Age-related changes in responsiveness to non-invasive brain stimulation neuroplasticity paradigms: A systematic review with meta-analysis

(cid:1) Thirty-nine studies explored the effect of age on responsiveness to non-invasive brain stimulation neuroplasticity paradigms. (cid:1) Pooled ﬁndings revealed age-dependent reduction in corticospinal excitability following certain neuroplasticity paradigms. (cid:1) Considerable heterogeneity within some paradigms was evident, limiting interpretations of pooled analyses.


Introduction
Neuroplasticity refers to the ability of the nervous system to reorganise in response to intrinsic and extrinsic stimulation, and is dependent on the characteristics of the person interacting with the presented stimulus (Park & Bischof, 2013).Neuroplasticity is thought to underpin learning and memory and relies on several processes to alter the structure and function of neuronal activity (Kempermann, 2011;Magee & Grienberger, 2020).Adult neuroplasticity enables fine-tuning of the production or alteration of neuronal activity to different situations, and harnessing neuroplasticity to enhance neural function may therefore offer a novel approach to tackle cognitive and behavioural changes that limit function in various conditions including healthy ageing.
It is a pervasive belief that age influences neuroplasticity, namely that age-related declines in neuroplasticity occur (Barnes, 2003;Ridding & Ziemann, 2010).Expectations of age-dependent effects on neuroplasticity stem predominantly from research showing young brains are very plastic and older ones are less so, the result of a trade-off between the need to rapidly learn in the early years and the need for stability of brain function for the duration of life (Kempermann, 2011).Supported by behavioural evidence that age is often associated with a slowing of movements, a decrease in the ability to learn new motor skills, and an inability to correct a movement plan once a movement has begun (Carmona & Michan, 2016), the evidence that age is a potent regulator of neuroplasticity seems compelling and in need of further investigation.
Neuroplasticity is difficult to directly measure non-invasively in humans.However, advances in non-invasive brain stimulation (NBS) techniques have allowed researchers to induce neuroplasticity-like effects in awake humans by applying precisely timed magnetic pulses and/or electrical currents to the scalp overlaying key brain regions.Moreover, other NBS techniques have also allowed the assessment or quantification of the effects of these neuroplasticity-inducing paradigms.One NBS technique capable of both inducing neuroplasticity-like effects and quantifying the effects of these paradigms is transcranial magnetic stimulation (TMS) (Barker et al., 1985;Hallett, 2007;Pascual-Leone et al., 1998).When stimulating areas overlaying the primary motor cortex, TMS pulses pass painlessly through the scalp and skull and activate the underlying descending cortico-motor neurons.If the stimulation intensity is sufficient, these descending neurons depolarise, sending multiple descending volleys down the corticospinal tract to the target muscles of interest.In response, the target muscles briefly contract.The size of this contraction can be quantified as a motor evoked potential (MEP) (Pascual-Leone et al., 1998).Changes in MEP peak-to-peak amplitude following NBS paradigms are considered an indirect measure of the neuroplasticity of corticospinal activity and are proposed to reflect features of motor cortex plasticity (Pascual-Leone et al., 1998;Wagner et al., 2007).
Many different NBS paradigms are currently used in both research and clinical settings to induce short lasting neuroplasticity-like effects.These techniques include paired associative stimulation (PAS), theta burst stimulation (TBS) and various types of repetitive transcranial magnetic simulation (rTMS and quadripulse stimulation; QPS).Other NBS paradigms such as transcranial direct current (tDCS) and transcranial alternating current stimulation (tACS) involve passing a weak electrical current between two or more electrodes attached to the scalp.Some NBS paradigms such as PAS are based on precise associative timing of stimulations between the primary motor cortex and the periphery, with timings based on animal models of spike timing dependent plasticity, whereas others are based on precise frequency (rTMS) and timing of stimulations (TBS) (Ridding & Ziemann, 2010).The effects of specific NBS paradigms can outlast the stimulation duration from a few minutes to a few hours (Liew et al., 2014).These extended effects of NBS on MEP amplitude are thought to be due to long term potentiation (LTP)-like and long term depression (LTD)-like mechanisms, dependent on N-methyl-D-aspartate (NMDA) receptor function (Bliss & Cooke, 2011;Cooke & Bliss, 2006;Ridding & Ziemann, 2010).Both LTP and LTD are forms of activity dependent synaptic plasticity, seen through the enhancement and reduction of synaptic transmission (Bliss & Cooke, 2011;Magee & Grienberger, 2020).LTP and LTD lead to lasting changes in the human central nervous system (Bliss & Cooke, 2011) and can be induced via high and low frequency NBS, respectively.
There is little consensus surrounding the influence of age on the neuroplastic capacity of the corticomotor system as measured using NBS plasticity paradigms, with only one previous review pooling data exploring the impact of age on PAS (Bhandari et al., 2016).From the two studies pooled (including data from 20 older and 29 younger participants), MEP amplitude change at 10 minutes post PAS was not significantly reduced with age (Hedges g = À0.528,95% CI [À1.157, 0.100], p = 0.099) and study heterogeneity was considerable (I 2 = 89.11).Given the small numbers of included studies in the previous review, and it being limited only to age-related differences following PAS, a follow-up review is warranted.Here we extend Bhandari and colleagues' work by including studies from any available NBS neuroplasticity paradigm, and we include synthesis of studies that include group comparisons between younger and older groups, as well as associations between age and NBSinduced neuroplasticity, regardless of the age range.The latter allows us to circumvent challenges inherent with small age differences between younger and older cohorts (e.g., younger cohort: <50 years; older cohort: !50 years).Increasing our understanding of NBS-induced neuroplasticity in older adults is an important step in furthering the field as it will provide direction for further research to target and optimise interventions that focus on increasing neuroplasticity in older adults.Therefore, the aim of our systematic review and meta-analysis was to summarise and critically appraise the available evidence for the effect of age on the corticospinal neuroplastic response to various NBS paradigms.

Methods
This systematic review and meta-analysis was conducted in accordance with PRISMA guidelines (Page et al., 2021).A protocol was developed a priori and registered with PROSPERO (CRD42017065809).

Eligibility criteria
Studies were included in the review if: they involved healthy human adults (!18 years old) without any physical or psychological impairment; used a NBS paradigm to induce neuroplasticity (i.e., rTMS, iTBS, cTBS, PAS, tDCS, tACS, QPS and iTMS) in the primary motor cortex; and assessed changes in corticospinal excitability via MEPs.Studies were required to either statistically compare corticospinal excitability between young and older participants within the same experimental design, or evaluate the correlation between age and neuroplasticity.
Studies that explored NBS-induced neuroplasticity paradigms in clinical populations were included only if they had also provided necessary data for healthy participants (control group), and only data from the control group were used for analysis.Studies that utilised priming (e.g., exercise or muscle contraction was performed prior to neuroplasticity paradigms) were included if they provided necessary data for an un-primed condition (control group), and only data from the un-primed conditions were used in analyses.
Studies were excluded if they: investigated NBS-induced neuroplasticity in children or adolescents ( 18 years old) or non-human participants; did not use a neuroplasticity paradigm; or used NBS to induce plasticity in cortical areas other than the primary motor cortex (e.g., NBS to posterior parietal cortex).Studies were also excluded if they were published in a language other than English or were editorials, case studies, opinion pieces or reviews.

Information sources
A systematic search was conducted in Medline, Embase, Psy-cInfo and Scopus from inception to July 30, 2021.The search was updated with same search strategy on February 7th, 2023.Key terms relating to NBS, neuroplasticity, and aging and their related subject headings (e.g., MeSH terms specific to database) were used to identify relevant studies.The search strategy was created with assistance from a research librarian.An example Medline search is provided in Table 1, with the search strategy for each of the other databases provided within supplementary material.

Study selection process
Studies identified by the search were uploaded to Endnote (EndNoteTeam, 2013) where duplicates were removed.Studies were then uploaded to Covidence (https://www.covidence.org)where duplicates were again removed automatically.Screening of titles and abstracts of studies was completed on Covidence by two independent reviewers (MS, with either JP, MLM, AR or SS) with irrelevant studies removed.Full texts were then retrieved for the remaining studies, with full eligibility criteria screened by the same two independent reviewers to determine eligibility.Any conflicts were resolved through discussion or, if required, via consultation with a third independent reviewer.

Data collection process
A detailed table was created in Microsoft Excel (Microsoft, 2023) to extract data relating to study details (title, author, year and study design), demographics (number of participants, agegroup, handedness), NBS neuroplasticity paradigm and parameters (stimulation paradigm intensity, duration), use of sham (yes/no), neuroplastic outcomes via TMS parameters (RMT/AMT, post TMS outcome measure points), and TMS data (type of MEP measure (in-dex, normalised, raw), mean and standard deviation of MEPs in each age group, or correlation values between age and MEP).When TMS outcomes were provided only in graphical form, authors were contacted a maximum of two times (within a one-month period) to request raw or summary data.If no response was received or data were unavailable, where possible, Web Plot Digitizer was used to extract data from available graphs, which was then checked for accuracy by one independent reviewer as recommended (Rohatgi, 2022).

Study quality check
The Chipchase et al. ( 2012) critical appraisal tool was used to assess the quality of included studies.This tool contains 26 criteria to evaluate relevant methodological variables for studies utilising TMS.

Synthesis of results
Studies were categorised according to the type of NBS paradigm used (e.g., rTMS, iTBS, etc.).The analyses were conducted in two steps.First, studies within a NBS paradigm (e.g., all studies which applied PAS) were narratively synthesised.Second, when two or more studies used similar NBS paradigms and measured post-NBS outcomes at similar timepoints, meta-analyses were considered (e.g., measures taken at 0-5 min were considered similar).For studies that investigated clinical populations or utilised priming prior to NBS paradigms, only data from healthy control groups and control variable conditions (without priming) were used.Data normalised to baseline were used for meta-analyses.If normalised data were not provided, data synthesis was undertaken, calculating normalised values using the following formula:

ðPostMEP À BaselineMEPÞ BaselineMEP
If a study evaluated two relevant MEP outcome measures within the same time period, participant numbers were halved in the meta-analysis, as recommended by Cochrane collaboration to prevent over-representation (Higgins et al., 2019).Meta-analyses used a random effects inverse variance model using Revman 5.4.1 software.Standardised mean difference (SMD) and 95% confidence intervals were calculated to facilitate overall comparisons across various timepoint outcome measures and across neuroplasticity inducing paradigms.Negative SMD would indicate reduced plasticity in the older cohort.Heterogeneity was evaluated using the I 2 statistic, when an I 2 > 50%, significant heterogeneity was considered present (Higgins et al. (2019); Cochrane Handbook, section 10.10).

Study selection
The database search yielded 9514 records, with hand searching identifying an additional 8 records.After duplicate removal, 7188 articles were screened at title and abstract level, and the resulting 777 articles were screened at full text level.Of these, 39 studies encompassing 40 experiments met eligibility criteria and were included in the review (See Fig. 1).Of the included studies, 19 recruited distinct younger and older cohorts, and 20 evaluated associations between age and NBS induced neuroplasticity.The review summarises data from a total of 496 young adults (mean age 29.28 ± 2.10 years) and 614 older adults (mean age: 65.75 ± 2. 18 years).Of the 39 included studies encompassing 40 experi- ments, 28 utilised excitatory NBS paradigms and 12 utilised inhibitory paradigms.

Risk of bias
Most studies (n = 36) were of moderate to high quality as per the Chipchase checklist (See Fig. 2).Out of a total of 25 points, the average score was 17.92 ± 2.02.There were 12 studies that scored below the average, and only 3 scored one standard deviation below the average (Fried et al., 2019;Motta et al., 2018;Young-Bernier et al., 2014).Most studies did not report if participants had any prior muscle activity, or did not report the level of relaxation of other muscles (non-target) throughout the stimulation.Further, little to no information was provided about participants' attention or arousal levels.Many studies did not provide clear eligibility criteria for their healthy participants, meaning that younger and older groups may have differed on other aspects besides age.In contrast, most studies provided detailed information regarding the type of TMS machine and the stimulation parameters (e.g., coil type, orientation, location, type of stimulator, simulation intensity, hotspot determination, method for determining threshold).

Effect of age on corticomotor excitability induced by PAS
Twelve studies investigated the effects of age on excitatory PAS, with one study also evaluating an inhibitory PAS paradigm (Delvendahl et al., 2012).
Of studies unable to be pooled, Fathi et al. (2010) and Kishore et al. ( 2014) investigated three age groups (younger, middle aged and older adults).Fathi et al. (2009) found no difference in PAS induced neuroplasticity between young and middle-aged cohorts and between middle-aged and older cohorts (Supplementary material 2).Kishore et al. (2014) showed a decrease in PAS induced neuroplasticity in the older cohort compared with the middle-aged cohort across the three time points (5, 15, & 30 min) (Supplementary material 2).Kishore also reported a difference between younger and middle-aged cohorts at 15 and 30 min, however in the opposite direction, whereby the middle-aged cohort saw an increase in plasticity compared to the younger cohort (Supplementary material 2).Minkova et al. (2019), List et al. (2013), and Bhandari et al. (2018) investigated associations between age and response to an excitatory PAS paradigm in an older cohort with limited age ranges (70.2 ± 5.5 years, 63.9 ± 6.2 years and 69.1 ± 8.88 years, respectively).List et al. (2013) and Bhandari et al. (2018) found no association between age and PAS-induced neuroplasticity (p < 0.05, no correlation coefficients provided).Minkova et al. (2019) found a weak, inverse, non-significant relationship between age and PAS-induced neuroplasticity (older age associated with reduced post PAS response, r = -0.22,p = 0.17).Muller-Dahlhaus et al. (2008) also investigated the association between age and response to PAS in a wider age range (22-71 years) and reported an inverse relationship between age and PAS-induced neuroplasticity (r = -0.57,p = 0.002).Delvendahl et al. (2012) investigated a younger cohort, utilising both an excitatory and inhibitory PAS paradigm and found no association with age (for either paradigm p = 0.84, p = 0.67, no correlation coefficients provided).

Effect of age on corticospinal excitability induced by rTMS
Two studies (Bashir et al., 2014;Todd et al., 2010) investigated the effects of inhibitory rTMS neuroplasticity paradigms on healthy adults.Whilst the studies reported a significant decrease in NBSinduced plasticity in the older cohort, when data were normalised to baseline, none of studies showed a reduction in neuroplasticity in older adults compared to the younger cohort (see Supplementary material 2).
Meta-analysis was considered for the studies, however, we decided against meta-analysing due to large discrepancies in the paradigm methodologies and post stimulation timepoints (see Table 2).
Guerra et al ( 2021) investigated the effects of tACS primed by iTBS and found older adults had reduced neuroplasticity (facilitation) compared to younger adults, and greater facilitation induced by iTBS primed tACS than iTBS combined with sham tACS in both groups.The older adult group showed a significant negative correlation with age (r = -0.51,p older = 0.02), while the younger group showed no significant correlation with age (r = -0.08,p young = 0.74).

Effect of age on corticomotor excitability induced by cTBS
Seven studies investigated the effect of age on response to cTBS neuroplasticity paradigms.Only Jannati et al., (2019) compared a younger and older cohort, reporting that older adults exhibited a smaller ratio of change (decrease in MEP amplitude) from baseline compared to younger adults (Supplementary material 2).We were unable to verify these findings (i.e., unable to reach the authorship team of Jannati et al. ( 2019)) and were unable able to extract data from graphs via Web Plot Digitizer.
Two studies investigated associations between age and response to cTBS paradigms in older participants with small age ranges (Smith et al. 2021 (66.
Data from other stimulation intensities and durations reported by Ghasemian-Shirvan et al. (2022) contained mixed results across post-cathodal time measurements.At certain stimulation intensities (1, 2 and 3 mA) administered for various time periods (15, 20 or 30 min) and post-cathodal tDCS timepoints, the older cohort demonstrated an increase in cathodal tDCS-induced neuroplasticity, whilst across others, a decrease in cathodal-tDCS induced neuroplasticity was observed (see Supplementary material 2 for specific stimulation intensity and post cathodal tDCS outcomes).

Effect of age on corticomotor excitability induced by tACS
Two studies investigated the effects of tACS (Fresnoza et al., 2018;Guerra et al., 2021) but were unable to be pooled due to differing neuroplasticity paradigms.Fresnoza et al. (2018) investigated the effect of tACS on healthy younger and older cohorts, and found older adults demonstrated a decrease in tACS-induced neuroplasticity compared to the younger cohort (Supplementary material 2).Guerra et al. (2021) investigated the effects of tACS and found that older adults had similar neuroplasticity (facilitation) compared with younger adults.

Polimanti et al. (2016)
• 3.9.Effect of age on corticomotor excitability induced by QPS Hanajima et al. (2017) was the only study to investigate the effects of age on an emerging patterned rTMS paradigm, QPS.Narrative synthesis of their results indicated older adults had a reduced plasticity response compared to younger adults at all time points from 10 to 25 minutes (Supplementary material 2).

Effect of age on corticomotor excitability induced by iTMS
Only one study by Opie et al (2018) investigated the effects of age on iTMS-induced plasticity.Narrative synthesis of their results indicated the ratio of difference in MEP amplitude post-iTMS to baseline was greater in young compared to old at both 4.1 ms and 4.9 ms ISI iTMS paradigms and all time points (10-30 min), with the exception of the 30 min measure point for 4.9 ms which showed no age differences (Supplementary material 2).

Discussion
This review synthesised the results from 39 studies (encompassing 40 experiments) consisting of eight different experimental NBS plasticity paradigms (PAS, rTMS, iTBS, cTBS, tDCS, tACS, QPS and iTMS) with the aim to determine if age affects the capacity to induce neuroplasticity in healthy humans.By incorporating all available NBS plasticity paradigms targeting the primary motor cortex, our findings significantly extend the previously published review on this topic (Bhandari et al., 2016).Importantly, with the addition of six further studies exploring PAS and by pooling the PAS data separately for post paradigm timepoints (0-5 min, 10-15 min, 20-25 min and 30-35 min), older adults appeared to be less responsive to a PAS plasticity intervention compared to younger adults.Across the eight different experimental paradigms, there were some differences in NBS-induced neuroplasticity, however it is important to note that in 20 studies (out of 39), the older cohort showed minimal to no changes in MEPs post NBS-compared to baseline.Eight out of the 13 studies that utilised inhibitory (or cathodal) paradigms and 12 out of 26 studies that utilised excitatory (or anodal) paradigms supported this finding.It should be noted that even where the pooled analyses suggested older participants had a reduced response to a NBS plasticity paradigm relative to younger participants, heterogeneity of the studies were often high, limiting our confidence in the findings.
Increased age is associated with many changes in both the central and peripheral nervous system.Within the brain these changes include (but are not limited to) decreased grey and white matter volume and widespread cortical thinning across multiple brain regions, including the primary motor cortex (Clark & Taylor, 2011;Dikmeer & Cankurtaran, 2022;Giorgio et al., 2010;Lee & Kim, 2022;Manini et al., 2013;Peters, 2006;Salat et al., 2004;Shaw et al., 2016;Verdú et al., 2000).These age-related anatomical brain changes likely lead to considerable increases in the distance between the TMS coil delivering the stimulation and the underlying brain tissue.Brain atrophy has been estimated at 5% per decade after 40 years of age, and further accelerates after 70 years, albeit with consider intraindividual variability (Peters, 2006).Stokes  Excitatory: A train of nerve electrical stimuli (6x 1 ms at 10 Hz) paired with a single pulse TMS 25 ms after the last stimulus in the train.This was repeated at 10 s intervals for 30 min (180 total pairs).YA n = 61 24.0 ± 3.9 Excitatory: 200 paired stimuli at 0.25 Hz.Electric nerve stimulation at 300% of threshold was applied 25 ms prior to the TMS stimuli applied at intensity required to produce peak-to-peak amplitude of 1 mV.Inhibitory: 200 paired stimuli at 0.25 Hz.Electric nerve stimulation at 300% of threshold was applied 10 ms prior to the TMS stimuli applied at intensity required to produce peak-to-peak amplitude of 1 mV.Excitatory: 600 pairs of stimuli delivered at 5 Hz.Electric nerve stimulation was applied at 250% threshold at 25 ms preceding the TMS stimuli, which was at 90% AMT.
OA n = 30 63.9 ± 6.2 Excitatory: 132 pairs of stimuli delivered at 0.2 Hz.Electric nerve stimulation was applied 300% threshold 25 ms preceding the TMS stimuli which was applied at 130% RMT.
YA n = 20 OA n = 20 22.9 ± 2.5 67.8 ± 2.7 3x 50 Hz TMS pulses repeated at 5 Hz for 2 s (10 bursts in total), repeated every 8 s until 600 stimuli were delivered at 70% RMT.2007) investigated the influence of increased coil-to-cortex distance on motor threshold and found that for an increase of every 1 mm, motor threshold increased 2.8%.Given the intra-individual variability in age-related brain atrophy it is plausible that this increased coil-to-cortex distance, could, in part, explain the variability in responsiveness to NBS neuroplasticity paradigms between young and old participants (Gomes-Osman et al., 2018).Emerging practice in TMS neuroplasticity studies is to adjust stimulation intensity for coil to cortex distance, although none of the studies included in this review did this.Aside from coil to cortex distance, other factors are relevant to consider to adequately disentangle the effects of biological age on responses to NBS neuroplasticity paradigms.Two factors likely to be relevant when comparing age groups are physical activity levels and the role of sex hormones.Physical activity and acute aerobic exercise have been shown to up-regulate neurotrophic and growth factors including brain derived neurotrophic factor (BDNF), which enhances neuronal function, supports neurogenesis, synaptogenesis, and has neuroprotective mechanisms against neurodegenerative diseases (Lin et al., 2018;Pickersgill et al., 2022).In general, older adults are less physically active compared to young (Barreto et al., 2013;Cerin et al., 2013;Lucas et al., 2011;Ribeiro et al., 2013).Only one included study by Smith et al. (2021) considered how daily behaviours across the 24-hour day (physical activity, sleep, and sedentary behaviour) impacted the effectiveness of NBS cTBS plasticity paradigms.Interestingly, more physical activity, and less sedentary behaviour and sleep were associated with greater neuroplasticity in the motor cortex, but age was not associated with the response in 41 participants with a narrow age range (60-75 years).Moreover, none of the included studies reported participant exercise habits immediately prior to the study.As, such we cannot rule out the influence of exercise on study outcomes.Additionally, fluctuating hormones across the course of the menstrual cycle have been linked to differences in the response to NBS plasticity paradigms (Inghilleri et al., 2004).Only (Polimanti et al., 2016) and Tecchio et al. (2008) reported collecting data on their young female participants during the early follicular phase, and reported older female participants were in menopause.Moreover, Polimanti et al., (2016) reported a distinct difference in hormonal regulation of PAS-induced plasticity, which was dependent on age, whereas Tecchio et al. (2008) reported a reduction in PAS-induced plasticity in females, particularly after menopause.Taken together, it is important for studies investigating age-related differences in response to NBS plasticity paradigms to adequately consider and control for important covariates such as physical activity levels and the role sex hormones including presence or absence of menopause.
To have the power to adequately consider and control for covariates, it is abundantly clear that sample sizes across the NBS plasticity field need to increase.Across the included studies, sample sizes ranged from n = 6 per group (Di Lazzaro et al., 2008) to n = 54/53 per group (Hanajima et al., 2017) with the majority of studies having sample sizes between 15 and 20.Only Bhandari et al. (2018) reported using a sample size power calculation.Small samples sizes and low powered studies often miss the true effects of the experiment.Mitra et al. (2019) conducted a meta-analysis on assessing the power of NBS studies given their samples sizes and found that in pursuit of statistically significant results with small sample sizes, NBS studies miss $ 50% of true positive results.Other fields including genomics and medical imaging have bene-fited from a shift in research practice towards larger open source datasets, harmonising of data collection methodologies and an increases in data sharing, through establishment of collaborative consortiums (Marek et al., 2022;Pomponio et al., 2020;Szucs & Ioannidis, 2020;Thompson et al., 2014).To ascertain the true effects of NBS, the field must move towards this approach.Certainly initiatives such as the Big TMS data collaboration are starting to do this and are very much needed (https://www.bigtmsdata.com/).
Our review highlights the ongoing heterogeneity across neuroplasticity inducing NBS paradigms.Even within paradigm subcategories (TBS, PAS, tDCS, rTMS) there is a lack of consensus on parameters, such as RMT thresholds, stimulus intensities, use of AMT vs RMT, frequency of stimuli, as well as post-stimulation assessments.This in turn makes pooling data and comparability across research groups difficult, representing a significant field lim-   itation.Paradigms such as PAS and rTMS that can both increase and decrease corticospinal excitability depending on the stimulating properties and can differ quite extensively.However, they also have common parameters such as stimulus intensity, post paradigm measures (timepoints post paradigm, number of MEPs measured per block) and application properties (i.e., utilising the same TMS coil, having the same researcher taking the participants through all TMS measures and paradigms) which can be standardised to increase consistency across the literature.In rTMS, the two studies differed in stimulus intensity, and post paradigm measures.Similarly, for cTBS, post-stimulation intensity varied across studies.tDCS and tACS studies reported the current stimulation along with duration, but not all studies reported if a ramp up and down was utilised.All studies also varied in post paradigm timing of measures.Standardising application factors such as stimulus intensity, stimuli train frequency, ramp up/down utilisation across NBS paradigm and setting up guidelines for more homogenous application will reduce variability due to the methodology, allow more studies in the space to be pooled, foster a greater environment for collaboration across research groups and increase the reliability, accuracy and validity of NBS as an experimental and clinical tool.Two newer plasticity paradigms were included within this review QPS and iTMS (Hanajima et al., 2017;Opie et al., 2018).Both studies, showed that the age of the participants influence the response to the paradigm, with older participants demonstrating a reduced response.Further replication studies are needed using gold standard methodology to confirm these results.
Several other approaches have emerged to overcome and reduce the heterogeneity problems across the field.These may offer exciting advancements for ageing research.For example, rapid PAS protocols have been developed which induce lasting excitability changes and negate the need for long-paradigms and sustained attentional controls (Quartarone et al., 2006).Other paradigms have utilized the combination of two methodologies electroencephalography (EEG) and TMS together -allowing the real-time millisecond resolution of EEG to trigger TMS (Zrenner et al., 2018).Recently, researchers combining QPS with PAS have demonstrated reduced interindividual variability albeit in healthy young volunteers (Wiratman et al., 2022).These innovative new approaches hold promise for advancing the field of NBS and the effects of ageing and should be explored.

Conclusion
Overall, this systematic review concludes that while a decrease responsiveness to certain NBS plasticity paradigms may be associated with age, there lies a significant amount of variability both within and between studies, further challenging the ability to draw meaningful conclusions from the meta-analysis.These findings highlight the growing necessity of comprehensive reporting, control of potential confounding factors, increasing sample size and standardization of NBS paradigms across research groups.Despite the large amount of variability within, between studies and across paradigms, PAS studies showed the most consistent decrease in NBS induced neuroplasticity in older cohorts.Due to this consistent outcome with PAS, we believe PAS should be further utilised within the field.

Fig. 2 .
Fig. 2. Chipchase et al. (2012) methodological checklist and critical appraisal assessment for included studies.Red Â symbol indicates a score of 0; Green filled symbol indicates a score of 1. y Indicates Guerra et al. (2021) was also included in tACS.

Fig. 3 .
Fig. 3. Meta-analysis comparing plasticity responses (changes in MEP amplitude) following excitatory PAS paradigm in healthy younger and older adults.Subgroup headings depict assessment timepoints post-PAS.
paired stimuli were administered with an interstimulus interval of 4-6 s.Each TMS pulse was paired with a peripheral nerve stimulation applied 25 ms prior to TMS pulse.pairs of stimuli delivered every 5 s for 20 min.Electric nerve stimulation was delivered at 110% threshold, 25 ms prior to the TMS stimuli pairs of stimuli were applied at an interval of 5 s.Electric nerve stimulation was applied at 300% threshold, pairs of stimuli applied at 0.2 Hz over 12 min.Electric nerve stimulation was applied at 300%, 25 ms preceding TMS stimuli.pairs of stimuli applied at 0.2 Hz over 12 min.Electric nerve stimulation was applied 25 ms preceding TMS stimuli.

Fig. 4 .
Fig. 4. Meta-analysis comparing plasticity responses (changes in MEP amplitude) post-iTBS paradigm in healthy young and older adults.Subgroup headings depict assessment timepoints post-iTBS.

Table 1
Medline Search Strategy.

Table 2
Summary of the NBS experimental parameters, participant demographics and post parameter findings included in this review.