[Excitability of the blink reflex during self-elicitation or elicitation by others]

Electrically evoked blink reflexes were studied in 24 healthy subjects in different conditions of elicitation. In one condition the electrical shock was delivered by the experimenter; in a second condition the subject triggered the stimulus himself by manually operating a switch. The different conditions were investigated relaxed or clenching the fist with left hand. Moreover, in 3 subjects the stimulus was given in various delays after the occurrence of the EMG activity of the forearm flexor muscles due to a ballistic wrist flexion. The latencies and peak to peak amplitudes of the ipsi- and contralateral early and late responses were analysed. Voluntary sustained contraction of the left hand (fist) caused no significant modification of the blink reflex. Self-triggering of the stimulus had a facilitating influence on the early components (R1, R1') and an inhibitory effect on the late components (R2, R2'). In EMG triggered self-stimulation the R2-inhibition could already be seen at a delay of 0 ms. From a delay of 10 ms on the suppression was still more pronounced and recovered over the following 500-1000 ms. We conclude, that the reflex alterations are not due to the outflow of the motor cortex, but to the conditioning effect of self-elicitation. The influence on the excitability of the blink reflex resemble that described in the literature for the conditioning effect of an acoustic or visual stimulus. We suggest involvement of the same interneuron network on the condition of self-eliciting.