The picture here is a computational model of a retinal ganglion cell’s ‘receptive field’.  This picture illustrates the areas, on the retina of the eye, where the presentation of light will excite the cell (‘+’ area) or inhibit the response of the cell (‘-’ area).  The diagram to the right is a cross section of this same receptive field, where the plus area is the peak, and the minus areas are the two troughs to the left and right.

Here’s a demonstration of how receptive fields work and are measured:

http://www.sinauer.com/wolfe/chap2/ganglionF.htm

and here’s anther video showing the first such measurements: