Modulation of initial leftward bias in visual search by parietal tDCS

Abstract

Transcranial direct current stimulation (tDCS) has the potential to modulate spatial attention by enhancing the activity in one hemisphere relative to the other. This study aims to inform neurorehabilitation strategies for spatial attention disorders by investigating the impact of tDCS on the performance of healthy participants. Unlike prior research that focused on visual detection, we extended the investigation to visual search and visual imagery using computerized neuropsychological tests. Forty-eight participants had to actively search for targets in space (visual search) and notice differences between two mental images (visual imagery). Anodal stimulation was administered over the left parietal cortex for half of the participants and over the right parietal cortex for the other half. The results showed that tDCS modulated spatial attention in visual search but not in visual imagery. In the sham condition, visual search was characterized by a leftward bias in the selection of the first target and a left asymmetry in the overall spatial distribution of cancelled targets. Parietal tDCS modulated the initial leftward bias, enhancing it (more lateral) during right anodal stimulation and reducing it (more central) during left anodal stimulation. However, this effect was not reflected in the spatial distribution of the cancelled targets. The overall visual search performance marginally improved during right anodal stimulation, as evidenced by a greater percentage of cancelled targets compared to sham. Finally, the results revealed no left-right asymmetries in the visual imagery task, either after sham or anodal stimulation. The specific effect of parietal tDCS on the initiation of visual search offers a new perspective for targeted neurorehabilitation strategies and provides further insight into the different sensitivity of visual search measures classically used in brain-lesioned patients.

Fig 1. Performance of one representative participant in the Mesulam cancellation test [44].

Red marks indicate all the stimuli cancelled by the participant within the 30 seconds allowed to do the task.

Fig 2. Stripcharts depicting the mean x-coordinate of the first cancelled target, as a function of tDCS condition (sham vs. active), stimulated hemisphere (left vs. right anodal), and test (Mesulam, Ota vs. Star).

Each of the 6 plots includes data from 24 participants. The black connected squares represent the estimated means derived from a Gamma GLMM for both sham and active tDCS conditions, with error bars indicating the SE. The grey connected dots represent the observed mean values for each participant in both the sham and active tDCS condition. The zero value corresponds to the center of the template, while negative and positive values indicate biases towards the left and right sides of the template, respectively. Asterisks represent significant differences between the sham and active tDCS conditions. In the Mesulam test, anodal stimulation of the left and right parietal cortex shifted the position of the first mark in the contralateral direction compared to sham stimulation. In the Star test, there was a contralateral bias for the anodal stimulation of the left but not right parietal cortex. No significant difference was observed in the Ota test.

Fig 3. Stripcharts of the centers of cancellation (CoC) in the cancellation tests as a function of tDCS condition (sham vs. active), and stimulated hemisphere (left vs. right anodal).

Each plot includes data from 24 participants. The black connected squares represent the estimated means derived from a LMM for the sham and active tDCS condition, with error bars indicating the SE. The grey connected dots represent the observed mean values for each participant in both sham and active tDCS conditions. The zero value corresponds to an absence of bias, while negative and positive values indicate biases towards the left and right sides of the template, respectively.

Fig 4. Stripcharts of the mean percentage of targets cancelled in the cancellation tests as a function of tDCS condition (sham vs. active), and stimulated hemisphere (left vs. right anodal).

Each plot includes data from 24 participants. The black connected squares represent the estimated means derived from a Gamma GLMM for the sham and active tDCS condition, with error bars indicating the SE. The grey connected dots represent the observed mean values for each participant in both sham and active tDCS conditions. The dagger symbol represents a marginal difference between the sham and active tDCS conditions. Anodal stimulation of right parietal cortex marginally improved overall accuracy compared to sham stimulation.

Fig 5. Stripchart of the mean laterality quotient (LQ) in the Cloud task as a function of tDCS condition (sham vs. active), and stimulated hemisphere (left vs. right anodal).

Each plot includes data from 24 participants. The black connected squares represent the estimated means derived from LMM for both sham and active tDCS conditions, with error bars indicating the SE. The grey connected dots represent the observed mean values for each participant in both sham and active tDCS conditions.

Fig 6. Stripcharts of the mean accuracy in the Cloud task as a function of tDCS condition (sham vs. active), and stimulated hemisphere (left vs. right anodal).

Each plot includes data from 24 participants. The black connected squares represent the estimated probabilities derived from a binomial GLMM for both sham and active tDCS conditions, with error bars indicating the SE. The grey connected dots represent the observed mean values for each participant in both sham and active tDCS conditions.