Abstract
It has been proposed that the retrosplenial cortex forms part of a ‘where/when’ information network. The present study focussed on the related issue of whether retrosplenial cortex also contributes to ‘what/when’ information, by examining object recency memory. In Experiment 1, rats with retrosplenial lesions were found to be impaired at distinguishing the temporal order of objects presented in a continuous series (‘Within‐Block’ condition). The same lesioned rats could, however, distinguish between objects that had been previously presented in one of two discrete blocks (‘Between‐Block’ condition). Experiment 2 used intact rats to map the expression of the immediate‐early gene c‐fos in retrosplenial cortex following performance of a between‐block, recency discrimination. Recency performance correlated positively with levels of c‐fos expression in both granular and dysgranular retrosplenial cortex (areas 29 and 30). Expression of c‐fos in the granular retrosplenial cortex also correlated with prelimbic cortex and ventral subiculum c‐fos activity, the latter also correlating with recency memory performance. The combined findings from both experiments reveal an involvement of the retrosplenial cortex in temporal order memory, which includes both between‐block and within‐block problems. The current findings also suggest that the rat retrosplenial cortex comprises one of a group of closely interlinked regions that enable recency memory, including the hippocampal formation, medial diencephalon and medial frontal cortex. In view of the well‐established importance of the retrosplenial cortex for spatial learning, the findings support the notion that, with its frontal and hippocampal connections, retrosplenial cortex has a key role for both what/when and where/when information.
Highlights
Retrosplenial cortex is densely interconnected with the hippocampal formation and the anterior thalamic nuclei
Lesions of retrosplenial cortex impair tests of spatial memory that involve allocentric cues, with examples coming from tasks in the water maze and radial-arm maze, as well as impairing tests that rely on egocentric directional information and path integration (Sutherland et al, 1988; Warburton et al, 1998; Pothuizen et al, 2008; Keene & Bucci, 2009; Miller et al, 2014)
It is already known that lesions in the rat hippocampus and anterior thalamic nuclei, along with prelimbic cortex, can disrupt the discrimination of nonspatial stimuli by their temporal order, i.e. ‘what/when’ information (Fortin et al, 2002; Hannesson et al, 2004a, b; Wolff et al, 2006; Barker et al, 2007; Barker & Warburton, 2011; DeVito & Eichenbaum, 2011; Albasser et al, 2012; Dumont & Aggleton, 2013)
Summary
Retrosplenial cortex is densely interconnected with the hippocampal formation and the anterior thalamic nuclei (van Groen & Wyss, 1990; van Groen & Wyss, 1992a,b; Van Groen & Wyss, 2003; Vann et al, 2009). Lesions of retrosplenial cortex impair tests of spatial memory that involve allocentric cues, with examples coming from tasks in the water maze and radial-arm maze, as well as impairing tests that rely on egocentric directional information and path integration (Sutherland et al, 1988; Warburton et al, 1998; Pothuizen et al, 2008; Keene & Bucci, 2009; Miller et al, 2014). ‘what/when’ information (Fortin et al, 2002; Hannesson et al, 2004a, b; Wolff et al, 2006; Barker et al, 2007; Barker & Warburton, 2011; DeVito & Eichenbaum, 2011; Albasser et al, 2012; Dumont & Aggleton, 2013) These findings prompted the need to know whether the rat retrosplenial cortex is involved in this form of temporal memory. Initial evidence comes from the finding that rats with retrosplenial cortex lesions are impaired at discriminating time intervals (Todd et al, 2015) and from the description of a patient with left retrosplenial pathology who showed poor recency judgements (Bowers et al, 1988)
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