Learning and Memory: Avians

The intermediate and medial part of the hyperstriatum ventrale (IMHV) is a restricted region in the chick forebrain that is crucial for the learning process of imprinting. Taken together, the evidence indicates that the IMHV is a site of storage of information about the imprinting stimulus (2). One hour after an hour's training with an imprinting stimulus, there is an increase, related to the strength of visual imprinting, in the number of neurons in the IMHV that are immunopositive for the immediate early gene product Fos. A learning-related change in Fos-like immunoreactivity was also detected in the hyperstriatum accessorium but not the other forebrain regions examined (lateral neostriatum, hippocampus, lobus parolfactorius and hyperstriatal component of the medial neostriatum/hyperstriatum ventrale [MNH]) (3). The MNH has been implicated in auditory imprinting (1). We have enquired whether the change in Fos expression induced in the IMHV by imprinting extends laterally within the hyperstriatum ventrale and whether similar learning-related changes occur in the neostriatal component of the MNH. Measurements were made on sections that contributed to the previous study of Fos expression (McCabe and Horn 1994). Chicks were trained by exposure for 1 h to an imprinting stimulus (a rotating, red box) and the maternal call of a hen. Control chicks remained in darkness. After training, the chicks were given a preference test in which they were exposed, with no maternal call, alternately to the training stimulus and to an alternative stimulus (a rotating, blue box). A preference score (percentage of approach during the test that was directed towards the training stimulus) was assigned to each trained chick as an index of the strength of imprinting to the visual stimulus. Trained chicks were classifed as 'good' and 'poor' learners according to whether their preference scores were > or < 65 respectively. The chicks were killed by decapitation 1 h after the end of training and brain sections stained for Fos. Fos-positive nuclei in the lateral hyperstriatum ventrale and neostriatal component of the MNH were counted by computer. Immunocytochemistry and counting were performed blind. In each region examined, there were no significant differences between numbers of Fos-positive nuclei in good learners, poor learners and untrained chicks. The learning-related change in the IMHV does not therefore extend to the adjacent lateral part of the hyperstriatum ventrale. The fact that no effect was detected in the MNH at this time may indicate that changes in the MNH …