Cortico-cortical paired associative stimulation highlights asymmetrical communication between rostral premotor cortices and primary motor cortex

cortico-cortical paired associative stimulation (ccPAS) is a transcranial magnetic stimulation (TMS) protocol designed to mimic neurostimulation patterns capable of inducing spike-timing-dependent plasticity (STDP). This protocol, based on the Hebbian principle, entails coupling of pre-and post-synaptic activity through TMS, targeting two interconnected brain areas. By tailoring stimulation parameters to the characteristics of the target pathway, ccPAS can modulate its connectivity strength [1Buch E.R. Johnen V.M. Nelissen N. O'Shea J. Rushworth M.F.S. Noninvasive associative plasticity induction in a corticocortical pathway of the human brain.J Neurosci. 2011; 31: 17669-17679Crossref PubMed Scopus (103) Google Scholar, 2Chiappini E. et al.Driving associative plasticity in premotor-motor connections through a novel paired associative stimulation based on long-latency cortico-cortical interactions.Brain Stimul. 2020; 13: 1461-1463Abstract Full Text Full Text PDF PubMed Scopus (25) Google Scholar, 3Johnen V.M. et al.Causal manipulation of functional connectivity in a specific neural pathway during behaviour and at rest.Elife. 2015; 4e04585Crossref PubMed Scopus (51) Google Scholar] and induce functional changes [[4]Fiori F. Chiappini E. Avenanti A. Enhanced action performance following TMS manipulation of associative plasticity in ventral premotor-motor pathway.Neuroimage. 2018; 183: 847-858Crossref PubMed Scopus (41) Google Scholar,[5]Turrini S. et al.Transcranial cortico-cortical paired associative stimulation (ccPAS) over ventral premotor-motor pathways enhances action performance and corticomotor excitability in young adults more than in elderly adults.Front Aging Neurosci. 2023; 15Crossref PubMed Scopus (10) Google Scholar]. Understanding the behavioral and physiological impact of ccPAS manipulation over different networks is key to developing clinical interventions. We have recently demonstrated that ccPAS over the ventral premotor cortex (PMv) and primary motor cortex (M1) induces changes in motor-evoked potentials (MEPs) [3Johnen V.M. et al.Causal manipulation of functional connectivity in a specific neural pathway during behaviour and at rest.Elife. 2015; 4e04585Crossref PubMed Scopus (51) Google Scholar, 4Fiori F. Chiappini E. Avenanti A. Enhanced action performance following TMS manipulation of associative plasticity in ventral premotor-motor pathway.Neuroimage. 2018; 183: 847-858Crossref PubMed Scopus (41) Google Scholar, 5Turrini S. et al.Transcranial cortico-cortical paired associative stimulation (ccPAS) over ventral premotor-motor pathways enhances action performance and corticomotor excitability in young adults more than in elderly adults.Front Aging Neurosci. 2023; 15Crossref PubMed Scopus (10) Google Scholar, 6Turrini S. et al.Gradual enhancement of corticomotor excitability during cortico-cortical paired associative stimulation.Sci Rep. 2022; 1214670Crossref Scopus (12) Google Scholar, 7Turrini S. et al.Cortico-cortical paired associative stimulation (ccPAS) over premotor-motor areas affects local circuitries in the human motor cortex via Hebbian plasticity.Neuroimage. 2023; 120027https://doi.org/10.1016/j.neuroimage.2023.120027Crossref PubMed Scopus (11) Google Scholar]. Forward PMv→M1 ccPAS, with subthreshold PMv conditioning, followed by suprathreshold M1 stimulation 8 ms later (ccPASPMv-M1), led to a gradual corticospinal excitability (CSE) increase during protocol administration, reflecting a progressive efficacy increase of excitatory PMv inputs to M1 via STDP [[7]Turrini S. et al.Cortico-cortical paired associative stimulation (ccPAS) over premotor-motor areas affects local circuitries in the human motor cortex via Hebbian plasticity.Neuroimage. 2023; 120027https://doi.org/10.1016/j.neuroimage.2023.120027Crossref PubMed Scopus (11) Google Scholar]. Conversely, reversed stimulation order during ccPAS (ccPASM1-PMv), expected to hinder connectivity between the two nodes [[1]Buch E.R. Johnen V.M. Nelissen N. O'Shea J. Rushworth M.F.S. Noninvasive associative plasticity induction in a corticocortical pathway of the human brain.J Neurosci. 2011; 31: 17669-17679Crossref PubMed Scopus (103) Google Scholar], tended to decrease CSE [[6]Turrini S. et al.Gradual enhancement of corticomotor excitability during cortico-cortical paired associative stimulation.Sci Rep. 2022; 1214670Crossref Scopus (12) Google Scholar]. While ccPAS has been applied to other premotor-motor networks [[8]Arai N. et al.State-dependent and timing-dependent bidirectional associative plasticity in the human SMA-M1 network.J Neurosci. 2011; 31: 15376-15383Crossref PubMed Scopus (101) Google Scholar], it remains unclear whether similar time-specific bidirectional effects characterize networks other than the PMv-M1. To clarify this issue, here, we compared 4 different ccPAS protocols targeting two premotor-motor circuits, i.e., the PMv-M1 and the supplementary motor area (SMA)-M1 pathways, and tested their physiological and behavioral effects (Fig. 1A). In 60 healthy adults, we administered 90 paired-pulses at a 0.1-Hz frequency, employing an 8-ms inter-stimulus interval (ISI), suitable for both PMv-M1 and SMA-M1 cortico-cortical interactions [[1]Buch E.R. Johnen V.M. Nelissen N. O'Shea J. Rushworth M.F.S. Noninvasive associative plasticity induction in a corticocortical pathway of the human brain.J Neurosci. 2011; 31: 17669-17679Crossref PubMed Scopus (103) Google Scholar,[9]Rurak B.K. Rodrigues J.P. Power B.D. Drummond P.D. Vallence A.-M. Reduced SMA-M1 connectivity in older than younger adults measured using dual-site TMS.Eur J Neurosci. 2021; 54: 6533-6552Crossref PubMed Scopus (8) Google Scholar]. We tested different ccPAS configurations, following the factorial combination of the targeted premotor Area (PMv; SMA) and stimulation Direction (“forward” premotor-M1; “reverse” M1-premotor), resulting in 4 groups of 15 participants each: ccPASPMv-M1, ccPASSMA-M1, ccPASM1-PMv, and ccPASM1-SMA (Fig. 1B; Supplementary Methods). We examined i) the online effect on MEPs recorded during the ccPAS protocol; ii) the impact of ccPAS on a choice reaction time (cRT) task before (pre), immediately (T0) and 30 minutes after the end of the ccPAS (T30) (Fig. 1a); and iii) the relationship between the observed neurophysiological and behavioral effects. We divided the 90 MEPs collected during the administration of ccPAS into 6 consecutive epochs, each containing 15 MEPs. Subsequently, we analyzed MEP amplitudes with an Area*Direction*Epoch general linear model (GLM), which revealed a significant 3-way interaction (F5,280 = 5.12;p < 0.001;ηp [[2]Chiappini E. et al.Driving associative plasticity in premotor-motor connections through a novel paired associative stimulation based on long-latency cortico-cortical interactions.Brain Stimul. 2020; 13: 1461-1463Abstract Full Text Full Text PDF PubMed Scopus (25) Google Scholar] = 0.08). Post-hoc analyses emphasized distinct patterns within the two targeted networks (see Fig. 1c for post-hoc comparisons). In the PMv-M1 pathway, we observed strong bidirectional effects, resulting in a gradual CSE increase during ccPASPMv-M1 and a decline during ccPASM1-PMv (Epoch-1 vs 6 comparisons: all p ≤ 0.001). In contrast, targeting the SMA-M1 circuit produced an increase in excitability regardless of the stimulation direction (Epoch-1 vs 6 comparisons: all p ≤ 0.05). Indeed, the ccPASPMv-M1, ccPASSMA-M1, and ccPASM1-SMA groups exhibited a comparable linear increase in MEPs, with no significant difference across time points (all p ≥ 0.18), while the ccPASM1-PMv group displayed the opposite pattern, differing significantly from all other groups at the end of the protocol (all p ≤ 0.03). Our findings point to dissociable features of the two targeted premotor-motor networks. Manipulating the PMv-M1 pathway with the same 8-ms ISI induced modulations of CSE with comparable magnitude but opposite directions depending on the order of the paired TMS pulses during ccPAS [[6]Turrini S. et al.Gradual enhancement of corticomotor excitability during cortico-cortical paired associative stimulation.Sci Rep. 2022; 1214670Crossref Scopus (12) Google Scholar]. This suggests that excitatory interactions that govern PMv influences on M17 can be enhanced or decreased by modulating the strength of the PMv input to M1, supporting the idea of a symmetrical organization and temporal features of the pathways between PMv and M1. Conversely, our study points to the distinctiveness of the SMA-M1 network, showing no such bidirectional modulations using the same 8-ms ISI. Paired-stimulation during ccPASSMA-M1 relied on an excitatory influence of SMA over M1 [[8]Arai N. et al.State-dependent and timing-dependent bidirectional associative plasticity in the human SMA-M1 network.J Neurosci. 2011; 31: 15376-15383Crossref PubMed Scopus (101) Google Scholar,[9]Rurak B.K. Rodrigues J.P. Power B.D. Drummond P.D. Vallence A.-M. Reduced SMA-M1 connectivity in older than younger adults measured using dual-site TMS.Eur J Neurosci. 2021; 54: 6533-6552Crossref PubMed Scopus (8) Google Scholar] and our study supports the idea that the protocol gradually enhanced excitatory SMA input to M1. Interestingly, we show a similar increase in CSE during ccPASM1-SMA using the 8-ms ISI, whereas a prior study showed non-significant facilitation and inhibition with a 10-ms and 15-ms ISI during ccPASM1-SMA8. Together with the evidence that TMS of the M1 elicits activity peaking at about 30 ms over the medial frontal cortex [[10]Leodori G. et al.Motor cortical network excitability in Parkinson's disease.Movem Disord. 2022; 37: 734-744Crossref PubMed Scopus (16) Google Scholar], these findings suggest that M1-SMA interactions could occur on a different timeframe compared to other directional pathways tested in this study. Reduced CSE following ccPASM1-SMA with a longer-latency (15-ms) ISI [[8]Arai N. et al.State-dependent and timing-dependent bidirectional associative plasticity in the human SMA-M1 network.J Neurosci. 2011; 31: 15376-15383Crossref PubMed Scopus (101) Google Scholar] aligns with the principles of STDP if one assumes M1-SMA interactions to recruit a slower, likely indirect, circuit. In contrast, shorter ISIs are found to be either ineffective (ISI = 10 ms8) or facilitatory (ISI = 8 ms; this study). This indicates that distinct mechanisms govern the interactions within the M1→PMv and M1→SMA networks as observed during reverse ccPAS: while ccPASM1-PMv acts by progressively weakening excitatory PM→M1 projections, resulting in a decreased CSE, ccPASM1-SMA induces a progressive increase in CSE, which may suggest that enhancing M1 input to SMA may either progressively attenuate SMA inhibitory influences on CSE or potentiate excitatory influences. At the behavioral level, the Area*Direction*Time GLM on inverse efficacy (IE, i.e., reaction times divided by task accuracy) index showed a general improvement in performance over time (F2,112 = 16.47;p < 0.0001;ηp [[2]Chiappini E. et al.Driving associative plasticity in premotor-motor connections through a novel paired associative stimulation based on long-latency cortico-cortical interactions.Brain Stimul. 2020; 13: 1461-1463Abstract Full Text Full Text PDF PubMed Scopus (25) Google Scholar] = 0.23), but no influence of the targeted Area (all p > 0.99) or ccPAS Direction (all p ≥ 0.35). Nonetheless, a further GLM tested the relation between behavioral and physiological changes and revealed that greater ccPAS-induced MEP facilitations predicted reduced performance gains in the cRT task, but exclusively for the two forward groups (ccPASPMv-M1, ccPASSMA-M1; F1,56 = 6.39;p = 0.014;ηp [[2]Chiappini E. et al.Driving associative plasticity in premotor-motor connections through a novel paired associative stimulation based on long-latency cortico-cortical interactions.Brain Stimul. 2020; 13: 1461-1463Abstract Full Text Full Text PDF PubMed Scopus (25) Google Scholar] = 0.10;adjR [[2]Chiappini E. et al.Driving associative plasticity in premotor-motor connections through a novel paired associative stimulation based on long-latency cortico-cortical interactions.Brain Stimul. 2020; 13: 1461-1463Abstract Full Text Full Text PDF PubMed Scopus (25) Google Scholar] = 0.092;Fig. 1d; see Supplementary Material). According to previous premotor-M1 ccPAS studies, forward ccPAS configurations increase connectivity within the targeted circuit [1Buch E.R. Johnen V.M. Nelissen N. O'Shea J. Rushworth M.F.S. Noninvasive associative plasticity induction in a corticocortical pathway of the human brain.J Neurosci. 2011; 31: 17669-17679Crossref PubMed Scopus (103) Google Scholar, 2Chiappini E. et al.Driving associative plasticity in premotor-motor connections through a novel paired associative stimulation based on long-latency cortico-cortical interactions.Brain Stimul. 2020; 13: 1461-1463Abstract Full Text Full Text PDF PubMed Scopus (25) Google Scholar, 3Johnen V.M. et al.Causal manipulation of functional connectivity in a specific neural pathway during behaviour and at rest.Elife. 2015; 4e04585Crossref PubMed Scopus (51) Google Scholar,[8]Arai N. et al.State-dependent and timing-dependent bidirectional associative plasticity in the human SMA-M1 network.J Neurosci. 2011; 31: 15376-15383Crossref PubMed Scopus (101) Google Scholar] while simultaneously decreasing connectivity in parallel and competing pathways to M13. Assuming this, we speculate that both ccPASSMA-M1 and ccPASPMv-M1 would entail reduced connectivity between the dorsal premotor cortex and M1, whose interaction is crucial for cRT performance, accounting for the observed relationship between increased SMA-M1/PMv-M1 connectivity and reduced gains in cRT. In sum, our ccPAS results highlight different physiological mechanisms underlying premotor-motor pathways. The PMv-M1 network exhibits opposite plastic effects (i.e., excitatory and inhibitory) depending on paired-stimulation order [[7]Turrini S. et al.Cortico-cortical paired associative stimulation (ccPAS) over premotor-motor areas affects local circuitries in the human motor cortex via Hebbian plasticity.Neuroimage. 2023; 120027https://doi.org/10.1016/j.neuroimage.2023.120027Crossref PubMed Scopus (11) Google Scholar], while the SMA-M1 shows enhanced excitability, irrespective of stimulation direction. Work supported by #NEXTGENERATIONEU (NGEU) and funded by the Ministry of University and Research (MUR), National Recovery and Resilience Plan (NRRP), project MNESYS (PE0000006) A Multiscale integrated approach to the study of the nervous system in health and disease (DN. 1553 October 11, 2022). This work was also supported by FISM – Fondazione Italiana Sclerosi Multipla (2022/R-Single/071) and financed or co-financed with the ‘5‰’ public funding, and by grants from the Bial Foundation (304/2022), Fondazione del Monte di Bologna e Ravenna (1402bis/2021), Universidad Católica Del Maule (CDPDS2022) awarded to Alessio Avenanti.