Effects of stimulus and task factors on Achilles tendon reflexes evoked early during a preparatory period.

The effects of intensity, duration and modality of a warning signal on tendon (T) reflexes evoked during the initial phase of a preparatory period of 4 sec were investigated. Reflexes were evoked simultaneously in both legs, from 0 to 350 msec after warning signal onset in steps of 50 msec. The required response was a planter flexion of the right foot. A facilitation of reflexes was seen within 150 msec after warning signal onset, showing a somewhat longer latency for visual as compared to auditory signals. An effect of intensity was found in the auditory modality only, where the louder of two warning signals yielded a clear peak at 100 msec while the softer stimulus caused no significant departure of Achilles tendon reflexes from baseline. The time course of facilitation in the auditory modality was influenced by warning signal duration as well, although this effect was only marginally significant. There were no effects of physical warning signal parameters on reaction time. A comparison with an experiment in which non-signal stimuli were presented alone, pointed to aspects of the preparatory process which were manifest at the spinal level as early as 200 msec after warning signal onset.