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Short-term memory for tap-habituation in the roundworm C. elegans : response-components & genetic dissection Bernardos, Aram Chaparian

Abstract

Habituation is a simple form of learning that manifests as a decrease in an innate response to a repeated stimulus that is not associated with important events. In this thesis, habituation-memory was studied using the tap-reversal response emitted by the roundworm C. elegans in response to a non-localized tap-stimulus. Delivery of 30 tap-stimuli produced a decline in the probability, duration, and speed of the reversal-response (i.e., learning). When stimulation was ceased for 10-minutes each of these response-components recovered partially but not completely, as evidenced by differences between training and test sessions (i.e., memory). Each of the components (probability, duration and speed) of the tap-reversal response showed different profiles of habituation-learning and habituation-memory in wild-type worms, with one not being predictive of the other. A genetic screen supported these findings and identified mutant strains that were deficient in habituation-memory for one component of the response but not the others. It was also shown that simply because a mutant strain showed more habituation-learning did not necessarily predict that it would show more habituation-memory. This suggested that different biological underpinnings likely underlie A) the persistence of habituation from stimulus-to-stimulus within a session of stimuli and B) the persistence of habituation from one session of stimuli to another. The genetic screen also identified mutant strains which supported a genetic dissociation of initial response-difference from overall average response-difference. This suggested that considering the phase of test-session is important for measuring memory. Finally, it was shown that the habituation-memory deficits shown by one mutant strain (pde-4) were selective to the frequency of training used and did not show deficits at all training frequencies. Together, these data suggest that habituation-memory is multidimensional and that considering both the components and time-scales of the response is important.

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