Field potentials evoked in the avian optic tectum by diffuse and punctiform luminous stimuli.

An investigation was made of the slow field potentials evoked in the pigeon optic tectum by visual stimulation. Diffuse illumination of the retina produces a polyphasic response which is negative-going in the surface layers and reverses polarity at 200--400 micron. It is suggested that this response reflects activity in radially disposed nerve cells. Local stimulation, produced by punctiform (small-spot) stimuli, results in a negative-going response in the superficial tectum, paired with a smaller and less widely distributed positive-going wave in the deeper layers. It is suggested that this response involves both radial neurones and the tangential neurones ramifying in layers c and d of the SGFS.