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

Abstract

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.

Keywords: Episodic memory; Landmarks; Navigation; Permanence; Retrosplenial cortex.

Fig. 1

Examples of the stimuli. Categories varied according to the number of permanent, ‘never moving’, items they contained. One example stimulus from each of the five permanence categories is shown here, ranging from no permanent items in the top stimulus, to all four items being permanent in the bottom stimulus.

Fig. 2

MVPA results. Mean classifier accuracy values for all 32 participants +/− 1 SEM, collapsed across hemispheres. Results for decoding of permanence (blue), size (yellow) and visual salience (purple) are shown for RSC, PHC and a control region (motor cortex). For RSC and PHC, five-way classification of the number of permanent items within each stimulus was not only significantly above chance (which was 20% – red dashed line) but also significantly greater than that for size and visual salience. *p < .05.

Fig. 3

Voxels carrying the greatest amount of permanence information. In these heatmaps, shown on the structural MRI scan of one participant chosen at random, the colours represent the percentage of all 32 subjects in which each voxel was identified by feature selection to carry large amounts of permanence information; RSC top panel, PHC lower panel.

Fig. 4

Results for good and poor navigators. Mean MVPA results +/− 1 SEM in good (green) and poor (red) navigators for each of the 3 item features in RSC and PHC. Permanence was the only feature that could be decoded significantly above chance (which was 50% – grey dashed line). Additionally, classification within the RSC of good navigators was significantly greater than that of poor navigators. RSC also contained significantly more permanence information than PHC within good navigators. *p < .05.