The Ecology of Humans (26-8) Hemispheres

 Hemispheres

Brain asymmetry enhances cognition. Directional biases of brain function are a putative adaptation to social behaviour. ~ Australian zoologist Lesley Rogers et al

Brain lateralization is ubiquitous in the animal kingdom. Honeybees have lateralized brains. Generally, the left side of the brain controls right-side body functions and vice versa.

The human cerebral cortex has 2 hemispheres: the left and right halves of the brain, connected by a tremendous number of pathways. The largest conduit connecting the left and right hemispheres is the corpus callosum, which facilitates interhemispheric communication via 200–250 million contralateral axonal connections.

The corpus callosum is the largest white matter (glial) structure in the human brain. Only placental mammals have a corpus callosum, though other animals have analogous fiber bundles for interhemispheric discourse.

Musicians that began learning their instruments before 7 years old have a larger corpus callosum than non-musicians, along with bulked-up auditory and motor areas of the brain.

Studying epilepsy, American neurobiologist Roger Sperry found that cutting the corpus collosum reduced seizures. The unsurprising side effect to this brain butchery was loss of mental integration. With a split brain, functions that predominately occur on one side of the cortex could no longer coordinate with the other.

In touching a recognizable object with the left hand without seeing it, a person with a severed corpus callosum would not be able to name it. Sensory-motor signals from the left hand are processed in the right hemisphere. Naming an object requires language processing, which is seated in the left hemisphere.

From such brain damage studies arose a left-brain/right-brain hypothesis: that the left hemisphere is rationally analytic, while the right hemisphere is emotive and creative. Despite the grain of truth in the brain processing input from the opposite side of the body, the left-brain/right-brain hypothesis overstates the situation.

Motor control, vision, and tactile brain processing for the left side of the body are handled on the right side of the brain, and conversely. The right side of the brain moves eyes to the left, while the left brain moves eyes to the right.

While vertical motion can be processed in one contralateral hemisphere, apparent horizontal movement requires integration between the hemispheres via the corpus callosum.

Individuals differ in the performance and quality of inter-hemisphere communication – more so than within a hemisphere. This affects subjective experience.

The left hemisphere handles routine behavior, while the right is responsible for unusual events and emergencies, along with associated intense emotions to provoke memory storage.

Foraging is a left-brain activity, while attacks are predominantly processed in the right hemisphere.

Small numbers are processed in the right side of the brain, while large numbers are handled in the left.

The right hemisphere is active in representing the space around the body and interacts with processes that maintain alertness of the environment. Resolving visual incongruities, language processing, and speech motor control stimulate the left hemisphere.

Language illustrates the importance of lateralization. Brains with weak lateralization struggle with literacy and language processing. In contrast, ideas and insight, rationalization of emotionally difficult decisions (emotional logic), appreciation of sound, and mental object manipulation invoke right hemisphere exertion.

Brain pathways that run longitudinally and laterally provide feedback mechanisms for behaviors. Speech generated largely via left hemisphere processing is regulated in other brain areas, including in the right hemisphere: especially emotive intonation.

Babies born without a corpus callosum – where the 2 brain hemispheres are separated – gain compensation by the brain growing alternate pathways which permit interhemispheric communication.

While certain localized areas correlate with certain functions, brain activity is entangled. Communications constantly course throughout the brain, operationally knitting it together.