Neuronal motion after-response induced in the tectal neurons of toads by moving textured background.

When an object is held stationary in the center of the receptive field of a tectal neuron in a toad and a textured background is moved for a period of time, some neurons produce a burst of discharges immediately after the movement of the background ceases. This effect was first found in a recent study and temporarily called 'neuronal motion after-response'. A total of 66 tectal neurons in toads were examined, and 29 out of them showed the effect. In the different neurons under investigation, the firing rate varied from a few spikes to discharges of very long duration. The appearance of the motion after-response was independent of either the object/background contrast (i.e. black against white vs. white against black) or the direction of the background movement. In order to induce this effect, however, the object must be of sufficient size, and the background must be moved for a sufficient length of time. For most tectal neurons, an 8 x 8 degree square was large enough to induce the motion after-response, but for several others, the size of the object had to be similar to that of the excitatory receptive field of the neuron. The duration of the background movement was also crucial: at least 20 s of background movement was necessary for the motion after-response to occur.