Effects of stress on memory
The effects of stress and trauma on the hippo- campus and on brain development described above suggest at least two hypotheses about the effect of stress on memory. Performance on tests of hippocampal-dependent memory should be impacted by exposure to high levels of stress because of the effect of stress hor- mones on cell birth and death in the hippo- campus. Work with animal models supports this hypothesis. Handling rats and placing them in novel environments reduces their level of performance in hippocampal-depen- dent maze learning tasks, but not in non-hip- pocampal dependent habit learning tasks (Di- amond, Fleshner, Ingersoll, & Rose, 1996). However, this effect is transient, and dissi- pated when the rats became acclimated to the new environment so that it was no longer stressful. Stress has also been shown to facili- tate some forms of learning that depend in part on the hippocampus, such as some forms of classical conditioning (Shors, Weiss, & Thompson, 1992). However, there are impor- tant differences between the types of hippo- campal memory that are facilitated and im- paired by stress. Although both forms of learning involve the hippocampus, the struc- tures involved in each beyond the hippocam- pus differ dramatically. Classical conditioning involves the cerebellum and assorted brain- stem nuclei, whereas declarative memory in- volves a complex circuit including the medial temporal lobe and cortical structures. More importantly, the stressor used by Shors et al. (inescapable shock) was very similar to the unconditioned stimulus used in the classical conditioning paradigm (preorbital shock paired with noise). It is not clear what the ex- act relation between the nature of the stressor and the nature of the memory trace is, or what is the effect of similarity between the two. However, given that intrusive, persistent, and heightened memory is one of the hallmark symptoms of PTSD, it is not altogether sur- prising that shock stress produces increased responsiveness to conditioned shock. Thus, is
it most likely the degree of stress and its dura- tion that determines whether and for how long it will impair memory.
The effect of stress on the developing memory system is far less well described. Early stress could lead to long term memory deficits if the stress occurred during a time when the hippocampus is not yet fully mature and is vulnerable. For example, the develop- ment of object permanence is delayed in mon- keys who were prenatally exposed to stress hormones, such as when their mothers were placed in stressful situations (Schneider, 1992). Specifically, pregnant monkeys were placed in cages in a darkened room and ex- posed to unpredictable loud bursts of noise. The infants of the stressed monkeys were older than control monkeys when they be- came able to find a hidden object on 80% of trials presented over two consecutive days, pointing to delayed development in the stressed animals. However, prenatal stress has a number of effects on monkeys’ behavior and physiology (Clarke, Wittwer, Abbott, & Schneider, 1994; Vallee, et al., 1997). For ex- ample, in prenatally stressed monkeys, hor- monal responsiveness to future shock is in- creased over nonstressed monkeys. As a result, it is difficult to conclude that the mem- ory deficits observed by Schneider were a re- sult of damage to the neurobiological memory system, or were a result of other behavioral consequences of prenatal stress exposure (such as increased release of acetylcholine in the hippocampus; see Day, Koehl, Deroche, Le Moal, & Maccari, 1998). In addition, Val- lee and colleagues (1997) failed to find mem- ory impairment in prenatally stressed adult rats. Nevertheless, it is clear from the results of work with prenatally stressed monkeys that there is an effect of early stress, either direct or indirect, on memory performance.