INDIVIDUAL VARIATION IN ASSOCIATIVE LEARNING OF THE NUDIBRANCH MOLLUSC HERMISSENDA CRASSICORNIS

¹ Section on Neural Systems, Laboratory of Biophysics, NINCDS-National Institute of Neurological and Communicative Disorders and Stroke, National Institutes of Health at the Marine Biological Laboratory, Woods Hole, Massachusetts 02543

Retention of learned suppression of positive phototaxis in the nudibranch mollusc Hermissenda crassicornis, induced by exposure to trials of paired light and rotation, was determined for individuals within groups trained in two, three, four, and six daily sessions of 100 trials each. Significant increases in latency to light (acquisition) were measured within all paired treatment groups when these were tested before treatment and 24 hours after the last session. No significant differences in latency were found within four unpaired and one random control group. Next, retention of phototactic suppression (increased latency to respond to light) for each individual was assessed by comparing its post-treatment suppression ratio (SR) scores to a population median score derived from the frequency distribution of scores from a naive group of animals repeatedly tested over a 31-day period. Retention, defined as the consecutive number of days post-treatment on which an animal's SR scores were suppressed below the population median score, was significantly longer in groups trained four and six days than in the two- and three-day paired treatment groups. When retention day score distributions from paired groups were compared to those from the unpaired and random control groups, a significant increase in phototactic suppression was found only for groups trained four and six days. Maximum retention, or resistance to extinction, was measured at 17-18 days (one animal) after 6 sessions. All paired treatments contained animals which did not acquire the association. Retention increased with experience (number of sessions) and the number of animals per group which showed no acquisition decreased.

Investigations on the neural correlates of this behavioral change in Hermissenda are currently in progress; an understanding of the relationship between the degree of phototactic suppression in a sample of animals and the number of training sessions will aid in design and interpretation of experiments in which biophysical and biochemical data are correlated with behavioral measures.

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