Abstract
Visual location is initially encoded in retinotopic co-ordinates but is later converted to a geocentric frame of reference, allowing stable perception despite eye movements. In primates, the first stage of the transformation involves neurons (in various areas) that have fixed receptive fields in retinotopic space but responsiveness that is strongly modulated by eye position. We provide evidence for the existence of such neurons in the human posterior parietal cortex. An MR adaptation paradigm was used. A circular, flickering, checkerboard pattern (diam. 10 deg) was presented in an otherwise-dark field. A fixation spot was provided in the middle of the pattern. Functional imaging was performed with a 1.5T MR scanner (GE Signa), using a purpose-built optical system to provide a large (80 deg) image. After an initial dark period to establish baseline activity, the stimulus appeared on the horizontal meridian at an eccentricity of 30 deg from the centre of the screen. The observer moved the eyes so as to foveate the fixation spot. The pattern remained at that location for 3 mins, during which time substantial BOLD signal adaptation occurred. It then disappeared and immediately reappeared at the same eccentricity on the opposite side. The observer made a single, large (60 deg) eye movement to re-establish fixation. Voxels that were sensitive to gaze direction were identified by correlating the activation timecourse with a model adaptation rebound response. Because retinal stimulation is invariant (except during the transition), there should be no rebound from adaptation in strictly retinotopic neurons. Rebound indicates a change in the active neuron population, reflecting sensitivity to gaze direction. A cluster of voxels showing strong rebound was observed immediately anterior to the parieto-occipital sulcus, close to the medial surface (possibly LIP; Sereno et al. Science 2001), suggesting that this region is sensitive to the geocentric location of stimuli.