The neural bases of written language processing

1. The neural bases of written language processing

2. Neural substrates of written language processing in the adult brain Evidence: (1) Lesion/deficit correlation (2) Functional neuro-imaging

3. Lesion/deficit correlation: The case of Monsieur C (Dejerine, 1892) Stroke 1: • Spoken language intact • Count read • Couldn’t name letters • No difficulties with objects or faces • Spelled normally

5. Monsieur C Lesion 1: • Left occipital areas as well as the splenium of the corpus collosum • spared the angular gyrus Hypothesis: • The left occipital lesion prevents the left hemisphere from processing the visual information • The splenial lesion prevents the information processed by the right hemisphere to contact reading areas in the left hemisphere • The angular gyrus contains visual word forms and it has been isolated from visual input for reading but still available for spelling

6. Monsieur C: Lesion 1

7. Monsieur C: Lesion 2 Stroke 2: • Impaired spoken language • Total agraphia • Lesion2: • Posterior and inferior left parietal-angular gyrus • Areas between the 2nd and 3rd temporal gyri and the first occipital gyrus • Conclusion: • Angular gyrus is the critical region involved in representing the visual word form (used in reading and spelling)

8. Standard neurological model of oral reading

9. Limitations of chronic lesion/deficit correlation • If there has been recovery of function then there may be functions that are intact but which are normally supported by infarcted tissue  Lesion/deficit correlation in acute stroke

10. MR-PWI • PWI: • Perfusion weighted image • Shows areas of poor blood flow or hypoperfusion • Method: • Inject a contrast agent • Record time-to-peak for each voxel (time to peak concentration of tracer in each voxel) • Compute delay in TTP relative to homologous voxel in the contralateral (normal) hemisphere

11. Hillis et al. (2001) • 40 patients • Reading evaluation • PWI within 24 hours of stroke onset • Allows identification of functional deficit and correlation with area of hypoperfusion • Presence or absence of a functional deficit is correlated with presence or absence of hypoperfusion

12. Criteria for determining functional deficits Orthographic input lexicon: Impairment criteria: 1 > 10% errors in: • Written lexical decision AND • Written word/pix verification AND • Oral reading of words 2 Production of visually and/or phonological related errors and/or letter-by-letter reading and/or omissions in readings No impairment criteria: • 0% errors in: • Written lexical decision OR • Written word/pix verification OR • Reading irregular words

13. Criteria for determining functional deficits OPC: Impairment criteria: Pseudoword reading < word reading No impairment criteria: 0% errors in: • Reading pseudowords

14. Results Orthographic input lexicon deficit: • BA 39 angular gyrus correlation =.46 (p<01) • BA 37 posterior MTG correlation =.43 (p<.01) OPC deficit: • BA 39 angular gyrus correlation = .60 (p<.002) • BA 40 supramarginal gyrus correlation = .43 (p<.04)

17. Neuroimaging: Reading Cohen et al (2002): An investigation of the visual word form area (VWFA) “Letter strings can be identified irrespective of their location in the visual field, of the color of the ink, of the case, size, type of font, etc.” Critical to this process is the abstract representation of letter strings. Hypothesis: “An area located in the midportion of the left fusiform gyrus, which activates whenever literate subjects read printed words, contributes crucially to the cerebral basis of the visual word form”. It should be active regardless of the hemisphere to which printed words are presented.

18. Fusiform gyrus

19. Faces, houses and other objects

20. Methods • Stimuli: • Words, consonant strings, checkerboards • Presentation: • To the right or left visual field (RVF/LVF) with central fixation cross (2 X 3 = 6 stimulus types) • Task: • Passive viewing of stimuli with eyes fixated on the fixation cross • 550 ms fixation, 200 ms target presentation

21. Stimuli

22. Experiments • Behavioral experiments (outside the scanner): for monitoring eye movements and oral reading response accuracy • Two fMRI experiments: • Once with presentation blocked by stimulus type • Once with random presentation of stimulus types • Why these two presentation modes?

23. fMRI Results 1 Contrast: • Alphabetic stimuli (words or consonant strings) vs. checkerboards; combining both LVF and RVF Results: • highly significant left fusiform activation (Detected in 15/16 subjects over the two experiments) • no activation in the homologous location in the right hemisphere (detected in only 2/16 subjects)

26. fMRI Results 2 Contrast: • Words vs. consonant strings within those voxels which were identified in Contrast 1 Results: • 56 voxels more active for words than consonant strings; effect was comparable whether stimuli were presented in the RVF or LFV • This result was observed in the same location in 6/7 subjects in Expt 1.

28. Behavioral results • Eye movements on only .5% of trials • More errors when reading aloud LVF vs. RVF words • Why might this be? • Structural reasons: RVF presentation provides more rapid access to the left hemisphere • Attentional reasons: Attentional “fixation” may be directed to the left side of a stimulus, placing most of the stimulus to the right of attention fixation

29. Conclusions(Cohen et al., 2002) • What is the function of the VWFA? • Influenced by language-dependent parameters rather than simply visual features • Does it correspond to the Orthographic Lexicon? • Given that words and consonant strings differ both in their lexical status as well as their orthographic legality, this cannot be determined • What contrast would be helpful in answering this question?

30. Discussion(Cohen et al., 2002) • Why is this particular region selected for processing letter strings? • “The fact that it is producible across subjects suggests that some initial properties intrinsic to this region and to its pattern of connectivity are the cause of its subsequent specialization for reading” • What are these properties? • Foveal vs. peripheral visual processing (Levy et al., 2001) • Local vs. global processing (Lerner et al., 2001) • Size and position invariance (Ito et al., 1995) • Anatomical links between the visual system and the left-hemispheric language areas

31. The relationship between reading and spelling • Are there shared components? • Orthographic lexicon? • Graphemic buffer? • PG-GP conversion? • This question has been difficult to address with cognitive psychology and cognitive neuropsychological methods • fMRI: common areas of activation may indicate shared mechanisms

32. Rapp & Hsieh, 2003 (Spelling) – (Case): Isolates processes dedicated to retrieval and inspection of a word’s spelling without engaging written production processes

33. Posterior, Inferior Temporal/Fusiform Gyrus Posterior, Inferior Temporal/Fusiform Gyrus Posterior, Inferior Temporal/Fusiform Gyrus Posterior, Inferior Temporal/Fusiform Gyrus Posterior, Inferior Temporal/Fusiform Gyrus Posterior, Inferior Temporal/Fusiform Gyrus Posterior, Inferior Temporal/Fusiform Gyrus