The Neuropsychological Implications of Interhemispheric Communication

The Neuropsychological Implications of Interhemispheric Communication

“I believe it then to be entirely unphilosophical, and tending to important errors, to speak of the cerebrum as one organ…The two hemispheres of the brain are really…two distinct and entire organs… as fully perfect in all its parts…as the two eyes. The corpus callosum, and the other commissures between them, can with no more justice be said to constitute the two hemispheres into one organ than the optic commissure can be called an union of the two eyes into one organ” (Wigan A.L., 1844, as cited in Devinsky, & D’esposito, 2004).

In the quoted passage A.L. Wigan beautifully articulates the importance of understanding lateralization of the brain. Implicit in the importance of brain lateralization is the importance of interhemispheric communication, or communication between the left and right halves of the brain. Interhemispheric communication is mediated by the commissures of the brain which include: the anterior commissure (precommissure), posterior commissure (epithalamic commissure), colossal commissure (corpus callosum), commissure of fornix (hippocampal commissure or psalterium), habenular commissure, and the supraoptic commissures (which consist of Gudden’s commissure and Meynert’s commissure) (“Commissures of the brain”, 2010). “Commissures functionally link the homologous areas of the hemispheres, providing enormous avenues for them to inhibit, evoke, and synchronize function” (Devinsky et al., 2004). The corpus callosum is by far the most prominent and thus the most studied of the commissures. Availability of research on the structure and function of the subsequent commissures varies. 

            The anterior commissure appears to primarily project white-matter tracts between the middle and inferior temporal lobes (Wilde, Bigler, Haider, Chu, Levin, Li, & Hunter 2006) but is also involved in communication between the right and left olfactory bulbs and entorhinal areas of the temporal lobe (Devinsky et al., 2004). Interestingly, while in rodents nearly all amygdaloid nuclei “have dense and direct interhemispheric connections via the anterior commissure” it appears that in primates “there are virtually no direct interhemispheric connections via the anterior commissure” (Irwin, Anderle, Abercrombie, Schaefer, Kalin, & Hunter, 2004). Wilde et al. (2006) have found that the anterior commissure plays an “important structural role in interhemispheric temporal lobe circuitry” and that pediatric traumatic brain injury is predictive of reduced anterior commissure volume. Despite a pop-culture belief that homosexual men share similar sexually dimorphic brain features with women, (particularly this structure) recent research indicates that this belief is completely unfounded (Lasco, Jordan, Edgar, & Petito, Byne , 2002).  

The largest of the commissures, and the “largest connecting fiber bundle in the human brain” (Rüsch, Luders, Lieb, Zahn, Ebert, Thompson, Toga, & Telbartz van Elst, 2007), is the corpus callosum. The corpus callosum “connects almost every part of one hemisphere with the corresponding part of the other hemisphere”(Prakash, & Nowinski, 2006) and as such has been implicated in a wide range of cognitive tasks. The collosal fiber tract coordinates bimanual tasks and bilateral axial movements, mediates sustained attention, as well as memory, sensory, motor and emotional functions (Devinsky et al., 2004). Further, approximately 40% of callosal fibers are unmyelinated, and these slow-conducting fibers primarily inhibit contralateral activity. The unmyelinated fibers of the corpus callosum predominantly form the connections for the frontal and temporal association areas while the secondary sensorimotor areas are connected by the faster myelinated axons (Aboitiz, & Montiel, 2003). Through this inhibitory capability the corpus callosum allows one hemisphere to inhibit the other, granting exclusive control during a specific behavioral function (Devinsky et al., 2004). As the function of the corpus callosum within the domains listed appears to be predominantly, if not exclusively, facilitating interhemispheric communication, efficiency of information transfer between the hemisphers, through the corpus callosum, appears to have profound implications for cognition. Efficient transfer and integration between hemispheres is associated with improvements to “performance on complex, multidimensional tasks, presumably because of the advantage of parallel processing” (Hiatt, & Newman, 2007). 

            Interhemispheric transfer time can be estimated utilizing right and left hand response times to stimuli presented in either the right or left visual field. Thus, “when information must be communicated across the hemispheres to initiate a motor response, an extra step of cross-callosal transmission is required, which should lead to increased [reaction times] relative to trials on which the stimulus is presented to the same hemisphere that controls the motor response” (Hiatt, & Newman, 2007). If a participant is tasked with a right hand response and the visual cue is presented in the right visual field, then the left hemisphere is processing both the visual information as well as the motor command. However, in the inverse situation, a right hand response with a left visual field cue, the right hemisphere must receive and interpret the cue and then pass a motor command through the corpus callosum to the left hemisphere to issue a command to the right hand to respond.  Reaction times on crossed stimuli/response trials are indeed longer than reaction times on uncrossed stimuli/response trials (Hiatt, & Newman, 2007). Interestingly, in research looking at reaction times during this task within a prison population of psychopaths, Hiatt et al. (2007) found that psychopaths had a reaction time more than twice as long as healthy populations. The authors note that this could suggest key deficits in complex processing within this population and could play a role in “known affective and cognitive processing abnormalities” within this population. 

            Abnormalities and deficiencies within the commissures can arise from a number of causes. Agenesis of the corpus callosum is an uncommon congenital disorder affecting 1-4 per 1000 children (Moes, Schilmoeller, & Schilmoeller, 2009). Agenesis of the corpus callosum has been identified to contribute to a wide range of motor, sensory and developmental deficits. When compared to siblings that have typical brain structure this population has a greater occurrence of issues with hearing, vision, reduced pain perception, and delayed motor development across a wide variety of motor tasks (particularly in activities that require left-right co-ordination). Handedness also appears to be effected by agenesis of the corpus callosum with affected individuals demonstrating a left hand preference at a significantly higher rate than their healthy siblings (Moes et al., 2009). 

            The corpus callosum will also sometimes be surgically severed in a callostomy. This is done in an effort to control seizure disorders  by reducing the incidence of seizures. Although knowledge of callosal function has been gathered through lesion studies, animal studies and patents recovering from strokes, the majority of our understanding stems from research examining patients following elective callostomy (Devinsky, & D’esposito, 2004). These studies support theories of lateralization providing evidence of “left hemisphere dominance for language, analytical functions, and skilled praxis movements and right hemisphere dominance for visuospatial functions, constructional (manipulative) tasks that rely on spatial concepts, facial recognition, and emotional responsiveness” (Devinsky, & D’esposito, 2004). 

            As described earlier,Prakash & Nowinski (2006) reportthat the corpus callosum “connects almost every part of one hemisphere with the corresponding part of the other hemisphere.” Devinsky, & D’esposito (2004) describe one of the exceptions to this statement noting that “The hand and foot areas of the human primary motor and somatosensory cortices have no callosal connections.” Despite this natural lack of direct connection between motor and somatosensory cortices, patients who have undergone a callostomy will display marked difficulty learning new tasks requiring complex cooperation of both hands. Curiously, tasks of this nature learned prior to the callostomy typically remain intact (Devinsky, & D’esposito, 2004). 

            Callostomy also has stark implications for attention, memory and emotion. The procedure has been found to impair nonverbal learning and memory, verbal memory, working memory, topographic memory and sustained attention. Emotionally, patients experience reductions in the range of negative affect (Devinsky, & D’esposito, 2004). Patients will generally not react to events that would otherwise be expected to illicit intense emotional responses. Experiences that might be expected to illicit “bitterness, sadness, hatred, anger, violence, or related negative emotions” (Zaidel, 1994) are reacted to in a manner Zaidel describes as “factual.” Facial expression remains intact and seems to follow the previously described pattern of emotional phenomena within this population. Overall, individuals who have undergone callostomy’s tend to have a “general personality characteristic” that is “’positive’ rather than ‘negative’” (Zaidel, 1994). 

            Although the daily behavior of patients who have undergone callostomy’s tend to give the appearance of unified consciousness (Zaidel, 1994) occasionally the procedure will result “in two spheres of independent volition” (Devinsky, & D’esposito, 2004). Alien hand syndrome can sometimes occur following a callostomy or as a result of anterior, posterior, or medial callosal lesions (Devinsky, & D’esposito, 2004). Alien hand syndrome as a result of callosal damage or surgical intervention typically results in one hand (left hand, right hemisphere) acting independently of the conscious wishes of the patient (left hemisphere) and often works directly against the actions of the opposite hand (Devinsky, & D’esposito, 2004). 

            Intehemispheric communication has implications for handedness as well. Mounting evidence indicates that “increased interaction between cognitive processes lateralised to the left versus right hemispheres”  is associated with mixed and inconsistent hand preference (Christman, Henning, Geers, Propper, & Niebauer, 2008). Mixed handedness is described by Christman et al. (2008) as “people who display any degree of mixed hand preference” indicated by a preference of performing any of a list of tasks with their non-dominant hand. 

Research appears to indicate strength of hand preference is more significant a distinction than direction of handedness. Christman et al. (2008) briefly discuss research indicating that while handedness does not appear to be overwhelmingly genetic in nature, strength of preference does appear to be heritable. It has been “found that stronger degrees of right-handedness were associated with smaller corpus callosum size” as well as the opposite of “larger corpus callosum size in mixed, relative to strong, handers have been reported” (Christman, Henning, Geers, Propper, & Niebauer, 2008). 

            Individuals of mixed hand preference appear to “display an increased tendency to update beliefs in response to information inconsistent with those beliefs. This has been interpreted as reflecting the fact that the left hemisphere maintains our current beliefs while the right hemisphere evaluates and updates those beliefs when appropriate” (Christman, Henning, Geers, Propper, & Niebauer, 2008). Christman et al. describe multiple studies supporting a notion that the left hemisphere is rigid and unchanging in its beliefs while the right hemisphere is “predisposed to generate and update new mental representations at the slightest provocation” Further, their own research branched from the concept of “belief updating” to the issues of persuasion and gullibility. Christman et al. found that individuals who are mixed handed tend to have “lower thresholds for persuasion and higher levels of gullibility.” This is attributed to increased size of the corpus callosum and thus enhanced interhemispheric communication within this population. This has implications regarding patients who have undergone callostomys. As increased interhemispheric communication results in more fluidity in beliefs (which comes along with the unfortunate side effect of being easily persuaded and more gullible) one would expect that individuals who have undergone a callostomy would be extremely rigid  in their beliefs, be very difficult to persuade, and be less gullible than the general population. It seems possible that issues with “belief updating” could play a role in some of the memory issues noted in callostomy patients. 

            Handedness and interhemispheric interaction also plays a role in risk perception and risk taking. As “right-hemisphere mechanisms are specifically sensitive to and averse to risk” (Christman, Jasper, Sontam, & Cooil, 2006) and “mixed degree of handedness is associated with increased access to right hemisphere processing” (Christman et al., 2006) patients that have undergone a callostomy could potentially be more likely to take risks as the access of their dominant left hemisphere will not have the same level of access to the right hemispheres risk aversion.  This seems a potential factor in alien hand syndrome. The right hemisphere, acting through the left hand, could be acting against the will of the individual in a manner it perceives as reducing some detected risk. 

            Interhemispheric communication plays a tremendous roll in cognition and functioning.  While patients with agenesis of the corpus callosum as well as patients who have undergone callostomy are capable of living relatively active lives there is much still to be learned about the brain through these individuals.  Despite the long accepted importance of the corpus callous “the functions of the corpus callosum remain incompletely defined” (Devinsky, & D’esposito, 2004).  It seems likely that further study, particularly on the nature of the corpus callosums inhibitory functioning will yield significant knowledge regarding the function of the brain. 

 

References 

Aboitiz, F., & Montiel, J. (2003). One hundred million years of interhemispheric communication: the history of the corpus callosum. Brazilian Journal of Medical and Biological Research, (36), 409-420. 

Christman, S.D., Henning, B.R., Geers, A.L, Propper, R.E., & Niebauer, C.L. (2008). Mixed-handed persons are more easily persuaded and are more gullible: interhemispheric interaction and belief updating. LATERALITY, 13(5), 403-426. 

Christman, S.D., Jasper, J.D., Sontam, V., & Cooil, B. (2006). Individual diverences in risk perception versus risk taking: handedness and interhemispheric interaction. Brain and Cognition, (63), 51-58. 

Commissures of the brain. (2010). Retrieved from http://radiopaedia.org/articles/commissures-of-the-brain 

Devinsky, O., & D’esposito, M. (2004). Neurology of cognitive and behavioral disorders. New York, NY: Oxford University Press. 

Hiatt, K.D., & Newman, J.P. (2007). Behavioral evidence of prolonged interhemispheric transfer time among psychopathic offenders. Neuropsychology, 21(3), 313–318. 

Irwin, W., Anderle, M.J., Abercrombie, H.C., Schaefer, S.M., Kalin, N.H., & Hunter, J.V. (2004). Amygdalar interhemispheric functional connectivity differs between the non-depressed and depressed human brain. Neuroimage, (21), 674– 686. 

Lasco, M.S., Jordan, T.J., Edgar, M.A., Petito, C.K., Byne, W., & , . (2002). A lack of dimorphism of sex or sexual orientation in the human anterior commissure. Brain Research, (936), 95–98. 

Moes, P., Schilmoeller, K., & Schilmoeller, G. (2009). Physical, motor, sensory and developmental features associated with agenesis of the corpus callosum. Child: care, health and development, 35(5), 656–672. 

Prakash, B.K.N., & Nowinski, W.L. (2006). Morphologic relationship among the corpus callosum, fornix, anterior commissure, and posterior commissure. Academic Radiology, 13(1), 24-35. 

Rüsch, N., Luders, E., Lieb, K., Zahn, R., & Ebert, D., Thompson, P.M., Toga, A.W., Telbartz van Elst, L., (2007). Corpus callosum abnormalities in women with borderline personality disorder and comorbid attention-deficit hyperactivity disorder. Journal of Psychiatry & Neuroscience, 32(6), 417–422. 

Wilde, E.A., Bigler, E.D., Haider, J.M., Chu, Z., Levin, H.S., Li, X., Hunter, J.V. (2006). Vulnerability of the anterior commissure in moderate to severe pediatric traumatic brain injury. Journal of Child Neurology, 21(9), 769–776. 

Zaidel, D.W. (1994). A view of the world from a split-brain perspective. 

 

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