Hemispheric asymmetry in visuotopic posterior parietal cortex emerges with visual short-term memory load

Abstract

Visual short-term memory (VSTM) briefly maintains a limited sampling from the visual world. Activity in the intraparietal sulcus (IPS) tightly correlates with the number of items stored in VSTM. This activity may occur in or near to multiple distinct visuotopically mapped cortical areas that have been identified in IPS. To understand the topographic and spatial properties of VSTM, we investigated VSTM activity in visuotopic IPS regions using functional magnetic resonance imaging. VSTM drove areas IPS0-2, but largely spared IPS3-4. Under visual stimulation, these areas in both hemispheres code the contralateral visual hemifield. In contrast to the hemispheric symmetry observed with visual stimulation, an asymmetry emerged during VSTM with increasing memory load. The left hemisphere exhibited load-dependent activity only for contralateral memory items; right hemisphere activity reflected VSTM load regardless of visual-field location. Our findings demonstrate that VSTM induces a switch in spatial representation in right hemisphere IPS from contralateral to full-field coding. The load dependence of right hemisphere effects argues that memory-dependent and/or attention-dependent processes drive this change in spatial processing. This offers a novel means for investigating spatial-processing impairments in hemispatial neglect.

Figure 1.

VSTM trial structure. A, Subjects encoded a set of briefly presented targets among distractors, and after a delay were asked whether the orientation of any of the targets had changed. During the experiment, targets and distractors were presented in red and blue, respectively (colors randomized between subjects). Here, targets are depicted in black, and distractors in white. B, Target location conditions. In trial blocks, targets appeared always in the right visual hemifield (remember-right), always in the left visual hemifield (remember-left), or could appear in either hemifield (remember-full-screen). The number of targets also varied across blocks (set sizes 1, 3, and 6), but the total number of stimuli remained constant.

Figure 2.

Average VSTM K score for each set size and location. K score plateaus for each memory location, with no significant differences between the locations.

Figure 3.

A, B, Activation in corresponding regions of the dorsal posterior parietal cortex for visuotopic mapping (A) and VSTM (B) for a representative subject (remember-full-screen for set sizes 3 and 6 vs fixation). Both activations overlap along the medial bank of the posterior branch of IPS in visuotopically defined areas IPS0/V7, IPS1, and IPS2. VSTM activity continued along the lateral/inferior bank of the anterior branch of IPS, showing decreased overlap with IPS3 and 4, which continued in a relatively more superior/medial bank of the anterior branch of IPS. C, Percentage of voxels in each visuotopic IPS area that exhibit memory load-dependent activation in the VSTM task.

Figure 4.

Effects of VSTM set size on IPS BOLD activity. A–C, Activation from the remember-full-screen condition (vs fixation) for a single subject for 1 (A), 3 (B), and 6 (C) targets. D, BOLD signal averaged across all 3 conditions exhibits a plateau that mirrors the behavioral (K score) performance shown in Figure 2. The thin lines represent IPS0–2 separately and the thick line reflects their combined ROI. Lat, Lateral; Ant, anterior.

Figure 5.

Group average contralateral bias (contralateral > ipsilateral). A, Group average of areas showing significantly greater activity for contralateral than ipsilateral locations for set sizes 3 and 6 combined. B, Activity for contralateral and ipsilateral conditions for each set size for IPS0–2 in each cortical hemisphere. Lat, Lateral; Ant, anterior.

Figure 6.

Comparison of contralateral bias during visuotopic mapping and VSTM. A, Preferred visual angle for voxels significantly active during visuotopic mapping in regions IPS0–2 for the left and right hemispheres. B, Contralateral bias [(contralateral_{set size} − ipsilateral_{set size})/(contralateral_{set size} + ipsilateral_{set size})] during VSTM for the same ROIs as in A.