Paired associative stimulation (PAS) protocols can be used to induce Hebbian plasticity in the human brain. A modified, cross-modal version, of the PAS (cross-modal PAS, cm-PAS) has been recently developed. The cm-PAS consists in the repetitive pairings of a transcranial magnetic stimulation (TMS) pulse over the primary somatosensory cortex (S1) and a visual stimulus depicting a hand being touched; a 20 ms of inter-stimulus interval (ISI) is required to affect S1 plasticity, in turn modulating tactile acuity and somatosensory evoked potentials. The present study explores the role of anticipatory simulation in the cm-PAS efficacy, which could be responsible for such a short ISI. To this aim, we compared the effect of the original, fixed-frequency, cm-PAS to that of a jittered version, in which the time interval between trials was not steady but jittered, hence avoiding the anticipation of the upcoming visual-touch stimulus. Moreover, in the jittered PAS, the ISI between the paired stimulations was varied: it could match the early, somatosensory-driven, activation of S1 (20 ms), or the mirror recruitment of S1 by touch observation (150 ms). Results showed that tactile acuity is enhanced by the fixed-frequency cm-PAS, with an ISI of 20 ms between paired stimulation (visual-touch stimulus and TMS pulse over S1), and also by the jittered cm-PAS but only if the ISI is of 150 ms. These findings suggest that the cm-PAS with a jittered frequency, by preventing an anticipatory pre-activation of S1, delays the timing of the interaction between the visual-touch stimulus and the cortical pulse. On a broader perspective, our study highlights the possible involvement of sensory anticipation, likely through mirror-like simulation mechanisms, in tactile mirroring, as well as its influence of the optimal interval between the afferent and the magnetic pulse during PAS protocols.

中文翻译：

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触摸预期介导初级体感皮层中的跨模式Hebbian可塑性。

配对联想刺激（PAS）协议可用于诱导人脑中的Hebbian可塑性。最近开发了PAS的修改过的交叉模式版本（交叉模式PAS，cm-PAS）。cm-PAS包含重复的配对：在主要体感皮层（S1）上的经颅磁刺激（TMS）脉冲和描述被触摸的手的视觉刺激；需要20毫秒的刺激间隔（ISI）来影响S1的可塑性，进而调节触觉敏锐度和体感诱发电位。本研究探讨了预期模拟在cm-PAS功效中的作用，这可能是造成如此短的ISI的原因。为此，我们将原始的固定频率cm-PAS与抖动版本的效果进行了比较，试验之间的时间间隔不是稳定的而是不稳定的，因此避免了即将到来的视觉触摸刺激的预期。此外，在抖动的PAS中，成对刺激之间的ISI有所不同：它可以匹配体感驱动的早期S1激活（20 ms），或者通过触摸观察（150 ms）恢复S1的镜像。结果表明，固定频率的cm-PAS增强了触觉敏锐度，配对刺激（视觉触摸刺激和S1上的TMS脉冲）之间的ISI为20 ms，并且cm-PAS抖动了，但仅当ISI是150毫秒。这些发现表明，具有抖动频率的cm-PAS通过防止S1的预激活而延迟了视觉触摸刺激与皮层脉冲之间相互作用的时间。从更广泛的角度来看，