In search of a recognition memory engram.
Bottom Line: Familiarity discrimination for individual visual stimuli seems to be effected by a system centred on the perirhinal cortex of the temporal lobe.The fundamental change that encodes prior occurrence within the perirhinal cortex is a reduction in the responses of neurones when a stimulus is repeated.A review is given of findings indicating that perirhinal cortex acts as a storage site for recognition memory of objects and that such storage depends upon processes producing synaptic weakening.
Affiliation: University of Bristol, School of Physiology and Pharmacology, Medical Sciences Building, Bristol BS8 1TD, UK. Electronic address: M.W.Brown@Bristol.ac.uk.Show MeSH
Mentions: As synaptic changes are hypothesised to be involved in the encoding of memory, and response changes have been recorded in perirhinal cortex in response to learning, it follows that pharmacological manipulations which impair memory should also impair synaptic plasticity if such plasticity mechanisms are responsible for memory storage. This relationship has been investigated using electrophysiological recordings of in vitro brain slice preparations. Synaptic enhancement (long-term potentiation: LTP) and synaptic weakening (long-term depression: LTD) can be produced in perirhinal cortical slices by appropriately patterned electrical stimulation (Liu and Bilkey, 1996; Ziakopoulos et al., 1999) (Fig. 6). For the most part those compounds which impair recognition memory have also been found to block induction or expression of synaptic plasticity in vitro, supporting the hypothesis that synaptic plasticity underlies recognition memory and informing us to some extent of the direction of plasticity affected by pharmacological intervention. However, one must be cautious not to use in vitro plasticity data as a proxy for recognition memory processes – the tone of afferent inputs to perirhinal cortical slices is significantly distorted. Furthermore, the stimulation patterns delivered to slices to induce activity-dependent synaptic plasticity are usually not consistent with those which occur in vivo, often stimulation lasting many seconds or minutes is applied to bring about synaptic changes which are thought to be expressed in a matter of milliseconds in behaving animals (Xiang and Brown, 1998), although exploration of novel object-place arrangements have been shown to affect induction of hippocampal plasticity in vivo using such protocols (Kemp and Manahan-Vaughan, 2004). Nevertheless, plasticity studies have proved useful tools in deciphering the molecular changes which may occur in the perirhinal cortex during object recognition memory formation. Electrophysiological studies of plasticity in chick IMM have similarly sought to make links to learning (e.g. Bradley et al., 1991a,b, 1992, 1999).
Affiliation: University of Bristol, School of Physiology and Pharmacology, Medical Sciences Building, Bristol BS8 1TD, UK. Electronic address: M.W.Brown@Bristol.ac.uk.