Locating primary somatosensory cortex in human brain stimulation studies: experimental evidence
Authors: Silvia Zeni
Alexander Stuart
Mariyana Rakova
Paisley Beeching
Mary Medford
Nicholas Paul Holmes
Luigi Tamè
Journal: Journal of Neurophysiology (2019)
Discipline: NEUROSCIENCE & BEHAVIOR
Keywords: S1; SI; TDCS; TMS; vibrotactile.
Affiliations: School of Psychology, University of Nottingham, University Park , Nottingham , United Kingdom., Department of Psychological Sciences, Birkbeck University of London , London , United Kingdom., School of Psychology and Clinical Language Sciences, University of Reading , Reading , United Kingdom.

Abstract:  Transcranial magnetic stimulation (TMS) over human primary somatosensory cortex (S1) does not produce immediate outputs. Researchers must therefore rely on indirect methods for TMS coil positioning. The "gold standard" is to use individual functional and structural magnetic resonance imaging (MRI) data, but the majority of studies don't do this. The most common method to locate the hand area of S1 (S1-hand) is to move the coil posteriorly from the hand area of primary motor cortex (M1-hand). Yet, S1-hand is not directly posterior to M1-hand. We localized the index finger area of S1-hand (S1-index) experimentally in four ways. First, we reanalyzed functional MRI data from 20 participants who received vibrotactile stimulation to their 10 digits. Second, to assist the localization of S1-hand without MRI data, we constructed a probabilistic atlas of the central sulcus from 100 healthy adult MRIs and measured the likely scalp location of S1-index. Third, we conducted two experiments mapping the effects of TMS across the scalp on tactile discrimination performance. Fourth, we examined all available neuronavigation data from our laboratory on the scalp location of S1-index. Contrary to the prevailing method, and consistent with systematic review evidence, S1-index is close to the C3/C4 electroencephalography (EEG) electrode locations on the scalp, ~7-8 cm lateral to the vertex, and ~2 cm lateral and 0.5 cm posterior to the M1-hand scalp location. These results suggest that an immediate revision to the most commonly used heuristic to locate S1-hand is required. The results of many TMS studies of S1-hand need reassessment. NEW &amp; NOTEWORTHY Noninvasive human brain stimulation requires indirect methods to target particular brain areas. Magnetic stimulation studies of human primary somatosensory cortex have used scalp-based heuristics to find the target, typically locating it 2 cm posterior to the motor cortex. We measured the scalp location of the hand area of primary somatosensory cortex and found that it is ~2 cm lateral to motor cortex. Our results suggest an immediate revision of the prevailing method is required. 

Retracted: No

PubPeer comments:
 This paper was criticised as part of a self-reflection posted on Twitter on 2nd-3rd April, 2021 (twitter TheHandLab status 1377760796130869248) Criticism: This is my best work, with Dr Luigi Tame many others. I know it s improvable, but it s too early to see more problems. E1 was underpowered messy (BSc project!); the fMRI data were not strong or high-res enough. ,