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Talk by Dr. Jarno Mikkonen

Department of Psychology, University of Jyväskylä, Finland

06 Jul 2012 15:00
06 Jul 2012 16:00

Intervening with hippocampal ripples disrupts learning in rabbit

Oscillatory state of the hippocampus in an immobilized awake rabbit alters spontaneously from highly regular rhythmic slow activity (~6 Hz, theta activity) to irregular state where sharp waves and transient fast ~200 Hz ripples are prominent. The current view on the role of hippocampal oscillations in the process of learning, mainly based on experiments on rats, is that during theta activity, the animal engages in behavior which is directed at the

surroundings and thus, represents the ‘read’ state of the process. In contrast, ripples occur when the animal is not paying specific attention to the surroundings and are hypothesized to mediate consolidation phase where recently acquired information is stored in areas relevant for the task in question. Direct evidence for this view has been gained in an experiment where hippocampus of a sleeping animal has been electrically stimulated so as to prevent ripples during the putative critical consolidation period after training session [Girardeau et al., 2010]. As a result, ripple-disrupted animals learned significantly slower. We aimed to show that hippocampal ripples in the awake state are necessary for learning. Specifically, we hypothesized that consolidation process can be disturbed by ripple-contingent intense natural stimulus during inter-trial intervals in trace eyeblink conditioning. We used a braincomputer interface to detect ripples and to flash a bright LED light contingent with them during the inter-trial intervals. The yoked control animals were also given the light stimulus, but irrespective of their hippocampal state, the normal control animals were not given any light stimulus during training. In eyeblink conditioning, the number of trials needed to reach the 5th conditioned response (TTC) has been used as a dependent measure for onset of learning. One-way ANOVA revealed a significant difference between groups in TTC. Post-hoc analysis showed that the TTC was significantly higher in ripple disrupted group (479 +/- 39) in comparison to both yoked control (258 +/- 73) and normal control (227 +/- 49) groups. The control groups did not differ significantly from each other. However, when ripple disrupted groups was trained without the light, they acquired the CRs to the same extent as their yoked controls. These results imply that awake-state ripples, not only those seen during sleep, are important in consolidation of hippocampus-mediated memory traces. It can also be noted that rabbit eyeblink conditioning is an ideal model for studying the effect of ripples on learning. In contrast with rats, where theta is usually seen when running and ripples when grooming, sleeping or eating, the oscillatory state of the hippocampus in rabbits fluctuates without clear changes in behavioral state.