Parietal contributions to visual working memory depend on task difficulty

Abstract

The nature of parietal contributions to working memory (WM) remain poorly understood but of considerable interest. We previously reported that posterior parietal damage selectively impaired WM probed by recognition (Berryhill and Olson, 2008a). Recent studies provided support using a neuromodulatory technique, transcranial direct current stimulation (tDCS) applied to the right parietal cortex (P4). These studies confirmed parietal involvement in WM because parietal tDCS altered WM performance: anodal current tDCS improved performance in a change detection task, and cathodal current tDCS impaired performance on a sequential presentation task. Here, we tested whether these complementary results were due to different degrees of parietal involvement as a function of WM task demands, WM task difficulty, and/or participants' WM capacity. In Experiment 1, we applied cathodal and anodal tDCS to the right parietal cortex and tested participants on both previously used WM tasks. We observed an interaction between tDCS (anodal, cathodal), WM task difficulty, and participants' WM capacity. When the WM task was difficult, parietal stimulation (anodal or cathodal) improved WM performance selectively in participants with high WM capacity. In the low WM capacity group, parietal stimulation (anodal or cathodal) impaired WM performance. These nearly equal and opposite effects were only observed when the WM task was challenging, as in the change detection task. Experiment 2 probed the interplay of WM task difficulty and WM capacity in a parametric manner by varying set size in the WM change detection task. Here, the effect of parietal stimulation (anodal or cathodal) on the high WM capacity group followed a linear function as WM task difficulty increased with set size. The low WM capacity participants were largely unaffected by tDCS. These findings provide evidence that parietal involvement in WM performance depends on both WM capacity and WM task demands. We discuss these findings in terms of alternative WM strategies employed by low and high WM capacity individuals. We speculate that low WM capacity individuals do not recruit the posterior parietal lobe for WM tasks as efficiently as high WM capacity individuals. Consequently, tDCS provides greater benefit to individuals with high WM capacity.

Keywords: PPC; individual differences; tDCS; task difficulty; working memory.

Figure 1

Example trials of each of the WM tasks used in Experiment 1: (top) sequential presentation WM task: in each trial a series of images were presented and after a delay a probe image appeared: (bottom) change detection WM task in which a visual array was presented and after a delay a probe image appeared. In both cases the response was to report whether the probe image was old or new.

Figure 2

Experiment 1 results. Performance is plotted as a difference index using performance accuracy values (tDCS − sham/tDCS + sham). Values above 0 indicate superior performance in the tDCS condition; values below 0 indicate impaired performance after tDCS. The sequential task is presented on the left and the change detection task on the right. The low WM span group is plotted in black and the high WM span group is plotted in white. Error bars represent the SEM. There was a significant between-group effect of tDCS across stimulation condition and WM task.

Figure 3

Experiment 2 results. The difference indices for anodal tDCS on WM accuracy. Error bars represent the SEM.

Figure 4

Experiment 2 results. The difference indices for cathodal tDCS on WM accuracy. Error bars represent the SEM.