Responding in the squirrel monkey under second-order schedules of shock delivery

Lever-pressing responses were maintained in the squirrel monkey when the only consequence of responding was the delivery of a response-produced electric shock, or alternatively, a brief visual stimulus that was occasionally followed by an electric shock. When shock was produced by the first response occurring after 8 min (8-min fixed-interval schedule), a period of no responding at the beginning of the interval was followed by a gradual increase in response rate during the interval. Similar rates and patterns of responding were maintained when a 1-sec visual stimulus was produced by the first response occurring after 8 min and shock delivery followed the brief stimulus. Subsequently, patterns of positively accelerated responding were engendered during individual fixed-interval components when the first response occurring after 4 min produced a 1-sec visual stimulus and shock delivery followed the second, and later the fourth, presentation of the 1-sec stimulus. When the duration of the brief stimulus was varied over a 100-fold range from 0.1 to 10.0 sec (1) mean response rates decreased monotonically as stimulus duration increased, and (2) patterns of positively accelerated responding were least variable and response rates during the initial part of each 4-min interval were lowest at a stimulus duration of 1 sec.