Behavioural basis of the dorsal bundle extinction effect.

Lesion to the fibres of the dorsal noradrenergic bundle arising from the locus coeruleus using the selective neurotoxin 6-hydroxydopamine has been shown to produce a resistance to extinction on a number of behavioural tests without affecting the acquisition learning process itself. The experiments reported here are aimed at elucidating the behavioural mechanisms involved in this resistance to extinction. Theories invoking general hyperactivity, internal inhibition and perseveration are tested in the following experiments and all are shown to be contrary to the observed facts. On the basis of other data it is tentatively concluded that the dorsal noradrenergic bundle may play a role in the filtering out of irrelevant stimuli or in the memory of non-reward.