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Integration of Information Between the
Cerebral Hemispheres
Marie T. Banich^
The Beckman Institute and Department of Psychology, University of Illinois at
Urbana-Champaign, Urbana, Illinois
Despite 30 years of research
clearly demonstrating fhe complementary functions of the cerebral
hemispheres, we have little infor-
;!Recommended Reading
1, M.T. (1995). Interhemispheric processing: Theoretical
• and empirical considerations. In
I RJ. Davidson & K. Hugdahl
(Eds.), Brain asymmetry (pp. 427\ 450). Cambridge, MA: MIT Press.
Helhge, J.B. (1993). Chapter 6: Vari, eties of interhemisplieric interac, tion. In J.B. Hellige, Hemispheric
asymmetry: Wfiat's right and what's
left (pp. 168-206). Cambridge,
MA: Harvard University Press.
Lassonde, M., & Jeeves, M.A. (Eds.).
(1994). Callosal agenesis: A natural
split brain? New York: Plenum
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Milner, A.D. (Ed.). (1995). Neuropsychological and developmental
studies of the corpus callosum
'
ISpecial lssuej
% 921-1007
Neuropsyihologia,
mation about how these two relatively distinct portions of the brain
interact to provide the seamless behavior we all exhibit in everyday
life. So striking are some of the
demonstrations of lateralization of
function in splif-brain patients (i.e.,
individuals in whom the corpus
callosum, which connects the cerebral hemispheres, has been severed) that philosophers and neuroscientists alike have paused to
consider whether humans might
have two separate and unique consciousnesses, rather than a single
mind. Recent work has helped to
expand our understanding of the
exquisite interplay between the
hemispheres that provides us with
unified thought. It has become
clear that interhemispheric interaction has some unanticipated functions, such as playing a role in perceptually binding together disparate
parts of an object or modulating attentional ability. Furthermore, in-
l-'ublished by Cambridge University Press
terhemispheric interaction appears
to have emergent properties, in
that under certain conditions, one
cannot deduce how the hemispheres
interact based solely on how each
hemisphere operates in isolation.
Researchers attempting to understand interhemispheric interaction have generally concentrated
on two major lines of inquiry. The
first examines how information is
represented as it is transferred
from one hemisphere to the other.
Thought of differently, this line of
inquiry attempts to understand the
"language" that the hemispheres
use to communicate with one another. The second line of inquiry
examines how transfer between the
hemispheres affects the brain's information processing capacities
and strategies. Thaf is, this line of
research attempts to understand
what mental processes are modulated or influenced by interhemispheric interaction.
Most interaction between the cerebral hemispheres occurs via a
very large neural band of fibers
known as the corpus callosum (see
Fig. 1), which is composed of more
than 200 million nerve fibers. Although there are other neural pathways by which information can be
transferred between the hemispheres (see Fig. 1), the vast major-
CURRi:\'T DIRIXTIONS SN PSYCHOiXXUCAL SCiFiNCB
Hippocatnpal commissure
Corpus callosum
Habenular commissure
Posterior
commissure
Anterior commissure
Massa intermedia
of the thalamus
Collicular
commissures
Fig. 1. A view of the brain cut down the middle so that the inside-most regions of
the right hemisphere are shown. Labeled in this figure are the various pathways
through which information can be communicated between the hemispheres. Notice
that the corpus callosum is by far the largest, and it is responsible for the vast
majority of information transfer between the cerebral hemispheres. From Banich
(1997). Copyright 1997 by Houghton Mifflin Company. Used with permission.
ity of information is transferred via
the callosum. As Reuter-Lorenz
and Miller (this issue) discuss, only
rudimentary information can be
transferred without a callosum.
Such information includes coarse
visual information regarding motion but not visual form, binary information (yes/no, odd/even),
general emotional tone (positive,
negative), and information that allows for the automatic orienting of
attention.
HOW IS INFORMATION
REPRESENTED WHEN IT IS
TRANSFERRED BETWEEN
THE HEMISPHERES?
One of the guiding principles of
interhemispheric interaction is that
different types of information are
transferred across different sections of the callosum. The callosum
is organized topographically, with
each section connecting nearby regions (i.e., the anterior callosum
connects anterior brain regions; the
posterior callosum connects posterior brain regions). Because each of
the major types of sensory information (e.g., visual, auditory, tactile)
is processed by a distinct brain region, and because different higher
order representations of information (e.g., abstract visual form vs.
meaning) are processed by different brain regions as well, one
might expect that the callosum consists of channels, each of w^hich is
responsible for transferring a distinct type of information.
For the most part, this expectation seems to hold. For example,
when a simple flash of light is directed to one hemisphere and the
motor response is controlled by the
other hemisphere, hoth sensory
(i.e., visual) and motoric information are transferred. Electrophysiological recordings suggest that the
motor signal is sent across middle
portions of the callosum that connect motor regions of the brain,
whereas the visual signal is sent
across posterior portions of the callosum that connect visual regions
of the brain (Rugg, Lines, & Milner,
1984). Furthermore, studies of patients with cailosal tumors or partial section of the callosum provide
evidence that, for the most part.
Copyright © 1998 American Psychological Society
visual, auditory, and somatosensory information are transferred
through different sections of the
callosum (e.g., Risse, Gates, Lund,
Maxwell, & Rubens, 1989). Intriguingly, there is evidence for an
asymmetry in the speed of transfer
of sensory information between the
hemispheres: Such transfer is faster
from the right hemisphere to the
left than from the left hemisphere
to the right (Marzi, Bisiacchi, &
Nicoletti, 1991).
Because not all information
transferred between the hemispheres is sensory in nature, researchers have also attempted to
determine how higher order (e.g.,
spatial, semantic) information is
communicated. This task can be
somewhat difficult because higher
order information could be transferred between the hemispheres in
a variety of ways. A word, for example, could be represented as a
visual pattern, as a series of letters,
as a series of sounds, or as a meaning. Hence, much of this research
has focused not on the exact nature
of information transferred, but
rather on whether the representation of the information involved in
interhemispheric communication is
similar to or distinct from the representation employed by each
hemisphere.
A commonly utilized method in
such endeavors is to com^pare the
processing that occurs when information is directed to only one
hemisphere with the processing
that occurs when both hemispheres
receive identical information. In
the case of visual information,
stimuli are presented to only one
visual field (i.e., only to the left or
right of fixation) on some trials and
to both visual fields (i.e., to both
sides of fixation) on other trials. Information presented to a given visual field is processed initially by
the opposite hemisphere. Thus, information presented to the right visual field (RVF; i.e., to the right of
fixation) is processed initially by
VOLUME?, \'i;VIi3i.-R !, FEfiRLiARY Vm
the left hemisphere, and information presented to the left visual
field (LVF; i.e., to the left of fixation) is processed initially by the
right hemisphere.
The major finding of this line of
inquiry is that the representation
invoked when both hemispheres
are involved in processing can vary
across situations (see Hellige, 1993,
chap. 6). In some cases, the representation employed when both
hemispheres receive information is
similar to the representation used
by one hemisphere, but not the
other. For example, Hellige, Jonsson, and Michimata (1988) asked
individuals to differentiate two
faces that varied by a single feature
(hair, eyes, mouth, jaw), and examined performance as a function of
which feature distinguished the
two faces. One might presuppose
that the representation employed
when both hemispheres saw the
faces (bilateral-visual-field, or BVF,
trials) would be similar to the representation used by the hemisphere generally considered specialized for the task (in this case,
the right hemisphere, because it is
usually superior at face processing). However, that was not the
case. The pattern of performance
on BVF trials was identical to that
observed on RVF (left-hemisphere)
trials, but different from that for
LVF (right-hemisphere) trials. In
fact, the representation employed
on BVF trials is frequently similar
to that of the hemisphere less adept
at the task. Perhaps when both
hemispheres are involved, the
more adept hemisphere has to
"dumb down" to meet the other
hemisphere's ability.
In other cases, the nature of processing when both hemispheres are
stimulated is a blend, or average, of
the processing that takes place
within each hemisphere. For example, when the task is naming
consonant-vowel-consonant sequences, the pattern of errors varies by visual field. On LVF trials.
errors are much more frequent on
the third letter of the sequence than
the first, whereas this difference is
much reduced on RVF trials. The
pattern of errors on BVF trials is
intermediate between that of RVF
and LVF trials (Luh & Levy, 1995).
These findings suggest that both
hemispheres contribute to performance so that the representation
employed in the interchange of information is a blend of the different
representations utilized by the two
hemispheres.
One of the m^ost interesting findings of such research is that the
representation employed when
both hemispheres receive information can be completely distinct
from the representation employed
by either hemisphere in isolation.
In a series of studies, Karol and I
instructed individuals to decide
whether either of two probe words
(which were positioned in the same
visual field on some trials and in
different visual fields on others)
rhymed with a previously presented target word. We found that
when a rhyme was present, performance on RVF trials, and to a lesser
degree on BVF trials, was influenced by whether the meaning of
the two probe words was identical
(e.g., bee and bee) or different (e.g.,
bee and sea), an effect not observed
for LVF trials. We then changed the
task slightly, presenting the two
words in different cases and fonts
(e.g., BEE and bee, BEE and sea) so
that they would no longer look
identical. This manipulation did
not affect performance on RVF and
LVF trials, but changed the pattern
for BVF trials (Banich & Karol,
1992, Experiments 4 and 5). Thus,
the words' meaning and form interacted to affect performance on
BVF trials, suggesting that the representation used when both hemispheres are involved in processing
is one in which physical form and
meaning are linked. In contrast, the
fact that font and case did not affect
either LVF or RVF performance
I'liblished by Cambridge University Press
suggests that the individual hemispheres employ a representation in
which meaning and form are separable.
HOW DOES
INTERHEMISPHERIC
INTERACTION
MODULATE
COGNITIVE PROCESSES?
Researchers have also made
some progress in understanding
how interaction between the hemispheres influences the ways in
which the brain processes information. Research with animals suggests that at least for the primary
sensory areas of the brain (i.e., the
region of the brain where information from sensor receptors, such as
the eye, is first received), integration of information across the callosum allows for a unitary sensory
world. Because the neural system
is organized so that information on
the left side of space goes to the
right half of the brain, and vice
versa, there is a need to fuse these
two perceptual half-worlds. The
callosum seems to play a critical
role in this regard. Callosal connections in primary sensory areas are
often limited to those regions of the
sensory world that fall along the
midline, such as where the RVF
and LVF meet.
Corroboration of the role of the
callosum in sensory midline fusion
comes from studies of individuals
who were born without a callosum,
a syndrome known as callosal
'agenesis. These individuals have
interesting sensory difficulties, including a poor ability to discern
which of two objects placed in central vision is closer or further away
(an ability that relies on the brain
computing slight differences in the
retinal images received by the two
eyes), a poor ability to determine
whether two points touched on either side of the trunk are different
or the same, and difficulties in localizing sounds in space (a skill
that relies on the brain computing
slight differences in the timing or
intensity of sounds received by the
two ears). All these difficulties rely
critically on the comparison of information received separately by
the two hemispheres, and hence require integration across the callosum (Lassonde, Sauerwein, & Lepore, 1995).
Recently, it has been proposed
that the pattern of activation across
the callosum may help bind together different parts of the visual
world into unitary objects. When
stimuli on different sides of the visual midline move in tandem, cells
in the two hemispheres fire synchronously. This synchrony relies
specifically on the callosum, as it
does not occur when the callosum
is severed. Because parts of an object generally move together in the
same direction and at the same
speed, such temporal aspects of
callosal firing may provide a
mechanism for binding together
parts of an object that appear on
either side of the midline (Engel,
Konig, Kreiter, & Singer, 1991). Because people typically move their
eyes so that an object of interest
falls in the center of gaze (and
hence spans the midline), such callosal connections may be important
for object recognition.
Interaction between the hemispheres not only influences sensory processing, but also modulates the processing capacity of the
brain. Belger and I (Banich & Belger, 1990) found that fhe ability of
inferhemispheric interaction to facilitate performance increases as
the computational complexity of
the task, which we define as fhe
number (and nature) of the steps
involved, increases. For example, if
it must be decided whether two
digits add to 10 (summation task),
performance is befter if one digit is
presented to each hemisphere (so
that the hemispheres musf interact
to make a decision) than if both
digits are presented to the same
hemisphere (in which case, no interaction is required). In contrast, if
the task is to decide whether two
digits are identical (physicalidentity task), interhemispheric interaction does not yield a performance advantage. The summation
task is more con:\plex because not
only is perceptual processing of the
digits required (as in the physicalidentity task), but then some identification and addition must be performed as well These results are
consistent with others demonstrating thaf performance is better
when operations are divided
across the hemispheres (e.g., directing a digit to be added to a target to one hemisphere, while directing another digit to be subtracted
from the target to the other hemisphere) than when bofh operations
must be performed by the same
hemisphere (Liederman, 1986).
We believe this effect occurs because the computational power of
the brain may be increased by dividing processing over as much
neural space as possible (in this
case, over the two hemispheres),
much the way that computational
power of computers is increased by
dividing a task over many sysfems.
Such a division is possible because
for most tasks (with the possible
exceptions of speech output and
phonetic processing), specialization of the hemispheres is relative
rafher than absolute. Thus, even
though one hemisphere may do a
particular task less capably or efficiently than the other, it nonetheless has the capacity to contribute
(see also Beeman & Chiarello and
Chabris & Kosslyn, this issue). As
tasks get more difficulf, any cost
overhead imposed by having to coordinate processing between the
hemispheres is more than offset by
the increased computational capacity provided by having both hemispheres involved.
Indirect support for such an idea
Copyright © 1998 American Psychological Society
has been provided by the recent
surge of brain-imaging studies. Although these studies are designed
to examine specific cognitive processes (e.g., memory), the manipulations employed often vary in
complexity (e.g., in how many
items musf be retrieved from
memory). As task demands are increased, often there is not only
greater activation of the brain region specialized for that task, but
greater activation over both hemispheres as well (e.g.. Braver et al.,
1997). Such findings are consistent
with the idea that as tasks get more
difficult to perform, more resources are recruited from both
hen:\ispheres.
Interhemispheric interaction
also seems to modulate fhe ability
to select certain information for
processing (e.g., the words on this
page) while filtering out other information (e.g., the background
noise that occurs while you are
reading). Using three well-known
paradigms, Alessandra Passarotti,
Joel Shenker, Daniel Weissman,
and I have demonstrated that interhemispheric interaction aids task
performance when a high degree,
but not a low degree, of selection is
required. In one of these paradigms, the individual must pay attention to the overall shape (global
form) of an item and ignore the
small shapes (local form) of which
it is composed (or vice versa); in
another, individuals must decide if
a pair of items matches, and that
decision must be made by attending to one attribute of the pair (e.g.,
their shapes) but not another (e.g.,
their colors); and in the third paradigm, the Stroop paradigm, an individual must pay attention to
(and name) the color of the ink in
which a word is presented while
ignoring the meaning of the word.
When there is no interference between the two types of information
and thus little need for selection,
interaction between the hemispheres does not aid performance.
For example, interhemispheric intefaction is not especially useful
when the global and local form of
an item are the same, when a pair
of items have the same form and
the same color, or when the color a
word names is concordant with its
ink color (e.g., "red" is printed in
red ink). However, when there is
conflicting information and individuals must select one type of information over another, the degree
of interference engendered by the
conflicting information can be reduced by interhemispheric interaction. For example, when an item's
global and local shape lead to different responses, when items
match in form but not color, and
when the word "blue" is printed in
red ink, interaction between the
hemispheres leads to superior performance. Furthermore, these results, for the most part, are relatively independent of hemispheric
asymmetries for the task, once
again suggesting that interhemispheric interaction may affect processing in a manner independent of
hemispheric specialization.
THE IMPORTANCE OF
INTERHEMISPHERIC
INTERACTION FROM A
CLINICAL OR
LIFE-SPAN PERSPECTIVE
The nature of interaction between the hemispheres may also be
important in a number of neuropsychological syndromes, and may
have implications for development
and aging. For example, the corpus
callosum seems especially vulnerable to damage caused by multiple
sclerosis (e.g., Rao et al., 1989) and
closed head injury (Gale, Johnson,
Bigler, & Blatter, 1995), and the
morphology and function of the
corpus callosum are different in
people with schizophrenia than in
neurologically intact individuals
(e.g., David, Minne, Jones, Harvey,
& Ron, 1995). The implications of
such findings remain unclear at
present, but it is possible that some
of the attentional difficulties observed in people with these syndronnes may be lir\ked to disrupted
interhemispheric interaction.
The interplay between the hemispheres may also be linked to
changes in cognitive processing
with development. During childhood, the speed with which information can be relayed between the
hemispheres increases because the
fatty insulation around neurons,
called myelin, continues to increase
in size around callosal neurons until some time during the late teen
years. In essence, the hemispheres
of young children are more functionally disconnected than those of
adults. Children do not exhibit the
same advantages of dividing processing across the hemispheres as
observed in older individuals, but
that changes as they grow older
(Liederman, Merola, & Hoffman,
1986). Interhemispheric interaction
may be disrupted in dyslexia (Markee, Brown, Moore, & Theberge,
1996), and may be atypical in attentional deficit disorder (Giedd et al.,
1994). Future work is likely to be
directed to more carefully explicating the nature of the relationship
between interhemispheric interaction and certain developmental
syndromes.
CONCLUSION
T
In sum, the functionally distinct
cerebral hemispheres appear to coordinate their performance in multiple ways via the corpus callosum,
which allows for dynamic interchange of information. The nature
of the representations used for
such communication is not well
elucidated at present, especially for
higher order information. However, it is clear that to further understand the implications of later-
Published by Cambridge University Press
alization of function, it will be
critical to understand interhemispheric integration, as research already suggests that the whole is
more than the sum of its parts.
Acknowledgments—Preparation of this
article was supported by National Institute of Mental Health Grant ROl
MH54217. I thank Mark Beeman, Christopher Chabris, Emanue! Donchin, and
Wendy Heller for helpful comments.
Note
1. Address correspondence to
Marie T. Banich, Beckman Institute and
Department of Psychology, University
of Illinois at Urbana-Champaign, 405
N. Mathews, Urbana, IL 61801; e-mail:
mbanich@s .psych.uiuc.edu.
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