Gaze-centered updating of visual space in human parietal cortex

Abstract

Single-unit recordings have identified a region in the posterior parietal cortex (PPC) of the monkey that represents and updates visual space in a gaze-centered frame. Here, using event-related functional magnetic resonance imaging, we identified an analogous bilateral region in the human PPC that shows contralateral topography for memory-guided eye movements and arm movements. Furthermore, when eye movements reversed the remembered horizontal target location relative to the gaze fixation point, this PPC region exchanged activity across the two cortical lobules. This shows that the human PPC dynamically updates the spatial goals for action in a gaze-centered frame.

Figure 1.

A, The delayed-movement task. Subjects fixated a central letter, S, P or F, referring to a delayed-saccade task (S), a delayed pointing task (P), or a fixation task (F), respectively. After a brief peripheral dot was presented, a horizontal band of distractors blinked for 2.5 sec. Subsequently, subjects made either a saccade or a pointing movement to the remembered target location and immediately back to center. Subjects made no movement when in fixation (F) task. B, The intervening saccade task. In this paradigm, two targets were briefly flashed sequentially, a green (the goal target) and red (refixation target for the first saccade) target. After a 6 sec delay, subjects made a saccade to refixate at the remembered location of the red target and, after a subsequent 12 sec period, made either a saccade (Saccade–Saccade Task) or a pointing movement (Saccade–Point Task) to the location of the remembered goal target and immediately back to center.

Figure 3.

The remembered location of a target is transferred from one cerebral hemisphere to the other within the PPC. A, RR, LL, RL, and LR signify four possible conditions of the intervening saccade paradigm; the first letter signifies initial location of the two targets (R, right hemifield; L, left hemifield), and the second letter refers to the remapped location of the remembered goal location. Left (B) and right (C) parietal activation (mean ± SE across 6 subjects) for each of the four conditions in the saccade–saccade task. SEs are plotted at the time point at which they were computed. All time courses are shifted to compensate for the fMRI hemodynamic lag. Dashed lines indicate presentation of stimuli, time of first saccade, and time of second saccade, respectively. Gray areas indicate the periods over which the differences between the LR and RL condition were taken. D, E, Saccade–Point Task, The remembered location of the goal target for hand pointing is transferred across cerebral hemispheres within the human PPC after an intervening saccade. Data in same format as B and C.

Figure 2.

A subset of areas within the PPC shows topography for both saccade and pointing target locations. A, The regions showing higher activation for saccades or pointing movements than for fixation (p < 10^-5) in one subject, rendered onto an inflated representation of the cortical surface. Orange, Voxels activated during saccades. Blue, Voxels activated during pointing movements. Purple, Voxels activated during both saccades and pointing movements. CS, Central sulcus; IPS, intraparietal sulcus. B, Areas that show left–right topography for saccades (p < 0.001). C, The same regions show left–right topography for pointing movements (p < 0.001). D, The centers of the parietal maps marked in two slice views: a coronal and sagittal view. Green cross indicates left hemisphere; red cross indicates right hemisphere. Talairach coordinates were as follows (in mm): x = -19, y = -58, z = 47 (left) and x = 17, y = -68, z = 55 (right).

Figure 4.

A comparison of activation before and after remapping in each subject. A, Goal is target for saccades. B, Goal is target for pointing movement. x-Axis, The difference (±SE) in the average activation between the RL and LR conditions just before the first saccade. y-Axis, Same but after remapping. Filled circles, Right PPC; open squares, left PPC. Before the first saccade (first delay period) activation should be contralateral (i.e., RL > LR for the left PPC, and LR > RL for the right PPC). After the remapping (second delay period), activation should switch hemispheres (i.e., LR > RL for left PPC, and RL > LR for right PPC). Gaze-centered remapping requires right PPC data in the second quadrant and left PPC data in the fourth quadrant (gray zones).