Post on 10-Feb-2016
description
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
-40-30-20-10
010203040
pos1 pos2 pos3 pos4 pos5 pos6 pos7
Word position
Stan
dard
ised
reac
tion
time
ms
5-letter words
-40-30-20-10
01020304050
pos1 pos2 pos3 pos4 pos5 pos6 pos7
Word position
Stan
dard
ised
reac
tion
time
ms
7-letter words
-40-30-20-10
0102030405060
pos1 pos2 pos3 pos4 pos5 pos6 pos7
Word postion
Stan
dard
ised
reac
tion
time
ms
-10
-8-6
-4
-2
02
4
68
10
-5 -4 -3 -2 -1 0 1 2 3 4 5 6 7 8 9 10
Laterality index (fTCD)
OVP
slo
pe a
vera
ged
over
all
wor
d le
ngth
s
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
-40-30-20
-100
1020
3040
pos_1 pos_2 pos_3 pos_4
Word position
Stan
dard
ised
reac
tion
time
ms
typical dominanceslopes: 6.77; 19.69
7-letter words
-40-30-20-10
01020304050
pos_1 pos_2 pos_3 pos_4
Word position
Stan
dard
ised
reac
tion
time
ms
Highly significant positive correlations :for the 4-letter/fMRI_LI
r = 0.85 for the 7-letter/fMRI_LI
r = 0.70
-4
-2
0
2
4
6
8
10
12
-1 -0.5 0 0.5 1
Laterality index (fMRI)
Stee
pnes
s of
OVP
cur
ve (S
lope
_4L)
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)