Abstract
The expression of the immediate-early gene c-fos was used to compare networks of activity associated with recency memory (temporal order memory) and recognition memory. In Experiment 1, rats were first familiarized with sets of objects and then given pairs of different, familiar objects to explore. For the recency test group, each object in a pair was separated by 110 min in the time between their previous presentations. For the recency control test, each object in a pair was separated by less than a 1 min between their prior presentations. Temporal discrimination of the objects correlated with c-fos activity in the recency test group in several sites, including area Te2, the perirhinal cortex, lateral entorhinal cortex, as well as the dentate gyrus, hippocampal fields CA3 and CA1. For both the test and control conditions, network models were derived using structural equation modeling. The recency test model emphasized serial connections from the perirhinal cortex to lateral entorhinal cortex and then to the CA1 subfield. The recency control condition involved more parallel pathways, but again highlighted CA1 within the hippocampus. Both models contrasted with those derived from tests of object recognition (Experiment 2), because stimulus novelty was associated with pathways from the perirhinal cortex to lateral entorhinal cortex that then involved both the dentate gyrus (and CA3) and CA1 in parallel. The present findings implicate CA1 for the processing of familiar stimuli, including recency discriminations, while the dentate gyrus and CA3 pathways are recruited when the perirhinal cortex signals novel stimuli.
Highlights
Recognition memory is the ability to discriminate novel from familiar stimuli
The immediate-early gene (IEG) c-fos was selected because the expression of this gene increases in perirhinal cortex when rats experience novel stimuli (Aggleton & Brown, 2005; Aggleton, Brown, & Albasser, 2012; Albasser et al, 2013; Albasser, Poirier, & Aggleton, 2010; Wan, Aggleton, & Brown, 1999, 2001; Warburton et al, 2005, 2003; Zhu, Brown, McCabe, & Aggleton, 1995, Zhu, McCabe, Aggleton, & Brown, 1996), but this expression is required for effective long-term recognition memory (Seoane, Tinsley, & Brown, 2012); that is, it has an integral role within this form of memory
Reflecting the majority of published studies on object recency by rodents, the present study focused on recency discriminations when the objects are separated by time and by a distinct event
Summary
Recognition memory is the ability to discriminate novel from familiar stimuli. Recency memory is the discrimination of familiar stimuli by their relative distance in time; that is, temporal order memory. The animal is placed back in the apparatus to select between objects A and B when they are presented together for the first time (e.g., Albasser et al, 2012; Barker et al, 2007; Hannesson, Howland, et al, 2004; Hannesson, Vacca, et al, 2004; Mitchell & Laicacona, 1998) This form of recency testing, in which the stimulus presentations include distinctive events that separate the items (see Templer & Hampton, 2013), can be compared with the ability to select between items previously presented in a single, continuous series; that is, without any specific intervening event (e.g., Agster, Fortin, & Eichenbaum, 2002; Fortin et al, 2002; Shaw & Aggleton, 1993). The bow-tie maze (Albasser, Chapman, Amin, Iordanova, Vann, & Aggleton, 2010; Albasser, Poirier, & Aggleton, 2010) was used for all behavioral testing because this apparatus makes it possible to deliver multiple trials without having to handle the rat
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