Ganglion cells feed on to thalamic cells in the lateral geniculate nucleus. These thalamic cells have receptive fields which are very similar to the fields of the ganglion cells which excite them. The major change here is perhaps an increase in contrast. Thalamic cells with small, overlapping "centre-surround" receptive fields, may converge on to cortical cells in the primary visual cortex, to produce cortical cells with larger receptive fields as illustrated here. These are termed simple cortical cells. They respond best, not to spots of light, but to slits (bars) of light on the retina, oriented at a particular angle. The cell in this Figure has a vertically oriented field, with an "ON" centre and an "OFF" surround. Horizontal slits of light (a & h, on left) which stimulate both ON and OFF areas simultaneously, have very little effect - The inhibition in the OFF area counterbalances the excitation in the ON area. A vertical slit in the ON area has the greatest stimulatory effect (e, on left). If this vertical slit were in the OFF area and the cell would not respond during 'light on', but would give a burst of excitation when the light is turned off. A spot of light in the centre (a, on right) gives a brief response at 'light on'; and in the surround (b, on right) a brief response at 'off'. Diffuse lighting through the whole field (c, on right) gives no response. This arrangement accentuates responses to discontinuities in lighting with particular orientations on the retina.

This response to light edges oriented at a given angle on the retina, is clearly an important feature of visual cortical cells. Cells that respond to light at a particular angle, tend to be associated together in definable cortical columns. Adjacent columns have cells which respond best to light at similar, but slightly different angles, and this optimal angle changes gradually and systematically as one traverses from one column to the next.

In addition to these orientation columns, the visual cortex is also divided into alternating columns influenced mainly by right or left eye. These ocular dominance columns alternate in a systematic way, and are roughly equal for each eye. If one eye becomes blind or is impaired in some way at an early stage, its ocular dominance columns become taken over by the good eye. If children suffering from strabismus are allowed to mature beyond about 5-6 years without fixing the problem, the faulty eye may become permanently blind, although optically and retinally sound. The blindness occurs because of the loss of its cortical domains to the contralateral eye. This is known as amblyopia (loss of vision in a functional eye).

Simple cortical cells in the primary projection area of the visual cortex, may in turn feed on to other cortical cells in various combinations, to produce cells with receptive fields of increasing complexity (complex cortical cells). Output from one cell may diverge to participate in several receptive field circuits. By this process of convergence and divergence to produce groups of cells with complex and varying response patterns, it is envisaged that the process of higher order cognition emerges.

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