I chose this topic from
of the neurosciences for a number of reasons. First of all,
because they are relatively rare (between 1985 and 1997, 563
craniotomies were performed, of which only 20 were hemispherectomies;
et al., 1999), the subject of cerebral hemispherectomies (CHs) is not
covered in a normal neuroscience courses. Second, CHs are, by
the direct application of neuroscience knowledge to human
Third, the necessity of this procedure in humans provides a pool of
participants who can answer the kinds of experimental and introspective
questions one could never address in non-human subjects. Four,
dramatic recovery in the vast majority of patients bears scientific
for issues of plasticity, particularly in relation to traumatic brain
This paper attempts to give an overview of the scientific perceptions
cerebral hemispherectomies and their significance to neuroscientific
Historical neglect of the concept
One could make the case
brain is who we are. However, with a cerebral hemispherectomy, an
individual can lose a significant portion of his brain mass and still
or recover a remarkable amount of the original function. This
of such incredible recovery following cerebral hemispherectomy
two of the central dogmas of neuroscience that happen also to be
Names such as Broca, Wernicke,
and many, many others are associated with the establishment of the
of localization of function within the central nervous system.
a newer dogma, plasticity, has come into its own in recent years, most
famously through the work of Hubel and Weisel (e.g., ocular dominance
in cats) and Kandel (e.g., habituation in Aplasia), among others.
While the second concept does not negate the first, it presents
when function is not consistently or rigidly localized.
In a review of the historical
Bach-y-Rita (1990) finds the plasticity concept was neglected for so
years for a number of reasons. Perhaps the most salient of these
is that many of the historically most prominent cases from the
period of neuroscientists helped to establish localization of
The majority of these, as with the aforementioned Broca, examined older
patients in whom plasticity yielded less sway than "hard-wired"
Thus, these patients did not typically recover and, indeed, this
left such patients without a call for therapy, thereby creating a
prophecy of permanent loss of function.
Arguably, this (mis)perception
to the present as is evident by the astonishment expressed in the case
of Christopher Reeve's progress following his spinal injury.
this famous case is not so usual as Franz (1915) noted that
recovery from paralysis resulting from cerebral injury could persist
late as 20 years after injury.” Most strikingly, he commented
“[w]e should probably not (always) speak of permanent paralysis, but of
uncared for paralysis” (Italics added). Returning to Reeve’s
his intense rehabilitation regimen has almost certainly made the
that has continued to garner him headlines, but the prevailing neglect
of the potential for plasticity has unfortunately made his level of
As a result of the fact that
has historically been underrepresented in the literature, Bach-y-Rita
contends that there has remained a poorly developed theoretical
available to guide researchers who might address these questions.
However, he does find that there is much evidence of plasticity in a
of forms in the literature and cites these as areas to consider in
such theoretical guideposts. Such areas include cellular and
phenomena (e.g., sensory convergence), rehabilitation following
injury (as covered above) and case studies in which recovery has been
Cerebral hemispherectomies represent opportunities to examine the
and mechanisms of recovery from several of these angles.
Issues surrounding cerebral
performed in patients with intractable seizures (e.g., those induced by
Rasmussen’s encephalitis, strokes, among other conditions). This
is a radical surgery, but one ultimately deemed necessary when all
therapies (e.g., drugs, special diets, etc.) have proven
In spite of the apparently extreme nature of this procedure, functional
recovery is commonly observed in these patients, particularly in the
where plasticity might be expected to be the greatest.
In the procedure, the
the focus of the seizure either is physically removed in what is known
as an anatomical hemispherectomy or it may be surgically isolated from
the rest of the brain. This latter case is labeled a functional
One might view this procedure as the ultimate lesion study.
because so much tissue is removed (or at least functionally
a number of confounds present themselves that might not otherwise be
in the controlled, focused lesions more commonly performed in order to
study specific function. For one thing, the serious nature of the
surgery demands a relatively lengthy recovery period, thus preventing
monitoring of the progress of functional recovery in the earliest,
most important stages.
Another problem with studying
for general answers to hypotheses is that, primarily because of their
scarcity, the majority of the literature tends to consist of case
highlighting typically only one or two patients and reviews and
of this literature are difficult for a number of reasons. To
with, because this procedure affects nearly every conceivable aspect of
the human condition, experiments address a plethora of areas (e.g., IQ,
motor skills, language proficiency) and each of these may be tracked by
any number of measures (particularly in the independently well-studied
area of language where hundreds of instruments have been
Contrast this with, say, the study of individuals with memory
The field is much more focused in those cases, aided as well by the
that their subjects may be otherwise “intact” and
By this last point, I consider the possibility of, for example,
in language production. Because an entire hemisphere was removed,
it may be very difficult to determine whether the deficit is in
language or the motor skills to vocalize and/or write (I admit this is
a flawed example, but I think the point is clear).
The issues of experimental
the patients themselves present even greater hurdles to pooling data by
virtue of the variance of the population and the idiosyncrasies
in their condition(s). For example, while Rasmussen’s
(RE) is most frequently associated with CH, it is not the only cause
the procedure. Different diseases may produce different effects
to the surgery, so those must be taken into account. In the case
of RE, patients’ IQ diminishes as the disease progresses. With a
cerebral stroke, there may be pre-surgical loss of function in speech
motor areas. Thus, the pathology of the original condition must
separated from the effects of the hemispherectomy.
Because CHs are performed most
on younger patients, developmental differences may also compound
The patient’s age at the onset of the disease, the severity of the
the age of the surgery, and the amount of post-surgical time that has
before the patient is examined must be taken into account when
data from this population.
The most obvious issue to
consider in studying
this CH patients is which hemisphere was excised. This alone, of
course, is not adequate to neatly partition data from subjects, as the
general population is heterogeneous with respect to hemispheric
of language (i.e., left, right, or even bilaterally). Handedness
is a general (though not perfect!) indicator of this factor, and is
noted in case studies. Even then, some patients may be bilingual
or have acquired a second language, further complicating standardized
Actually, some of these “complications” may represent opportunities to
address questions about language acquisition and some case studies have
exploited some of these subpopulations to examine, for example,
Cerebral hemispherectomy research
The obvious solution to
of a limited human population and their inherent heterogeneity is to
animal research. Surprisingly, as limited as the research on CH
in general, the vast majority of the publications on this topic tend to
be case studies in humans. This is likely due to a number of
First, CH is a relatively uncommon procedure, thus not attracting the
attention of more generalized topics like learning and memory.
a parallel situation exists in traumatic spine injury research.
her seminar last year at UNT, Dr. Lisa Rosenberg related that drug
in general have little interest in this topic because the number of
patients/customers for this type of injury is miniscule compared to
returns for developing, say, a new allergy medication.)
Animal research may also be
CH is most relevant to humans, therefore they are studied directly in
settings primarily by physicians who naturally have the most contact
them. Finally, as alluded to above, a CH does not target one
region, thus making it difficult to draw conclusions about loss and/or
recovery of function.
Cerebral hemispherectomy patients
patients generally have poor or virtually no use of contralateral
although this tends to improve over time, with the extent of recovery
on the age of the patient, course of the illness, and other factors
Patients often (thought not
"mirror movements" during the early stages of recovery. These are
movements made by the limbs contralateral to the excised hemisphere
mirror those performed by the unaffected limbs. This mirroring is
almost always of a lesser magnitude than the movements performed
with the “working” limbs, and the phenomenon itself diminishes over a
brief period as the patient begins to acquire control of contralateral
limbs. These mirror movements have also been demonstrated in rats
following CH while undergoing electrical stimulation (Machado et al.,
Language is usually impaired to
in patients, with the typical pattern of recovery leaning to the notion
of “younger is better.” However, contrary to the conventional
of language acquisition, Curtiss et al. (2001) find that there may be
developmental windows for recovery with respect to language, and that
plasticity underlying the recovery may change in a nonlinear fashion.
Regardless, there is initially
a more pronounced
deficit which diminishes across time. Generally, some deficits do
persist, but patients several years beyond their surgeries often have
coping strategies for avoiding, for example, complex grammar by
statements into smaller sentences. Thus, they appear high
to the casual observer. Interestingly, deficits tend to be
reductions in scores with respect to population norms across multiple
of language, rather than the aphasias of the more specific variety as
famously described by Broca and Wernicke. This is all the more
given that, rather than a single, isolated lesion, an entire cerebral
has been removed!
Also surprising is that no
or primary sensory impairment are reported. One might expect that
perception would be altered permanently by such a sweeping alteration
brain architecture, but no such deficits are reported even in the short
Proposed mechanisms of recovery
in cerebral hemispherectomy patients
remarkable recovery of patients following CH remain elusive.
that the main population studied under these conditions are human
it is not likely that the more invasive experimental manipulations
DNA chip gene screens, electrophysiological recordings, etc.) required
to address the most of the necessary questions will be performed in the
immediate future. However, work certainly continues in other,
areas that may shed light on this apparently miraculous recovery of
The overwhelming view of the
function following CH refers to the admittedly unknown mechanism as
However, plasticity (as it is presently understood as the physical
of existing synaptic connections and the establishment of new ones)
certainly cannot completely explain the dramatic recovery of language,
motor skills, and other aspects of function in such incredibly short
spans. For example, the nearly complete (re)acquisition of
in the non-dominant hemisphere occurs over a period days. One
Chomsky’s arguments against associative learning as the primary basis
of language acquisition; that several words an hour would be learned,
too fast for a behaviorist model. Similarly, “transference” of
to the remaining hemisphere happens much faster than even long term
consolidation can realistically account for.
Based on this and other
idea of “transference” (though the term is routinely applied to this
is clearly a misnomer in the implication that information is moved out
of the damaged (and ultimately removed) hemisphere. Instead,
have proposed that latent circuitry resides in each hemisphere,
to step in to restore function where the primary circuitry is rendered
nonfunctional. The existence of such hypothetical networks
have yet to be demonstrated, although other hypotheses similarly lack
evidence to support them or refute this one.
review is any comment on the effects on memory post-hemispherectomy,
in retention of pre-surgery memories or in the ability acquire new
in any form. This seemingly obvious area was neglected in the
Why this is not tested directly and/or reported is not clear, but the
emphases of this area of research tends to be on language and motor
perhaps because of well-developed rehabilitation programs, but this is
Though less tangible in terms
measures, one might also expect the concept of personality to have been
addressed in the literature, even anecdotally given the single-subject
case study nature of many of the papers on these patients.
this was never addressed. Granted, the majority of patients were
children when the surgeries were performed, but all the literature
with then several years post-CH. One might expect that the
of “left-brained” and “right-brained” personalities would be
exaggerated in these patients, as it has been reported that individuals
have been known to undergo a personality change following a stroke or
unilateral brain injury, in which the personality tends to resemble
expected of the healthy, unaffected hemisphere.
While the subject of CH may
to the general neuroscience community, it is more than an obscure
procedure. Rather, these patients possess a unique set of data
a potential model system for certainly traumatic brain injury and
mechanisms for dramatic feats of plasticity.
Brain plasticity as a basis for recovery of function in humans. Neuropsychologia.
Muller RA, Chugani
DC. (1996). Functional brain reorganization in children. Brain
Curtiss S, de
Bode S, Mathern
GW. (2001 ). Spoken language outcomes after
factoring in etiology. Brain Lang. 79:379-96.
Sheetz M, Wilson
A. (1915). The possibility of motor function recovery in
hemiplegia. J. Am. Med. Assoc. 65:2150-2154.
Gemba H, Miki
N, Sasaki K.
(1995). Cortical field potentials preceding vocalization and
of -erebellar hemispherectomy upon them in monkeys. Brain Res.
Crawley AP, Hwang PA. (1998). Regionalized sensorimotor
after hemispherectomy fMRI evaluation. Pediatr Neurol.
Linke DB, Elger CE. (1997). Patterns of language dominance
in focal left and right hemisphere epilepsies: relation to MRI
EEG, sex, and age at onset of epilepsy. Brain Cogn.
Gadian DG, Vargha-Khadem
F, Porter DA, Boyd SG, Connelly A. (2000). The
of sensorimotor function in children after hemispherectomy. A
MRI and somatosensory evoked potential study. Brain. 123
Simos PG, Wheless
JW, Baumgartner JE, Kim HL, Davis RN, Zhang W, Papanicolaou AC.
Multimodality functional imaging evaluation in a patient with
encephalitis. Brain Dev. 24:239-44.
Clinical disorders of brain plasticity. Brain Development (In
available on-line only at the time of this writing).
Shoji A, Ballester
G, Marino R Jr. (2003). Mapping of the rat's motor area
hemispherectomy: The hemispheres as potentially independent motor
Smith JR, Lee
PD, King DW, Murro AM, Park YD, Lee GP, Meador KJ, Loring DW.
A 13-year experience with epilepsy surgery. Stereotact Funct
(1997). Follow-up study of a right- and a left-hemispherectomized
child: implications for localization and impairment of language in
(2000). Developmental neuroplasticity in a model of cerebral
and stroke. Neuroscience. 95:625-37.