Effects of 10Hz and 40Hz Transcranial Alternating Current Stimulation (tACS) on Visual Search

PURPOSE:Previous researches have identified parietal regions involved in the visuospatial orienting and revealed the important roles of alpha- and gamma-band neural oscillations in spatial attention. Here we investigated how modulation of neural oscillations in alpha and gamma bands on right inferior parietal cortex may bias spatial attention. METHODS： 24 participants were randomly divided into 3 tACS stimulation groups (10 Hz, 40 Hz, or sham). The experiment consisted of four sessions, each taking approximately 12 min. Reaction time was recorded while they performed a modified version of the additional singleton task in the four sessions. Upon completion of session 1, 15 min tACS (or sham stimulation) started. The stimulation was applied to the right inferior parietal lobe (P6 & Cz) while participants went through session 2. There was no tACS in sessions 3 and 4. RESULTS： Distractor effect (DE = RT_distractor – RT_non-distractor) was calculated as an index of attention captured by a colored singleton, and then submitted to a repeated-measures ANOVA with  within-participants factors of time (sessions 1 to 4), side (target in the left vs. right hemifield), and  a between-participant factor of treatment (10Hz vs. 40Hz vs. sham). There was a side × treatment interaction. For the sham group, the DE in the left visual field was significantly larger than that in the right one, exhibiting a spatial attention bias towards the right hemifield. However, such a bias was not observed in both the 10Hz and 40Hz stimulation groups. Furthermore, for both the stimulation groups, the DEs in sessions 2 and 3 were smaller than that in session 1, suggesting an enhanced inhibition of salient task-irrelevant distractors. This effect was found in the online (2nd) session and even continued in the offline (3rd) session. It could persist in session 4 in the 40 Hz group, but not in the 10 Hz group. CONCLUSIONS: Our results provide new evidence for the roles of gamma and alpha oscillations in altering spatial attention bias and enhancing the inhibition function of attention. We also reveal the possibility of affecting visual search performance by tACS at alpha and gamma bands.