[Small-field receptor monocular neurons in the optic tectum of frogs: analytical properties and problems of classification].

Small field (less than 10 degrees) monocular neurons were electrophysiologically recorded from the rostral part of the frog's optic tectum. When tested with conventional stimuli, these neurons displayed rather common properties. Two main groups could nevertheless be defined. In group I neurons (30%), habituation to repetitive stimulations was present but the responses never disappeared completely. These neurons also showed brief OFF-ON responses to light spot stimulations. Group II neurons (70%) only responded to the first presentations of moving targets. Responses to light spot stimulation were different: 25% of them (group IIa) showed OFF-ON responses, 19% (group IIb) presented only OFF responses and 26% (group IIc) gave no response. The receptive fields had no null region and extended about equally in all directions. There was no correlation between RF diameters and response types. Small field visual neurons represented approximately 37.7% of the whole neuronal population. The problem of their place in the usually used classification is discussed.