Reinhart & Nguyen, 2019, Nat. Neuro.
Review prepared by: Low, Sock Ching (@yokosocko)
Against a backdrop of studies showing the effectiveness of transcranial alternating current stimulation (tACs) in modulating neural oscillations and, consequently, working memory (WM), Reinhart & Nguyen examine the persistency of such a WM boost in older adults who exhibit typical age-related cognitive decline. They used a simple task which required either a purely perception-based response (control) or one based on a stimulus that was presented 3s prior (experimental). Through HD-tACs using individually-tuned theta they restored not only behavioural performance of older adults in the memory task, but also significantly increased previously compromised EEG measures to a level similar to younger adults. Their behavioural gains were maintained till the end of the experiment 50mins post-stimulation, suggesting that tACs had caused the improvement through synaptic changes rather than other, more transient, ways.
Nonetheless, while it is valid to claim that the older adults’ measures were improved to levels similar to younger adults, it remains unknown if this is partially due to the ease of the task for younger adults creating a ceiling effect. If the behavioural performance was more different between younger and older adults at baseline, the older adults’ revived performance may be more dissimilar to the younger adults’. In addition, reaction time was consistently not affected by tACs in experiments 1 and 2, but in experiment 3 tACs-induced WM deficits included an increase in participants’ reaction time. This was not explicitly mentioned nor discussed. Lastly, the frequencies declared to fall in the theta and gamma ranges are not entirely within the accepted range in literature; they take 8Hz to be theta when it is normally considered as alpha and 30Hz to be gamma when it could also be considered in the beta range. That the key frequencies in this paper sit on the fringes of the canonical band thresholds could provide insight as to how the brain communicates between regions, but it was not discussed.
The findings of this paper give hope for the development of a way to mitigate cognitive decline due to aging, or at least to lessen its impact. Even with its shortcomings, it makes a strong argument for WM being indexed in the brain through PAC and theta synchronisation and is presented in a clear manner. Hence, it is certainly a paper worth delving into in detail.