Saccadic reaction times during isometric voluntary contraction of the shoulder girdle elevators and vibration stimulation to the trapezius.

We investigated the effect of increases in muscle afferent information from the shoulder girdle elevators on saccadic reaction time. Saccadic reaction time was measured under conditions of isometric voluntary contraction of the shoulder girdle elevators and vibratory stimulation of the trapezius. Saccadic reaction time was defined as the latency until the beginning of eye movement toward the lateral target, which was moved at random time-intervals in jumps of 20 degress amplitude. Eye movement was measured using the electro-oculogram technique. Muscle contraction force was set in 10% increments from 0% to 30% of maximal voluntary contraction (MVC), and vibration frequency was maintained at 100 Hz. Under voluntary contraction, the saccadic reaction time gradually shortened up to 30% MVC. Under vibration stimulation at 0% MVC, the reaction time shortened to the same degree as that under voluntary contraction at 30% MVC. Under conditions of combined vibratory stimulation and voluntary contraction, the reaction time was essentially identical to these values; namely, no additive effect in shortening of the reaction time was recognized. The results demonstrated that saccadic reaction times were remarkably shortened by increases in muscle afferent information from the neck extensors. We have discussed reasons for the lack of an additive effect and factors limiting shortening of reaction times.