Visual imprinting and the neural mechanisms of recognition memory.

To understand the neural bases of memory it is necessary to localize the regions storing information. Part of the hyperstriatum ventrale (IMHV) serves such a function for the learning process of imprinting in domestic chicks. Chicks exposed to an object learn its characteristics, and in doing so, the responsiveness of IMHV neurones to that object is selectively enhanced. Imprinting is associated with both pre- and postsynaptic changes in the region. Postsynaptic changes involve increases in the length of the postsynaptic density on dendritic spines and in the numbers of NMDA receptors; presynaptically, converging evidence points to an early and persistent enhancement of neurotransmitter release. Increases in the amounts of certain neural cell adhesion molecules a day after training might serve to stabilize the synaptic changes associated with a particular memory by strengthening pre- to postsynaptic adhesion, and by more strongly interconnecting the cytoskeletal frameworks of the dendritic spine and the synaptic terminal. Learning-related increases in the number of neurones staining positive for the transcription factor Fos in the IMHV give promise of identifying the neurones engaged in memory functions and of analysing their connections.