Compensatory response capabilities of the labial system in relation to variation in the onset of unanticipated loads

Abstract

Loads were applied to the lower lip during production of bilabial consonants at several selected intervals prior to and following the onset of Orbicularis Oris Inferior (OOI) EMG associated with bilabial closure to determine (1) if patterns of compensation differed as a function of when the “error” was introduced, and (2) the critical time after which “corrections” to the closing gesture could no longer be accomplished. EMG signals were recorded from OOI, Orbicularis Oris Superior (OOS), Mentalis (MTL), and Depressor Labii Inferior (DLI), and Depressor Angulis Oris (DAO) muscles to discern lower and upper lip compensation patterns. Results indicated that feedback and feedforward compensations (i.e., upper and lower lip muscle changes) both operated for early loads, while feedforward compensations (upper lip muscle compensations) were more predominant for later loads. These observations are consistent with findings in the oculomotor system and engineering analyses that suggest that feedforward is more flexible than feedback in its compensatory capabilities and also may suggest differential access to the labial muscles depending upon the compensatory pathways involved. Regarding critical correction time, it appeared that compensations could be accomplished even if unanticipated disturbances were introduced as much as 40–50 ms after the onset of OOI EMG. [Research supported in part by NINCDS grant.]