Acoustic facilitation of the Hoffmann reflex.

The facilitative effect of a pure tone on the amplitude of the Hoffmann reflex was tested in 11 subjects under four conditions: (i) trying not to respond to the tone, (ii) voluntary abduction of the right index finger in response to the tone, (iii) voluntary plantarflexion of the left leg, and (iv) right plantarflexion in response to the tone. The acoustic stimulus was a 1000-Hz pure tone (80 dB, re 0.0002 dyne/cm2), presented on a random set of trials. Tone onset occurred 100 ms prior to the electrical stimulus that evoked the Hoffmann reflex. The Hoffmann reflex was larger for "tone" trials than for "silent" trials. The facilitative effect was equal in four cases: (i) making no response (NR), (ii) finger abduction (FA), (iii) noninvolved muscle (NIM), and (iv) involved muscle during treadle pressing (IM). The effect of the first tone presented was larger than that of subsequent tones, indicating some habituation of the acoustic effect on the H-reflex. Tests for spontaneous recovery and stimulus generalization were not significant. The results supported the existence of a purely auditory facilitation of spinal excitability.