Changes in cortical plasticity induced by paired associative stimulation in people with mild cognitive impairment and Alzheimer's disease: a systematic review and meta-analysis.

BACKGROUND

Paired associative stimulation (PAS) is a non-invasive neuromodulation paradigm capable of inducing long-term potentiation (LTP)- or long-term depression (LTD)-like plasticity. It has been used to probe neuroplasticity and corticospinal excitability alterations in Alzheimer's disease (AD) and mild cognitive impairment (MCI). However, existing studies report inconsistent directions and magnitudes of PAS-induced plasticity changes across the AD continuum.

METHODS

We searched PubMed, Web of Science, Embase, and the Cochrane Library from database inception to December 2025. Eligible studies were case-control studies and randomized controlled trials (RCTs) that assessed PAS-induced cortical plasticity in individuals with AD or MCI, with healthy older adults as controls. Study quality was evaluated using the Newcastle-Ottawa Scale (NOS) for non-randomized studies and the Cochrane Risk of Bias tool (RoB 2) for RCTs. This review was registered in PROSPERO (CRD420251178441).

RESULTS

Five studies met the inclusion criteria. Four studies quantified motor cortical plasticity using changes in motor-evoked potential (MEP) amplitude (total N = 135; 64 MCI/AD and 71 controls). The pooled analysis showed no significant difference in PAS-induced MEP changes between participants with MCI/AD and healthy controls [mean difference (MD) = 0.06, 95% CI (-0.05, 0.17), p = 0.32]. This pooled estimate was restricted to MEP-based outcomes and should be interpreted as an exploratory synthesis of motor-system readouts rather than definitive evidence of preserved cortical plasticity in MCI or AD. The remaining RCT, which assessed dorsolateral prefrontal cortex (DLPFC) plasticity using a repetitive PAS (rPAS) intervention without MEP outcomes, also found no significant improvement in prefrontal plasticity relative to control stimulation.

CONCLUSION

Based on the currently available MEP-based evidence, PAS-induced motor-system plasticity findings in individuals with MCI or AD remain inconclusive. Given the limited number of studies and heterogeneity in experimental designs, the pooled negative results should not be interpreted as evidence that cortical plasticity is preserved. Corticospinal hyperexcitability and network-level PAS findings should be regarded as preliminary, hypothesis-generating observations requiring validation in larger, longitudinal, and biomarker-characterized cohorts.