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.

The effect of oppositional parietal transcranial direct current stimulation on lateralized brain functions

“…This type of interaction between brain stimulation and cognitive load has only previously been reported in the context of working memory, to the authors’ knowledge (Jones & Berryhill, ; Sandrini et al ., ; Wu et al ., ). For example, using an oppositional montage, Sandrini et al .…”

Brain state and polarity dependent modulation of brain networks by transcranial direct current stimulation

“…A relationship between cognitive brain state and stimulation effects is also suggested by the small number of behavioral studies which demonstrate that manipulations of task difficulty can influence the behavioral modulations seen with tDCS (Gill, Shah‐Basak, & Hamilton, ; Jones & Berryhill, ; Li et al, ; Sandrini, Fertonani, Cohen, & Miniussi, ; Wu et al, ). Transcranial alternating current stimulation also shows effects on cortical network activity and connectivity that are dependent on the cognitive brain state (Neuling, Rach, Herrmann, & Schwiedrzik, ; Violante et al, ; Vosskuhl, Huster, & Herrmann, ).…”

Direct current stimulation of prefrontal cortex modulates error‐induced behavioral adjustments

“…This suggests that the DLPFC might be more involved in modulating behavior when the participants had to resolve cognitively demanding problems. Previous studies have shown that the effects of tDCS over parietal cortex (Jones & Berryhill, ) or over the DLPFC (Gill et al ., ) were related to the difficulty of the cognitive tasks.…”