Neurobiological Basis for the Recognition and Localization of Environmental Signals: How Does a Toad Brain Recognize Prey and Enemy?

Abstract

When a toad sits motionless at twilight in front of its hiding place, very little or even no information is fed to its brain from the static visual environment (via retino-tectal projection paths, see p. 101) [17,35]. This has been demonstrated in experiments where, with the aid of a relatively sophisticated electrophysiological recording technique (Fig. 161), the neuronal responses from single optic nerve fibers were recorded in the optic tectum of a quietly sitting animal facing a stationary pattern. Result: responses were absent. Neuronal responses were observed as soon as the pattern started moving, and they continued in certain neuronal classes (R2) for a couple of seconds after the movement was stopped. In this important respect we, for example, differ from the toad. Although the static images on our retinae are also extinguished by local adaptation, we are able to counteract the adaptation by moving the image over an area of the retina with the aid of involuntary saccadic eye movements, thereby transforming it into a moving stimulus.