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To study cross-habituation of the eyeblink reflex in humans, subjects were repetitively exposed to a blink-eliciting stimulus in one modality and then shifted to a second stimulus in a different modality. One group of subjects began with a tactile stimulus and switched after 40 trials to an acoustic stimulus. The other began with the acoustic and switched to the tactile. No evidence of cross-habituation was found in the shift from tactile to acoustic, suggesting that habituation takes place in the sensory limb of the reflex. Cross-habituation was found, however, in the shift from acoustic to tactile stimuli. This result was expected because the act of blinking to the acoustic stimulus activates trigeminal afferents from the eyelid; as a result, the tactile stimulation from the blink to the acoustic stimuli had already begun habituation in the tactile pathway before the first external tactile stimulus was received.

The effect of two schedules of steroid hormone administration on the reward effectiveness of lateral hypothalamic stimulation was examined in ovariectomized Long-Evans female rats. In Experiment 1, estrogen and progesterone were administered on a 4-day schedule so as to mimic the hormonal profile of the estrous cycle; in Experiment 2, silastic implants of progesterone and estrogen were used to induce steroid hormone levels similar to those observed in mid- and late pregnancy. In both experiments, hormone administration was sufficient to induce characteristic behaviors: lordosis in Experiment 1 and a change in sucrose preference and body weight in Experiment 2. In contrast, hormone administration did not change the reward effectiveness of the electrical stimulation in either Experiment 1 or Experiment 2.

Rats were trained on a nonrecurring-items delayed nonmatching-to-sample task, using a newly designed apparatus and a training protocol similar to that used in experiments on nonspatial working memory in humans and monkeys. On each trial, the rats were briefly presented with a sample object, which was presented again along with another object after a delay; the rats were rewarded with food if they chose the novel object. New stimuli were used on each trial. With delays of 4 sec between the sample and choice runs, the rats learned the task to 90% accuracy in less than 250 trials. When the delay was subsequently increased to 15, 60, 120, and 600 sec, the rats scored approximately 91%, 81%, 77%, and 57%, respectively. These results establish that rats are capable of excellent performance on a nonspatial working-memory task that is comparable to those commonly used in monkey models of amnesia, and they suggest that the nonrecurring-items delayed nonmatching-to-sample paradigm may prove valuable in modeling brain-damage-produced amnesia in rats.

Three hundred forty female jewel wasps were reared as larvae on blowfly pupae and then separately exposed as adults to varying numbers and combinations of previously parasitized and unparasitized housefly pupae. Each adult was tested with 6 fresh housefly pupae. Only foraging experience on unparasitized housefly pupae increased host acceptance for subsequent housefly pupae, producing curvilinear and convex host-acceptance functions; foraging experience on preparasitized housefly pupae had no effect upon subsequent host acceptance. Labile host- acceptance thresholds are thus adjusted by foraging experience to track local unparasitized host abundances for different species by modulating host-species cue response biases. These presum- ably kairomonal response biases were found to be sensitive to psychophysical “subjective proba- bilities of stimulus presentation,” representing expected unparasitized host-species encounter rates, not altered “payoff matrices” of “observer decision outcomes,” representing modified host- species profitabilities.

The acoustic startle reflex, a short-latency motor response to a loud noise, is modifiable by experience. Here we test whether contextual fear conditioning potentiates startle responses, in addition to two other indices of fear: freezing and defecation. First, we trained rats with zero, one or three 0.6-mA footshocks. Upon reexposure to the same context, startle responses, amount of time freezing, and defecation were increased in the one- and three-shock groups, but not in the zero-shock group, compared with pretraining baseline levels. Second, to test whether these increases in fear responses were context specific, we trained and tested rats in a context discrimination paradigm. Rats were trained to discriminate between two contexts, one in which three footshocks where delivered, the other in which no footshocks were delivered. Startle amplitudes and time spent freezing were increased in the paired as opposed to the unpaired context. These results suggest that the acoustic startle reflex can be modified by a specific memory of an environmental context in which an aversive event has occurred.

The connections that the dorsomedial (MD) thalamus makes with the frontal cortex and hippocampus suggest that it should share in their functions. This study examined the role of the dorsomedial nucleus (MD) of the thalamus in learning tasks that require the use of compound cues and in amphetamine-induced locomotion. The cell-specific neurotoxins ibotenic acid and quinolinic acid were injected into MD, and the animals were tested on a configural tactile—olfactory task, on a matching-to-place task in a swimming pool, and on spontaneous and amphetamine-induced locomotion. The ibotenic acid lesions produced cell death that was relatively confined to MD, while the quinolinic acid lesions produced widespread thalamic cell death and calcium deposits. The control and the ibotenic acid groups were not different, but the quinolinic acid lesions produced significant impairments on all of the tasks. The results suggest that although the frontal cortex and hippocampus may participate in solving the tasks used in this study, the MD, despite its reciprocal connections, does not share their involvement. The quinolinic acid results suggest either that very large lesions are required to produce deficits or that other thalamic nuclei are involved in the behaviors that were measured.