Visual receptive fields of single cells in the pigeon's optic tectum

Abstract

Three hundred and ninety-four units within the optic tectum of pigeons were studied using extracellular recording. According to their receptive field properties, they were classified in 4 major classes. (1) Concentrically organized fields (6%), composed of a center ‘on’ and an antagonistic surround ‘off’, or the reverse. The center and the surround had identical spectral sensitivities. Motion of a white or black spot across the center-surround evoked equally strong responses for all directions of movement. (2) Cells with homogeneous ‘on’ or ‘off’ fields (1.3%) respond well to flashes of light, but have no antagonistic surround. (3) Movement selective cells (72%) respond preferentially to moving stimuli, and poorly or not at all to static spots. According to the size of their receptive fields these were classified as ‘small’ (2°), ‘middle’ (5°) and ‘large field’ (31°). Cells with ‘large fields’ were always found at the deepest part of the electrode penetration (1100 μm to 1400 μm). (4) Directionally selective cells (21%) respond optimally to motion of the spot in one direction ‘preferred’ and not to movement in the opposite direction ‘null’. They were always recorded in the superficial range of penetration, above 1000 μm. Although the cells in the pigeon's optic tectum seem to be preferentially stimulated by moving objects, directional selectivity appears to be a characteristic of the superficial tectal layers. These results, together with other studies, suggest that the ‘output’ of the pigeon's tectum might carry directionally coded signals.