PSY 2C 08: BRAIN, BODY AND
UNIT–I: BRAIN AND BEHAVIOUR
Behavioural Effect of Disconnection
BEHAVIOURAL EFFECT OF DISCONNECTION
Neuro anatomical disconnection occurred when a focal process disrupts pathways connecting one brain area to
another. A disconnection syndrome is a cluster of clinical finding produced by the separation. Historically
disconnection syndrome most often refers to higher brain functions. Disturbances of sensory or motor functions
are not routinely classed as disconnections, thought some are best explained by a disconnection mechanism.
While long acknowledged disconnections became a major influences in neuroscience only after the publication of
“Disconnections syndromes in animal and man” a series of paper written by Norman Geschwind reinvented
disconnections, stimulated an era of research based on disconnection paradigm, and provided a foundation for
network theories of brain functions.
Disconnection syndrome is a general term for a collection of neurological symptoms caused via lesions
to associational or commissural nerve fibres by damage to the white matter axons of communication pathways in
the cerebrum (not to be confused with the cerebellum) independent of any lesions to the cortex. The behavioural
effects of such disconnections are relatively predictable in adults. Disconnection syndromes usually reflect
circumstances where two regions and still have their functional specializations except in domains that depend on
the interconnections between the two regions.
Callosal syndrome or split-brain, is an example of a disconnection syndrome from damage to thecallosum between the two hemispheres of the brain. Disconnection syndrome can also lead to aphasia, left-
sided apraxia, and tactile aphasia, among other symptoms. Other types of disconnection syndrome include
conduction aphasia (lesion of the association tract connecting Boca’s area and Wernicke’s) agnosia, apraxia, pure
Anatomy of cerebral connection
Theodore Meynert, a neuroanatomist of the late 1800s, developed a detailed anatomy of white matter pathways.
He classified the white matter fibers that connect the neocortex into three important categories projection fibers,
commissural fibers and association fibers. Projection fibers are the ascending and descending pathways to and
from the neocortex. Commissural fibers are responsible for connecting the two hemispheres while the association
fibers connect cortical regions within a hemisphere. These fibers make up the interhemispheric connections in the
Many studies have shown that disconnection syndromes such as aphasia, agnosia apraxia pure alexia and many
others are not caused by direct damage to functional neocortical regions. They can also be present on only one
side of the body which is why these are categorized as hemispheric disconnections. The cause for hemispheric
disconnection is if the interhemispheric fibers, as mentioned earlier, are cut or reduced. An example is commissural
disconnect in adults which usually results from surgical intervention, tumour, or interruption of the blood supply to
the corpus callosum or the immediately adjacent structures. Callosal disconnection syndrome is characterized by
left ideomotor apraxia and left-hand agraphia and/or tactile anomia, and is relatively rare.
Other examples include commissurotomy, the surgical cutting of cerebral commissures to treat epilepsy and
callosal agenesis which is when individuals are born without a corpus callosum. Those with callosal agenesis can
still perform interhemispheric comparisons of visual and tactile information but with deficits in processing complex
information when performing the respective tasks.
Behavioural effect of disconnection
The clinical effects of callosal disconnection were first seriously considered by Carl Wernicke in 1874 and became
a prominent part of early neurology. Wernicke predicted the existence of an aphasic syndrome, conduction
aphasia, that would result from severing fiber connections between the anterior and the posterior speech zones.
In this condition, speech sounds and movements are retained, but speech is impaired because it cannot be
conducted from one region to the other. In 1892, Joseph Dejerine was the first to demonstrate a distinctive
behavioural deficit resulting from pathology of the corpus callosum.
In a series of papers published around 1900, Hugo Liepmann most clearly demonstrated the importance of severed
connections as an underlying factor in the effects of cerebral damage. Having carefully analysed the behaviour of
a particular patient, Liepmann predicted a series of neocortical disconnections that could account for the behaviour.
In 1906, after the patient died, Liepmann published the post-mortem findings, which supported his hypothesis.
Liepmann wrote extensively on the principle of disconnection, particularly about the idea that some apraxias might
result from disconnection. He reasoned that, if a patient was given a verbal command to use the left hand in a
particular way, only the verbal left hemisphere would understand the command. To move the left hand, a signal
would then have to travel from the left hemisphere through the corpus callosum to the right hemispheric region that
controls movements of the left hand.
Interrupting the part of the corpus callosum that carries the command from the left hemisphere to the right would
disconnect the right hemisphere’s motor region from the command. Thus, although the subject would comprehend
the command, the left hand would be unable to obey it. This apraxia would occur in the absence of the weakness
or incoordination of the left hand that would develop in the presence of a lesion of the right-hemisphere motor
cortex, which controls the actual movement of the left hand.
Liepmann’s deduction, although brilliant, was ignored for a number of reasons. For one, it was published in German
and so was not widely read by English-speaking neurologists. Additionally, except in the extremely unusual case
of a patient with a natural lesion of only the corpus callosum, any observed behavioural deficits should be attributed
to damage of gray matter itself without reference to connections. Finally, the results of numerous animal studies
consistently implied that no significant behavioural effects followed the cutting of the corpus callosum. Not until the