Hemispheric Interactions Are Different in Left-Handed Individuals
Nicolas Cherbuin and Cobie Brinkman
Australian National University
In a previous study, N. Cherbuin and C. Brinkman (2006) showed that in right-handed participants,
interhemispheric transfer time (measured with A. T. Poffenberger’s, 1912, paradigm) was a signiﬁcant
predictor of the efﬁciency of hemispheric interactions (measured with a split visual ﬁeld, letter-matching
task). No effect was found for degree of handedness in this study. This was surprising because
handedness has been shown to be associated with differences in the morphology and the structure of the
corpus callosum, and cerebral anatomical lateralization, as well as functional lateralization both in
behavioral and scanning studies. Because these ﬁndings were found in a large sample, but one limited to
right-handed participants, the aim of the present study was to determine whether a similar relationship
was present between interhemispheric transfer time and hemispheric interaction in left-handed partici-
pants (using identical measures) and to assess whether the analysis of a larger sample that comprised both
left- and right-handed participants might reveal an effect of handedness. Results demonstrate signiﬁcant
handedness effects, suggesting that left-handed individuals tend to have more efﬁcient hemispheric
Keywords: corpus callosum, interhemispheric transfer time, hemispheric interactions, left-handedness,
Small but signiﬁcant differences between left-handed and right-
handed individuals have been demonstrated in numerous neuro-
anatomical (e.g., Westerhausen et al., 2004), morphological
(Amunts, Jancke, Mohlberg, Steinmetz, & Zilles, 2000; Tuncer,
Hatipo, & O
¨zate, 2005), and behavioral studies (Haude, Morrow-
Thucak, Fox, & Pickard, 1987; Schmidt, Oliveira, Rocha, &
Abreu-Villaca, 2000; Tremblay, Monetta, & Joanette, 2004); how-
ever, other studies have failed to show any handedness effects
(Beaton, 1997; Piccirilli, Finali, & Sciarma, 1989; Steinmetz et al.,
1992). As a whole, the results suggest that slight neuroanatomical
differences between handedness groups may nevertheless be in-
dicative of differences in interhemispheric conductivity and in
patterns of functional lateralization that are likely to affect hemi-
At least ﬁve morphological studies have shown that left-handed
individuals differ from right-handed individuals in the size of their
corpora callosa. Witelson (1985, 1989) found, in two postmortem
studies, that left-handed and mixed-handed individuals had a larger
corpus callosum (CC) than right-handed individuals. This differ-
ence was mainly present in the isthmus, which connects regions of
the parietal association cortices that are known to be involved in
asymmetrical brain processes (e.g., language). In a magnetic res-
onance imaging (MRI) study, Habib et al. (1991) conﬁrmed that
nonconsistent right-handers (including left-handed participants)
had larger CCs, although this time it was demonstrated for the
anterior part. Further support for such a difference was provided by
Tuncer et al. (2005), who found that left-handers had larger CC
sections in the anterior body, posterior body, and the isthmus.
Finally, Moffat, Hampson, and Lee (1998) found, in an MRI study,
that left-handed participants with left-hemisphere speech functions
had a larger CC (isthmus, splenium, and genu) than both left-
handed participants with right-hemisphere speech functions or
An interaction of sex and handedness was also found in three of
these studies (Habib et al., 1991; Tuncer et al., 2005; Witelson,
1989), suggesting that men tend to have a larger CC than women
(although Jancke & Steinmetz, 2002, have argued that these ﬁnd-
ings might be methodological artefacts), with left-handed men
having a larger CC than right-handed men, whereas women do not
seem to differ in CC size as a function of handedness.
A larger CC size in left-handed individuals is not in itself proof
that interhemispheric connectivity is greater in this group because
ﬁber density and size could vary between handedness groups.
However, a recent diffusion-tensor MRI study (Westerhausen et
al., 2004) indicates that ﬁber density might be greater in the CC of
left-handed people, which suggests a greater interhemispheric con-
nectivity in this group, particularly in the posterior third of the CC
(isthmus and splenium).
These ﬁndings are consistent with the results of three behavioral
studies. Fagard and Corroyer (2003) showed that in small children,
handedness is associated with interhemispheric transfer, with less
right-handed children performing better on a bimanual coordina-
tion task. Another study involving adults (Potter & Graves, 1988)
also showed that non-right-handed participants performed signif-
icantly better than right-handed participants in a motor task (draw-
ing a line with each hand while blindfolded) and a tactile task
(determining whether left and right hands were touched in the
same location). Finally, Schmidt et al. (2000) showed that left-
handed men beneﬁted most from learning transfer between hands
in a ﬁne motor task.
Left-handed individuals have also been shown to differ from
right-handed individuals in measures of anatomical and functional
Nicolas Cherbuin and Cobie Brinkman, School of Psychology, Austra-
lian National University, Canberra, Australian Capital Territory, Australia.
Correspondence concerning this article should be addressed to Nicolas
Cherbuin, School of Psychology, Australian National University, Build-
ing 39, Canberra, Australian Capital Territory, 0200, Australia. E-mail:
Neuropsychology Copyright 2006 by the American Psychological Association
2006, Vol. 20, No. 6, 700–707 0894-4105/06/$12.00 DOI: 10.1037/0894-4126.96.36.1990
lateralization. Left-handers tend to have more anatomically sym-
metrical hemispheres and, speciﬁcally, a more symmetrical pla-
num temporale (for a review, see Beaton, 1997; Sequeira et al.,
2006) as well as a reversed pattern of asymmetry of the motor
cortex (Amunts et al., 2000). Because more symmetrical brains
have been shown to be associated with a greater number of callosal
ﬁbers, these ﬁndings are also consistent with greater interhemi-
spheric connectivity in left-handers (Rosen, Sherman, & Gala-
burda, 1989). This view is supported by an electro-physiological
study showing that event-related potentials across the CC are faster
and have a greater amplitude in left-handed compared with right-
handed individuals (Hoffman & Polich, 1999).
It has also been demonstrated that left-handed individuals vary
in their pattern of functional lateralization. More left-handed indi-
viduals (15% compared with 1%– 8% in right-handers) present
with bilateral or reversed representation of the language centers
(Knecht et al., 2000; Tzourio, Crivello, Mellet, Nkanga-Ngila, &
Mazoyer, 1998). Left-handers have also been shown to be less
functionally lateralized than right-handers (e.g., Eviatar, Hellige,
& Zaidel, 1997).
As a whole, the ﬁndings presented above suggest that left-
handed individuals have greater interhemispheric connectivity,
which may be associated with more efﬁcient levels of hemispheric
interaction in this population; however, tests of this hypothesis
have produced mixed results. Two studies that used digit- and
letter-matching tasks (Banich, Goering, Stolar, & Belger, 1990) or
mental calculation tasks (Hatta & Yoshizaki, 1996) failed to ﬁnd
signiﬁcant differences in hemispheric interaction between left- and
right-handers. A third study found that left-handed participants did
not beneﬁt from hemispheric interactions in a task involving
matching letters across and within visual ﬁelds (Eviatar et al.,
1997), whereas right-handed participants demonstrated a signiﬁ-
cant across visual ﬁeld advantage in this task.
A fourth study (Belger & Banich, 1998) found that handedness
affected hemispheric interaction, but that this effect was modulated
by an interaction with hemispheric arousal.
The inconsistent ﬁndings of the four studies listed above cannot
be attributed to a lack of power because they all used large
numbers of participants (86, 66, 106, and 57, respectively). Fur-
thermore, these ﬁndings cannot be attributed to obvious differ-
ences in the type of tasks used because they were very similar—
particularly the tasks used by Banich et al. (1990) and Eviatar et al.
(1997; letter and digit matching)—and used comparable parame-
ters (e.g., eccentricity, presentation time, stimulus size).
These ﬁndings are surprising because, as reviewed earlier, mor-
phological, neurophysiological, and behavioral data point to left-
handed individuals having greater interhemispheric connectivity.
Furthermore, a previous study (Cherbuin & Brinkman, 2006) has
shown that greater interhemispheric connectivity (interhemi-
spheric transfer time [IHTT] measured with Poffenberger’s, 1912,
paradigm) is associated with more efﬁcient hemispheric interac-
tions (measured with a letter-matching task similar to that used by
Banich et al., 1990, and Eviatar et al., 1997). It is possible that
some limitations of previous studies might have obscured subtle
differences between handedness groups. For instance, three of the
four studies reported did not take into account the magnitude of
left- or right-handedness. Rather, they relied on direction only,
although Hatta and Yoshizaki (1996) split the left-handed group
into familial and nonfamilial left-handers. This is important be-
cause there is substantial evidence that handedness is probably a
continuous measure and certainly not a homogeneous one (see
Beaton, 1997). It is also possible that the task used by Banich et al.
(1990) might not have been difﬁcult enough to reveal differences
between handedness groups. This possibility is supported by the
fact that Eviatar et al. (1997), using a similar but more complex
task (a name rather than a shape letter-matching task), have dem-
onstrated a handedness group difference, whereas Hatta and Yo-
shizaki have found trends that suggest a difference between famil-
ial and nonfamilial left-handers in their more complex task but not
in the simpler one.
On the basis of this evidence, in this study we aimed to assess
the efﬁciency of hemispheric connectivity and of hemispheric
interactions in left-handed individuals and to compare them with
previous ﬁndings in right-handed individuals (Cherbuin & Brink-
man, 2006) using identical measures. Because Cherbuin and
Brinkman (2006) have shown that interhemispheric connectivity is
related to the efﬁciency of hemispheric interactions (for the accu-
racy measure) in right-handed individuals, the ﬁrst aim of this
study was to test whether a similar relationship was also present in
a left-handed population when an identical design was used. The
second aim was to determine whether the lack of a handedness
effect in the relationship between IHTT and hemispheric interac-
tion found by Cherbuin and Brinkman might be due to a limited
handedness range. This was done by merging Cherbuin and Brink-
man’s and the present data sets so that a multiple regression
analysis could be conducted on the basis of left- and right-handed
participants’ measures. Conducting a multiple regression analysis
has the advantage of enabling the assessment of not only the
direction but also the strength of handedness measures and deter-
mining whether other factors—such as age, sex, attention, and
overall performance—also inﬂuence hemispheric interactions.
IHTT was measured with Poffenberger’s (1912) paradigm.
The efﬁciency of hemispheric interactions was assessed with a
letter-matching task in which participants decided whether two of
four letters were matching. Matches could occur either within the
left or right visual ﬁelds (and performance on these trials was
thought to reﬂect within-hemispheres performance), or across vi-
sual ﬁelds (requiring interactions between the two hemispheres).
An index of interhemispheric interaction (IHI), often called the
bilateral distribution advantage, was then computed by subtracting
across from within visual ﬁeld performance. For reaction time
(RT), a positive measure of IHI indicated a more efﬁcient level of
hemispheric interaction, whereas for accuracy the reverse was true.
It was predicted that left-handed individuals would present with a
similar relationship between IHTT and hemispheric interaction
compared with right-handed individuals but that left-handed
individuals would demonstrate more efﬁcient hemispheric
It has been shown that some level of hemispheric interactions also
occur in within-hemispheres trials (Pollmann, Zaidel, & Cramon, 2003).
Therefore, it is not suggested that a condition in which no interaction takes
place is compared with another condition in which hemispheric interac-
tions do take place. Instead, two conditions with varying levels and quality
of hemispheric interactions are compared.
701HEMISPHERIC INTERACTION AND LEFT-HANDEDNESS
Twenty left-handed participants (10 women, 10 men) between the ages
of 18 and 42 years with normal or corrected-to-normal vision completed
this study (2 participants’ data were excluded because they did not com-
plete all sessions). They were students who either participated to fulﬁll
undergraduate psychology course requirements or were recruited on the
Australian National University campus, and they received $35 as compen-
sation for their time and travel. Handedness was assessed with a question-
naire adapted from the Edinburgh Handedness Inventory (Oldﬁeld, 1971).
Participants who reported a history of neurological or motor disease were
excluded from the study. The study received approval from the Human
Research Ethics Committee.
Stimuli and Procedure
For each session, participants were seated in a dimly lit room, had their
chins in a chinrest, and were positioned 40 cm from the computer screen
with their eyes focused on a ﬁxation cross. Stimuli were presented on a
Daewoo (Chung-gu, Seoul, Korea) 17-in. (43.18-cm) cathode ray tube
monitor controlled by a Pentium III computer. The Inquisit software
package (Draine, 2003) was used for stimulus presentation and response
recording. Testing was spread over six half-hour sessions, with the ﬁrst two
sessions dedicated to Poffenberger’s task and handedness questionnaires,
and the last 4 sessions dedicated to the letter task.
Poffenberger’s task. The stimuli were white dots on a black back-
ground, with a size of 0.6° of visual angle and displayed with their
center 4.0° to the left or right of a central ﬁxation cross. Stimuli were
presented for 47 ms. During each trial, a ﬁxation cross appeared for 500 ms
or 800 ms (to discourage anticipatory responses) after which a dot was
ﬂashed randomly to the left or the right of the cross. The participant’s task
consisted of responding to detection of the stimulus by pressing with the
index ﬁnger the button of a centrally positioned computer mouse. Each
session consisted of 12 blocks of 50 trials (25 trials per visual hemiﬁeld, in
random order). Left and right hand responses were randomized across
blocks. Participants were tested in two sessions, for a total of 1,200 trials.
Letter task. The stimuli were seven capital letters and their lower case
counterparts (“Aa,” “Bb,” “Ee,” “Ff,” “Gg,” “Hh,” “Tt”), which were
displayed in Arial 34-point bold font. Arrays of four letters were displayed
in a square format, with the top two letters in upper case and the lower two
letters in lower case. Stimuli subtended a maximum of 1.0° of visual angle
and were displayed as white letters on a black background to decrease
eyestrain. Each letter was presented with 2.0° of visual angle to the left or
right of the central ﬁxation cross and 2.0° above or below the central
ﬁxation cross for 200 ms. In the match condition, two letters were match-
ing, whereas in the no-match condition, none of the letters matched. The
match always occurred between a lower case letter in the lower part of the
visual ﬁeld and an upper case letter in the upper part of the display. For half
of the match trials, the match occurred within the left or right half of the
visual ﬁeld (within-hemispheres condition), and for the other half, the
match occurred across the two hemiﬁelds (across-hemispheres condition,
see Figure 1). In the across-hemispheres condition, matches always oc-
curred on one of the diagonals (upper left and lower right letters or lower
left and upper right letters). This was done to decrease the inﬂuence of
scanning habits that would increase for horizontal matches. The partici-
pant’s task consisted in deciding whether two letters in the display were
matching. In the match condition (half of the trials), a mouse button had to
be pressed with the index ﬁnger, whereas in the no-match condition, no
response was to be made. The responding hand was randomized across
blocks of trials. Feedback (average RT and percentage correct) was given
at the end of each block of trials and after each trial for wrong responses
(“wrong”) to encourage higher accuracy. Participants were tested in four
sessions, for a total of 2,304 trials. Each session consisted of 12 blocks
of 48 trials, amounting to 288 trials per condition [2 Hands (left/right) ⫻2
Match Conditions (match/no-match) ⫻2 Hemisphere Conditions (within-
hemispheres match/across-hemispheres match)].
Participants had a mean age of 25 years and 4 months
(SD ⫽7.20 years) and an average handedness coefﬁcient (Edin-
burgh Inventory) of ⫺0.72 (SD ⫽0.26).
IHTT Using Poffenberger’s Paradigm
RTs to the presentation of a dot in the left or right visual ﬁeld
were measured for the two hands, and an estimate of IHTT or
crossed– uncrossed difference was computed by subtracting RTs
for the direct route (visual ﬁeld ipsilateral to responding hand)
from RTs for the indirect route (visual ﬁeld contralateral to re-
sponding hand). RTs smaller than 100 ms, which are considered to
be anticipatory responses, and larger than 1,000 ms (2.3%), which
are considered to be because of attentional lapses, were excluded.
The average RT in Poffenberger’s task in all conditions ranged
from 231 ms to 321 ms, with average valid response rates of
98%. The average crossed– uncrossed difference was 1.56 ms
A 2 Hand ⫻2 Visual Field within-subjects analysis of variance
(ANOVA) was applied to the RT measures to conﬁrm that re-
sponses by the hand contralateral to the hemisphere perceiving the
Figure 1. Exemplars of the different visual ﬁeld conditions (with matching letters): within left visual ﬁeld,
within right visual ﬁeld, and across visual ﬁeld.
702 CHERBUIN AND BRINKMAN
stimulus were faster than those of the ipsilateral hand. A signiﬁcant
Hand ⫻Visual Field interaction, F(1, 19) ⫽5.428, p⫽.031,
supported this premise. Cell means are presented in Table 1.
Hemispheric Interaction in a Letter-Matching Task
RT and accuracy for matching letter pairs were measured for
four visual ﬁeld/hemisphere conditions: within left visual ﬁeld,
within right visual ﬁeld, and across visual ﬁelds (lower left, upper
right; and upper left, lower right). RTs smaller than 250 ms and
larger than 1,500 ms (3.3% of responses) were excluded.
Average RT and accuracy for the within- and across-hemi-
spheres conditions were computed and the difference between
these measures was calculated as indexes of IHI with the following
formula: within-hemispheres performance minus across-hemi-
These results are presented in Table 2. The
RT measure of IHI was not signiﬁcantly different from zero,
t(19) ⫽0.760, ns, but the accuracy measure of IHI was,
t(19) ⫽2.495, p⬍.05, indicating that participants performed
more accurately in the within visual ﬁeld trials.
The correlation between the accuracy and RT measure of IHI
was not signiﬁcant (r⫽⫺.032, ns). To determine whether one
hemisphere was more proﬁcient at the letter task than the other, we
conducted paired sample ttests, but no signiﬁcant difference was
found for the RT measure, t(19) ⫽0.997, ns, or for the accuracy
measure, t(19) ⫽0.037, ns.
IHTT–Hemispheric Interaction Relationship
A previous study (Cherbuin & Brinkman, 2006) showed that in
right-handed individuals, IHTT and the accuracy measure of IHI
were signiﬁcantly correlated (r⫽.354, p⬍.001), whereas IHTT
and RT measures of IHI were not (r⫽⫺.078, ns). A correlational
analysis between IHTT and the measures of IHI was conducted to
determine whether these variables were also associated in left-
handed individuals. IHTT was not found to be signiﬁcantly cor-
related with the RT measure of IHI (r⫽.041, ns), but a correlation
of similar magnitude as that found in right-handed individuals was
found between IHTT and the accuracy measure of IHI (r⫽.401,
p⫽.08), although this relationship was marginally insigniﬁcant.
Handedness Effect in IHTT–IHI Relationship
To further investigate the effect handedness has in the IHTT–
IHI relationship, we pooled the data of the present study with that
of right-handed individuals collected during an identical study
(Cherbuin & Brinkman, 2006).
Because the variability of the measure of IHI could, apart from
IHTT and possibly handedness, also be explained by other factors,
such as sex, age, hemispheric lateralization, or attention, a multiple
regression analysis was conducted on these variables. Lateraliza-
tion indexes for the letter task were computed by subtracting the
performance to matches presented in the left visual ﬁeld from the
performance to matches in the right visual ﬁeld and adjusting for
overall performance with the following formula [(right visual ﬁeld
– left visual ﬁeld)/(right visual ﬁeld ⫹left visual ﬁeld)] for the RT
and accuracy measures (across visual ﬁeld matches were not used
in the computation of these measures). Correct RT and accuracy
rates of Poffenberger’s task were used as indexes of attention.
Because both handedness coefﬁcients and age were negatively
skewed, these variables were transformed with an inverse function.
Average accuracy rate for the letter-matching task was also in-
cluded in the analysis to exclude the possibility of an effect of
A backward deletion multiple regression analysis was con-
ducted (the ﬁrst and last models are shown in Table 3) and showed
that neither sex, age, lateralization, attention, and overall perfor-
mance signiﬁcantly accounted for the variability in IHI, leaving
IHTT and handedness as the only signiﬁcant variables, accounting
for 16.1% of the variance in IHI.
To better describe how handedness affects the accuracy measure
of IHI, we split the combined sample into four groups: extreme
left-handed (handedness coefﬁcient between –1.0 and ⫺.8), left-
handed (handedness coefﬁcient between ⫺.8 and 0), right-handed
(handedness coefﬁcient between 0 and .9), and extreme right-
handed (handedness coefﬁcient between .9 and 1.0). Because
left-handed individuals live in a right-handed world and are there-
fore biased toward using more of their right hand, the cutoff
chosen for extreme left-handedness was lower than that for ex-
treme right-handedness. Proportion of men and women and age of
these groups are shown in Table 4. Figure 2 shows the accuracy
measure of IHI for each group. To determine whether differences
between the handedness groups were signiﬁcant, we conducted a
one-way ANOVA with handedness category as the independent
variable and accuracy measure of IHI as the dependent variable;
however, no main effect was detected, F(3, 96) ⫽1.010, ns.
Handedness and RT Measures of IHI
Because no signiﬁcant correlation was found between IHTT and
RT measures of IHI, there was no rationale for analyzing this
relationship further. However, this does not exclude the possibility
that handedness is a signiﬁcant predictor of the RT measure of IHI.
Therefore, a similar regression analysis was conducted with the RT
measure of IHI as that with the accuracy measure of IHI. The same
variables were included, with the exception of IHTT and the
average accuracy measure of the letter-matching task, which was
replaced with the average RT measure of this task (overall
The regression analysis showed that sex, age, and the RT
measure of attention did not signiﬁcantly predict the RT measure
of IHI. The regression analysis also showed that (inverse) hand-
It is important to note that because across-hemispheres performance is
subtracted from within-hemispheres performance, a larger IHI measure for
RT is interpreted as a more efﬁcient hemispheric interaction, whereas a
larger IHI measure for accuracy is interpreted as a less efﬁcient hemi-
Poffenberger’s Paradigm Mean Reaction Times (in milliseconds)
for the Hand and Visual Field Conditions
Condition Left visual ﬁeld Right visual ﬁeld
Left hand 277.79 (24.46) 278.76 (24.29)
Right hand 279.76 (25.43) 277.61 (25.78)
Note. Standard deviations are presented in parentheses.
703HEMISPHERIC INTERACTION AND LEFT-HANDEDNESS
edness, t(99) ⫽⫺3.758, p⬍.001, ␤⫽⫺.338; overall perfor-
mance (RT of the letter-matching task), t(99) ⫽3.490, p⬍.01,
␤⫽.315; and attention (average RT of Poffenberger’s paradigm),
t(99) ⫽⫺1.997, p⬍.05, ␤⫽⫺.181, were signiﬁcant predictors
of the RT measure of IHI, F(3, 96) ⫽9.928, p⬍.001, accounting
for 23.7% (21.3% adjusted) of its variance. Correlations between
these variables, the standardized regression coefﬁcients, and the
semipartial correlations are presented in Table 5. The handedness
groups used in the previous analysis were then used to further
describe the effect of handedness on the RT measure of IHI. Figure
3 shows the differences in RT measure of IHI between these
groups. To determine whether differences between the handedness
groups were signiﬁcant, we conducted a one-way ANOVA with
handedness category as the independent variable and RT measure
of IHI as the dependent variable. A signiﬁcant effect of handedness
category was detected, F(3, 96) ⫽9.785, p⬍.001. Post hoc
contrasts showed that the RT measure of IHI of the extreme
left-handed group was signiﬁcantly larger than all other handed-
ness groups and that the difference between the left-handed group
measure of IHI and the extreme right-handed group measure of IHI
almost reached signiﬁcance ( p⫽.077). A trend analysis also
revealed that the relationship between handedness category and
RT measure of IHI was linear ( p⬍.001).
Finally, because of the different relationships found between
IHTT, handedness, and accuracy and RT measures of IHIs, a
correlational analysis between accuracy and RT measures of IHI
was conducted to exclude the possibility of a speed–accuracy
trade-off. The correlation between accuracy and RT measures of
IHI was signiﬁcant (r⫽⫺.275, p⬍.01), indicating that partici-
pants who had more efﬁcient IHIs based on the accuracy measure
also had more efﬁcient RT interaction based on the RT measure.
The aims of this study were two-fold. First, we wanted to
establish whether the relationship found in a right-handed popu-
lation (Cherbuin & Brinkman, 2006), showing an association
between faster IHTT and more efﬁcient hemispheric interactions,
was also present in left-handed participants. Second, we wanted to
determine whether left-handed individuals differed from right-
Mean Accuracy and Reaction Time (RT) Measures (in milliseconds) for the Different Conditions
of the Letter-Matching Task and Measures of Interhemispheric Interaction (IHI) for the
Left-Handed Participants (This Study) and Right-Handed Participants (Cherbuin & Brinkman, 2006)
Visual ﬁeld conditions
RT 879 (109) 899 (90) 915 (94) 942 (70) 10 (60)
% correct 90.6 (7.2) 90.8 (6.8) 89.3 (8.6) 89.4 (6.4) 1.48 (2.7)
RT 829 (94) 849 (112) 876 (99) 870 (93) ⫺35 (34)
% correct 92.4 (5.9) 91.5 (6.3) 92.5 (5.6) 90.5 (6.0) 0.5 (2.7)
Note. Standard deviations are presented in parentheses. For RT, a positive IHI indicates more efﬁcient
hemispheric interactions; the reverse is true for the accuracy measure.
Correlations Between IHTT, Handedness, and Accuracy Measures of IHI and Standardized
Regression Coefﬁcients (␤) and Semipartial Correlations (sr
) of the Final Regression Analysis
Predictor variable B␤Partial Part
⫽.206 Adjusted R
IHTT** .126 .341 .337 .319
Handedness* .006 .248 .248 .229
Age .536 .197 .193 .175
Lateralization (accuracy) .146 .137 .121 .109
Lateralization (RT) .042 .094 .081 .073
Attention (accuracy) .100 .069 .067 .060
Attention (RT) .001 .026 .026 .024
Sex .001 .001 .001 .001
⫽.190 Adjusted R
IHTT*** .003 .366 .372 .361
Handedness** .007 .274 .281 .264
Age .474 .174 .186 .170
Note. IHTT ⫽interhemispheric transfer time; IHI ⫽interhemispheric interaction; RT ⫽reaction time.
F(8, 91) ⫽2.357, p⫽.026.
F(3, 96) ⫽7.527, p⬍.001.
*p⬍.05. ** p⬍.01. *** p⬍.001.
704 CHERBUIN AND BRINKMAN
handed individuals in the efﬁciency of their hemispheric interac-
tions. This latter hypothesis was based on ﬁndings showing that
left-handed individuals tend to differ in their cerebral morphology
(Amunts et al., 2000; Tuncer et al., 2005) and asymmetry (for a
review, see Beaton, 1997; Eviatar et al., 1997; Sequeira et al.,
2006) as well as in their functional asymmetry compared with their
right-handed counterparts, and that these variations are thought to
indicate greater interhemispheric connectivity in left-handed
The present ﬁndings show that, as in right-handers (r⫽.354),
the measure of interhemispheric connectivity (IHTT measured
with Poffenberger’s paradigm) was correlated with a measure
(accuracy) of hemispheric interaction (letter-matching within and
across visual ﬁelds) in left-handed individuals (r⫽.401), such that
individuals with faster IHTT demonstrated a greater degree of
hemispheric interaction (for the accuracy measure a larger measure
of IHI denotes less efﬁcient hemispheric interactions). As previ-
ously discussed by Cherbuin and Brinkman (2006), other expla-
nations cannot be completely excluded. In children, higher cogni-
tive abilities have been shown to be associated with more efﬁcient
IHI. Because in the present study cognitive abilities were not
measured, this hypothesis cannot be tested. However, contrary to
Singh and O’Boyle (2004), who compared mathematically gifted
children with age-matched controls, the present sample is con-
cerned with adults and is a lot more homogeneous, with most
participants being undergraduate students. Therefore, we believe
that the present ﬁndings are unlikely to be due to differences in
cognitive abilities. Other explanations—such as differences in
cerebral architecture, or the efﬁciency of within- and across-
hemispheres resources allocation—are also possible but cannot be
tested in the present study beyond the inclusion in our analysis of
an attentional measure and a measure of functional lateralization,
which can only partly index these variables. Further research will
need to be conducted to clarify this issue.
The effect of direction and degree of handedness was examined
by merging the data set of the present study with that of an
identical study in right-handed participants (Cherbuin & Brink-
man, 2006). This analysis revealed that handedness was signiﬁ-
cantly associated with both the RT and accuracy measures of
hemispheric interaction. A surprising ﬁnding is that left-handed-
ness was associated with reduced hemispheric interaction efﬁ-
ciency when the accuracy measure was considered but with a
greater hemispheric interaction efﬁciency when the RT measure
was considered. This suggests the presence of a dissociation be-
tween neural substrates underlying accuracy and RT performance
across the hemispheres.
To further deﬁne the relationship between handedness and
hemispheric interaction, participants were allocated to four hand-
edness groups. Post hoc tests showed that at least for the RT
measure of IHI, handedness category was a signiﬁcant predictor of
hemispheric interaction efﬁciency. Furthermore, the relationship
between handedness category and RT measure of IHI was shown
Figure 2. Accuracy measures of interhemispheric interaction (IHI) for
the four handedness groups. Error bars show the standard error of the
mean. LH ⫽left-handed; RH ⫽right-handed.
Figure 3. Reaction time (RT) measures of interhemispheric interaction
(IHI) for the four handedness groups. Error bars show the standard error of
the mean. LH ⫽left-handed; RH ⫽right-handed.
Handedness, Sex Ratio, and Average Age for the Four Handedness Groups
Group Handedness coefﬁcient Men/Women ratio
Extreme left-handed ⬎⫺1.0, ⬍⫺.8 4/3 25.0 (8.0)
Left-handed ⬎⫺.8, ⬍0.0 6/7 25.5 (7.0)
Right-handed ⬎0.0, ⬍.9 23/12 21.3 (5.0)
Extreme right-handed ⬎.9, ⬍1.0 17/28 23.4 (8.0)
705HEMISPHERIC INTERACTION AND LEFT-HANDEDNESS
to be linear, with greater efﬁciency of hemispheric interaction
being associated with more extreme left-handedness. Opposite
trends were found for the accuracy measure of IHI.
It is difﬁcult to explain the opposite direction of the relationship
between accuracy and RT measures of IHI and handedness. Par-
ticularly because, in the whole sample, those participants who
demonstrated an across-hemispheres advantage for the accuracy
measure also demonstrated an across-hemispheres advantage for
the RT measure. One possibility is that one of these relationships
is mediated by one factor, perhaps some properties of interhemi-
spheric transfer, whereas the other might be mediated by another
factor, such as the degree of anatomical/functional lateralization.
One ﬁnding lending some support to this hypothesis is the fact that
our measure of interhemispheric transfer was a signiﬁcant predic-
tor of the accuracy measure of IHI but not of the RT measure of
IHI. Thus, it appears that the RT measure of IHI must be mediated
by other factors, with the stronger contender being the varying
degree of an individual’s cerebral lateralization.
Another possibility is that the reliability of the accuracy and/or
the RT measures of IHI were not high enough. Because the RT
measure of IHI had a good spread and because a robust effect was
found between this measure and handedness, this might suggest
that the accuracy measure of IHI was less reliable. We do not favor
this explanation, however, because in this and in the previous
study in right-handed individuals it was the accuracy measure of
IHI that showed a strong relationship with IHTT and not the RT
measure, which suggests that it is a sensitive measure.
In any case, the positive relationship between handedness and
RT measure of IHI was strong, with extreme left-handed individ-
uals presenting an across visual ﬁeld advantage of 43 ms, whereas
extreme right-handed individuals performed better in the within-
visuals condition by 39 ms. In contrast, the handedness–accuracy
measure of IHI relationship was much weaker with extreme left-
handed individuals performing slightly better (1.7%) in the within-
visual ﬁeld condition compared to the across-visual ﬁeld condition,
whereas extreme right-handers performed equally well in the with-
in- and across-hemispheres conditions.
A surprising result is that no effect of sex was found. A number
of studies discussed in the introduction (Habib et al., 1991; Tuncer
et al., 2005; Witelson, 1989) reported handedness effects in men
but not in women. It might therefore have been expected that an
interaction between sex and handedness would be found. The fact
that it was not emphasizes the importance of using large sample
sizes, controlling covariates, and replicating studies involving het-
erogeneous variables. It is also important to treat handedness as a
continuous variable or at least to use more than two handedness
groups because subtle differences are often found between
strongly and mildly left-handed individuals, between strongly and
mildly right-handed individuals, and between individuals with or
without familial left-handedness.
A near signiﬁcant effect of age ( p⫽.067, ␤⫽.174) as a
predictor of IHI was also found. This suggests that, as described in
previous studies (Cabeza, Anderson, Locantore, & McIntosh,
2002; Reuter-Lorenz, 2002), the efﬁciency of hemispheric inter-
actions might increase with age possibly to compensate for the
effect of decreased within-hemispheres resources associated with
In conclusion, these ﬁndings conﬁrm our prediction of an in-
creasing efﬁciency of hemispheric interactions with increasing
left-handedness. Furthermore, they reinforce the view that hemi-
spheric interactions are mediated by a number of factors subtly
interacting. Finally, they replicate previous ﬁndings showing that
greater hemispheric interaction efﬁciency is associated with faster
Amunts, K., Jancke, L., Mohlberg, H., Steinmetz, H., & Zilles, K. (2000).
Interhemispheric asymmetry of the human motor cortex related to hand-
edness and gender. Neuropsychologia, 38, 304 –312.
Banich, M., Goering, S., Stolar, N., & Belger, A. (1990). Interhemispheric
processing in left- and right-handers. International Journal of Neuro-
science, 54, 197–208.
Beaton, A. (1997). The relation of planum temporale asymmetry and
morphology of the corpus callosum to handedness, gender, and dyslexia:
A review of the evidence. Brain and Language, 60, 255–322.
Belger, A., & Banich, M. (1998). Costs and beneﬁts of integrating infor-
mation between the hemispheres: A computational perspective. Neuro-
psychology, 12, 380 –398.
Cabeza, R., Anderson, N., Locantore, J., & McIntosh, A. (2002). Aging
gracefully: Compensatory brain activity in high-performing older adults.
NeuroImage, 17, 1394 –1402.
Cherbuin, N., & Brinkman, C. (2006). Efﬁciency of callosal transfer and
hemispheric interaction. Neuropsychology, 20, 178 –184.
Draine, S. (2003). Inquisit (Version 1.32) [Computer software]. Seattle,
WA: Millisecond Software.
Eviatar, Z., Hellige, J., & Zaidel, E. (1997). Individual differences in
lateralization: Effects of gender and handedness. Neuropsychology, 11,
Fagard, J., & Corroyer, D. (2003). Using a continuous index of laterality to
determine how laterality is related to interhemispheric transfer and
bimanual coordination in children. Developmental Psychobiology, 43,
Correlations Between Handedness, Overall Performance, Attention and Reaction Time (RT)
Measures of Interhemispheric Interaction (IHI) and Standardized Regression Coefﬁcients (␤)
and Semipartial Correlations (sr
)of the Final Regression Analysis
Handedness Overall performance Attention IHI (RT) ␤sr
Handedness 1 ⫺.094 ⫺.113 ⫺.347 ⫺.338 ⫺.335
Overall performance ⫺.094 1 .139 .322 .315 .311
Attention ⫺.113 .139 1 ⫺.099 ⫺.181 ⫺.178
706 CHERBUIN AND BRINKMAN
Habib, M., Gayraud, D., Olivia, A., Regis, J., Salamon, G., & Khalil, R.
(1991). Effects of handedness and sex on the morphology of the corpus
callosum: A study with brain magnetic resonance imaging. Brain and
Cognition, 16, 41– 61.
Hatta, T., & Yoshizaki, K. (1996). Interhemispheric cooperation of left and
right-handers in mental calculation tasks. Laterality, 4, 299 –313.
Haude, R., Morrow-Thucak, M., Fox, D., & Pickard, K. (1987). Differen-
tial visual ﬁeld–interhemispheric transfer: Can it explain sex and hand-
edness differences in lateralization? Perceptual and Motor Skills, 65,
Hoffman, L., & Polich, J. (1999). P300, handedness, and corpus callosal
size: Gender, modality, and task. International Journal of Psychophys-
iology, 31, 163–174.
Jancke, L., & Steinmetz, B. (2002). Brain size: A possible source of
interindividual variability in corpus callosum morphology. In E. Zaidel
& M. Iacoboni (Eds.), The parallel brain: The cognitive neuroscience of
the corpus callosum (pp. 51– 63). Cambridge, MA: MIT Press.
Knecht, S., Deppe, M., Drager, B., Bobe, L., Lohmann, H., Ringelstein,
E.-B., & Henningsen, H. (2000). Language lateralization in healthy
right-handers. Brain, 123, 74 – 81.
Moffat, S., Hampson, E., & Lee, D. (1998). Morphology of the planum
temporale and corpus callosum in left handers with evidence of left and
right hemisphere speech representation. Brain, 121, 2369 –2379.
Oldﬁeld, R. (1971). The assessment and analysis of handedness: The
Edinburgh Inventory. Neuropsychologia, 9, 97–113.
Piccirilli, M., Finali, G., & Sciarma, T. (1989). Negative evidence of
difference between right- and left-handers in interhemispheric transfer of
information. Neuropsychologia, 27, 1023–1026.
Poffenberger, A. T. (1912). Reaction time to retinal stimulation with
special reference to the time lost in conduction through nerve centers.
Archives of Psychology, 23, 1–73.
Pollmann, S., Zaidel, E., & Cramon, D. (2003). The neural basis of the
bilateral distribution advantage. Experimental Brain Research, 221,
Potter, S., & Graves, E. (1988). Is interhemispheric transfer related to
handedness and gender? Neuropsychologia, 26, 319 –325.
Reuter-Lorenz, P. (2002). Parallel processing in the bisected brain: Impli-
cations for callosal function. In E. Zaidel & M. Iacoboni (Eds.), The
parallel brain: The cognitive neuroscience of the corpus callosum (pp.
341–354). Cambridge, MA: MIT Press.
Rosen, G., Sherman, G., & Galaburda, A. (1989). Interhemispheric con-
nections differ between symmetrical and asymmetrical brain regions.
Neuroscience, 33, 525–533.
Schmidt, S. L., Oliveira, R. M., Rocha, F. R., & Abreu-Villaca, Y. (2000).
Inﬂuences of handedness and gender on the grooved pegboard test.
Brain and Cognition, 44, 445– 454.
Sequeira, S., Woerner, W., Walter, C., Kreuder, F., Lueken, U., Wester-
hausen, R., et al. (2006). Handedness, dichotic-listening ear advantage,
and gender effects on planum temporale asymmetry—A volumetric
investigation using structural magnetic resonance imaging. Neuropsy-
chologia, 44, 622– 636.
Singh, H., & O’Boyle, M. (2004). Interhemispheric interaction during
global-local processing in mathematically gifted adolescents, average-
ability youth, and college students. Neuropsychology, 18, 371–377.
Steinmetz, H., Jancke, L., Kleinschmidt, A., Schlaug, G., Volkmann, J., &
Huang, Y. (1992). Sex but no hand difference in the isthmus of the
corpus callosum. Neurology, 42, 749 –752.
Tremblay, T., Monetta, L., & Joanette, Y. (2004). Phonological processing
of words in right- and left-handers. Brain and Cognition, 55, 427– 432.
Tuncer, M., Hatipo, E., & O
¨zate, M. (2005). Sexual dimorphism and
handedness in the human corpus callosum based on magnetic resonance
imaging. [Electronic version]. Surgical and Radiologic Anatomy, 27,
Tzourio, N., Crivello, F., Mellet, E., Nkanga-Ngila, B., & Mazoyer, B.
(1998). Functional anatomy of dominance for speech comprehension in
left handers vs. right handers. NeuroImage, 8, 1–16.
Westerhausen, R., Kreuder, F., Sequeira, S. D. S., Walter, C., Woerner, W.,
Wittling, R. A., et al. (2004). Effects of handedness and gender on
macro- and microstructure of the corpus callosum and its subregions: A
combined high-resolution and diffusion-tensor MRI study. Cognitive
Brain Research, 21, 418 – 426.
Witelson, S. (1985, August 16). The brain connection: The corpus callo-
sum is larger in left-handers. Science, 229, 665– 668.
Witelson, S. (1989). Hand and sex differences in the isthmus and genu of
the human corpus callosum: A postmortem morphological study. Brain,
112, 799 – 835.
Received November 28, 2005
Revision received June 20, 2006
Accepted July 6, 2006 䡲
707HEMISPHERIC INTERACTION AND LEFT-HANDEDNESS