Long-term consequences of exposure to an imprinting stimulus on ‘spontaneous’ impulse activity in the chick brain

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Young chicks were exposed in a running wheel to an imprinting object, a rotating flashing red box (n = 36 chicks) or a rotating stuffed jungle fowl (n = 18). The net number of revolutions of the wheel toward the imprinting object provided a measure of approach activity during training. The chicks were subsequently anaesthetized and the ‘spontaneous’ firing rates of units were recorded simultaneously in two regions of the left forebrain hemisphere. One microelectrode penetration was made through a region, the intermediate and medial part of the hyperstriatum ventrale (IMHV) known to be critical for imprinting; simultaneously another microelectrode penetration was made through the visual Wulst. The electrodes were advanced in steps of 250 μm and the spontaneous discharge of multiple units at each site recorded. The mean firing rate from ⩾ 3 sites was calculated for IMHV and for the hyperstriatum accessorium of the Wulst. There was a negative correlation between mean firing rate of units in IMHV and approach activity for chicks trained on the box. The effect was: (i) stimulus specific since it was not found in chicks trained on the jungle fowl; and (ii) regionally specific since approach activity, depending on conditions which are described, was either not correlated or positively correlated with mean firing rate for neurones in the hyperstriatum accessorium. The positive correlation was not dependent on the training stimulus but on the presence of a visually responsive lamina deep to the hyperstriatum accessorium.