Spatial localization and distribution of the TMS-related 'hotspot' of the tibialis anterior muscle representation in the healthy and post-stroke motor cortex

Transcranial magnetic stimulation (TMS) is a type of noninvasive brain stimulation used to study corticomotor excitability of the intact and injured brain. Identification of muscle representations in the motor cortex is typically done using a procedure called 'hotspotting', which involves establishing the optimal location on the scalp that evokes a maximum TMS response with minimum stimulator intensity. The purpose of this study was to report the hotspot locations for the tibialis anterior (TA) muscle representation in the motor cortex of healthy and post stroke individuals. A retrospective data analyses from 42 stroke participants and 32 healthy participants was conducted for reporting TMS hotspot locations and their spatial patterns. Single pulse TMS, using a 110 mm double cone coil, was used to identify the motor representation of the TA. The hotspot locations were represented as x and y-distances from the vertex for each participant. The mediolateral extent of the loci from the vertex (x-coordinate) and anteroposterior extent of the loci from the vertex (y-coordinate) was reported for each hemisphere: non-lesioned (XNLes, YNLes), lesioned (XLes, YLes) and healthy (XH, YH). We found that the mean hotspot loci for TA muscle from the vertex were approximately: 1.29 cm lateral and 0.55 cm posterior in the non-lesioned hemisphere, 1.25 cm lateral and 0.5 cm posterior in the lesioned hemisphere and 1.6 cm lateral and 0.8 cm posterior in the healthy brain. There was no significant difference in the x- and y-coordinates between the lesioned and non-lesioned hemispheres. However, the locations of the XNLes (p = 0.01) and XLes (p = 0.004) were significantly different from XH. The YNLes and YLes showed no significant differences from YH loci. Analyses of spatial clustering patterns using the Moran's I index showed a negative autocorrelation in stroke participants (NLes: Moran's I = −0.09, p < 0.001; Les: Moran's I = −0.14, p = 0.002), and a positive autocorrelation in healthy participants (Moran's I = 0.16, p < 0.001), suggesting that individuals with stroke demonstrated a more dispersed pattern of hotspot locations than healthy individuals. Our results suggest that the hotspot loci show different spatial patterns in healthy and stroke individuals. The hotspot locations from this study has the potential to provide a guideline for optimal stimulation locations for the TA muscle in healthy and post stroke individuals for neuromodulation procedures such as transcranial direct current stimulation.