P184 Individual transcranial alternating current stimulation (tACS) within the beta range during verbal working memory

Abstract

Introduction Beta band brain oscillations (13–30 Hz) correlate with cognitive manipulations during language processing ( Weiss and Muller, 2012 ). In particular, they are associated with the active maintenance of stimuli in verbal working memory ( Weiss and Rappelsberger, 2000 ). Therefore, beta oscillations are suited for tACS during verbal memory encoding. Objectives We examined whether 18 Hz tACS stimulation modulates verbal WM performance in a healthy participant. We did this single-case study since most tACS results are based on group studies and we want to test tACS in individual patients with disturbances of verbal WM. Materials & Methods A right-handed female (27 y) had to memorize 600 visually presented concrete words (150 nouns, 150 verbs; randomly presented two times) and do subsequent free recall. tACS in the beta (18 Hz) and alpha (9 Hz) band as well as a sham stimulation was applied in a double-blind multiple treatment design (ABCBCA) on 6 different days with at least 24 h between sessions. tACS-intensity was 350 μA (below individual phosphene and discomfort threshold). 20 min stimulation was applied at left temporo-parietal and right fronto-central sites. Results A significant difference between the stimulation and sham condition (Pearson χ2 = 12.0, p = 0.03, two-tailed) was found which differed for nouns and verbs. During beta stimulation the participant showed a 48% increase of recalled nouns (8% for verbs), during alpha stimulation a 28% (26 % for verbs) and during sham a 20% (36% for verbs) increase. Overall error rate was higher for verbs (Pearson χ2 = 11.73, p = 0.008, two-tailed). The influence of beta vs. alpha stimulation could not be reliably distinguished with the current protocol. Conclusion Although, beta oscillations are important for linking past and present input into a coherent representation of memorized contents ( Weiss and Rappelsberger, 2000 , Kopell and Whittington, 2011 ), no significant difference was found between alpha and beta stimulation. The word category difference is in accordance with previous findings ( Fiori and Cipollari, 2013 ) who revealed better performance for verb versus noun processing when stimulating left frontal in contrast to temporal areas.