NOCTURNALITY AS A DEFENSIVE BEHAVIOR IN THE RAT - AN ANALYSIS IN TERMS OF SELECTIVE ASSOCIATION BETWEEN LIGHT AND AVERSIVE-STIMULATION

One striking aspect of the rat's daily activity pattern is its strong nocturnal tendency. The present research tests the assumption that nocturnality in the rat serves a defensive function. If nocturnality represents a form of defense, the rat may associate light with aversive events more readily than it associates dark with aversive events. Experiment 1 examined this hypothesis in terms of the rat's foraging behavior while living in an operant chamber. Electric footshock delivered to the grid floor near the response lever simulated the influence of predation during feeding. The results indicated that foraging behavior in the rat is sensitive to the risk of predation. Rats that had shock paired with either the light or dark phase reorganized their meal taking and reduced time spent at risk. However, the finding that the largest changes in foraging occurred when shock was delivered exclusively during the dark phase did not support the hypothesis that nocturnal feeding is a defensive behavior and was not indicative of a selective association between light and aversive events. Similar conclusions were reached when the defensive behavior freezing was used as the discriminative response in differential conditioning studies. Rate of differentiation was faster when shock was paired with a dark phase CS, relative to pairings with a light phase CS (Experiment 4). Rats failed to show differential freezing in a light S+ chamber and a dark S- chamber (Experiment 2), but did so with S+ and S- conditions reversed. Similar results with a light or dark chamber paired with a food US (Experiment 3) suggested two things: first, rats that acquired the discrimination when light phase signaled shock may have relied on endogenous cues rather than light/dark per se; second, the results obtained with pairings of darkness and shock were not indicative of a selective association between darkness and aversive events.