Clinical School Services Recent Findings in the Neurobiology & Neuropsychology of Reading Processes -Part c- A. Maerlender, Ph.D. Clinical School Services & Learning Disorders

Visual Processing and Reading A. Maerlender, Ph.D. 1

Is there visual Dyslexia? 3/26/99 Is there visual Dyslexia? Sea-change in thought Overwhelming evidence of phono. processing does not rule out other difficulties But visual skills don’t correlate with reading ability (Olson, 1989) A small subset complain of moving letters, etc. often called orthographic problems do not recognize words but can sound out A. Maerlender, Ph.D.

Orthographic Processing the ability to identify visual patterns of symbol strings dual route theory phonological processing first visual-orthographic processing later conditioned response automatic word identification use phonemic system when encountering new words

Binocular instability and Fixation instability suspected to be related to magnocellular function (Stein, 1996) Binocular stability (Dunlop test) - a test of a lateralized reference point as vision fuses Fixation instability – the actual eye movements are greater in dyslexics However, erratic eye movements not part of processing difficulty Known to exist Most likely a symptom as the deficits appear t be in the processing of the word (K. Rayner, 1996)

Low Level Visual Problems Visual Processing in Reading 3/26/99 Low Level Visual Problems physiological studies over the last 15 years MRI electrophysiological psychophysical experiments variety of stimulus conditions detection of coherent motion in random dot kinematograms uniform field flicker flickering sinewave gratings A. Maerlender, Ph.D. 6

Primary Visual Cortex Visual Processing in Reading A. Maerlender, Ph.D. 13

Primary Visual Cortex Area 17 analysis of visual form and feature Visual Processing in Reading Primary Visual Cortex Area 17 analysis of visual form and feature depends on input from LGN 12 A. Maerlender, Ph.D. 12

Visual Pathways

Two Visual Streams from primary visual association to polymodal convergence zones dorsal via parietal “where” system ventral via temporal “what” system

Striate Cortex: V1

Magnocellular visual system includes lateral geniculate nucleus of thalamus (LGN) corresponds with parvocellular system projects to specific layers of primary visual cortex (area 17 of occipital lobe)

M-system responsible for fast processing of visual info allows disinhibition - shifting high temporal sensitivity transmits information on change faster conduction velocities some control of eye movements Disengage visual gain

Parvocellular system smaller cell bodies sustained response stationary targets identification of patterns resolution of fine detail

Visual Processing in Reading 3/26/99 Thalamic Relays Thalamus is primary relay station from association areas to higher level processing in cortex Auditory: medial geniculate body Visual: lateral geniculate body thalamus A large mass of gray matter deeply situated in the forebrain. There is one on either side of the midline. Sometimes they are interconnected at the massa intermedia. It relays to the cerebral cortex information received from diverse brain regions. Sort of a requisite 'last pit stop' for information going to cortex. Axons from every sensory system (except olfaction) synapse here as the last relay site before the information reaches the cerebral cortex. There are other thalamic nuclei that receive input from cerebellar-, basal ganglia- and limbic-related brain regions. A. Maerlender, Ph.D. 8

Thalamic Nuclei

Dysfunctional magnocellular system Visual Processing in Reading 3/26/99 Dysfunctional magnocellular system impedes smooth, rapid transmission of visual information impaired motion detection (Cornelissen) anomalies found in dyslexics (Galaburda) smaller and fewer cells than normals possibly impedes timing mechanisms (Stein & Walsh, 1997) destabilize binocular vision (?) A. Maerlender, Ph.D. 20

Does reduced magnocellular function affect reading? Visual Processing in Reading 3/26/99 Does reduced magnocellular function affect reading? fluent reading involves rapid, alternating patterns of fixation and saccadic eye movements. visual system samples text Livingstone (1991) compared histology of dyslexics and normals ventral magnocellular layers of LGN (mLGN) in dyslexic brains had fewer and smaller cells than normals no differences in pLGN A. Maerlender, Ph.D. 22

Livingstone’s Studies with Dyslexics Visual Processing in Reading 3/26/99 Livingstone’s Studies with Dyslexics dyslexics have smaller LGN cells did poorly on tests of rapid visual processing diminished visual ERP with rapid low contrast info, but normal ERP with slow, high contrast info A. Maerlender, Ph.D. 26

Studies of Motion Detection, LGN and Reading Visual Processing in Reading 3/26/99 Studies of Motion Detection, LGN and Reading Cornelissen & Hansen, 1998; Demb et al 1998 Several studies have shown that dyslexics - on average - have greater trouble on visual tasks found an association between coherent motion detection & a task requiring accurate information of letter position poor letter position encoding predicted group membership motion detection more impaired than contrast sensitivity Motion detection seems to relate to reading rate A. Maerlender, Ph.D. 85 32

Why does the M-system impact fine-grained analysis? assumed that during the saccades between fixations, the M-system suppressed the P activity generated during the periods of fixation. M-system appears to dominate in selective attention: An attentional spotlight. However, recent electrophysiological studies have indicated a function for the magnocellular dominated dorsal stream in selective attention (macaques). An attentional spotlight.

M-system deficits impact P-system If M-system gates visual input through V1, P-functions will be impacted Particularly in tasks of intense competition for resources Efficient use of the attentional spotlight may be vitally important for normal reading and it may be this function that is compromised in ‘visual dyslexia’ Vidyasagar 1999 If the M system is indeed involved in gating all visual input going through the striate cortex, a deficit in this system can be apparent as a deficit not only in pure M mediated functions such as movement perception, but also in functions subserved by the P system. This would become manifest in tasks in which there is intense competition for attentional resources and the supposed M-mediated attentional spotlight is essential for good performance. efficient use of the attentional spotlight may be vitally important for normal reading and it may be this function that is compromised in visual dyslexia.

Visual Searching & Focusing Serial rapid attention reading impaired children had more difficulty than age-matched normal readers in a search task Serial rapid attention in a cluttered visual scene, the magnocellular (M) pathway is crucial for focusing attention serially on the objects in the field. reading impaired children had more difficulty than age-matched normal readers in a search task conjunction of two features processed by the parvocellular system dyslexic group's performance significantly poorer than controls when a large number of distractor items present with form & color stim. (P-system).

Additional Physiological Evidence for M-System Deficits Visual Processing in Reading 3/26/99 Additional Physiological Evidence for M-System Deficits ERP evidence of motion detection deficits Kubova et al, 1996 Support from fMRI Demb, 1998b dyslexics had reduced activity (relative to controls) in M input areas primary visual cortex (V1) adjacent motion sensitive areas (MT+) A. Maerlender, Ph.D. 41

Disagreement: General Perceptual Deficit Subgroup specific magnocellular deficits were not characteristic of a dyslexia subgroup Amatay et al 2000; RD group: perceptual deficits in both visual and auditory tasks pattern of impairments inconsistent with a magnocellular deficit or specific deficit in processing brief stimuli.

‘M’ deficits plus perceptual deficits 2 groups of RD subjects based on magno tasks performance was worse than that of the controls (six of 30), performance was within the range of the controls Low-M participants difficulties in all the visual and auditory psychophysical tasks; visual and auditory perceptual difficulties on tasks unrelated to magnocellular functions. [Magno. z-scores correlated with Coding (!)] Both in the control and in the RD group, the magnocellular Z-score was not significantly correlated with phonological decoding (reading non-words, NW-read) or with orthographic skills (ORTH). However, the magnocellular Z-score was significantly correlated with cognitive measures such as the Block Design subtest of the WAIS-III. Digit Symbol-Coding and the magnocellular Z-score were also significantly correlated in RD subjects and marginally correlated in controls. Furthermore, this subtest was the only parameter for which the magnocellular Z-score made a unique contribution to its variance, after the variance accounted for by Block Design had been removed (17.4%, P = 0.001, in the entire population; and 21.4%, P = 0.002, in the RD population). Thus, while the magnocellular Z-score is not a predictor of reading skills, it is correlated with performance in a task requiring fine visual-motor coordination.