Neural encoding of subject-object distance in a visual recognition system.

Domestic chicks follow a familiar (imprinted) object when it recedes. In behaving, imprinted chicks with no experience of objects at different distances, neuronal activity was recorded from the intermediate and medial part of the hyperstriatum ventrale (IMHV), a brain region crucial for the recognition memory underlying imprinting. We found that (i) some neurones responded equivalently, irrespective of the subject-object distance d (d-invariant); (ii) other neurones responded differently at different values of d (d-sensitive); (iii) these response types were found in monocular chicks and in chicks with both eyes exposed; (iv) the action potential shape of d-invariant neurones was different from that of other neurones and (v) the spontaneous firing rate of some neurones was correlated with locomotor activity. Taken together with previous findings, the results raise the possibility that IMHV has a major role to play in the sensory and motor-control aspects of imprinting in addition to its mnemonic functions.