Potential role for peripheral nerve stimulation on learning and long-term memory: A comparison of alternating and...

In the past decade, a rising interest in transcranial electrical stimulation has emerged owing to its advantageous capacity to facilitate the extraction of casual links between neuromodulation and the obtained behavioral effects in cognitive performance. However, an insufficient number of direct comparative studies between transcranial alternating current stimulation (tACS) and transcranial direct current stimulation (tDCS) effects on associative memory have caused optimal parameters and procedural application to remain undefined.

The current study aimed to comparatively investigate the effects of tDCS and tACS applied to the occipital nerve (ON), targeting the locus coeruleus, on associative memory performance.

We employed a randomized, double-blind, two-visit, active-controlled study design. 85 cognitively normal adults were assigned to receive either active ON-tDCS, 40 Hz ON-tACS, sham ON-tDCS, or 1 Hz ON-tACS during encoding of a 50-word Swahili-English associative memory recall task. To evaluate the effects of electrical stimulation, we measured the cumulative rate of learning on Day 1 and to assess possible long-term effects, we measured the number of words recalled on Day 7.

Results presented two notable findings: (1) participants who received 40 Hz ON-tACS learned significantly more words on Day 1 (F_3,81 = 4.37, p = .007, η² = 0.14), and (2) participants who received 40 Hz ON-tACS or active ON-tDCS recalled significantly more words on Day 7 (F_3,81 = 11.08, p < .001, η² = 0.29).

The evidence from this study alludes to 40 Hz ON-tACS and active ON-tDCS inducing distinct behavioral effects, whereby 40 Hz ON-tACS generated an effect during memory encoding via enhanced attention, however, active ON-tDCS elicited an offline effect transpiring during consolidation. Further neuroimaging studies are needed to validate these findings and proposed mechanisms of action.