Previous experiments have shown that a restricted part of the forebrain, the intermediate and medial part of the hyperstriatum ventrale (IMHV), is involved in the learning process of imprinting. Through this process chicks selectively approach, and so recognise, a visually conspicuous object to which they have been exposed. Previous work has also demonstrated that the left and right IMHV regions play different roles in the recognition memory of imprinting, and that exposure to an object results in changes in synaptic organisation in the left, but not in the right IMHV. We have enquired whether training has a differential effect on spontaneous multiple-unit impulse activity in the two regions. Thirty-two chicks were reared in darkness. Sixteen were trained by exposing them to a flashing, rotating box; the remaining chicks serving as dark-reared controls. The trained chicks were anaesthetised either immediately after training (0-h trained chicks) or returned to the dark incubator and anaesthetised 6 h after the end of training (6-h trained chicks). A single microelectrode penetration was made through the left IMHV simultaneously with a single penetration through the right IMHV; a single penetration was also made through a visual projection area, the left hyperstriatum accessorium. Recordings were made from between 3 and 7 sites in each penetration, and the mean multiple-unit firing rate for these sites was calculated. Spontaneous activity in the right IMHV changed in respect of activity in the left IMHV with time after training; no such effects were found in the left hyperstriatum accessorium. These findings are discussed in relation to other aspects of hemispheric asymmetries in chick brain. Multiple-unit brusts (greater-than-or-equal-to 3 spikes with an interspike interval of less-than-or-equal-to 10 ms) activity was also studied, data from right and left IMHV being taken together since there were no significant differences between them in this measure of neuronal activity. Compared to the frequency of bursting in dark-reared chicks, training led to a reduction of bursts both in the hyperstriatum accessorium and in IMHV. For the hyperstriatum accessorium the reduction was significant in both the 0-h and 6-h trained chicks; for IMHV the reduction was present only in the 6-h trained birds. These findings are discussed in terms of arousal effects of training.
