Assessing the mechanism of response in the retrosplenial cortex of good and poor navigators

Stephen D Auger,Eleanor A Maguire

doi:10.1016/j.cortex.2013.08.002

Stephen D Auger, Eleanor A Maguire

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https://doi.org/10.1016/j.cortex.2013.08.002

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Journal: Cortex	Publication Date: Aug 12, 2013
Citations: 84	License type: cc-by

Affiliation: Wellcome Centre for Human Neuroimaging

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The retrosplenial cortex (RSC) is consistently engaged by a range of tasks that examine episodic memory, imagining the future, spatial navigation, and scene processing. Despite this, an account of its exact contribution to these cognitive functions remains elusive. Here, using functional MRI (fMRI) and multi-voxel pattern analysis (MVPA) we found that the RSC coded for the specific number of permanent outdoor items that were in view, that is, items which are fixed and never change their location. Moreover, this effect was selective, and was not apparent for other item features such as size and visual salience. This detailed detection of the number of permanent items in view was echoed in the parahippocampal cortex (PHC), although the two brain structures diverged when participants were divided into good and poor navigators. There was no difference in the responsivity of the PHC between the two groups, while significantly better decoding of the number of permanent items in view was possible from patterns of activity in the RSC of good compared to poor navigators. Within good navigators, the RSC also facilitated significantly better prediction of item permanence than the PHC. Overall, these findings suggest that the RSC in particular is concerned with coding the presence of every permanent item that is in view. This mechanism may represent a key building block for spatial and scene representations that are central to episodic memories and imagining the future, and could also be a prerequisite for successful navigation.

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In order to compare good and poor navigators in an unbiased fashion, it was necessary to define a set of voxels to be used for classification in all participants. We identified this set of voxels based upon data from a completely independent cohort of participants in our previous functional MRI (fMRI) study (Auger et al, 2012); the voxels which showed increased activity for items with greater permanence which fell within the anatomical region of interest (ROI) for retrosplenial cortex (RSC) and parahippocampal cortex (PHC)
In a previous fMRI study, we found that the RSC responded in a highly selective manner to only the most permanent items when stimuli were presented singly (Auger et al, 2012)
We found that in a situation that was more akin to real life, with multiple items in view, the RSC coded for the specific number of permanent items contained in a visual array

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The retrosplenial cortex (RSC) comprises Brodmann areas 29/ 30 and is part of an extended network of brain regions engaged during fMRI studies of autobiographical memory, spatial navigation, imagining fictitious and future experiences and scene processing (Addis, Wong, & Schacter, 2007; Epstein, 2008, 2011; Maguire, 2001a, 2001b; Hassabis, Kumaran, & Maguire, 2007; Spreng, Mar, & Kim, 2009; Svoboda, McKinnon, & Levine, 2006; Troiani, Stigliani, Smith, & Epstein, 2012). When responses to the most permanent items were examined using fMRI, poor navigators had significantly reduced responses in RSC. This suggested that the RSC’s contribution may be to provide input regarding permanent items upon which other brain areas can build effective spatial and scene representations (Auger et al, 2012)

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