Comparative threshold studies of the acoustic pinna, jaw and startle reflex in the rat

Electromyograms of M. Levator auris and M. Temporalis and movement produced by whole body startle were recorded simultaneously in awake, freely moving rats. Thresholds were 78 db SPL for the L. auris, 80 dB SPL for the ballistic and 81 dB SPL for the Temporalis. The rank ordering of the three thresholds was extremely strict, 188 suprathreshold M. L. auris responses could be observed without M. Temporalis responses, but only once was a M. Temporalis response observed without a M. L. auris response. Thresholds as well as amplitudes and latencies measured by the different methods show correlated fluctuations. While the rise in amplitude which accompanies increasing stimulus intensity is similar in the three measures, the latency decrease is not. The latency difference between M. Temporalis EMG and M. L. auris EMG is intensity dependent, increasing from 0 msec at 78 dB SPL to 1.1 msec at 115 dB SPL, with a faster response for the M. L. auris.