Hemispheric Interactions Are Different in Left-Handed Individuals

Abstract

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 participants (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 interactions.

Full text

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
interactions.
Keywords: corpus callosum, interhemispheric transfer time, hemispheric interactions, left-handedness,
functional lateralization
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-
spheric interactions.
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
right-handed participants.
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:
n.cherbuin@anu.edu.au
Neuropsychology Copyright 2006 by the American Psychological Association
2006, Vol. 20, No. 6, 700–707 0894-4105/06/$12.00 DOI: 10.1037/0894-4105.20.6.700
700

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).
1
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
interactions.
1
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

Method
Participants
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)].
Results
Participants’ Variables
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
(SD ⫽2.99).
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-
spheres performance.
2
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
overall performance.
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
performance).
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-
2
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-
spheric interaction.
Table 1
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.
Discussion
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-
Table 2
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)
Group
Visual field conditions
Within
left
Within
right
Across
up
Across
down IHI
Left-handed
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)
Right-handed
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.
Table 3
Correlations Between IHTT, Handedness, and Accuracy Measures of IHI and Standardized
Regression Coefficients (␤) and Semipartial Correlations (sr
i
2
) of the Final Regression Analysis
Predictor variable B␤Partial Part
Initial model
a
R⫽.454 R
2
⫽.206 Adjusted R
2
⫽.136
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
Final model
b
R⫽.436 R
2
⫽.190 Adjusted R
2
⫽.165
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.
a
F(8, 91) ⫽2.357, p⫽.026.
b
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
people.
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.
Table 4
Handedness, Sex Ratio, and Average Age for the Four Handedness Groups
Group Handedness coefficient Men/Women ratio
Age, years
M(SD)
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
aging.
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
interhemispheric transfer.
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Received November 28, 2005
Revision received June 20, 2006
Accepted July 6, 2006 䡲
707HEMISPHERIC INTERACTION AND LEFT-HANDEDNESS