FIGURE 1.
Hypothetical stimulus–response relationships for a sensory neuron. The red circle represents the boundary of the set of permissible stimuli. (A) Stimulus x0 is a firing rate peak which corresponds to the intuitive notion of the optimal stimulus where any perturbation away from x0 results in a decrease in the firing rate. (B) This neuron is tuned to one stimulus dimension but is insensitive to the second dimension. Instead of a single optimal stimulus x0 there is a continuum of optimal stimuli (green line). (C) A neuron whose stimulus–response function around the point x0 is saddle-shaped. Along one stimulus dimension x0 is a firing rate maximum, and along the other stimulus dimension x0 is a minimum.