Inhibition of tectal neurons from telencephalic visual areas in pigeons

The author investigated the influence of efferent projections of visual telencephalic areas upon the optic tectum, in locally anesthetized, paralyzed and artificially ventilated pigeons, using extracellular unit recording and electrical stimulation by means of stereotaxically implanted bipolar electrodes. Despite the fact that some neurons located at superficial tectal layers could be driven by telencephalic shocks, the most consistent data are the inhibitory effects observed throughout the tectum, but mainly at deep layers. This effect was observed either on spontaneous activity or on electrically elicited spikes, by stimulating the optic tract. In the latter case, a critical interval (25--60 msec) was necessary to obtain maximal inhibition, when testing Wulst shocks previous to optic tract stimuli. These results show that the higher telencephalic visual area (the so-called Wulst) can modulate tectal activity, and so control the output of this region, by an example, to thalamic nucleus rotundus, second station of the tectofugal visual pathway. The Wulst-tectal relationship is discussed in its possible role in visual information processing and visually guided behaviours in pigeons, and also in comparison to mammalian corticotectal systems.