Effects of stimulus intensity on laryngeal long latency responses in awake humans

Percutaneous electrical stimulation applied to the internal branch of the superior laryngeal nerve (ISLN) results in two long latency laryngeal adductor responses in awake humans: an ipsilateral thyroarytenoid (TA) R1 muscle response at 16 ms, and later bilateral TA R2 muscle responses at 60 ms. The purpose of this study was to determine whether a functional relationship existed between the R1 and R2 responses by gradually increasing the level of electrical stimulation from threshold to supramaximal levels. R1 amplitude increased linearly with stimulation intensity in 9 of the 11 subjects, whereas R2 only had a positive linear relationship in 3 subjects and a negative relationship with stimulation intensity in 1 subject. Significant negative relationships were found between response latency and stimulation intensity in 3 subjects for the R1 responses and 3 other subjects for the R2 responses. Overall, R1 amplitudes increased systematically, whereas R2 responses varied in latency and amplitude with increasing stimulus intensity. Neither the latencies nor the amplitudes of the two responses were related after adjusting for stimulation intensity within subjects by using partial correlation coefficients. The R1 and R2 responses were functionally unrelated and most likely have different neural components.