The primitive, 3-cell-layered, olfactory processing area (paleopallium or paleocortex, p) becomes associated with an area called the archicortex (a), which receives the processed olfactory information, and produces approach/ avoidance responses: revulsion/fear or attraction/like. This area also is linked to learning and memory based upon these "emotional" responses. In mammals a new, 6-cell-layered, processing area develops, between the archi- and paleo-cortices. This neocortex (n) attracts sensory input from all sensory systems (visual, auditory, somatic sensory).
Amongst more advanced mammals, the neocortex expands greatly. It pushes the paleocortex to the underside of the
hemispheres, and the archicortex towards the midline. As these cortices expand, they move from the primitive position near to the ventricle, to a more superficial position, overgrowing the ventral basal ganglia (b), which are motor control areas. Axon tracts growing to and from the neocortex traverse the basal ganglia giving them a striated appearance (and hence the name corpora striata) (ganglion = a definable cluster of nerve cells). Further expansion in advanced mammals, throws the neocortex into folds (gyri) and grooves (sulci). Right & left neocortices are linked by fibres (axons) crossing between corresponding points, to form the corpus callosum (cc). (v = ventricle).
Specific neocortical areas for receiving somatic sensory, auditory and visual inputs develop.
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