The impact of typical and The impact of typical and atypical language dominance atypical language dominance

on visual word recognitionon visual word recognition

Marc BrysbaertMarc Brysbaert

Language and the brainLanguage and the brain

Does the brain organisation have an effect Does the brain organisation have an effect on the ways in which language is on the ways in which language is processed, or is language “machine-processed, or is language “machine-independent”, like a computer program?independent”, like a computer program?Functionalism vs. dualism or materialismFunctionalism vs. dualism or materialism

Language dominanceLanguage dominance

Already known since the mid 19th century Already known since the mid 19th century that most people have language lateralised that most people have language lateralised to the left (Broca, Dax)to the left (Broca, Dax)

Concluded on the basis of brain lesionsConcluded on the basis of brain lesions Also contribution from neurosurgery studies Also contribution from neurosurgery studies

(epilepsy; WADA test)(epilepsy; WADA test) Now with brain imaging techniques, this Now with brain imaging techniques, this

becomes possible to study in healthy becomes possible to study in healthy participantsparticipants

Pujol et al. (1999)Pujol et al. (1999)

50 lefthanders and 50 righthanders50 lefthanders and 50 righthanders fMRI scanningfMRI scanningWord fluency task: silently generate words Word fluency task: silently generate words

that start with an “F”that start with an “F”

Knecht’s work in MünsterKnecht’s work in Münster

Knecht et al.(2000): language dominance Knecht et al.(2000): language dominance defined with functional transcranial defined with functional transcranial Doppler ultrasonography (fTCD)Doppler ultrasonography (fTCD)

188 righthanders188 righthandersWord generation task (verbal fluency)Word generation task (verbal fluency)

Knecht’s work in Münster (cont.)Knecht’s work in Münster (cont.)

188 righthanders + 138 lefthanders188 righthanders + 138 lefthandersWord generation taskWord generation task

Does language dominance have Does language dominance have an effect on word recognition?an effect on word recognition?

General assumption: probably for General assumption: probably for parafoveal word recognition but not for parafoveal word recognition but not for foveal word recognitionfoveal word recognition

Hypotheses about the foveal Hypotheses about the foveal representation in the brainrepresentation in the brain

1. Fovea projects bilaterally1. Fovea projects bilaterally

Problem: Corballis & Trudel (1993) : split-brain Problem: Corballis & Trudel (1993) : split-brain patientpatient

Hypotheses about the foveal Hypotheses about the foveal representation in the brainrepresentation in the brain

2. Fast interhemispheric transfer2. Fast interhemispheric transfer

Dehaene, Cohen, Sigman, and Vinckier (2005, p. 338): “It Dehaene, Cohen, Sigman, and Vinckier (2005, p. 338): “It has been proposed that ‘foveal splitting’, whereby the left has been proposed that ‘foveal splitting’, whereby the left and right halves of a centrally fixated word are initially sent and right halves of a centrally fixated word are initially sent to distinct hemispheres, has important functional to distinct hemispheres, has important functional consequences for reading. However, beyond V1, callosal consequences for reading. However, beyond V1, callosal projections have the precise structure required to guarantee projections have the precise structure required to guarantee the continuity of receptive fields across the midline and the continuity of receptive fields across the midline and allow convergence to common visual representations. We allow convergence to common visual representations. We believe that these connections minimize the functional believe that these connections minimize the functional impact of the initial foveal split.”impact of the initial foveal split.”

Hypotheses about the foveal Hypotheses about the foveal representation in the brainrepresentation in the brain

3. Split fovea3. Split fovea Brysbaert (2004, p. 260):Brysbaert (2004, p. 260): “ “I have come to view the two I have come to view the two

arguments in favor of a distinction between foveal and arguments in favor of a distinction between foveal and parafoveal word recognition as seductive simplifying parafoveal word recognition as seductive simplifying assumptions rather than as firm foundations of a coherent assumptions rather than as firm foundations of a coherent theoretical framework. They have allowed researchers of theoretical framework. They have allowed researchers of visual word recognition to ignore the vast literature of visual word recognition to ignore the vast literature of cerebral asymmetry, and they have allowed laterality cerebral asymmetry, and they have allowed laterality researchers to ignore the fine details and controversies researchers to ignore the fine details and controversies within computational models of visual word recognition. within computational models of visual word recognition. There was no gain to be found for either camp in There was no gain to be found for either camp in questioning the assumptions.”questioning the assumptions.”

Ellis & Brysbaert (Neuropsychologia, 2010)Ellis & Brysbaert (Neuropsychologia, 2010)

OVP-curve and cerebral OVP-curve and cerebral dominance (Münster study)dominance (Münster study)

20 participants from the original Knecht et 20 participants from the original Knecht et al. studies contacted againal. studies contacted again

13 male; 28 years old; 12 left-handed13 male; 28 years old; 12 left-handedRetested fTCDRetested fTCD12 LD (+1.4 to +7.8); 8 RD (-1.2 to -4.9)12 LD (+1.4 to +7.8); 8 RD (-1.2 to -4.9) fTCD test-retest correlation r = .78fTCD test-retest correlation r = .78

OVP-curve and cerebral OVP-curve and cerebral dominance (Münster study)dominance (Münster study)

German nouns of 3-, 5- German nouns of 3-, 5- and 7-letters and 7-letters (controlled for (controlled for freq. and neighbourhood size) freq. and neighbourhood size)

Presentation:Presentation:

7 possible fixation 7 possible fixation locations shifted across locations shifted across the screenthe screen

630 stimuli 630 stimuli

randomised orderrandomised order Presentation 180 msPresentation 180 ms

3-letter words

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5-letter words

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7-letter words

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Word postion

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-5 -4 -3 -2 -1 0 1 2 3 4 5 6 7 8 9 10

Laterality index (fTCD)

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r = .55

OVP-curve and cerebral OVP-curve and cerebral dominance (RHUL study)dominance (RHUL study)

See whether this type of study is feasible See whether this type of study is feasible on an individual level when you do not on an individual level when you do not have access to 100s of participantshave access to 100s of participants

More detailed information about the More detailed information about the degree of laterality (fMRI)degree of laterality (fMRI)

26 lefthanders started the study tested 26 lefthanders started the study tested with VHF tasks (picture naming and word with VHF tasks (picture naming and word naming)naming)

fMRI studyfMRI study

10 individuals (4 male, 6 female; M age 19.8) 10 individuals (4 male, 6 female; M age 19.8) Mental word generation taskMental word generation task

10 letters with highest beginning of word frequency

Pre-processing and analysis with SPM

Levels of activation compared in LH and RH in predefined anatomical regions of interest (ROI) encompassing BA 44/BA 45 = Broca’s area

LI > +0.4 were classed as left-dominant>> 6 participants

LI < -0.4 were classed as right-dominant >> 2 participants

-0.4 > LI < +0.4 were classed as bilateral >> 2 participants

VHF picture naming and fMRI_LI: r = 0.77, p < 0.01

VHF word naming and fMRI_LI: r = 0.63, p < 0.1

OVP task with fMRI subgroup

4 letter words fixated on each position4 letter words fixated on each position7 letter words fixated on each odd position 7 letter words fixated on each odd position

(1, 3, 5, 7)(1, 3, 5, 7)All words seen at all positions by each All words seen at all positions by each

participantparticipant

OVP task with fMRI OVP task with fMRI subgroupsubgroup

atypical dominanceslopes: -2.6; 3.41

4-letter words

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typical dominanceslopes: 6.77; 19.69

7-letter words

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Highly significant positive correlations :

for the 4-letter/fMRI_LIr = 0.85

for the 7-letter/fMRI_LIr = 0.70

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-1 -0.5 0 0.5 1

Laterality index (fMRI)

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ConclusionsConclusions

1.1. We can predict with near 100% accuracy the We can predict with near 100% accuracy the laterality of speech production by looking at laterality of speech production by looking at the slope of the OVP in a word naming taskthe slope of the OVP in a word naming task

2.2. This pattern is already present for 4-letter This pattern is already present for 4-letter words, subtending a width of slightly more words, subtending a width of slightly more than 1.5 degreesthan 1.5 degrees

3.3. Same results (though slightly worse) are Same results (though slightly worse) are obtained for the VHF tasks we usedobtained for the VHF tasks we used

4.4. Clear that IHTT is involved in foveal word Clear that IHTT is involved in foveal word recognition and that it has a substantial cost, recognition and that it has a substantial cost, even in healthy adultseven in healthy adults

ConclusionsConclusions

5.5. Does interhemispheric transfer happen early Does interhemispheric transfer happen early (i.e., before word recognition starts as in (i.e., before word recognition starts as in SERIOL) or late (i.e., do both hemispheres SERIOL) or late (i.e., do both hemispheres start word processing on the basis of the start word processing on the basis of the information received as in Shillcock et al.)information received as in Shillcock et al.)

In all likelihood it happens early (Van der Haegen In all likelihood it happens early (Van der Haegen et al., 2009; McCormick et al., in preparation)et al., 2009; McCormick et al., in preparation)