Modifiability of responsiveness in a visual projection area of the chick brain: visual experience is only one of several factors involved.

The effects of visual experience on neuronal responsiveness in the hyperstriatum accessorium, a visual projection area, were investigated in 4 groups of domestic chicks, each comprising dark-reared (total = 44) and visually experienced (total = 34) birds. Visually experienced birds were placed singly in running wheels facing a flashing red light for 3 h; wheel revolutions were used as a measure of the chicks' locomotor activity. At approximately equal to 48 h after hatching each chick was anaesthetized and a microelectrode advanced in 250 micron steps through the left Wulst. After each step the responsiveness of units to diffuse retinal illumination (light flashes) was tested. Each recording site at which responses greater than or equal to 5 successive flashes could be evoked was classified as a response site. In some chicks at least one site responding briskly to greater than or equal to 15 successive flashes was found deep to the hyperstriatum accessorium. The zone of markedly increased responsiveness is referred to as the visually responsive lamina. The effect of visual experience on the response of units in the hyperstriatum accessorium varied between the groups of chicks. Visual experience did not significantly effect neuronal responsiveness in this region for chicks without a visually responsive lamina. For chicks with a lamina there was an effect of visual experience, but the effect again varied between groups. When data from inactive, visually experienced chicks were excluded, the group-to-group variation ceased to be significant. Thus visual experience alone was not adequate consistently to bring about long-term changes in the responsiveness of neurones in the hyperstriatum accessorium. Such experience was likely to increase neuronal responsiveness provided the chicks: were behaviourally active; and possessed a visually responsive lamina.