Some effects of combined S-DS.

When the individual S(D) components of a multiple schedule were combined, their control over a response summated, thus increasing the response probability to a point over that controlled by either of the S(D)s independently. Summation was concluded to be a phenomenon relevant for operant as well as respondent stimulus control (Pavlov, in Kimble, 1960; Hull, 1943). The results of the present study appear to be a special case of the general S(D) enhancement effect demonstrated by Hanson (1959) and by Pierrel and Sherman (1960).