BC Children's Hospital Research Institute (BCCHR) Research Find a Researcher

Debbie Giaschi

(she/her)

PhD, MA, BSc

Investigator, BC Children's Hospital

Visual perceptual abilities develop rapidly in infants, but considerable maturation occurs in preschool and elementary school-aged children. Several things can go wrong during the course of this development and a child may be left with visual problems that interfere with different aspects of life. Of particular interest are motion and depth perception, reading, and hand-eye coordination. The goal of our research is to understand the typical development of the underlying brain structures that control these perceptual abilities and to discover the changes in the brain that result in perceptual impairments. We focus specifically on a developmental visual disorder called amblyopia, or lazy eye, and a developmental reading disorder called dyslexia.

Academic Affiliations

Professor, Department of Ophthalmology and Visual Sciences, Faculty of Medicine, University of British Columbia
Research Theme: Brain, Behaviour & Development
Research Group(s): Mental Health and Behaviour; Neurodevelopmental and Neurological Disorders; Origins of Child Health and Disease; Pathways to Healthy Birth; Visualizing the Brain

Contact Information

Location

4500 Oak St, Vancouver, BC, Canada, V6H 3N1

(604) 875-2345 ext. 7807

deborah.giaschi@ubc.ca

Visual Neuroscience Lab

Evaluating visuomotor coordination in children with amblyopia

Developmental Psychobiology

Sabrina W. Hou and Yan Zhang and Lisa Christian and Ewa Niechwiej-Szwedo and Deborah Giaschi

DOI: 10.1002/dev.22270

05 / 2022

Effect of Visual Field Location on Global Motion Perception: A Developmental Study

Perception

Yousef M. Shahin and Kimberly Meier and Deborah Giaschi

DOI: 10.1177/0301006620930901

07 / 2020

Concurrent maturation of visuomotor skills and motion perception in typically-developing children and adolescents

Developmental Psychobiology

Ewa Niechwiej-Szwedo and Kimberly Meier and Lisa Christian and Mina Nouredanesh and James Tung and Pamela Bryden and Deborah Giaschi

DOI: 10.1002/dev.21931

04 / 2020

Longitudinal outcomes of an individualized and intensive reading intervention for third grade students

Dyslexia

Marita Partanen and Linda S. Siegel and Deborah E. Giaschi

DOI: 10.1002/dys.1616

08 / 2019

Impaired Fellow Eye Motion Perception and Abnormal Binocular Function

Investigative Opthalmology & Visual Science

DOI: 10.1167/iovs.19-26885

08 / 2019

Effect of reading intervention and task difficulty on orthographic and phonological reading systems in the brain

Neuropsychologia

DOI: 10.1016/j.neuropsychologia.2018.07.018

07 / 2019

Fellow Eye Deficits in Amblyopia

Journal of Binocular Vision and Ocular Motility

Eileen E. Birch and Krista R. Kelly and Deborah E. Giaschi

DOI: 10.1080/2576117X.2019.1624440

07 / 2019

Continuous theta burst TMS of area MT+ impairs attentive motion tracking

Arijit Chakraborty and Tiffany T. Tran and Andrew E. Silva and Deborah Giaschi and Benjamin Thompson

DOI: 10.1101/601807

04 / 2019

Reading ability of children treated for amblyopia

Vision Research

Laveniya Kugathasan and Marita Partanen and Violet Chu and Christopher Lyons and Deborah Giaschi

DOI: 10.1016/j.visres.2019.01.001

03 / 2019

The Effect of Stimulus Area on Global Motion Thresholds in Children and Adults

Vision

Kimberly Meier and Deborah Giaschi

DOI: 10.3390/vision3010010

03 / 2019

Dichoptic Attentive Motion Tracking is Biased Toward the Nonamblyopic Eye in Strabismic Amblyopia

Investigative Opthalmology & Visual Science

DOI: 10.1167/iovs.18-25236

09 / 2018

Neural Correlates of Speed-Tuned Motion Perception in Healthy Adults

Perception

Kimberly Meier and Marita Partanen and Deborah Giaschi

DOI: 10.1177/0301006618771463

06 / 2018

Disparity configuration influences depth discrimination in nave adults, but not in children

Vision Research

Wilcox, L.M. and Hartle, B. and Solski, A. and Mackenzie, K.J. and Giaschi, D.

DOI: 10.1016/j.visres.2016.12.010

Unilateral amblyopia affects two eyes: Fellow eye deficits in amblyopia

Investigative Ophthalmology and Visual Science

Meier, K. and Giaschi, D.

DOI: 10.1167/iovs.16-20964

Effect of spatial and temporal stimulus parameters on the maturation of global motion perception

Vision Research

Meier, K. and Giaschi, D.

DOI: 10.1016/j.visres.2017.04.004

A case of Bilateral Perisylvian Syndrome with reading disability

Cortex

Eckert, M.A. and Berninger, V.W. and Hoeft, F. and Vaden, K.I. and Beaulieu, C. and Berninger, V. and Castellanos, X. and Chiarello, C. and Conway, T. and Cutting, L. and Dehaene-Lambertz, G. and Eden, G. and Frye, R. and Giaschi, D. and Gilger, J. and Hoeft, F. and Kibby, M. and van Krigstein, K. and Kronbichler, M. and Leonard, C. and Milham, M. and Odegard, T. and Poldrack, R. and Pugh, K. and Richards, T. and Rollins, N. and Schneider, K. and Talcott, J. and Wandell, B.

DOI: 10.1016/j.cortex.2016.01.004

Global motion perception in children with amblyopia as a function of spatial and temporal stimulus parameters

Vision Research

Meier, K. and Sum, B. and Giaschi, D.

DOI: 10.1016/j.visres.2016.06.011

The effect of occlusion therapy on motion perception deficits in amblyopia

Vision Research

Giaschi, D. and Chapman, C. and Meier, K. and Narasimhan, S. and Regan, D.

DOI: 10.1016/j.visres.2015.05.015

Using Colors to Assess Pain in Toddlers: Validation of The Rainbow Pain Scale"A Proof-of-Principle Study

Journal of Pediatric Oncology Nursing

Mahon, P. and Holsti, L. and Siden, H. and Strahlendorf, C. and Turnham, L. and Giaschi, D.

DOI: 10.1177/1043454214555197

Flicker adaptation or superimposition raises the apparent spatial frequency of coarse test gratings

Vision Research

Kaneko, S. and Giaschi, D. and Anstis, S.

DOI: 10.1016/j.visres.2015.01.005

The maturation of global motion perception depends on the spatial and temporal offsets of the stimulus

Vision Research

Meier, K. and Giaschi, D.

DOI: 10.1016/j.visres.2013.12.007

On the typical development of stereopsis: Fine and coarse processing

Vision Research

Giaschi, D. and Narasimhan, S. and Solski, A. and Harrison, E. and Wilcox, L.M.

DOI: 10.1016/j.visres.2013.07.011

Sparing of coarse stereopsis in stereodeficient children with a history of amblyopia

Journal of Vision

Giaschi, D. and Lo, R. and Narasimhan, S. and Lyons, C. and Wilcox, L.M.

DOI: 10.1167/13.10.17

The effect of dot speed and density on the development of global motion perception

Vision Research

Narasimhan, S. and Giaschi, D.

DOI: 10.1016/j.visres.2012.02.016

The Processing of Motion-Defined Form

Seeing Spatial Form

Giaschi, D.

DOI: 10.1093/acprof:oso/9780195172881.003.0008

Quantitative measurement of interocular suppression in children with amblyopia

Vision Research

Narasimhan, S. and Harrison, E.R. and Giaschi, D.E.

DOI: 10.1016/j.visres.2012.06.007

Cortical basis for dichotic pitch perception in developmental dyslexia

Brain and Language

Partanen, M. and Fitzpatrick, K. and Mdler, B. and Edgell, D. and Bjornson, B. and Giaschi, D.E.

DOI: 10.1016/j.bandl.2012.09.002

Effects of speed, age, and amblyopia on the perception of motion-defined form

Vision Research

Hayward, J. and Truong, G. and Partanen, M. and Giaschi, D.

DOI: 10.1016/j.visres.2011.08.023

Neural correlates of the multiple-object tracking deficit in amblyopia

Vision Research

Secen, J. and Culham, J. and Ho, C. and Giaschi, D.

DOI: 10.1016/j.visres.2011.10.011

Combined functional MRI and diffusion tensor imaging analysis of visual motion pathways

Journal of Neuro-Ophthalmology

Lanyon, L.J. and Giaschi, D. and Young, S.A. and Fitzpatrick, K. and Diao, L. and Bjornson, B.H. and Barton, J.J.S.

DOI: 10.1097/WNO.0b013e3181a58ef8

The role of low-spatial frequencies in lexical decision and masked priming

Brain and Cognition

Boden, C. and Giaschi, D.

DOI: 10.1016/j.bandc.2008.12.002

Low- and high-level motion perception deficits in anisometropic and strabismic amblyopia: Evidence from fMRI

Vision Research

Ho, C.S. and Giaschi, D.E.

DOI: 10.1016/j.visres.2009.07.012

Low- and high-level first-order random-dot kinematograms: Evidence from fMRI

Vision Research

Ho, C.S. and Giaschi, D.E.

DOI: 10.1016/j.visres.2009.04.018

Navigational skills correlate with hippocampal fractional anisotropy in humans

Hippocampus

Iaria, G. and Lanyon, L.J. and Fox, C.J. and Giaschi, D. and Barton, J.J.S.

DOI: 10.1002/hipo.20400

M-stream deficits and reading-related visual processes in developmental dyslexia.

Psychological Bulletin

Catherine Boden and Deborah Giaschi

DOI: 10.1037/0033-2909.133.2.346

03 / 2007

Deficient motion-defined and texture-defined figure-ground segregation in amblyopic children

Journal of Pediatric Ophthalmology and Strabismus

Wang, J. and Ho, C.S. and Giaschi, D.E.

Stereopsis-dependent deficits in maximum motion displacement in strabismic and anisometropic amblyopia

Vision Research

Ho, C.S. and Giaschi, D.E.

DOI: 10.1016/j.visres.2007.07.008

The role of cortical area V5/MT+ in speed-tuned directional anisotropies in global motion perception

Vision Research

Giaschi, D. and Zwicker, A. and Young, S.A. and Bjornson, B.

DOI: 10.1016/j.visres.2006.12.017

Abnormal spatial selection and tracking in children with amblyopia

Vision Research

Ho, C.S. and Paul, P.S. and Asirvatham, A. and Cavanagh, P. and Cline, R. and Giaschi, D.E.

DOI: 10.1016/j.visres.2006.03.029

The effects of optical blur on motion and texture perception

Optometry and Vision Science

Zwicker, A.E. and Hoag, R.A. and Edwards, V.T. and Boden, C. and Giaschi, D.E.

DOI: 10.1097/01.opx.0000222919.21909.1e

Deficient maximum motion displacement in amblyopia

Vision Research

Ho, C.S. and Giaschi, D.E.

DOI: 10.1016/j.visres.2006.09.025

Deficient motion perception in the fellow eye of amblyopic children

Vision Research

Ho, C.S. and Giaschi, D.E. and Boden, C. and Dougherty, R. and Cline, R. and Lyons, C.

DOI: 10.1016/j.visres.2004.12.009

The maturation of form and motion perception in school age children

Vision Research

Parrish, E.E. and Giaschi, D.E. and Boden, C. and Dougherty, R.

DOI: 10.1016/j.visres.2004.10.005

Sensory and nonsensory influences on children's performance of dichotic pitch perception tasks

Journal of the Acoustical Society of America

Edwards, V.T. and Giaschi, D.E. and Low, P. and Edgell, D.

DOI: 10.1121/1.1861599

Children with dyslexia: Evidence for visual attention deficits in perception of rapid sequences of objects

Vision Research

Visser, T.A.W. and Boden, C. and Giaschi, D.E.

DOI: 10.1016/j.visres.2004.05.010

The effect of disrupting the human magnocellular pathway on global motion perception

Vision Research

Chapman, C. and Hoag, R. and Giaschi, D.

DOI: 10.1016/j.visres.2004.06.003

Psychophysical indexes of temporal processing abnormalities in children with developmental dyslexia

Developmental Neuropsychology

Edwards, V.T. and Giaschi, D.E. and Dougherty, R.F. and Edgell, D. and Bjornson, B.H. and Lyons, C. and Douglas, R.M.

DOI: 10.1207/s15326942dn2503_5

Warning: Attending to a mask may be hazardous to your perception

Psychonomic Bulletin and Review

Tata, M.S. and Giaschi, D.E.

Conscious visual abilities in a patient with early bilateral occipital damage

Developmental Medicine and Child Neurology

Giaschi, D. and Jan, J.E. and Bjorson, B. and Young, S.A. and Tata, M. and Lyons, C.J. and Good, W.V. and Wong, P.K.H.

DOI: 10.1017/S0012162203001439

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

Journal of Vision

Hoag, R.A. and Chapman, C.S. and Giaschi, D.E.

DOI: 10.1167/3.9.274

Attention deficits in children with anisometropic amblyopia

Journal of Vision

Paul, P.S. and Giaschi, D. and Cavanagh, P. and Cline, R.

DOI: 10.1167/1.3.80

Cortical mechanisms of central pitch processing revealed by dichotic pitch stimuli

NeuroImage

Giaschi, D. and Bjornson, B. and Dougherty, R. and Young, S.A.

Non-invasive mapping of sensorimotor cortex in a child with a cavernous angioma: FMRI and high-resolution EEG compared with surgical mapping

NeuroImage

Bjornson, B. and Giaschi, D. and Cochrane, D. and Au Young, S. and Rootman, D. and Boden, C. and Connolly, M. and Poskitt, K. and Boyle, C. and Smith, S. and Akagemi, R. and Wong, P.K.H.

High resolution EEG & seizure localization

NeuroImage

Wong, P.K.H. and Bjornson, B. and Connolly, M. and Steinbok, P. and Young, S.A. and Giaschi, D. and Gregory, D. and Smith, S. and Lee, R. and Man, A. and Cochrane, D.

Dichotic pitch: A new stimulus distinguishes normal and dyslexic auditory function

NeuroReport

Dougherty, R.F. and Cynader, M.S. and Bjornson, B.H. and Edgell, D. and Giaschi, D.E.

Comparison of visual activation measured by fMRI and high resolution EEC

NeuroImage

Dougherty, R.F. and Au Young, S.M.W. and Giaschi, D.E. and Bjornson, B.H. and Wong, P.K.H.

The effect of stationary noise on global motion perception

Investigative Ophthalmology and Visual Science

Giaschi, D.E. and Simpson, W.A. and Ferdinandi, A.K. and Tata, M.S.

In search of the best psychophysical indicator of an M-stream deficit in developmental dyslexia

Investigative Ophthalmology and Visual Science

Dougherty, R.F. and Giaschi, D.E. and Bjornson, B.H. and Chamul, S.L. and Edgcll, D. and Lyons, C.J.

Development of motion-defined figure-ground segregation in preschool and older children, using a letter-identification task

Optometry and Vision Science

Giaschi, D. and Regan, D.

DOI: 10.1097/00006324-199709000-00024

Reversible dissociation of sensitivity to dynamic stimuli in Parkinson's disease: Is magnocellular function essential to reading motion-defined letters

Vision Research

Giaschi, D. and Lang, A. and Regan, D.

DOI: 10.1016/S0042-6989(96)00316-1

Loss of sensitivity to motion-defined form in patients with primary open-angle glaucoma and ocular hypertension

Journal of the Optical Society of America A: Optics and Image Science, and Vision

Giaschi, D.E. and Trope, G.E. and Kothe, A.C. and Hong, X.-H.

Crowding and contrast in amblyopia

Optometry and Vision Science

Giaschi, D.E. and Regan, D. and Kraft, S.P. and Kothe, A.C.

Measurement of glare susceptibility using low-contrast letter charts

Optometry and Vision Science

Regan, D. and Giaschi, D.E. and Fresco, B.B.

The time course of direction-selective adaptation in simple and complex cells in cat striate cortex

Journal of Neurophysiology

Giaschi, D. and Douglas, R. and Marlin, S. and Cynader, M.

Measurement of glare sensitivity in cataract patients using low-contrast letter charts

Ophthalmic and Physiological Optics

Regan, D. and Giaschi, D.E. and Fresco, B.B.

Defective processing of motion-defined form in the fellow eye of patients with unilateral amblyopia

Investigative Ophthalmology and Visual Science

Giaschi, D.E. and Regan, D. and Kraft, S.P. and Hong, X.-H.

Motion-defined letter detection and recognition in patients with multiple sclerosis

Annals of Neurology

Giaschi, D. and Regan, D. and Kothe, A. and Hong, X.H. and Sharpe, J.A.

Method for identifying amblyopes whose reduced line acuity is caused by defective selection and/or control of gaze

Ophthalmic and Physiological Optics

Regan, D. and Giaschi, D.E. and Kraft, S.P. and Kothe, A.C.

Visual processing of motion-defined form: Selective failure in patients with parietotemporal lesions

Journal of Neuroscience

Regan, D. and Giaschi, D. and Sharpe, J.A. and Hong, X.H.

Suppression of OKN and VOR by afterimages and imaginary objects

Experimental Brain Research

Howard, I.P. and Giaschi, D. and Murasugi, C.M.

DOI: 10.1007/BF00248537

The less you see it, the faster it moves: Shortening the "on-time" speeds up apparent motion

Vision Research

Giaschi, D. and Anstis, S.

DOI: 10.1016/0042-6989(89)90082-5

Adaptation to apparent motion

Vision Research

Anstis, S. and Giaschi, D. and Cogan, A.I.

DOI: 10.1016/0042-6989(85)90093-8

Improving treatment success in children with amblyopia

Amblyopia (lazy eye) is a common developmental disorder in which vision is impaired in one eye and cannot be corrected with glasses. An eye turn (strabismus) or an optical problem can cause amblyopia to develop in an otherwise healthy eye. If these problems are detected and treated when a child is young enough, amblyopia can often be prevented or reversed by patching the unaffected fellow eye; however, not all children respond well to this patching therapy. Treatment failures may occur because patching treats only some of the problems in amblyopia. Most amblyopia treatment and research focuses on problems with form perception, such as visual acuity (the smallest character that can be seen on an eye chart). In contrast, our research focuses on problems with motion perception, depth perception, and eye gaze control. We are using customized computer games, naturalistic movie watching, virtual-reality displays, and functional magnetic resonance imaging (fMRI) to modernize how amblyopia is viewed and treated.

Visuomotor adaptation in children

Being able to adapt movements to changes in the environment is essential for children to learn and carry out everyday tasks such as feeding themselves and brushing their teeth. This learning requires visuomotor adaptation, in which motor commands are adjusted in response to visual feedback. We are using a reaching task with a computer mouse to study the typical maturation of visuomotor adaptation in children and its disruption by amblyopia. We are also using magnetoencephalography (MEG), a quiet and safe brain imaging method, to examine how and when the visual and motor systems communicate during successful versus unsuccessful movements and how excitatory and inhibitory connections among different brain regions coordinate goal-directed actions.

Improving treatment success in children with dyslexia

Dyslexia is a common developmental disorder in which children have unexpected difficulty learning to read. Children with dyslexia have poor school performance, but they may also develop mental health problems. Therefore, increasing reading skills in childhood is very important to their overall health as adults. We are using computer games and fMRI to determine the effect of dyslexia on perceptual and cognitive skills, in addition to reading. We are also using fMRI to examine the brain networks associated with reading, reading difficulty, and reading improvement following an intensive school-based reading intervention program.

Honours & Awards

UBC Killam Teaching Prize - 2018

BC Research Institute for Children’s & Women’s Health Investigatorship Award – 1999-2002

Our Research

At BC Children’s, we are making discoveries that save lives and transform health care for children in our province and around the world. Our research portfolio includes basic, clinical, population, and public health research.

EXPLORE OUR RESEARCH

Debbie Giaschi

(she/her)

PhD, MA, BSc

Investigator, BC Children's Hospital

Academic Affiliations

Contact Information

Location

Publications

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Evaluating visuomotor coordination in children with amblyopia

Effect of Visual Field Location on Global Motion Perception: A Developmental Study

Concurrent maturation of visuomotor skills and motion perception in typically-developing children and adolescents

Longitudinal outcomes of an individualized and intensive reading intervention for third grade students

Impaired Fellow Eye Motion Perception and Abnormal Binocular Function

Effect of reading intervention and task difficulty on orthographic and phonological reading systems in the brain

Fellow Eye Deficits in Amblyopia

Continuous theta burst TMS of area MT+ impairs attentive motion tracking

Reading ability of children treated for amblyopia

The Effect of Stimulus Area on Global Motion Thresholds in Children and Adults

Dichoptic Attentive Motion Tracking is Biased Toward the Nonamblyopic Eye in Strabismic Amblyopia

Neural Correlates of Speed-Tuned Motion Perception in Healthy Adults

Disparity configuration influences depth discrimination in nave adults, but not in children

Unilateral amblyopia affects two eyes: Fellow eye deficits in amblyopia

Effect of spatial and temporal stimulus parameters on the maturation of global motion perception

A case of Bilateral Perisylvian Syndrome with reading disability

Global motion perception in children with amblyopia as a function of spatial and temporal stimulus parameters

The effect of occlusion therapy on motion perception deficits in amblyopia

Using Colors to Assess Pain in Toddlers: Validation of The Rainbow Pain Scale"A Proof-of-Principle Study

Flicker adaptation or superimposition raises the apparent spatial frequency of coarse test gratings

The maturation of global motion perception depends on the spatial and temporal offsets of the stimulus

On the typical development of stereopsis: Fine and coarse processing

Sparing of coarse stereopsis in stereodeficient children with a history of amblyopia

The effect of dot speed and density on the development of global motion perception

The Processing of Motion-Defined Form

Quantitative measurement of interocular suppression in children with amblyopia

Cortical basis for dichotic pitch perception in developmental dyslexia

Effects of speed, age, and amblyopia on the perception of motion-defined form

Neural correlates of the multiple-object tracking deficit in amblyopia

Combined functional MRI and diffusion tensor imaging analysis of visual motion pathways

The role of low-spatial frequencies in lexical decision and masked priming

Low- and high-level motion perception deficits in anisometropic and strabismic amblyopia: Evidence from fMRI

Low- and high-level first-order random-dot kinematograms: Evidence from fMRI

Navigational skills correlate with hippocampal fractional anisotropy in humans

M-stream deficits and reading-related visual processes in developmental dyslexia.

Deficient motion-defined and texture-defined figure-ground segregation in amblyopic children

Stereopsis-dependent deficits in maximum motion displacement in strabismic and anisometropic amblyopia

The role of cortical area V5/MT+ in speed-tuned directional anisotropies in global motion perception

Abnormal spatial selection and tracking in children with amblyopia

The effects of optical blur on motion and texture perception

Deficient maximum motion displacement in amblyopia

Deficient motion perception in the fellow eye of amblyopic children

The maturation of form and motion perception in school age children

Sensory and nonsensory influences on children's performance of dichotic pitch perception tasks

Children with dyslexia: Evidence for visual attention deficits in perception of rapid sequences of objects

The effect of disrupting the human magnocellular pathway on global motion perception

Psychophysical indexes of temporal processing abnormalities in children with developmental dyslexia

Warning: Attending to a mask may be hazardous to your perception

Conscious visual abilities in a patient with early bilateral occipital damage

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

Attention deficits in children with anisometropic amblyopia

Cortical mechanisms of central pitch processing revealed by dichotic pitch stimuli

Non-invasive mapping of sensorimotor cortex in a child with a cavernous angioma: FMRI and high-resolution EEG compared with surgical mapping

High resolution EEG & seizure localization

Dichotic pitch: A new stimulus distinguishes normal and dyslexic auditory function

Comparison of visual activation measured by fMRI and high resolution EEC

The effect of stationary noise on global motion perception

In search of the best psychophysical indicator of an M-stream deficit in developmental dyslexia

Development of motion-defined figure-ground segregation in preschool and older children, using a letter-identification task

Reversible dissociation of sensitivity to dynamic stimuli in Parkinson's disease: Is magnocellular function essential to reading motion-defined letters

Loss of sensitivity to motion-defined form in patients with primary open-angle glaucoma and ocular hypertension

Crowding and contrast in amblyopia

Measurement of glare susceptibility using low-contrast letter charts

The time course of direction-selective adaptation in simple and complex cells in cat striate cortex

Measurement of glare sensitivity in cataract patients using low-contrast letter charts

Defective processing of motion-defined form in the fellow eye of patients with unilateral amblyopia

Motion-defined letter detection and recognition in patients with multiple sclerosis

Method for identifying amblyopes whose reduced line acuity is caused by defective selection and/or control of gaze

Visual processing of motion-defined form: Selective failure in patients with parietotemporal lesions

Suppression of OKN and VOR by afterimages and imaginary objects