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 laryng eal adductor responses in awake humans: an ipsilateral thyroarytenoid (TA) R1 muscle response at 16 ms, and later bilateral TA R2 muscle res ponses at 60 ms. The purpose of this study was to determine whether a functional relationship existed between the R1 and R2 responses by gra dually increasing the level of electrical stimulation from threshold t o supramaximal levels. R1 amplitude increased linearly with stimulatio n intensity in 9 of the 11 subjects, whereas R2 only had a positive li near relationship in 3 subjects and a negative relationship with stimu lation intensity in 1 subject. Significant negative relationships were found between response latency and stimulation intensity in 3 subject s for the R1 responses and 3 other subjects for the R2 responses. Over all, R1 amplitudes increased systematically, whereas R2 responses vari ed in latency and amplitude with increasing stimulus intensity. Neithe r the latencies nor the amplitudes of the two responses were related a fter adjusting for stimulation intensity within subjects by using par tial correlation coefficients. The R1 and R2 responses were functional ly unrelated and most likely have different neural components.