Some effects of hippocampal lesions on the behaviour of pigeons

The behavioural effects of hippocampal lesions in birds have not previously been investigated, although there is now considerable evidence, which is reviewed here, of structural and histochemical similarities between the avian and the mammalian hippocampus. Therefore, a series of experiments were carried out to study the effects of hippocampal lesions in pigeons, and it was found that they performed more efficiently on both the acquisition and reversal of a 70:30 colour probability discrimination, confirming a prediction derived from the cognitive mapping theory of hippocampal function (O'Keefe and Nadel, 1978). Hippocampal pigeons were also impaired on the serial reversal of a spatial discrimination and on a DRL 10 schedule of reinforcement, but not on the acquisition or the reversal of a simultaneous visual form discrimination, a delayed spatial alternation task, or a delayed colour alternation task. Furthermore, they did not show increased resistance to extinction, except following DRL 10 training, or increased response perseveration in reversal. These effects show many similarities to those that have been found to occur in hippocampal mammals in comparable tasks, and it is proposed, therefore, that the results of the experiments reported in this thesis provide good evidence that the avian hippocampus and the mammalian hippocampus are behaviourally homologous. These results extend the findings by others of structural similarities between the hippocampus in birds and mammals and therefore lend considerable support to the proposal that they are homologous structures. Moreover, in common with much of the mammalian hippocampal data, the present results do not support the response-inhibition, response-shift, or selective attention theories of hippocampal function, but it is argued that they support instead the hypothesis that the hippocampus is involved in the processing of spatial information, and that they are consistent with the cognitive mapping model of the hippocampus proposed by O'Keefe and Nadel (1978),