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

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

3. 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.

4. 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

5. 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)

6. 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
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7. Primary Visual Cortex Visual Processing in Reading
A. Maerlender, Ph.D. 13

8. 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

9. Visual Pathways

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

11. Striate Cortex: V1

12. 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)

13. 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

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

15. 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.
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16. Thalamic Nuclei

17. 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

18. 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
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19. 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
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20. 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
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21. 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.

22. 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.

23. 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).

24. 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+)
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25. 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.

26. ‘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.