Trunk and Hand-Centred Spatial Coordinate Frames

Abstract

Neurons in early visual cortex represent space within a retinocentric coordinate frame, whereas downstream in motor cortex object location is coded with respect to the effector. The reference frame used by parietal neurons for the spatial analysis required to create movement plans is still contested. One dominant view is that parietal neurons use a retinal-centred reference frame at the single-cell level, but that other coordinate schemes can be ‘read out’ when information is pooled from a population of neurons gain-modulated by the head, hand, or other body part. More recently, there has been a surge of evidence for higher-order reference frames existing at the single-cell level, predominantly within non-human primate research. Furthermore, the range of coding typologies appears to be wide and complex, with the emergence of hybrid and dimension dependent response profiles. The research presented here investigates explicit body-centred and hand-centred coordinate systems in a two-part study using a memory-guided saccade paradigm. In the neuroimaging experiment, time-series analysis was used to test for the reorganisation of topographic maps along the intraparietal sulcus (IPS), following left and right, and near and far changes to hand position. In the behavioural experiment, eye-tracking data from the right eye was used to test for differences in error, following 90 degree torso rotations coupled with left and right hand placements. In both experiments, the retinal coordinates of saccade targets remained constant, and significant differences between conditions would provide evidence for the contribution of hand and body-centred spatial coding. This evidence would support the recently emerging evidence from the non-human primate literature and strengthen the argument for using posterior parietal cortex (PPC) as the source of command signals for the real-time control of neural prosthetics.