Secondary imprinting in the domestic chick: Binocular and lateralized monocular performance

Abstract

Newly-hatched chicks were reared with a coloured imprinting object on day 1 of life (primary imprinting) and then with an object of a different colour (secondary imprinting) on day 2. They were then tested on day 3 for preferences between the primary and the secondary imprinting object in binocular and in monocular conditions. The main results were that (1) left-eyed chicks usually showed clearer choice than right-eyed chicks;(2) there were colour preferences that appeared to affect choice differently in left-and right-eyed chicks; (3) eye asymmetries were in general more pronounced in males than in females. Experiments using composite stimuli (that contained simultaneously the colours of both the primary and the secondary imprinting objects) and experiments in which retention of memories for the primary and secondary imprinting objects were tested against the preference for novel objects showed that the eye asymmetries cannot be explained neither by hemispheric differences in response to novelty nor by different rates of forgetting of primary and secondary imprinting objects in the two hemispheres. It is argued that properties of single-units responses in neural structures involved in imprinting in the left and right hemisphere can account for these behavioural results.The domestic chick (Gallus gallus) has proved to be an ideal model for studies of imprinting (for recent reviews see Bolhuis & Honey, 1998;Horn, 1998). One unexpected observation that has arisen from such studies concerns the differential role played by the left and right brain hemispheres in imprinting. When newly hatched chicks are exposed to a visually conspicuous object, they approach it, learn its characteristics, and form a social attachment to it. (In the natural environment, the object is usually the hen, but in the laboratory imprinting can be easily obtained using a variety of simple artificial objects.) The intermediate part of the hyperstriatum ventrale (IMHV), an associative forebrain structure, is part of a memory system that encodes a representation of the imprinting object (Horn, 1990). The right and the left IMHV appear, however, to have different roles in memory of the imprinting object. Evidence from lesion and electrophysiological studies suggest that both the right and the left IMHV act as long-term stores (Nicol, Brown, & Horn, 1995), but the right IMHV is also crucial in establishing another store, outside the IMHV region, and referred to as S' (McCabe, We thank Elena da Roit for help in testing the animals and Fabio Marconato for drawing Figure 1.