Non-invasive brain stimulation in the treatment of post-stroke aphasia: a scoping review.

PURPOSE: Aphasia is an acquired language impairment that commonly results from stroke. Non-invasive brain stimulation (NIBS) might accelerate aphasia recovery trajectories and has seen mounting popularity in recent aphasia rehabilitation research. The present review aimed to: (1) summarise all existing literature on NIBS as a post-stroke aphasia treatment; and (2) provide recommendations for future NIBS-aphasia research. MATERIALS AND METHODS: Databases for published and grey literature were searched using scoping review methodology. 278 journal articles, conference abstracts/posters, and books, and 38 items of grey literature, were included for analysis. RESULTS: Quantitative analysis revealed that ipsilesional anodal transcranial direct current stimulation and contralesional 1-Hz repetitive transcranial magnetic stimulation were the most widely used forms of NIBS, while qualitative analysis identified four key themes including: the roles of the hemispheres in aphasia recovery and their relationship with NIBS; heterogeneity of individuals but homogeneity of subpopulations; individualisation of stimulation parameters; and much remains under-explored in the NIBS-aphasia literature. CONCLUSIONS: Taken together, these results highlighted systemic challenges across the field such as small sample sizes, inter-individual variability, lack of protocol optimisation/standardisation, and inadequate focus on aphasiology. Four key recommendations are outlined herein to guide future research and refine NIBS methods for post-stroke aphasia treatment.

A comprehensive review of non-invasive brain stimulation (NIBS) post-stroke aphasia literature, including all study designs, was required.Review of this literature revealed that anodal transcranial direct current stimulation is the mostly commonly used type of NIBS in aphasia treatment research.Systemic challenges across the field hinder prospective translation of NIBS into aphasia practice.Aphasia rehabilitation professionals should note that further research is required before NIBS is suitable for translation into clinical practice.

Do age-related differences in aperiodic neural activity explain differences in resting EEG alpha?

Alpha-band oscillatory activity in human electroencephalography (EEG) becomes slower and lower in amplitude with advanced age. However, the influence of aperiodic activity on these measures has received little consideration. We investigated whether age-related differences in aperiodic activity explains differences in resting EEG peak alpha frequency and power. We assessed aperiodic activity in 85 younger and 92 older adults by fitting the 1/f-like background activity evident in EEG power spectra using the spectral parameterization ("specparam") algorithm. Across the scalp, the aperiodic exponent and offset were smaller in older compared to younger participants, reflecting a flatter 1/f-like slope and a downward broadband shift in power spectra with age. After correcting for aperiodic activity, peak alpha frequency remained slower in older adults; however, peak alpha power no longer differed statistically between age groups. The large sample size utilized in this study, as well as the depth of analysis, provides further evidence that the aperiodic component of the resting EEG signal is altered with aging and should be considered when investigating neural oscillatory activity.

Does predictive cueing of presentation time modulate alpha power and facilitate visual working memory performance in younger and older adults?

Selective attention and working memory (WM) are vulnerable to age-related decline. Older adults perform worse on, and are less able to modulate alpha power (8-12 Hz) than younger adults in tasks involving cues about 'where' or 'when' a memory set will appear. However, no study has investigated whether alpha power is modulated by cues predicting the presentation time of a memory set. Here, we recorded electroencephalography while 24 younger (18-33 years) and 23 older (60-77 years) adults completed a modified delay match-to-sample task where participants were cued to the duration of a memory set (0.1 s or 0.5 s). We found: (1) predictive cues increased WM storage; (2) no differences in preparatory alpha power between predictive and neutral cue types, but preparatory alpha suppression was weaker in older adults; (3) retention period oscillatory power differed between presentation times, but these differences were no longer present when comparing trial types from the onset of the memory set; and (4) oscillatory power in the preparatory and retention periods were unrelated to performance. Our results suggest that preparatory alpha power is not modulated by predictive cues towards presentation time, however, reductions in alpha/beta power during visual WM retention may be linked to encoding, rather than retention.

Load-dependent modulation of alpha oscillations during working memory encoding and retention in young and older adults.

Working memory (WM) is vulnerable to age-related decline, particularly under high loads. Visual alpha oscillations contribute to WM performance in younger adults, and although alpha decreases in power and frequency with age, it is unclear if alpha activity supports WM in older adults. We recorded electroencephalography (EEG) while 24 younger (aged 18-35 years) and 30 older (aged 50-86) adults performed a modified Sternberg task with varying load conditions. Older adults demonstrated slower reaction times at all loads, but there were no significant age differences in WM capacity. Regardless of age, alpha power decreased and alpha frequency increased with load during encoding, and the magnitude of alpha suppression during retention was larger at higher loads. While alpha power during retention was lower than fixation in older, but not younger adults, the relative change from fixation was not significantly different between age groups. Individual differences in alpha power did not predict performance for either age groups or at any WM loads. We demonstrate that alpha power and frequency are modulated in a similar task- and load-dependent manner during WM in both older and younger adults when WM performance is comparable across age groups. IMPACT STATEMENT: Aging is associated with a marked decrease in the power and frequency of alpha oscillations. Here, we demonstrate that when verbal working memory performance is matched across age groups, alpha power and frequency are modulated in a similar task- and load-dependent manner in both young and older adults.

The Role of Alpha Power in the Suppression of Anticipated Distractors During Verbal Working Memory.

As working memory (WM) is limited in capacity, it is important to direct neural resources towards processing task-relevant information while ignoring distractors. Neural oscillations in the alpha frequency band (8-12 Hz) have been suggested to play a role in the inhibition of task-irrelevant information during WM, although results are mixed, possibly due to differences in the type of WM task employed. Here, we examined the role of alpha power in suppression of anticipated distractors of varying strength using a modified Sternberg task where the encoding and retention periods were temporally separated. We recorded EEG while 20 young adults completed the task and found: (1) slower reaction times in strong distractor trials compared to weak distractor trials; (2) increased alpha power in posterior regions from baseline prior to presentation of a distractor regardless of condition; and (3) no differences in alpha power between strong and weak distractor conditions. Our results suggest that parieto-occipital alpha power is increased prior to a distractor. However, we could not find evidence that alpha power is further modulated by distractor strength.