VI operant study

[From Bruce Abbott (971230.1545 EST)]

Rick Marken (971230.1200) --

Bruce Abbott (971230.1310 EST)

(At the moment I'm 40+ sessions into a study of VI schedule

Cool! How about telling us about the study and what you've
found so far?

For those who may be unfamiliar with operant conditioning terminology, a VI
(variable interval) schedule of reinforcement arranges a contingincy between
a response (in this case a lever-press) and reinforcer (in this case the
delivery of a 45 mg food pellet) such that reinforcement is delivered
immediately following the first response to occur after a certain interval
of time as passed since the previous reinforcement. The length of the
interval varies in an unsystematic way but averages a specified length in
the long run. For example, a VI 10-s schedule would arrange for
reinforcement an average of 10 seconds after the previous reinforcement.
Such a schedule tends to promote a relatively steady, moderate rate of
responding. Responses that occur before an interval has elapsed are
recorded but are otherwise ignored by the program.

The experiment involves testing individual rats on a series of VI schedules
differing in average interval. PCT predicts that, if the rats are
controlling the rate of pellet delivery, then lengthening the schedule
parameter (e.g., from 10-s to 20-s) should result in an increased rate of
responding as the rats attempt to compensate for the reduction of pellet
delivery rate imposed by the lengthening of average interval. Reinforcement
theory predicts a decrease in rate of responding owing to the less frequent

I have had two rats on VI 10-s for over 40 sessions now and performance has
yet to stabilize. (One of the things I am doing in this experiment is to
evaluate the stability of such data over sessions.) One animal in
particular is continuing to produce a slow increase its rate of responding
over sessions; today it produced over 3700 lever-presses in 40 minutes and
earned 207 pellets. I have been holding this rat's body weight constant by
adjusting supplemental feeding; it is possible that the slow increase
reflects a slowly increasing error in some variable(s) related to body
weight. However, an undeprived control animal from the same batch has been
holding a steady body weight since the study began. The slow increase in
rate is undesirable, but knowing its slope, I can take this into account if
necessary when assessing any change in rate that follows the schedule
change. I plan to switch to VI 20-s sometime this week.

Another part of this study will involve training rats on a multiple VI-VI
schedule. In a multiple schedule, two (or more) components are identified
by different stimuli (e.g., houselight on, houselight off) and different
schedules of reinforcement are programmed in each component. In this case
the two schedules will be VI with different interval lengths (e.g., mult VI
20-s VI 40-s). The components will alternate at irregular intervals
averaging about 10 minutes. This will enable us to examine the dynamics of
the transition from one schedule value to the other and vice versa.