Retroactive Raising of a Sensory Threshold by a Contralateral Stimulus

Abstract

In an experiment designed to extend Howarth and Treisman's recent observations on the lowering of the visual and auditory threshold produced by a warning stimulus, it has been found that the threshold to electrical stimulation of one forearm is raised by a similar stimulus on the other forearm. As in the earlier experiments the “warning” was effective if it came 100 millisec. before or after the threshold stimulus. The effect increased with increasing warning strength over the range of five near-threshold strengths used. These results appear to be inconsistent with the explanation previously proposed to account for the retroactive lowering of threshold. This suggested that the warning was acting simply as a marker, reducing the subject's uncertainty as to when the stimulus was coming. The fact that the rise in threshold was also induced retroactively shows that some interaction must be occurring in the central processes concerned in perception, and that it cannot be explained in terms of attenuation of the afferent volley.