The enhancement of contrast at the edge of an object is the result of: lateral inhibition in the retina. the color of the object. fatigue of the rods and cones. the diffraction of light from the edge's surface.

lateral inhibition in the retina

Explanation:

Horizontal cells are in contact with photoreceptor cells through the synapse. One photoreceptor forms an excitatory synapse with horizontal cell in the adjacent regions of the retina while the horizontal cell forms an inhibitory synapse with another photoreceptor.  

Hence, as one photoreceptor activates the horizontal cell, the horizontal cell, in turn, inhibits the other photoreceptor cell and thereby reduces the quantity of neurotransmitter released at the bipolar cell. In this way, horizontal cells transmit lateral inhibitory stimulus to the photoreceptor which in turn improves the contrast at the edges of the object.  

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