Grouping of Stimuli and Apparent Exceptions to the Psychological Refractory Period

When a subject is called upon to respond independently to two stimuli, the second of the two responses is often delayed if the stimuli follow closely on one another, and this has led to the suggestion that in making decisions the human operator accepts and organizes the available input information intermittently in the manner of a discontinuous servo. According to this view two nearly simultaneous stimuli can only be dealt with equally fast if they are grouped into a single decision to respond to both stimuli; otherwise one will have to wait for the attention of the central mechanism until the other has been dealt with. In the present experiment it is shown that delays in the second response are not necessary or invariable, and that the pattern and timing of the second responses when they are performed without delay differ in important respects from those to be expected of grouped responses. It is concluded that the central mechanisms concerned in the response do not possess the limitations that the single channel theory would suggest.