Reaction time as a measure of intersensory facilitation.

In measuring reaction time (RT) to simultaneously presented stimuli, Todd (1912) found facilitation when sound and electric shock were paired but no effect when light was paired with either sound or shock. These results may be understood using the scheme suggested by Hilgard (1933) to account for facilitation of the eyeblink reflex. Hilgard used light and sound stimuli slightly offset in time and found that the magnitude of the eyeblink to sound was augmented when the light preceded the sound by intervals of 25 and 50 msec. Thus, he maintained, there was a summation of "reaction tendencies" when the stimuli were presented at an interval equal to the differences in the latencies of their respective responses. Since the latency for light is greater than that for sound, the light must precede the sound in order for "response tendencies" to occur simultaneously. Indeed, Hilgard suggests that the difference between the latencies for the single stimuli may be the point of maximum facilitation. When taken together these two studies present a coherent picture. Since sound and shock yielded very