PSY 2C 08: BRAIN, BODY AND
UNIT–I: BRAIN AND BEHAVIOUR
TOPIC: Introduction to Disconnection
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 regions A and B 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 the corpus callosum
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 Broca’s area and Wernicke’s), agnosia, apraxia, pure alexia, etc...
Anatomy of cerebral connections
Theodore Meynert, a neuroanatomist of the late 1800s, developed a detailed anatomy of white matter pathways.
He classified the white matter fibres that connect the neocortex into three important categories – projection fibres,
commissural fibres and association fibres. Projection fibres are the ascending and descending pathways to and
from the neocortex. Commissural fibres are responsible for connecting the two hemispheres while the association
fibres connect cortical regions within a hemisphere. These fibres make up the interhemispheric connections in the
Callosal disconnection syndrome is characterized by left ideomotor apraxia and left-hand agraphia and/or tactile
anomia, and is relatively rare.
HEMISPHERIC DISCONNECTIONS SYSTEMS
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 fibres, 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.
Hemispheric disconnection has impacted behaviours relating to the sensory and motor systems. The different
systems affected are listed below:
➢ Olfaction – The olfactory system is not crossed across hemispheres like the other senses, which means
that left input goes to the left hemisphere and right input goes to the right hemisphere. Fibres in the
anterior commissure control the olfactory regions in each hemisphere. A patient who lacks an anterior
commissure cannot name odours entering the right nostril or use the right hand to pick up the object
corresponding to the odour because the left hemisphere, responsible for language and controlling the
right hand, is disconnected from the sensory information.
➢ Vision – Information from one visual field travels to the contralateral hemisphere. Therefore, with a
commissurotomy patient, visual information presented in the left visual field travelling to the right
hemisphere would be disconnected from verbal output since the left hemisphere is responsible for
➢ Somatosensory – If the two hemispheres are disconnected, the somatosensory functions of the left and
right parts of the body become independent. For example, when something is placed on the left hand of
a blindfolded patient with the two hemispheres disconnected, the left hand can pick the correct object
within a set of objects but the right hand cannot.
➢ Audition – Though most of the input from one ear would go through the same ear, the opposite ear also
receives some input. Therefore, the disconnection effects seem to be reduced in audition compared to
the other systems. However, studies have shown that when the hemispheres are disconnected, the
individual does not hear anything from the left and only hears from the right.
➢ Movement – Apraxia and agraphia may occur where responding to any verbal instructions by movement
or writing in the left hand is inhibited because the left hand cannot receive these instructions from the right