Abstract

The notion that memory consists of a trace left in the brain by a past, learned experience goes back several hundred years, yet the effort to characterise properties of the putative trace continues. I shall review some of the progress that has been made in this effort through a study of visual imprinting in the domestic chick. The young chick learns to recognise and form a filial attachment to its mother or to a surrogate mother, which may be a conspicuous artificial object. An advantage of studying imprinting is that a visually inexperienced animal may learn the features of the first object to which it is exposed; so the putative trace is inscribed in a nervous system in which there are no previous inscriptions of learned visual experiences. Evidence will be presented that a localised area (the ‘IMM”) of the cerebral hemispheres stores information acquired through imprinting. The IMM is a polysensory region receiving inputs from the hippocampus and the avian equivalent of the mammalian amygdala. I shall describe some of the cellular and molecular changes that occur in the region after imprinting, focussing on the neurophysiological changes, and the role of sleep in stabilizing them. Some of these changes were quite surprising. Chicks have a predisposition to approach objects resembling conspecifics. This predisposition may guide them to one individual conspecific, usually their mother, whose features they learn. The IMM is necessary for this learning to occur, but not for the predisposition, which can be manipulated experimentally.