Prestimulation effects on blink and cardiac reflexes of 15-month human infants.

Reduction of reflex startle by brief changes in prestimulation is a robust phenomenon in adults of several species. Although the phenomenon does not require structures above midbrain, it has a long and uneven developmental course. This study of human infants assessed prestimulus effects at 15 months, within a period which has failed, in past work, to show the usual inhibitory modulation. Magnitude and onset latency of the startle blink and concurrent changes in heart rate were measured under four conditions: 2 single-stimulus conditions, 25-msec, 84-dB, 1000-Hz tone or 50-msec, 109-dB white noise; 2 paired-stimulus conditions, noise bursts preceded by tone at lead times of 125 msec or 225 msec. Compared to noise-alone, paired conditions elicited insignificant increases in blink size, significant shortening of blink onset latency, and significant attenuation of heart rate responses. The findings add to growing evidence of a dissociation between modulating effects on blink magnitude and latency and of a dissociation between modulating effects on somatic and autonomic reflexes.