Neuropsychologia 163 (2021) 108041
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Neuropsychologia
journal homepage: www.elsevier.com/locate/neuropsychologia
Can the mind be split? A historical introduction
Michael C. Corballis *, Paul M. Corballis
School of Psychology, University of Auckland, Science Centre, 23 Symonds Street, Auckland Central, New Zealand
A R T I C L E I N F O
A B S T R A C T
Keywords:
Commissurotomy
Consciousness
Split brain
Subcortical connections
The idea that the mind might be composed of distinct conscious entities goes back at least to the mid-19th
century, and was at first based on the bilateral symmetry of the brain, with each side seemingly a mirrorimage replica of the other. This led to early speculation as to whether section of the forebrain commissures
might lead to separate, independent consciousnesses. This was not put to the test until the 1960s, first in
commissurotomized cats and monkeys, and then in humans who had undergone commissurotomy for the relief of
intractable epilepsy. Initial results did indeed suggest independent consciousness in each separated hemisphere,
but later findings have also revealed a degree of mental unity, especially in some perceptual functions and in
motor control. Some of these findings might be interpreted in terms of subcortical connections or external crosscuing, and also address questions about the nature of consciousness in a more concrete way.
1. Introduction
"two minds"
The idea that the two sides of the brain might sustain separate
conscious minds can be traced to 1844, with the publication of the book
The Duality of Mind, by the English physician Arthur Wigan. Based on the
seemingly identical features of the two sides of the brain, he argued that
they must operate as separate minds. He nevertheless thought that these
two minds had to be coordinated through “exercise and moral cultiva­
tion,” and that failure to do so could lead to pathology. Lack of coor­
dination might explain such phenomena as déjà vu, with an experience
recorded in one side of the brain felt to be familiar by the other side, but
not actually recalled. The two minds might also alternate, giving rise to
different “personae,” each unaware of the existence of the other, as later
popularized in Robert Louis Stevenson’s (1886) thriller The Strange Case
of Dr Jekyll and Mr Hyde. What purported to be a real-life example was
described in Morton Prince’s (1906) book Dissociation of a Personality,
the case of a Miss Beaumont who occasionally appeared in the person­
ality of a very different person known as Sally B. direita/esquerda
Wigan did not attribute different mental qualities to the two sides of
the brain. Based on the prevalence of right-handedness, though, he did
suggest that the left side was generally “superior in power” to the right,
which “aids the left and corroborates its fellow as an assistant aids a
workman” (Wigan, 1844, p. 313). Broca’s (1861) later discovery of a
left-brained dominance for speech, though, led to much speculation
about fundamental differences between the two sides. For example, the
French physician Gaetan Delaunay (1874) thought that the two sides
NOT APPROVED
direita/esquerda
had different kinds of dreams, and that the left brain was associated with
maleness and reason, and the right side with femaleness and emotion. VOID
The historian Anne Harrington, 1985 remarks, “It is interesting that,
once one has given the two hemispheres sexual identities, the idea of
cerebral dominance becomes a rather apt metaphor for the social and
economic domination of men over women in 19th-century Europe” (p.
624). Much of the dual-brain speculation evaporated after the turn of the
century, but was revived in different guises following the split-brain
studies of the 1960s. one mind
It was also well understood in the 19th century that the two cerebral
cortices were connected by several commissures, of which by far the
largest and most important was the corpus callosum. This gave rise to
early speculation as to what would happen if the corpus callosum were
severed. The experimental psychologist Fechner (1860) argued that the
mind would then indeed be split, and essentially duplicated, with each
half carrying the same moods, predispositions, knowledge and mem­
ories. This was later challenged, though, by the British psychologist
William McDougall (1911), a dualist who believed that the mind would
remain unified even if the two halves were separated. He even asked the
famous physiologist Sir Charles Sherrington to cut his corpus callosum if
he were to contract an incurable disease. “If I am right,” he is reported to
have said, “my consciousness will remain a unitary consciousness”
(quoted by Zangwill, 1974, p. 265).
In the first half of the 20th century, studies of damage to the corpus
callosum provided some evidence as to its function, but little of conse­
quence to the understanding of what would happen if it were split. Based
* Corresponding author.
E-mail addresses: m.corballis@auckland.ac.nz (M.C. Corballis), p.corballis@auckland.ac.nz (P.M. Corballis).
https://doi.org/10.1016/j.neuropsychologia.2021.108041
Received 21 November 2020; Received in revised form 20 August 2021; Accepted 12 September 2021
Available online 25 September 2021
0028-3932/© 2021 Published by Elsevier Ltd.
M.C. Corballis and P.M. Corballis
Neuropsychologia 163 (2021) 108041
1967; Gazzaniga et al., 1962, 1965).
As in the 19th century, this again led to often extreme speculation as
to fundamental differences between the two sides of the brain. Joseph E.
Bogen (1989), one of the surgeons who carried out the split-brain
operation, characterized the left brain as propositional and the right
side as appositional, a duality elaborated by Ornstein (1972). Bogen
supposed that the duality was evident even in the intact brain, and
coined the term neo-Wiganism in recognition of Wigan (1844) earlier
claims.1
The idea that the two sides of the brain process the world differently
has gained some support. For example, Wolford et al., (2000) found that
the two sides form different hypotheses as to what will occur next in a
sequence of events, with left hemisphere seeking patterns in the se­
quences and the right hemisphere matching past frequencies. They take
this as evidence for the left brain as the “interpreter,” generating hy­
potheses, whereas the right hemisphere relies on simple associations
(Gazzaniga, 1995). Other evidence, though, suggests the right brain is
the “interpreter” when it comes to visual processing (P.M. Corballis,
2003). More generally, the notion of a global difference, as proposed by
Bogen and elaborated in popular views of “left-brain” and “right-brain”
modes of thought, is an oversimplification. There are many respects in
which the two sides of the brain operate in like fashion rather than
differently (M.C. Corballis, 2020), and where differences occur they are
not on a single continuum, but rather occur on several independent
dimensions (Häberling et al., 2016; Liu et al., 2009).
The discovery of specialized processing within the hemispheres need
not mean that they are separately conscious. Sperry’s Nobel Prize was
“for his discoveries concerning the functional specialization of the ce­
rebral hemispheres,” but his more dramatic claim was that the discon­
nected hemispheres function as separate minds, in apparent denial of
McDougall’s dualism. In a lecture delivered in Stockholm when he
received his prize, and later published in Science, Sperry (1982) sum­
marized as follows:
on the effects of callosal tumors, Raymond et al. (1906) described what
they called a mental callosal syndrome, involving a loss of connectedness
of ideas, problems of recent memory, a bizarreness of manner, and
swings of mood. However, it was difficult in individual cases to distin­
guish effects of callosal damage from those of neighboring damage,
although the surgeon Walter Dandy in 1936 reported on a case in which
the corpus callosum was split in an operation to remove a pineal tumor
and wrote that " … no symptoms follow its division. This simple
experiment puts an end to all of the extravagant hypotheses on the
functions of the corpus callosum” (Dandy, 1936, p. 40).
In the 1940s, William van Wagenen, a surgeon in Rochester NY,
severed the corpus callosum for the relief of epilepsy in a series of pa­
tients, and examination by the psychiatrist Andrew Akelaitis did in fact
reveal some of the psychological effects that were later to attract
widespread interest (Akelaitis, 1941, 1942, 1945; Akelaitis et al., 1942),
but at the time these did not seem to attract interest. Although epileptic
seizures were relieved in seven of the 10 patients, the operation was not
considered successful enough to warrant continuation, and in a review
of the evidence Tomasch (1954) concluded that "’the corpus callosum is
hardly connected with any psychological function” (p. 119).
Things changed dramatically in the 1960s.
2. The California series
Roger W. Sperry began his split-brain research using cats and mon­
keys. By splitting the optic chiasm as well as the corpus callosum, he and
his colleagues were able to present visual information to one side of the
brain by presenting it to just one eye. A visual input to the left eye would
therefore be projected only to the left brain, and input to the right eye
only to the right brain. The results revealed remarkable disconnections.
If the split-brain animal learned to make a visual discrimination, such as
responding to a cross but not to a circle, when viewing with one eye, it
was then unable to make that discrimination when tested with the other
eye. Each side of the brain could even be taught conflicting discrimi­
nations, with one eye and therefore one side of the brain learning to
respond to a cross but not to a circle, and the other eye–brain combi­
nation learning the opposite. In monkeys, by using polarized filters,
conflicting information could be presented to the two eyes simulta­
neously, and the animal learned both discriminations without any
apparent conflict. When later given a free choice, the animals would
alternate periodically between the two, as though each side of the brain
would take control for a while before yielding to the other. As Sperry
(1961) put it, “The split-brain cat or monkey is thus in many respects an
animal with two separate brains that may be used either together or in
alternation” (p. 133).
Michael S. Gazzaniga was aware of split-brain surgery on humans for
the relief of epilepsy, and became interested in testing whether humans
might show similar dissociations. He at first approached van Wagenen in
Chicago, but this initiative failed, and he then enrolled as a PhD student
in Sperry’s laboratory. Two Los Angeles surgeons, Philip J. Vogel and
Joseph E. Bogen, spurred by Sperry’s earlier work on cats and monkeys,
decided again to sever the corpus callosum, along with the anterior
commissure and hippocampal commissure, for the relief of intractable
epilepsy. This time, the operation was judged successful, and has been
repeated in a number of settings around the world, typically with section
of the corpus callosum alone, and with decreasing frequency due to
more effective alternative treatments.
Gazzaniga and Sperry, aided by Bogen, seized the opportunity to
examine these patients, with a view to determining the extent to which
the two hemispheres operated independently. For obvious ethical rea­
sons, it was not feasible to section the optic chiasm in human patients,
but Gazzaniga and Sperry were able to project information to a single
hemisphere by flashing it quickly in one visual hemifield, or by having
shapes felt by one hand. They soon established a left-hemisphere
dominance for speech along with some evidence of right-hemisphere
specialization for nonverbal processing (e.g., Gazzaniga and Sperry,
Each disconnected hemisphere behaved as if it were not conscious of
cognitive events in the partner hemisphere … Each brain half, in
other words, seemed to have its own largely separate cognitive
domain with its own private perceptual, learning, and memory ex­
periences, all of which were seemingly oblivious to corresponding
events in the other hemisphere. (p. 1224).
Separate consciousness within each disconnected hemispheres was
implied in a study by Luck et al. (1989). In searching for a specified
target in visual arrays that covered both visual fields, two split-brain
patients scanned twice as quickly as did control participants, implying
that they undertook separate but simultaneous searches in each hemi­
sphere. Independent consciousness is also implied by the phenomenon
of the “alien hand” (Feinberg et al., 1992). For a period after the com­
missurotomy, the patient’s nondominant hand may act in opposition to
what the patient intends with the dominant hand; Gazzaniga et al.
(1962) note such instances, as when the right hand picks up the evening
paper, the left hand puts it down, and the right hand picks it up again.
They write:
It was as if the control of the left hand were strongly centered in the
minor hemisphere at such times and hence isolated from the main
intent and prevailing directorship of the dominant hemisphere (p.
1267).
Because this implies an element of intention, Bogen (1993) has
preferred the term “anarchic hand.”
There are also occasional indications that patients do transfer in­
formation between hemispheres. In one study, the patient L.B. seemed
1
Bogen also arranged for Wigan’s book to be republished in 1985, and
owned a car with the licence plate WIGAN.
2
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Neuropsychologia 163 (2021) 108041
able to name digits projected to the left hemifield (and right hemi­
sphere), and had also been shown to recognize rhymes shown to the left
hemifield (Zaidel and Peters, 1981). Myers and Sperry (1985) charac­
terized the transferred information as " … neither precise nor complete
nor unprocessed. It appears to consist rather of limited arousal or
orientational cues and partial, contextual or ambient impressions anal­
ogous to ‘mental block’ or ‘tip of the tongue’ sensations in which there is
available some relevant information which is yet insufficient to trigger
precise identification” (p 256).
It was also possible that the patient L.B. had limited speech in the
right hemisphere, so his ability to give verbal responses to digits in the
left hemifield need not imply transfer. Another study revealed L.B.,
along with two other patients, to be unable to judge whether digits,
letters, line drawings of faces, and colors flashed on either side of the
vertical meridian were the same or different (Johnson, 1984). A fourth
patient (N.G.) did show ability to make these decisions, as she again did
a study by Myers and Sperry (1985), but in later studies she was also at
chance, as were two other patients (M.C. Corballis, 1994; Corballis and
Corballis, 2001). The occasional ability of patients to make interhemi­
spheric comparisons may be due in part to cross-cueing, or to imperfect
control of visual fixation, but the results generally show little if any
ability to judge input in opposite hemispheres to be same or different.
Nevertheless, it has long been observed that split-brained patients
maintain a sense of unity in their everyday lives, further hinting that
consciousness may not be quite so divided as originally claimed. Bogen
(1993) wrote of their “social ordinariness” (p. 3), and Sperry (1968)
remarked that “a person with complete section of the forebrain com­
missures would usually go undetected as a rule in a casual first meeting
or conversation or even through an entire routine medical exam” (p.
1223). It was perhaps observations such as these that led to earlier be­
liefs that the corpus callosum was of little importance to psychological
function.
Sergent (1987) showed that split-brained patients were well above
chance at deciding whether sloping lines in the two hemifields were
aligned or not, or whether an arrow in one field pointed to a dot in the
other, again suggesting subcortical processing of location and orienta­
tion. Such results suggest that subcortical processing is not restricted to
peripheral vision or to prolonged viewing, as suggested by Trevarthen
and Sperry (1973).
There is other support for the integration of motion perception across
hemifields. Ramachandran, Cronin-Golomb and Myers (1986) reported
that three patients perceived apparent motion when a light presented in
one hemifield was followed by a light in the other. This was later
confirmed in a study with added controls designed to rule out inferential
strategies (Naikar and Corballis, 1996). Another study showed that a
line-motion illusion (Hikosaka et al., 1993) triggered by a flash of light
was induced when the flash and perceived motion were in opposite
hemifields (M.C. Corballis et al., 2004).
The corpus callosum itself is a structure limited to eutherian mam­
mals, and was preceded phylogenetically by other commissures,
including the anterior commissure and various subcortical commissures
including the tectal commissure connecting the superior colliculi.
Interhemispheric transfer seems well developed in non-eutherian spe­
cies such as birds (e.g., Manns et al., 2017) and fish (e.g., Hemsley and
Savage, 1987). One possibility is that the corpus callosum enables
interhemispheric integration of higher-order cortical functions such as
object recognition and language, while subcortical systems have to do
with more basic visual functions such as visual attention, and the
perception of location, orientation, and movement. The collicular sys­
tem may play a regular role in mammals, and link to cortical areas, as
suggested by Trevarthen and Sperry (1973).
Based on her own findings, Sergent (1987) wrote as follows:
This subcortical coordination of hemisphere activity may thus un­
derlie the behavioural integration displayed by commissurotomized
patients in their daily activities, allowing them to relate different
parts of the visual field and to maintain a unity of purpose in their
action (p. 1389).2
3. Subcortical transfer?
Trevarthen and Sperry (1973) had also suggested that visual transfer
might be accomplished subcortically through what they called the
ambient visual system, as distinguished from the cortical focal system.
While the focal system was dedicated primarily to vision in the foveal
region and dedicated to detailed vision and the recognition of objects,
the ambient system is sensitive to peripheral vision and to moving
stimuli. It is largely subcortical, involving projections via the superior
colliculus to the pulvinar nucleus of the thalamus, where it interacts
with visual association areas of the cortex (e.g., Ingle, 1967; Schneider,
1967). This system has also been referred to as the “second” visual
system (Bogen, 1990).
To demonstrate the role of the ambient system in interhemispheric
transfer, Trevarthen and Sperry (1973) moved large shadowcast stimuli
into peripheral vision and required the subjects to hold fixation while
they viewed them. Three split-brained patients were able make quite
accurate judgments about the relative motion of disks and lines located
some 45◦ from fixation in the two hemifields, and were also able to make
crude verbal statements about stimuli in the left hemifield. They also
responded quite well to changes in size and brightness, but descriptions
of shapes were poor, although they could describe simple properties
such as whether a shape was elongated or square, or whether it was
single or composed of two parts. The subjects could also crudely identify
colors in the left hemifield, at a level approximating that of a dichromat.
Later studies also showed transfer of motion, location, and control of
spatial attention, functions thought to be processed in the superior
colliculi. Holtzman (1984) found that split-brain patients could direct an
eye movement to a specific location in one visual hemifield on the basis
of a locational probe flashed briefly in the other, but could not do so if
the probe was a shape rather than a location; for example, if the probe
was a cross, the patients were unable to move their eyes to the cross in a
2 × 2 matrix that also contained a circle, a square, and a triangle.
In that respect, split-brain patients appear to be of one mind, rather
than two.
Savazzi et al. (2007) provided an elegant demonstration of the role of
the superior colliculus in cross-hemispheric transfer. Simple reaction
time is longer when the signal is on the side opposite the responding
hand (Poffenberger, 1912), and this effect was considerably prolonged
in split-brained patients when the signal was presented in
short-wavelength light undetectable by superior-colliculi neurons. This
effect was not evident in individuals with intact commissures.
The findings to this point seemed to suggest that the two hemispheres
do sometimes function as separate conscious entities, but also sometimes
combine in a unified manner. To the extent that it occurs, separate
consciousness in the two hemispheres seems to rule out theories that
propose a global workspace (e.g., Baars, 2005; Dehaene and Naccache,
2001) or an integration across different circuits (e.g., Tononi and Koch,
2015). They support instead the view that consciousness depends on
local recurrent circuits (e.g., Block, 2007; Lamme, 2006), which can be
contained within one or the other hemisphere. This further implies that
consciousness need not be accessible to speech, as when conscious be­
haviors are initiated by the right hemisphere. In the case of Sperry’s
earlier experiments on cats and monkeys, of course, neither hemisphere
could produce speech, and there still seemed clear evidence of inde­
pendent consciousness in the two hemispheres.
In some cases, though, split-brain patients do behave in integrated
fashion. This might involve cases where existing commissures, such as
2
Justine Sergent took her own life in 1994, which may have dampened in­
terest in her findings. Her main findings on the split-brain, though, have been
largely confirmed (M. C. Corballis, 2016).
3
M.C. Corballis and P.M. Corballis
Neuropsychologia 163 (2021) 108041
the tectal commissure, retain a degree of sensory and perhaps emotional
and semantic integration. Integration sometimes takes time to emerge,
as when the alien hand ceases to emerge and patients begin to display
more of what Bogen called “social ordinariness.” Patients may also
develop integrated strategies, such as cross-cuing. That is, each hemi­
sphere can signal to the other externally. One hand may touch the other,
or visibly tap a finger so that either hemisphere can see it. Even directing
gaze to particular objects or scenes can alert one hemisphere to what is
being processed by the other.
Interest in the split brain gradually diminished over the past half
century, as effective alternative treatment for epilepsy were developed,
and fewer cases of commissurotomy were available. Against this trend,
though, was a flurry of interest in the late 2010s, and it was this that
prompted this special issue.
Whether a parallel system operates in primates remains to be seen.
There may be truth to all of these accounts, and the question remains
not so much that of whether consciousness is divided in the split brain as
of when it is divided and when it is not. As de Haan et al. (2020) point
out, this must depend in part on what defines consciousness – there are
certainly some respects in which processing is divided in the split brain
and some in which they are not. Precise description of the varying ca­
pacities of split-brain patients should help us understand what con­
sciousness is – or perhaps question whether we really need the concept
at all.
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4. Revival
The renewed interest began with a claim by Pinto et al., (2017a, b)
that consciousness is unified in the split brain, but perception is divided.
They based this on a split-brained patient who could respond with either
hand or verbally to the location, orientation or identity of an object
across the entire visual field, but were unable to compare stimuli across
hemifields. Because the patient could respond with either hand and
claimed awareness of the entire visual field, the authors suggested that
he retained unitary consciousness. With perception divided, the ques­
tion is then how consciousness is unified. Following from the study by
Pinto et al., 2017a, 2017b, de Haan et al. (2021) suggest that, in the
normal brain as in the split brain, conscious perception can be localized
in independent circuits, but that the impression of unity depends on the
fact that the body itself is a single entity in an external world with
ever-changing constraints. The apparent singularity of consciousness
then resides in the execution and planning of bodily action.
Volz and Gazzaniga (2017) responded to the article by Pinto et al.
(2017a, 2017b) by retaining the strong view that consciousness is
indeed divided in the split brain, but that when interhemispheric
transfer is seen to occur, it may be achieved through cross-cuing, as
suggested earlier. Even facial expressions might serve as informational
cues. Volz and Gazzaniga also suggest that mirror neurons within each
hemisphere may play a role, enabling one hemisphere to perceive the
goal of an action produced by the other (Rizzolatti and Sinigaglia, 2010).
If the two sides of the brain do indeed have separated minds, they have
shared the one body, and it would not be surprising if they had devised
intimate strategies for communication.
M. C. Corballis et al. (2018) reiterated the possible role of subcortical
exchange between hemisphere, as reviewed earlier, in establishing at
least a degree of visual unity between hemispheres in the split brain.
Pinto and colleagues do recognize this possibility, but suggest that their
results probably do not depend on the anterior or posterior commissures.
These structures were intact in the patient they investigated, but had
been severed along with the corpus callosum in a number of earlier
patients, and disconnection effects seem to be independent of whether or
not they are intact. This has little bearing, though, on the role of the
collicular system, because the tectal commissure, which connects the
superior colliculi appears to have been unaffected in the patients tested
in the Californian series, as well as in subsequent patients. The collicular
system is likely to have remained functional bilaterally in all split-brain
patients so far examined. Integration via this system might explain why
some elementary visual features are integrated, while higher-order
cortical processing of objects or words is not. It might also explain
why split-brain patients seem to act as though the visual and social
worlds are unified.
Some new but indirect support for a subcortical contribution comes
from a study by Beltramo and Scanziani (2019), who identified an area
in the postrhinal cortex of the mouse that receives input from the su­
perior colliculi independently of the geniculo-cortical system, and out­
performs the primary visual cortex in the discrimination of movement.
4
M.C. Corballis and P.M. Corballis
Neuropsychologia 163 (2021) 108041
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