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Neuropsychologia. 2017 Feb;96:9-18. doi: 10.1016/j.neuropsychologia.2016.12.030. Epub 2016 Dec 29.

Pre-stimulus alpha oscillations over somatosensory cortex predict tactile misperceptions.

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University of Leeds, Leeds, UK.
University of Manchester, Manchester, UK.
University of Manchester, Manchester, UK; University of Valparaiso, Valparaiso, Chile.
University of Leeds, Leeds, UK. Electronic address:


Fluctuations of pre-stimulus oscillatory activity in the somatosensory alpha band (8-14Hz) observed using human EEG and MEG have been shown to influence the detection of supra- and peri-threshold somatosensory stimuli. However, some reports of touch occur even without a stimulus. We investigated the possibility that pre-stimulus alpha oscillations might also influence these false reports of touch - known as tactile misperceptions. We recorded EEG while participants performed the Somatic Signal Detection Task (SSDT), in which participants must detect brief, peri-threshold somatosensory targets. We found that pre-stimulus oscillatory power in the somatosensory alpha range exhibited a negative linear relationship with reporting of touch at electrode clusters over both contralateral and ipsilateral somatosensory regions. As pre-stimulus alpha power increased, the probability of reporting a touch declined; as it decreased, the probability of reporting a touch increased. This relationship was stronger on trials without a somatosensory stimulus than on trials with a somatosensory stimulus, although was present for both trial types. Spatio-temporal cluster-based permutation analysis also found that pre-stimulus alpha was lower on trials when touch was reported - irrespective of whether it was present - over contralateral and ipsilateral somatosensory cortices, as well as left frontocentral areas. We argue that alpha power may reflect changes in response criterion rather than sensitivity alone. Low alpha power relates to a low barrier to reporting a touch even when one is not present, while high alpha power is linked to less frequent reporting of touch overall.


Alpha oscillations; EEG; Perception; Pre-stimulus; Signal detection; Somatosensory

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