Decoding egocentric space in human posterior parietal cortex using fMRI

In our subjective experience, there is a tight link between covert visual attention and ego-centric spatial attention. One key difference is that the latter can extend beyond the visual field, providing us with an accurate mental representation of an object’s location relative to our body position. A neural link between visual and ego-centric spatial attention is suggested by lesions in parietal cortex, that lead not only to deficits in covert visual attention, but frequently also to a disorder of ego-centric spatial awareness, known as hemi-spatial neglect. While parietal involvement in covert visual spatial attention has been much studied, relatively little is known about mental representations of the unseen space around us. In the present study we examined whether also unseen spatial locations beyond the visual field are represented in parietal activity, and how they are related to retinotopic representations. We employed a novel virtual reality (VR) paradigm during functional magnetic resonance imaging (fMRI), whereby observers were prompted to draw their spatial attention to the position of one of eight possible objects located around them in an octagonal room. By changing the observers’ facing direction every few trials, the egocentric location of objects was disentangled from their absolute position and from the objects’ identity. Thus, mental representations of egocentric space surrounding the observer were sampled eight-fold. De-coding results of a multivariate pattern analysis classifier (MVPA), but not univariate results, showed that egocentric spatial directions were specifically represented in parietal cortex. These representations overlapped only partly with visually driven retinotopic activity. Our results thus show that parietal cortex codes not only for retinotopic and visually accessible space, but also for egocentric locations of the three-dimensional space surrounding us, including unseen space