There is lateralization of sensory processing and motor output. The contralateral control of movement for the motor cortex is not absolute. Proximal muscle control is largely bilateral and distal muscle control is largely contralateral. The lateralization of language can be tested by using the WADA-test (temporarily anaesthetize left hemisphere). Lateralization of language can be left, right or bilateral.
The following anatomical differences between the two hemispheres can be observed: a flatter Sylvian fissure on the left (1), difference in size in Heschl’s gyrus (2), difference in size in Planum Temporale (3), larger pyramidal cell bodies in the left Heschl’s gyrus (4) and more distance between patches in Wernicke’s area (5).
Connections between cells are organized in patches. Anatomical differences do not correlate with language lateralization. Macroscopic and microscopic anatomical differences and functional differences exist between the two hemispheres, but are not absolute.
The two hemispheres cooperate because the corpus callosum connects the two hemispheres, together with the anterior commissure and the posterior commissure.
There is interhemispheric synchrony between neurons that respond to the same stimulus that activates cells with receptive fields in both hemifields.
For split-brain patients, stimuli that are only presented to the right hemisphere cannot be verbally reported if there is a full split of the corpus callosum. When the posterior part of the callosum is sectioned, higher order, abstract information about the stimulus can be reported, but not the stimulus itself.
Two hemispheres can search for a visual target independently if the corpus callosum is cut. Search time increases with half the slope of regular subjects when items are presented to two hemispheres simultaneously.
There are differences between the left- and the right hemisphere. The right-hemisphere: is better at spatial and geometrical tasks (1), is better at recognizing faces (2), is better at perceptual organization (3), is better at processing global information (4), has a more prominent fusiform face area (5), uses the maximizing strategy (6). The left-hemisphere: is better at processing details (1), can recognize faces if verbal descriptions can be made (3), is biased towards recognizing the self (4), seeks patterns and sequences in events (5), uses the matching strategy (6).
The difference in spatial and geometrical tasks occurs because of a difference in sensory to motor mapping. Post-hoc rationalization refers to the left hemisphere trying to explain actions performed by the right hemisphere.
