INDIVIDUAL VARIATION IN ASSOCIATIVE LEARNING OF THE NUDIBRANCH MOLLUSCHERMISSENDA CRASSICORNIS

Abstract

Retention of learned suppression of positive phototaxis in the nudibranch mollusk H. crassicornis, induced by exposure to trials of paired light and rotation, was determined for individuals within groups trained in 2, 3, 4 and 6 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 h after the last session. No significant differences in latency were found within 4 unpaired and 1 random control group. 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 31 days. 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 4 and 6 days than in the 2- and 3-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 4 and 6 days. Maximum retention, or resistance to extinction, was measured at 17-18 days (1 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.