Reply to Han et al.

Letter to the EditorReply to Han et al.Mads Alexander Just Madsen, Lasse Christiansen, and Hartwig Roman SiebnerMads Alexander Just MadsenDanish Research Centre for Magnetic Resonance, Department for Radiology and Nuclear Medicine, Copenhagen University Hospital - Amager and Hvidovre, Copenhagen, DenmarkDepartment of Anatomy and Neurosciences, MS Center Amsterdam, Amsterdam Neuroscience, Amsterdam University Medical Center, Location VUmc, Vrije Universiteit Amsterdam, Amsterdam, The Netherlands, Lasse ChristiansenDanish Research Centre for Magnetic Resonance, Department for Radiology and Nuclear Medicine, Copenhagen University Hospital - Amager and Hvidovre, Copenhagen, DenmarkDepartment of Neuroscience, Faculty of Health and Medical Sciences, University of Copenhagen, Copenhagen, Denmark, and Hartwig Roman SiebnerDanish Research Centre for Magnetic Resonance, Department for Radiology and Nuclear Medicine, Copenhagen University Hospital - Amager and Hvidovre, Copenhagen, DenmarkDepartment of Neurology, Copenhagen University Hospital Bispebjerg and Frederiksberg, Copenhagen, DenmarkDepartment of Clinical Medicine, Faculty of Health and Medical Sciences, University of Copenhagen, Copenhagen, DenmarkPublished Online:01 Oct 2025https://doi.org/10.1152/jn.00411.2025MoreSections PDF (305 KB) ToolsExport citationAdd to favoritesTrack citations ShareShare onFacebookXLinkedInWeChat reply: We thank Han et al. (1) for their letter to the editor in which they endorse the results of our sulcus-shape based mapping study comparing the corticomotor representations of single- and paired-pulse transcranial magnetic stimulation (TMS) in healthy participants. We found that the center of gravity (CoG) of short-interval cortical facilitation (SICF) maps was consistently shifted posteriorly relative to single-pulse maps, with a larger shift for the first SICF peak (SICF1) than for the second SICF peak (SICF2) (2). In their letter, Han et al. (1) offer a thoughtful and detailed interpretation of our findings, relating them to another key effect of single-pulse TMS over the primary motor cortex (M1): the generation of descending indirect waves (I-waves) in fast-conducting corticospinal axons (3, 4). I-wave generation in M1 has historically been explained by two contrasting within-M1 intracolumnar models. One model invokes a large recurrent network of simplified neurons (5), whereas the other model is based on a local feedforward cascade onto a detailed single pyramidal cell (6, 7). Han et al. (1) present a series of arguments favoring the latter model, in which I-waves arise from the dendritic arborization of layer-5 pyramidal cells, driven by synchronized transsynaptic excitation of a population of layer-2/3 excitatory and inhibitory neurons (6). Both models include inhibitory interneurons as an integral component. Although their letter raises an important mechanistic question, we contend that several of their arguments rest on incomplete or inaccurate premises. Before addressing their specific points, we wish to clarify two central issues. First, Han et al. (1) note that in our original article (2), we referred to the traditional view of I-wave generation via chains of interneurons. Our study, however, was neither designed nor sufficiently suited to resolve such microscale mechanisms. Therefore, we made no claims about whether interneuronal chains, single pyramidal neurons, or tightly defined columnar circuits are the origin of SICF peaks. Given the spatial resolution of our TMS mapping and the biophysics of TMS, our conclusions concern macroscopic corticomotor representations, not single-cell or microcircuit generators. When we report spatially distinct maps for single- and paired-pulse stimulation, we describe population-level effects arising from converging cortical sources, potentially involving multiple principal cell and interneuron types, rather than a single canonical neuron or specific dendritic sites of pyramidal neurons. A comprehensive perspective on SICF and I-wave generation should also consider contributions from pericentral non-M1 regions projecting to M1, such as adjacent premotor and somatosensory cortex. Surface stimulation of these regions in nonhuman primates has been shown to evoke multiple I-waves, likely through activation of cortico-cortical projections to M1 (8, 9). Second, the commentary by Han et al. (1) appears to be motivated by the phenomenological similarities between SICF and I-waves, particularly in their temporal patterning. Although these similarities are notable, the precise physiological relationship between specific SICF peaks and descending I-waves remains unresolved. It is uncertain which peak corresponds to which I-wave and whether SICF and I-waves are underpinned by the same mechanisms. We therefore caution against assuming direct equivalence between the two phenomena or treating SICF map positions as a reflection of cortical sources of I-wave generation in the classical sense. We respond to each of Han et al.'s (1) arguments point by point as follows: 1) Han et al. (1) make highly specific assumptions about the preferentially targeted cortical site and cell population in the pericentral cortex. With the peri-threshold intensities used [110% and 90% resting motor threshold (RMT)], we agree that our shape-based mapping induced the strongest field in the crown and posterior lip of the precentral gyrus. However, the biphasic pulse also effectively stimulated the anterior lip of the precentral gyrus and both the anterior lip and crown of the postcentral gyrus—meaning three lip regions (posterior precentral, anterior precentral, and anterior postcentral) and two crown regions (precentral and postcentral) were within the targeted volume. Within this volume, bends, branch points, terminals, and diameter changes of myelinated axons are low-threshold sites for activation (4). Computational modeling using fully detailed cell morphologies shows that layer 5 and layer 2/3 pyramidal neurons in the hand area of M1 have similar thresholds, with excitation extending to the top of the crown (10). Pyramidal neurons in the gyral crown are readily activated by fields perpendicular to their main axis, as axonal branches and terminals, along with those of projecting myelinated axons (projecting onto the dendritic tree of pyramidal cells), are oriented in multiple directions. It is therefore untenable to claim that biphasic TMS at the intensities applied in the study by Madsen et al. (2) selectively recruited tangentially oriented layer 5 Betz cells in posterior M1 [Broadmann's area (BA)4p]. Rather, we expect concurrent activation of Betz cells and other pyramidal neurons in anterior M1 (BA4a), dorsal premotor cortex (BA6), and postcentral somatosensory cortex (BA3a, BA2), where pyramidal cells are less strictly tangentially oriented. 2) Han et al. (1) argue that the anterior site of single-pulse TMS is well compatible with a generation locus of single-pulse TMS at the axon initial segment or even the axonal bend of the corticospinal tract. However, this implies that the observed single-pulse response is generated by direct (D)-waves. This proposal seems unlikely given the evidence provided by epidural recordings which do not show D-waves from biphasic TMS with a posterior-anterior or anterior-posterior current direction until much higher stimulation intensities (11). This would also imply a latency shift in the motor evoked potential (MEP), which we did not observe (2). The authors further argue that I-waves originate in the layer 1 dendritic arborizations of layer 5 pyramidal cells and attribute the posterior shift in the center of gravity of the SICF corticomotor map to activation of these dendrites. Although this is an interesting hypothesis, we hesitate to draw such specific microscale conclusions from macroscopic TMS mapping. Given the spatial resolution and biophysical constraints of TMS, the observed posterior CoG shifts are more likely due to combined population-level dynamics than the laminar anatomy of a single cortical column. 3) Han et al. (1) postulate that both TMS pulses used to probe SICF excite the whole dendritic arborization, producing successive I-waves. We believe that it is more plausible to assume that TMS-induced stimulation of presynaptic axons only excites a fraction rather than the entire dendritic tree. We therefore propose that the second pulse also acted on presynaptic axonal elements that had been partially depolarized by the first pulse, producing strong temporal summation of excitatory post-synaptic potentials (EPSPs), recruiting additional dendritic elements that had not been activated by the first pulse. This notion is supported by the time-constants of the partially depolarized neuronal membranes of pyramidal cells, which are longer than the interstimulus intervals applied in our study (1–3 ms) (12, 13), but see Ref. 14. Furthermore, the spatial dendritic recruitment pattern induced by the two TMS pulses can be expected to differ between the first and second pulse because of intrinsic stochastic fluctuations and conditioning effects of the first pulse, resulting in differences in axonal threshold, refractory recovery, or facilitation of excitability. In our protocol, we used a suprathreshold pulse (S1 at 110% RMT) followed by a subthreshold conditioning pulse (S2 at 90% RMT) to elicit SICF. This difference in stimulus intensity between the first and second pulse additionally contributed to the excitation of different cortical elements by the first and second TMS pulses. We therefore interpret the paired-pulse effects as arising from a combination of presynaptic, postsynaptic, and population recruitment mechanisms, which go beyond reexcitation of the same presynaptic inputs via a single obligatory process. 4) Han et al. (1) mention degeneration of layer 1 dendritic arborizations in amyotrophic lateral sclerosis (ALS) and associated prolongation of SICF latency as the evidence supporting their hypothesis. Although interesting, degeneration of apical dendrites would likely reduce later SICF peak amplitudes, rather than delay conduction. Moreover, it should be noted that corticomotor pathology of ALS is complex, involving loss of inhibitory interneurons and broader network changes (15, 16). Thus, clinical SICF changes in ALS can be explained by multiple competing hypotheses and do not uniquely support a single-cell dendrite model over population-level interpretations. In summary, we thank the authors for engaging with our work and providing the opportunity to clarify our findings. Our results show that single- and paired-pulse TMS recruit overlapping, but spatially distinct corticomotor representations at the macroscopic level. However, they offer no basis for attributing the observed differences in CoG to specific cortical cell types, such as inhibitory interneurons, given the inherent resolution limits of TMS and its engagement of diverse local and cortico-cortical circuits. Although parsimonious computational models are valuable, we argue that single-neuron columnar models likely oversimplify the complex mechanisms underlying TMS responses. Innovative neurophysiological approaches capable of resolving the spatiotemporal cascades of neural events evoked by TMS in the human cortex are urgently needed to produce biologically validated models that link noninvasive signatures of cortical excitability, such as SICF, to their generators at the cellular and local circuit levels.GRANTS This work was supported by the Novo Nordisk Foundation Interdisciplinary Synergy Program ("Biophysically adjusted state-informed cortex stimulation; Grant No. NNF14OC0011413). M.A.J.M. is supported by the European committee for treatment and research in multiple sclerosis and the European magnetic resonance imaging in multiple sclerosis network (ECTRIMS-MAGNIMS) research fellowship and the Danish Multiple Sclerosis Society (Grant No. A44804). L.C. holds a personal grant from the Lundbeck Foundation (Grant No. R322-2019-2406). H.R.S. received funding as principal investigator for the project "Precision Brain-Circuit Therapy - Precision-BCT" from Innovation Funds Denmark (Grant No. 9068-00025B) and the project "ADAptive and Precise Targeting of cortex-basal ganglia circuits in Parkinson's Disease - ADAPT-PD" from Lundbeckfonden (collaborative project grant, Grant No. R336-2020-1035).DISCLAIMERS The content and any opinions expressed in this article are those of the authors and do not necessarily represent the views of the American Physiological Society.DISCLOSURES M.A.J.M. serves on the board of the Danish Society for Research in Multiple Sclerosis (DAREMUS). H.R.S. has received honoraria as speaker and consultant from Lundbeck AS, Denmark, and as editor (Neuroimage Clinical) from Elsevier Publishers, Amsterdam, The Netherlands. He has received royalties as book editor from Springer Publishers, Stuttgart, Germany; Oxford University Press, Oxford, UK; and Gyldendal Publishers, Copenhagen, Denmark. None of the other authors has any conflicts of interest, financial or otherwise, to disclose.AUTHOR CONTRIBUTIONS M.A.J.M. drafted manuscript; M.A.J.M., L.C., and H.R.S. edited and revised manuscript; M.A.J.M., L.C., and H.R.S. approved final version of manuscript.REFERENCES1. Han T, Paulus W, Möhwald K. Posterior shifts of I-waves generated in tangentially oriented pyramidal cells of the motor cortex. J Neurophysiol 134: 815–816, 2025. doi:10.1152/jn.00234.2025. Link | Web of Science | Google Scholar2. Madsen MAJ, Christiansen L, Chung C, Jonsson MG, Siebner HR. Single and paired TMS pulses engage spatially distinct corticomotor representations in human pericentral cortex. J Neurophysiol 133: 1423–1434, 2025. doi:10.1152/jn.00459.2024. Link | Web of Science | Google Scholar3. Di Lazzaro V, Profice P, Ranieri F, Capone F, Dileone M, Oliviero A, Pilato F. I-wave origin and modulation. Brain Stimul 5: 512–525, 2012. doi:10.1016/j.brs.2011.07.008. 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Previous Back to Top Download PDF FiguresReferencesRelatedInformation Related ArticlesSingle and paired TMS pulses engage spatially distinct corticomotor representations in human pericentral cortex 30 Apr 2025Journal of NeurophysiologyPosterior shifts of I-waves generated in tangentially oriented pyramidal cells of the motor cortex 25 Aug 2025Journal of Neurophysiology More from this issue > Volume 134Issue 4October 2025Pages 1115-1117 Crossmark Copyright & PermissionsCopyright © 2025 The Authors.Licensed under Creative Commons Attribution CC-BY-NC 4.0. Published by the American Physiological Society.https://doi.org/10.1152/jn.00411.2025History Received 18 August 2025 Accepted 11 September 2025 Published online 1 October 2025 Published in print 1 October 2025 Keywordscortical mappingI-wavesshort interval intracortical facilitationsomatotopytranscranial magnetic stimulation Metrics