Differential effects of exposure to an imprinting stimulus on 'spontaneous' neuronal activity in two regions of the chick brain.

Young chicks were placed in running wheels and exposed for a total of 3 h to an imprinting stimulus: a rotating flashing red light. Revolutions made towards the stimulus minus revolutions made away from it were totalled and are referred to as approach counts. After exposure, chicks were anaesthetized and two vertical electrode penetrations were made, one through the visual projection area, hyperstriatum accessorium (HA), and one through the intermediate and medial part of hyperstriatum ventrale (IMHV), a region which has been implicated in the imprinting process. 'Spontaneous' impulse activity was recorded from neurones in these areas. In some of the chicks, after penetrating through HA, discharges evoked by diffuse retinal illumination were monitored and the zona hyperstriatum intercalatus/hyperstriatum dorsale (HI/HD) below HA was classified as being a visually responsive lamina (VRL) if responses were obtained to greater than or equal to 20 consecutive light flashes. In HA there was a positive correlation between spontaneous activity and approach counts in those chicks with a VRL; there was no significant correlation in those chicks without a VRL. In contrast, for IMHV the correlation between spontaneous activity and approach counts was negative and appeared to be independent of the presence or absence of a VRL.