Trends in Neurosciences
Mechanisms of emotional arousal and lasting declarative memory
Section snippets
Modulation of memory storage
Most studies of brain mechanisms of memory focus on the neural events mediating memory and the anatomical locus of the `memory trace'. Equally important, however, are neurobiological systems that regulate, or modulate, long-term memory storage. Long-term memories are not made instantaneously: they consolidate over time after learning10, 11, 12. Recent evidence concerning memory consolidation comes from many domains of investigation, including studies of synaptic plasticity13, 14, 15 and behavior
Role of stress-released hormones in memory consolidation
The fact that recently formed memories are susceptible to exogenous modulatory treatments provides the opportunity for endogenous modulation of memory storage for emotional events. Stress hormones are a priori candidate endogenous modulators (Fig. 1). Stress-hormone systems activated by emotional situations serve the immediate adaptive needs of an organism[22]. Additionally, extensive evidence suggests they influence memory storage8, 9, 23, 24. Initial studies examined the effects of
Role of the amygdaloid complex in memory modulatory mechanisms
The hypothesis that the amygdala modulates declarative-memory storage is rooted in several lines of research. Early studies demonstrated that electrical stimulation of the amygdaloid complex (AC) elicits behavioral arousal (the `orienting reflex') and activates cortical EEG ([38]). It is of interest for reasons discussed below that the cortical arousal response elicited by AC stimulation is mediated by the stria terminalis (ST), a major AC pathway[38]. Goddard's research[39] was the first to
Fractionation of amygdala nuclei function in memory
The `amygdala' is a heterogeneous collection of distinct nuclei. As long ago as 1915, Johnston noted that the AC `is a complex of many diverse elements which have been brought together by mechanical forces and have no primary functional unity' (quoted by Goddard[51]). Although Johnston might have overstated the case, the evidence indicates that nuclei of the AC have different functions in learning33, 35, 52.
The basolateral AC (BL) appears to be the nucleus most crucially involved in the
AC modulation of memory storage in other brain regions
The evidence briefly summarized above strongly suggests that the amygdala is not the neural site of long-term memory for declarative information. It is equally evident that the amygdala, particularly the BL nucleus, is involved in modulating memory storage processes in other brain regions. Other recent findings strongly support these implications. Packard et al.[60] reasoned that if the AC modulates memory in a particular brain structure, stimulation of the AC should influence formation of the
The AC and emotionally influenced, long-term declarative memory in humans
There has been controversy concerning the role of the AC in declarative memory73, 74. Scoville and Milner[73] examined memory in ten patients (including H.M.) who had received medial temporal-lobe surgery, often including removal of the AC, and concluded that `Removal of the amygdala bilaterally does not appear to cause memory impairment'. A recent study investigating a rare patient with AC damage concluded that, although the AC is important for classical conditioning of autonomic responses, it
Concluding remarks
An impressively broad array of experimental evidence either directly supports, or is consistent with the hypothesis that stress-hormone systems and the AC are key components of an endogenous memory modulating system. Generally inactive in unemotional learning situations, this system is activated during and after an emotionally arousing event and appears to regulate declarative-memory storage processes in other brain regions (Fig. 5). This mechanism aids in the selection of long-term memories on
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