Recent Findings in the Neurobiology & Neuropsychology of Reading Processes -Part c-

1. 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

3. 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

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 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

7. Primary Visual Cortex

8. Primary Visual Cortex • Area 17 • analysis of visual form and feature • depends on input from LGN 12

9. Visual Pathways

10. Two Visual Streamsfrom 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. Thalamic Relays • Thalamus is primary relay station from association areas to higher level processing in cortex • Auditory: medial geniculate body • Visual: lateral geniculate body

16. Thalamic Nuclei

17. 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 (?)

18. 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

19. 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

20. 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

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.

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

23. Visual Searching & Focusing • Serial rapid attention • reading impaired children had more difficulty than age-matched normal readers in a search task

24. 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+)

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 bothvisual and auditory tasks • pattern of impairmentsinconsistent with a magnocellular deficit • or specificdeficit 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 thecontrols (six of 30), • performance was withinthe range of the controls • Low-M participants • difficulties in all the visual and auditory psychophysicaltasks; • visual and auditory perceptual difficulties on tasks unrelatedto magnocellular functions. [Magno. z-scores correlated with Coding (!)]