Transcript Brain laterality and visual word recognition

The impact of typical and
atypical language dominance
on visual word recognition
Marc Brysbaert
Language and the brain

Does the brain organisation have an effect
on the ways in which language is
processed, or is language “machineindependent”, like a computer program?

Functionalism vs. dualism or materialism
Language dominance
Already known since the mid 19th century
that most people have language
lateralised to the left (Broca, Dax)
 Concluded on the basis of brain lesions
 Also contribution from neurosurgery
studies (epilepsy; WADA test)
 Now with brain imaging techniques, this
becomes possible to study in healthy
participants

Pujol et al. (1999)
50 lefthanders and 50 righthanders
 fMRI scanning
 Word fluency task: silently generate words
that start with an “F”

Knecht’s work in Münster
Knecht et al.(2000): language dominance
defined with functional transcranial
Doppler ultrasonography (fTCD)
 188 righthanders
 Word generation task (verbal fluency)

Knecht’s work in Münster (cont.)
188 righthanders + 138 lefthanders
 Word generation task

Does language dominance have
an effect on word recognition?

General assumption: probably for
parafoveal word recognition but not for
foveal word recognition
Hypotheses about the foveal
representation in the brain
1. Fovea projects bilaterally

Problem: Corballis & Trudel (1993) : split-brain
patient
Hypotheses about the foveal
representation in the brain
2. Fast interhemispheric transfer

Dehaene, Cohen, Sigman, and Vinckier (2005, p. 338): “It
has been proposed that ‘foveal splitting’, whereby the left
and right halves of a centrally fixated word are initially sent
to distinct hemispheres, has important functional
consequences for reading. However, beyond V1, callosal
projections have the precise structure required to guarantee
the continuity of receptive fields across the midline and
allow convergence to common visual representations. We
believe that these connections minimize the functional
impact of the initial foveal split.”
Hypotheses about the foveal
representation in the brain
3. Split fovea


Brysbaert (2004, p. 260): “I have come to view the two
arguments in favor of a distinction between foveal and
parafoveal word recognition as seductive simplifying
assumptions rather than as firm foundations of a coherent
theoretical framework. They have allowed researchers of
visual word recognition to ignore the vast literature of
cerebral asymmetry, and they have allowed laterality
researchers to ignore the fine details and controversies
within computational models of visual word recognition.
There was no gain to be found for either camp in
questioning the assumptions.”
Ellis & Brysbaert (Neuropsychologia, 2010)
OVP-curve and cerebral
dominance (Münster study)
20 participants from the original Knecht et
al. studies contacted again
 13 male; 28 years old; 12 left-handed
 Retested fTCD
 12 LD (+1.4 to +7.8); 8 RD (-1.2 to -4.9)
 fTCD test-retest correlation r = .78

OVP-curve and cerebral
dominance (Münster study)

German nouns of 3-, 5and 7-letters (controlled for
freq. and neighbourhood size)

Presentation:

7 possible fixation
locations shifted across
the screen

630 stimuli

randomised order
Presentation 180 ms

5-letter w ords
50
Standardised reaction time ms
40
30
20
10
40
30
20
atypical:
10 -3.67
0
0 1.42
typical:
-10
-20
-30
-40
-10
-20
-30
-40
pos1 pos2 pos3 pos4 pos5 pos6 pos7
pos1 pos2 pos3
Word position
pos4 pos5
pos6 pos7
Word position
7-letter w ords
60
Standardised reaction time ms
Standardised reaction time ms
3-letter w ords
50
40
30
20
atypical: 4.08
10
typical: 10.99
0
-10
-20
-30
-40
pos1 pos2 pos3 pos4 pos5 pos6 pos7
Word postion
OVP slope averaged over all word
lengths
10
8
6
4
2
0
-2
-4
-6
-8
-10
-5
-4
-3 -2
-1
0
1
2
3
4
5
Laterality index (fTCD)
r = .55
6
7
8
9
10
OVP-curve and cerebral
dominance (RHUL study)
See whether this type of study is feasible
on an individual level when you do not
have access to 100s of participants
 More detailed information about the
degree of laterality (fMRI)
 26 lefthanders started the study tested
with VHF tasks (picture naming and word
naming)

fMRI study
10 individuals (4 male, 6 female; M age 19.8)
 Mental 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
participants
LI < -0.4 were classed as right-dominant
participants
-0.4 > LI < +0.4 were classed as bilateral
participants
>> 6
>> 2
>> 2

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 position
 7 letter words fixated on each odd position
(1, 3, 5, 7)
 All words seen at all positions by each
participant

OVP task with fMRI subgroup
typical dominance
slopes: 6.77; 19.69
4-letter w ords
Standardised reaction time ms
atypical dominance
slopes: -2.6; 3.41
40
30
20
10
0
-10
-20
-30
-40
Highly significant positive
correlations :
for the 7-letter/fMRI_LI
r = 0.70
pos_2
pos_3
pos_4
Word position
7-letter w ords
Standardised reaction time ms
for the 4-letter/fMRI_LI
r = 0.85
pos_1
50
40
30
20
10
0
-10
-20
-30
-40
pos_1
pos_2
pos_3
Word position
pos_4
Steepness of OVP curve (Slope_4L)
12
10
8
6
4
2
0
-2
-4
-1
-0.5
0
0.5
Laterality index (fMRI)
1
Conclusions
1.
2.
3.
4.
We can predict with near 100% accuracy the
laterality of speech production by looking at
the slope of the OVP in a word naming task
This pattern is already present for 4-letter
words, subtending a width of slightly more
than 1.5 degrees
Same results (though slightly worse) are
obtained for the VHF tasks we used
Clear that IHTT is involved in foveal word
recognition and that it has a substantial cost,
even in healthy adults
Conclusions
Does interhemispheric transfer happen early
(i.e., before word recognition starts as in
SERIOL) or late (i.e., do both hemispheres
start word processing on the basis of the
information received as in Shillcock et al.)
5.

In all likelihood it happens early (Van der Haegen
et al., 2009; McCormick et al., in preparation)