The effect of closely spaced percutaneous stimuli (81 and 82) on the early (HI) component of the human blink reflex was studied in three experiments. In all experiments, successive Rl components (called Rl, and Rl, here) were frequently elicited by 81-82 pairs with stimulus onset asynchronies (80As) as brief as 4msec. The successive Rls appeared to be under the separate control of81 and 82, respectively. Experiment 1 (in which 81 was set at each subject's threshold for Rl elicitation and 82 was

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... set at 80% of that threshold) and Experiment 2 (in which 81 was set at 80% of each subject's threshold for Rl elicitation and 82 was set at the threshold level) showed that 82 had no effect on either the probability or the amplitude of Rl, but that 81 in• creased both the probability and the amplitude ofRl, above control levels established by presentation of 82 alone. Augmentation of Rl, was greatest at the shortest 80A tested (4 msec) and decayed to zero at an 80A of about 11-12 msec. Paired stimuli produced no systematic effect on the latency of either Rl, or Rl,. Experiment 3 showed that the probability and amplitude of Rl, increased with 81 intensity but were unaffected by 82 intensity over the range tested. The probability and amplitude ofRl, increased with 82 intensity and were increased by the presence of 81; the augmentation produced by 81 was greater at the 4-than at the 8-msec 80A, and increased with 82 intensity. There were no systematic effects on the latency of either Rl. Reflex augmentation in these experiments is not consistent with the model of temporal integration of stimulus energy. Instead, a percutaneous stimulus appears to evoke a transient excitation of the Rl pathway; a second percutaneous stimulus presented before the excitation decays is more likely to elicit an Rl than otherwise, and the Rl elicited will on the average be larger.