Topography and elimination of slow EEG responses related to tongue movements.

This study aimed to characterize the topography and mechanism of slow EEG potentials related to tongue movement (TMP), and to devise a method for their elimination. Eight adult subjects were recorded with 8-channel direct current (DC) EEG (n = 5) or with a 256-channel, dense array EEG (n = 4). Subjects were requested to push their tongue toward the incisors in a closed mouth. We examined the time course and topography of the ensuing TMP. The underlying mechanism and elimination of TMP were studied by electrical isolation of the tongue with a latex sheet, and by short-circuiting of tongue to gingival pouch. Forward movement of the tongue caused global changes in scalp potentials, with frontal areas more positive, and the strongest gradients between the mastoid region and other scalp areas. Scalp current source density was highest near ear canals and orbital fossae. Electrical isolation of the tongue tip with a latex sheet resulted in a near-complete elimination of TMPs, while short-circuiting resulted in high amplitude responses, suggesting that TMPs are caused by a change in the geometry of conductive pathways. Our results indicate that significant scalp potentials are caused by even modest tongue movements, which may occur subconsciously during various cognitive tasks. The topography and small amplitude of TMPs make their off-line rejection difficult, while an isolation method of the kind used here may substantially diminish TMP artifacts in studies on slow cognitive potentials.