Signalling and incentive processes in instrumental reinforcer devaluation

We have previously reported that conditioning an aversion to the reinforcer using an isotonic lithium chloride (LiCl) solution following instrumental training reduces performance in a subsequent extinction test only if animals are re-exposed to the reinforcer prior to the test. Rescorla (1992), in contrast, reported an immediate devaluation effect using a hypertonic LiCl solution that did not depend upon re-exposure. In two experiments we examined the effect of using a hypertonic LiCl solution to condition the aversion to the reinforcer on subsequent instrumental performance in extinction, with and without re-exposure. In Experiment 1 thirsty rats were trained to press a lever for a sucrose solution before being injected with 0.6 M LiCl either immediately or after a delay. Half of the immediate and delay groups were then re-exposed to the sucrose in the absence of the lever, with the remainder being exposed to water. Contrary to the previously reported effects of isotonic LiCl, a hypertonic solution induced a reinforcer devaluation effect in all the immediately poisoned animals, which did not depend upon re-exposure to the reinforcer. In Experiment 2 the possibility that this devaluation effect was induced by the discomfort associated with the hypertonicity of the solution was assessed by replicating Experiment 1 but, in addition, using two immediately poisoned groups given the LiCl injection under anaesthesia. In the absence of anaesthesia, the devaluation effect observed without re-exposure to the reinforcer in Experiment 1 was replicated. When the injection was given under anaesthesia, however, a reinforcer devaluation effect was observed only in animals that were re-exposed to the reinforcer prior to the extinction test. These results were interpreted as evidence that a reinforcer devaluation effect induced by pairing the reinforcer with illness depends upon a process of incentive learning, whereas a devaluation effect mediated by learning a signalling relationship between the reinforcer and somatic discomfort does not.     

Bar holding during escape conditioning

To analyze bar-holding behavior during escape conditioning three rats were trained to escape from shock by pressing a bar, and three were trained to escape by releasing a bar. Bar holding behavior was stronger and more stable under the release condition. These effects were related to the relative shock duration of onbar versus offbar shocks.     

Heading and path information from retinal flow in naturalistic environments

In four experiments, we explored the heading and path information available to observers as we simulated their locomotion through a cluttered environment while they fixated an object off to the side. Previously, we presented a theory about the information available and used in such situations. For such a theory to be valid, one must be sure of eye position, but we had been unable to monitor gaze systematically; in Experiment 1, we monitored eye position and found performance best when observers fixated the designated object at the center of the display. In Experiment 2, when we masked portions of the display, we found that performance generally matched the amount of display visible when scaled to retinal sensitivity. In Experiments 3 and 4, we then explored the metric of information about heading (nominal vs. absolute) available and found good nominal information but increasingly poor and biased absolute information as observers looked farther from the aimpoint. Part of the cause for this appears to be that some observers perceive that they have traversed a curved path even when taking a linear one. In all cases, we compared our results with those in the literature.