Introduction

The retina contains millions of light-sensitive cells. One prominent type of cell is the center-surround cell, which has two receptive fields. One is circular and in the center, and the other is a surrounding ring-shaped field. Each receptive field is either on 'ON' field or on 'OFF' field. 'ON' receptive field produce an increase in neural activity when light is present, while 'OFF receptive field inhibit neural activity when light is present. Some cells are 'ON-center OFF-surround' and produce maximum nearal activity when light hits the center of the cell and no light hits the surround. Others are 'OFF-centre ON-surround', and produce maximum neural activity when light hits the surround and no light reached the centre. However, because 'ON' area and 'OFF area are stimulately by light, the net output is little or no change in neural activity. Similarly, when neither area receives light, there is little or no change in neural activity. This is what makes these cells so good at detecting edges.

Look around you. Note that anywhere you see on edge of an objext, there is usually a change in the light level. Centre-surround cells respond the most in areas in the visual field where there is contract and a sharp difference in the amount of light.

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