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Brad Wyble
Computing Laboratory, University of Kent

Wednesday 20 April 2005


B10 Seminar Room
Alexandra House, 17 Queen Square, London


The Hippocampus as a Link Between Memory and Behaviour: Theta Mechanisms


The hippocampus is commonly known as a memory organ, but its descending inhibitory projections to the hypothalamus are just as numerous as its projection through the traditional DG->CA3->CA1 trisynaptic path.  This pattern of connectivity puts the hippocampus in a privileged position to manipulate the behavioural state of the organism with the output of its computations.  Hippocampal theta rhythms comprise a vital part of the reciprocal behavioural control loop between the hippocampus and brainstem/hypothalamic areas.  This talk will describe correlative studies of theta oscillations in two experimental paradigms.  One is a visual discrimination task, in which theta is shown to increase prior to the expected arrival of a salient stimulus, indicative of theta's traditional role in attending to salient stimuli.  What is also demonstrated is a rapid shutdown of theta prior to bar-pressing behaviour leading to reward, but not bar-presses to initiate a trial.  These data answer the question of whether theta accompanies discrete motor activities: it depends on the behavioural context of the activity.


A similar behavioural contingency of theta is demonstrated in a runway task in which rats run to opposite ends of the track, expecting reward at one end and not the other.  Theta decreases sharply 200-300 msec prior to arrival at the rewarded end of the track while the rat is decelerating.  Together, these results indicate that theta generation mechanisms may be incompatible with hypothalamic brain activity responsible for initiating feeding behaviour.  It may be the case that the hippocampus modulates tonic levels of fear-related behavioural processes based on incoming sensory stimuli and spatio-temporal context through its descending projections to the hypothalamus.  The deactivation of theta mechanisms prior to feeding support a theory that tonic levels of fear are present during exploratory behaviour, but are sharply reduced to permit a class of behaviours to be expressed (eating/drinking/grooming/copulation).