations of ongoing activity during baseline have an impact on perceptual decisions reported for an ambiguous visual stimulus (Hesselmann et al., 2008). To test whether this result generalizes from the visual object domain to other perceptual and neural systems, the current study investigated the effect of ongoing signal fluctuations in motion-sensitive brain regions on the perception of coherent visual motion. We determined motion coherence thresholds individually for each subject using a dynamic random dot display. During functional magnetic resonance imaging (fMRI), brief events of subliminal, supraliminal, and periliminal coherent motion were presented with long and variable interstimulus intervals between them. On each trial, subjects reported whether they had perceived "coherent" or "random" motion, and fMRI signal time courses were analyzed separately as a function of stimulus and percept type. In the right motion-sensitive occipito-temporal cortex (hMT+), coherent percepts of periliminal stimuli yielded a larger stimulus-evoked response than random percepts. Prestimulus baseline activity in this region was also significantly higher in these coherent trials than in random trials. As in our previous study, however, the relation between ongoing and evoked activity was not additive but interacted with perceptual outcome. Our data thus suggest that endogenous fluctuations in baseline activity have a generic effect on subsequent perceptual decisions. Although mainstream analytical techniques used in functional neuroimaging do not capture this nonadditive effect of baseline on evoked response, it is in accord with postulates from theoretical frameworks as, for instance, predictive coding.

Sensitivity for reverse-phi motion

Low-level contrast information in the primary visual pathway is represented in two different channels. ON-center cells signal positive contrasts and OFF-center cells signal negative contrasts. In this study we address the question whether initial motion analysis is performed separately in these two channels, or also through combination of signals from ON and OFF cells. We quantitatively compared motion coherence detection for regular and for reverse-phi motion stimuli. In reverse-phi motion the contrast of a pattern flips during displacements. Sensitivity is therefore based on correlating positive and negative contrasts, whereas for regular motion it is based on correlating similar contrasts. We compared tuning curves for step size and temporal interval for stimuli in which motion information was limited to a single combination of step size and interval. Tuning for step size and temporal interval was highly similar for the two types of motion. Moreover, minimal coherence thresholds for both types of motion matched quantitatively, irrespective of dot density. We also measured sensitivity for so-called no-phi motion stimuli, in which the contrast of displaced dots was set to zero. Sensitivity for no-phi motion was low for stimuli containing only black or only white dots. When both dot polarities were present in the stimulus, sensitivity was absent. Thus, motion information based on separate contrasts was effectively cancelled by a component based on different contrasts. Together these results show equal efficiency in correlating dots of opposite contrast and of similar contrast, which strongly suggests efficient detection of correlations across ON and OFF channels.

Robust processing of magnetotelluric data in the AMT dead band using the continuous wavelet transform

The energy sources for magnetotellurics (MT) at frequencies above [Formula: see text] are electromagnetic waves generated by distant lightning storms propagating globally within the earth-ionosphere waveguide. The nature of the sources and properties of this waveguide display diurnal and seasonal variations that can cause significant signal amplitude attenuation, especially at [Formula: see text] frequencies — the so-called audiomagnetotelluric (AMT) dead band. This lack of energy results in unreliable MT response estimates; and, given that in crystalline environments ore bodies located at some [Formula: see text] depth are sensed initially by AMT data within the dead band, this leads to poor inherent geometric resolution of target structures. We propose a new time-series processing technique that uses localization properties of the wavelet transform to select the most energetic events. Subsequently, two coherence thresholds and a series of robust weights are implemented to obtain the most reliable MT response estimates. Finally, errors are estimated using a nonparametric jackknife algorithm. We applied this algorithm to AMT data collected in northern Canada. These data were processed previously using traditional robust algorithms and using a telluric-telluric magnetotelluric (TTMT) technique. The results show a significant improvement in estimates for the AMT dead band and permit their quantitative interpretation.

Motion integration deficits are independent of magnocellular impairment in Parkinson’s disease

Motion processing involves multiple hierarchical steps, from the magnocellular pathway, sensitive to high temporal frequency modulations, to subsequent motion integration within the visual cortical dorsal stream. We have tested whether motion integration deficits in mild Parkinson disease (PD) can be explained by visual deficits in earlier processing nodes. Contrast sensitivity deficits in the magnocellular pathway, were compared with speed discrimination of local dots moving in random directions, speed and direction discrimination of moving surfaces and motion integration as measured by 2D coherence thresholds (n=27). We have found that low-level magnocellular impairment in PD does not explain deficits in subsequent steps in motion processing. High-level performance was abnormal in particular for tasks requiring perception of coherently moving surfaces. Motion coherence deficits were predictive of visuomotor impairment, corroborating a previous magnetic stimulation study in normal subjects. We conclude that dorsal stream deficits in PD have a high-level visual cortical basis independent of low-level magnocellular damage.