Discussion
memory-related disorders, TLE
TLE, CORTEX
The present study investigated whether anodal tDCS and theta tACS over the left temporal lobe improved episodic memory performance in healthy adults. Unlike many previous studies comparing the effects of tDCS and tACS on memory, we applied stimulation to the temporal lobe, as encoding and recall of information is typically associated with temporal lobe structures (e.g., the hippocampus and entorhinal cortex). We anticipated that both stimulation types would result in improved memory performance compared to sham stimulation, with theta tACS resulting in superior performance compared to tDCS.In terms of memory performance, our findings did not support our initial hypotheses. Instead, they demonstrated the opposite effect, with participants recalling significantly fewer words during tDCS and tACS, compared to sham stimulation. Furthermore, participants performed significantly worse during tACS compared to tDCS, a finding that was unexpected given the breadth of literature demonstrating the enhancing effect of tACS on memory performanceParticipants made significantly more memory errors (i.e., total intrusions and repetitions) during theta tACS compared to sham stimulation, with the comparison to tDCS trending towards significance. After separating intrusion and repetition errors, we found that tACS resulted in more intrusion errors, compared with sham stimulation, and more repetition errors compared to both sham and tDCS. While both intrusion and repetition errors are typically associated with fronto-subcortical patterns of impairmentThere was no significant difference in stimulation impedance between groups, suggesting comparable conductivity levels between stimulation types. Perceived stimulation intensity and overall side-effects of stimulation were however higher in the tDCS group compared to both tACS and sham stimulation. While the perceived intensity and side-effects of stimulation may have impacted performance, it is worth noting that tACS (which did not significantly differ from sham stimulation on these factors) was associated with a greater impact on memory performance. The effectiveness of the neuroConn sham protocol has previously been investigated and was found to be an adequate blinding method, whereby 73% of the sham group in a double-blinded study believed they had received active tDCSTherapeutic transcranial electrical stimulation is of growing interest, particularly in the context of memory-related disorders (e.g., AD and TLE). However, there are several different stimulation modalities available. Additional research is required to understand which stimulation modalities produce reliable improvements in cognition. Our findings highlight how different stimulation methods can differentially impact cognition, as theta tACS was associated with worse memory performance, compared to tDCS. Furthermore, the target of stimulation often varies between studies, with the majority of tDCS research focussing on the stimulation of the DLPFC. Our study applied current to the temporal lobe to engage mesial temporal structures that are typically associated with encoding and recall of information. Our findings appear to support the role of these structures within memory processes; however, contrary to our hypotheses, we did not demonstrate an enhancing effect—instead reducing memory performance during stimulation. These finding have major implications for how we utilise therapeutic stimulation in future studies, as they indicate that some stimulation modalities, namely tACS, may only be beneficial in certain cohorts (i.e., older adults) or under certain conditions (i.e., where specific brain activity is expected to be aberrant or diminished).There were a few limitations to our study. First, given our study did not include individual MRIs, it is difficult to confidently ascertain whether the current flow of stimulation engaged our region of interest. Nevertheless, the use of an MRI-derived standard head model (i.e., New York Head) is a well validated method for predicting current flow in the absence of individual MRI scans