INTEGRATION SHOWN IN THE LATE COMPONENT OF THE HUMAN BLINK REFLEX

The effect of closely spaced stimulus pairs (S1-S2), one at or above t hreshold for eliciting the second electromyographic burst of the cutan eous blink reflex (R2) and one below threshold intensity, on the laten cy, duration, and area of R2 was examined in four experiments. The siz e of the R2 elicited by a threshold S1 (but not by an S1 at three time s threshold intensity) was augmented by threshold or near-threshold S2 s that appeared after stimulus onset asynchronies of 4, 8, and 16 msec . More intense S2s had a greater effect on R2 size and led also to a s light shortening of R2 latency. Although R2 augmentation was accompani ed by a modest increase in response duration, this increase in duratio n was by itself insufficient to account for the augmentation of respon se amplitude. Subreflexogenic S1s (at 0.1, 0.2, 0.4, and 0.8 times R2 threshold intensity) did not affect the R2 elicited by an S2 at thresh old intensity or two times threshold intensity. Unlike the simple brai nstem pathway which mediates R1, the more complex brainstem pathway wh ich mediates R2 is capable of summing stimulus energy over time.