Abstract
The retrosplenial cortex supports navigation, with one role thought to be the integration of different spatial cue types. This hypothesis was extended by examining the integration of nonspatial cues. Rats with lesions in either the dysgranular subregion of retrosplenial cortex (area 30) or lesions in both the granular and dysgranular subregions (areas 29 and 30) were tested on cross-modal object recognition (Experiment 1). In these tests, rats used different sensory modalities when exploring and subsequently recognizing the same test objects. The objects were first presented either in the dark, i.e., giving tactile and olfactory cues, or in the light behind a clear Perspex barrier, i.e., giving visual cues. Animals were then tested with either constant combinations of sample and test conditions (light to light, dark to dark), or changed “cross-modal” combinations (light to dark, dark to light). In Experiment 2, visual object recognition was tested without Perspex barriers, but using objects that could not be distinguished in the dark. The dysgranular retrosplenial cortex lesions selectively impaired cross-modal recognition when cue conditions switched from dark to light between initial sampling and subsequent object recognition, but no impairment was seen when the cue conditions remained constant, whether dark or light. The combined (areas 29 and 30) lesioned rats also failed the dark to light cross-modal problem but this impairment was less selective. The present findings suggest a role for the dysgranular retrosplenial cortex in mediating the integration of information across multiple cue types, a role that potentially applies to both spatial and nonspatial domains.
Highlights
Research into the functions of the retrosplenial cortex has often considered its potential roles in spatial memory (Vann et al 2009), reflecting its dense interconnections with the hippocampus and anterior thalamic nuclei (Van Groen and Wyss 1990, 1992, 2003)
Many studies have demonstrated the involvement of the retrosplenial cortex in spatial memory problems, including when the tasks involve cue switching
Previous studies have shown that rats with retrosplenial cortex lesions are unimpaired on standard object recognition tasks (Ennaceur et al 1997; Vann and Aggleton 2002; Parron and Save 2004)
Summary
Six rats in the dysgranular retrosplenial lesion group (RSdysg) were excluded as the lesions were either only present in one hemisphere or because there was a high level of sparing of the dsygranular retrosplenial cortex. The dark–dark and light –light trials is dependent on the posterior parietal cortex (Winters and Reid did not differ in the amount of time that the rats spent exploring 2010) Such findings raise the question of whether the retrospleobjects in these two conditions (F , 1) and no differences were nial cortex might be involved in non-visual object recogniseen between the exploration times of the two groups (F , 1) tion, despite the lack of any consistent lesion effects when (see Table 2). The Sham animals performed above chance levels in the cross-modal dark–light trials, consistent with previous studies (Winters and Reid 2010) Both the RSdysg and RScomb animals failed to solve this trial type, i.e., when there was a switch in cue modalities from tactile to visual between sampling the objects and being tested for recognition. Such findings highlight the value of examining further the roles of the retrosplenial cortex in stimulus integration and translation, while trying to narrow down the specific nature of its contribution
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