Visual Areas of the Cerebral Cortex

Visual Areas

        The classical visual area has been described before. This area receives fibres of the optic radiation. It is also called the striate cortex. In addition to the striate cortex additional areas of cortex responding to visual inputs are described. Area 18 (peristriate area) is the second visual area; and area 19 (parastriate area) is the third visual area.

        A modified nomenclature recognising five visual areas has been described as follows.

a)              First visual area (V1) in area 17.

b)             Second visual area (V2) occupying the greater part of area 18, but not the whole of it.

c)              Third visual area (V3) occupying a narrow strip over the anterior part of area 18.

d)             Fourth visual area (V4) within area 19.

e)              Fifth visual area (V5) at the posterior end of the superior temporal gyrus.

        The visual areas give off efferent fibres also. These reach various parts of the cerebral cortex in both hemispheres. In particular, they reach the frontal eye field which is concerned with eye movements. Like other ‘sensory’ areas the visual areas are, therefore, to be regarded as partly motor in function. This view is substantiated by the fact that movements of the eyeballs and head can be produced by stimulation of areas 17 and 18 which constitute an occipital eye field. Efferents from the visual areas also reach the superior colliculus, the pretectal region, and the nuclei of cranial nerves supplying muscles that move the eyeballs. There is physiological evidence of a corticogeniculate projection. Fibres also reach the thalamus (pulvinar).

        The total number of neurons to be seen in delimited vertical areas of cortex is remarkably constant in different regions. The cortex of the visual area is an exception and has a much greater density of neurons than other parts of the cortex.