Behavioural and neurophysiological aspects of sexual imprinting in zebra finches.

Sexual imprinting has been defined as the process by which young animals learn the characteristics of their future sexual partners. It is a two stage process including an acquisition period where features of the social environment are learnt, and a stabilization process by which, under the guidance of the previously acquired social information, a preference for a sexual partner is established and stabilized, so that it cannot be altered again subsequently. The stabilization process is short (1 h) and can be controlled experimentally. This allows for the design of experiments to examine the physiological events accompanying the imprinting process. During the stabilization process, four areas of the forebrain are more activated than in any other behavioural context. These are the hyperstriatum accessorium/dorsale (HAD), the archi-neostriatum caudale (ANC), the medial neo/hyperstriatum (MNH) and the lateral neo/hyperstriatum (LNH). Isolation during development reduces the spine density of neurons in HAD and ANC and enhances it in MNH and LNH. Subsequent exposure to a female (which stabilizes the previously acquired preference in behavioural experiments) for 1 week leads to an enhancement of spine densities in HAD and ANC, and to a reduction in MNH and LNH. The enhancement in HAD and ANC is reversible by a second isolation period after the exposure to a female, the reduction within MNH and LNH is not. This irreversibility indicates that the reduction process within MNH and LNH may be the anatomical manifestation of the imprinting process. The examination of spine densities in the four brain areas after two experiments which have been shown previously to affect the stabilization process in behavioural experiments, confirms this idea.