Changes in the firing rate of a single neuron moves the current sensory+motivational context in one dimension out of n. This can be enough to change behaviour if it moves the sensory+motivational state from one Voronoi cell to another. More often, the firing of a single neuron contributes little or nothing to perception or action. Disparity sensitive neurons and their response to correlated or anti-correlated patterns are an informative example. Dorsal and ventral streams generate a lot of debate but the signals in these streams seem well suited to navigating across a surface of images. I have included a page on psychophysical paper that is relevant to some neurophysiological findings on stereoacuity.