Charles F. Hinderliter

Amnesia induced by hypothermia as a function of treatment-test interval and recooling in rats

In Experiment I, a repeated tests procedure was employed to assess hypothermia-induced amnesia of a footshock experience. Rats tested 4 h after training treatment showed no memory loss, but amnesia was present at 24 h. Although recovery of memory was obtained when the same animals were cooled 2 h prior to a 50-h test, repeated testing also tended to attenuate amnesia. In Experiment II, independent groups were tested at 6 or 50 h after training treatment. Again, memory of the footshock was present at the short, but not at the long, interval. Recooling shortly prior to the 50-h test eliminated amnesia. Experiment III indicated that the return of memory produced by recooling did not persist if testing was delayed. These findings suggested that hypothermia may function as an important contextual cue for memory retrieval.

Severity of amnesia and the effectiveness of reactivation treatment: Evidence for a retrieval process

Reactivation of memory following retrograde amnesia treatment may be based on additional learning (storage model) or on gaining access to previously unavailable information (retrieval model). If further learning at reactivation summates with the residual memory, then the effectiveness of reactivation treatment should be inversely related to the degree of amnesia. The present experiment addressed this issue. Severity of retrograde amnesia was manipulated in adult rats by varying the delay (.5, 5, or 10 min) between punishment training and amnestic treatment (hypothermia). Prior to testing 24 h later, one half of the subjects received reactivation treatment (recooling). Passive avoidance latencies from the nonrecooled subjects showed the typical temporal gradient of amnesia. Recooling significantly improved retention in the strong amnesia group (.5 min), marginally improved retention in the moderate amnesia group, and failed to improve retention in the weak amnesia group. Controls for systemic stress demonstrated that the reactivation effect was not a performance artifact. In addition, test scores from a trained-only group were shown to reliably exceed the reactivation scores, suggesting that the magnitude of reactivation was not constrained by either functional or arbitrary ceiling limits. These findings were interpreted as consistent with a recently proposed “contextual cues” model as well as other retrieval-oriented interpretations of amnesia.

Effects of artificial rewarming upon hypothermia-induced retrograde amnesia

Three experiments were conducted to examine the relationship between recovery rate from deep body cooling and later memory loss. In each of these three investigations, a passive avoidance trial was followed immediately by hypothermia and recovery was manipulated by exposing rats to warm water. In Experiment I, rats returned to near normothermic levels by immersion in 36.4°C water for 20 min following the training-hypothermia treatment showed good retention, while rats allowed to recover under room temperature conditions showed the typical RA. Results from Experiment II indicated that extent of amnesia was inversely related to the duration of immersion in warm water following training-hypothermia treatment. In Experiment III. the prevention of RA by rewarming was shown to follow a time-dependent course. Initiation of rewarming 0, 5, or 10 min after hypothermia treatment eliminated RA, but the same rewarming given 30 min after treatment had no protective effect. The implications of these findings for consolidation and retrieval models of RA were considered.

Hypothermia: Amnesic agent, punisher, and conditions sufficient to attenuate amnesia

In three experiments, the effects of prior experience with hypothermia as a manipulation to attenuate amnesia were examined. In Experiment 1, prior experience with one hypothermia treatment did not appear to influence the amnesic effects of hypothermia. In Experiment 2, hypothermia treatment was made contingent upon stepping into a darkened chamber of a passive avoidance apparatus daily for 8 days. By Day 8, latencies indicated that subjects could remember the previous hypothermia treatments. The values of hypothermia found to produce learning in Experiment 2 were then used to evaluate several retrieval hypotheses concerning the prevention of amnesia in Experiment 3. Rats which received eight daily hypothermia treatments, as well as rats restrained without immersion, showed little evidence of amnesia following a train-hypothermia treatment. Results were interpreted as supporting the view that amnesia may be produced because important contextual cues necessary for memory retrieval are usually absent during testing.

Hypothermia-produced retrograde amnesia in young and adult rats

Although the phenomenon of retrograde amnesia has received extensive investigation in adult animals, relatively little attention has been given to the effects of amnestic agents on retention performance of young animals. In the present experiment, 50-66- and 25-31-day-old rats were given one-trial passive avoidance training followed immediately by hypothermia treatment. Adult and young rats cooled to a temperature at or below 21

A reduction of ECS-produced amnesia through post-ECS sensory isolation

#x00B0; C within 8-9 min showed little evidence of retention 24 h later. In contrast, 25-31-day-old rats cooled in half the time to the same temperature showed evidence of good retention. The implications of these findings for further studies examining the relationship between age and short-term changes in memory traces were considered.

Acquisition and retention of active avoidance behavior in previsual rats

Previous research has shown that, following training-ECS treatments, rats kept in a sensory isolation chamber during the entire retention interval show little evidence of retrograde amnesia. In this experiment, rats trained on a passive avoidance task and kept in a sensory isolation chamber for only 1 h after training-ECS treatments also showed attenuated anmesia. In contrast, retrograde amnesia was complete in animals similarly trained but returned to home cages immediately after ECS. These results indicate that environmental manipulations within the first hour after training-ECS treatments can alter the amnesic effect of ECS, suggesting that critical events determining whether or not retrograde amnesia is produced may occur within that time period.

Preventing and alleviating hypothermia-induced amnesia in weanling and young adult rats.

The ability of previsual rats to acquire and retain an active avoidance response at intervals ranging from 0 min to 48 hr was examined in five experiments. Experiments 1 and 2 demonstrated that improvement in avoidance responding over trials was a training effect. Experiments 3 and 4 demonstrated that there was no evidence of retention of the avoidance response over retention intervals ranging from 15 min to 48 hr. Testing at intervals of 0-30 min in Experiment 5 indicated that 10-day-old rats could retain the response over intervals ranging from 0-15 min, but not over a 30-min interval. However, even at the very short intervals (5-15 min), there was evidence of a retention deficit. In general, the results suggest that previsual rats have sufficient memory capabilities to acquire an active avoidance response and to retain it over a 15-min interval. However, without intervention, e.g., reactivation, a retention deficit appears almost immediately, and retention loss seems complete within 30 min. This procedure and phenomenon may prove useful in allowing an immediate assessment of variables believed to alleviate retention loss and in eliminating the influence of many extraneous variables that could alter the retention performance of developing animals.

Ontogeny of memory in the neonatal mouse.

Reminder (Experiment 1) and familiarization (Experiment 2) treatments were found to have similar effects on the 24-hr retention performance of 24- to 26- and 90- to 100-day-old rats that either did or did not undergo an amnesic treatment (hypothermia) immediately after training. Similar degrees of retrograde amnesia and normal forgetting were evident in both trained age groups that were not subjected to familiarization or reminder treatments. These results suggest that memory processes in weanling and adult rats are similar in susceptibility to disruption by an established amnesic treatment (hypothermia) and in the ease of prevention of and recovery from amnesia by recognized preventive (familiarization) and alleviation (reminder) measures. The similarity of the effects of these preventive and alleviation treatments on normal forgetting and induced amnesia suggests that experimentally induced amnesia may be a fruitful approach to studying the ontogeny of memory processes and, more specifically, to studying factors that influence infantile amnesia.