Within-session changes in the VI response function: separating food density from elapsed session time.
Food density—not just session time—bends VI response curves, so track how hungry your client is.
01Research in Context
What this study did
The team worked with rats on variable-interval (VI) schedules.
They wanted to know what changes the shape of the response curve.
Was it just clock time, or how much food the rat actually got?
They held food density steady in one condition and let it drop in another.
What they found
Food density, not minutes on the clock, bent the response curve.
When the same number of pellets kept coming, the curve stayed flat.
When food thinned out, the curve dipped even though the session was young.
How this fits with other research
Rodewald (1974) saw the same food-linked twist earlier.
That study raised pigeons’ body weight and watched their pecks scatter more on VI schedules.
Together the papers say: if you change anything that alters how food feels—less weight, fewer pellets—the VI curve moves.
Davison et al. (1989) give us the ruler: use rate for speed, bounce for scatter, celeration for bends—tools you need to spot these shifts.
Why it matters
For BCBAs running reinforcement schedules, this is a wake-up call.
Check the client’s deprivation level before you trust your VI data.
A skipped lunch or a dense token board can reshape the whole curve.
Measure food density or satiation first, then blame the schedule.
Want CEUs on This Topic?
The ABA Clubhouse has 60+ free CEUs — live every Wednesday. Ethics, supervision & clinical topics.
Join Free →Note the client’s last meal or token rate before each VI session and graph it beside response rate.
02At a glance
03Original abstract
Previous studies examining the relationship between response rate and reinforcement rate on variable-interval schedules (the variable-interval response function) have confounded elapsed session time with within-session changes in food density. The present experiments attempted to manipulate these factors independently and thus isolate their effects on responding. In Experiment 1, 7 rats pressed a bar for food on a series of four variable-interval schedules (7.5 s, 15 s, 30 s, and 480 s). Elapsed session time was held constant while food density was manipulated via a presession feeding. Changes in food density altered the form of the variable-interval response function, independently of elapsed session time. In Experiment 2, 8 rats responded on the same series of variable-interval schedules as in Experiment 1, but food density was held constant and elapsed session time was manipulated via the use of timeout periods. The results revealed no evidence for an effect of elapsed session time independent of food density. The present results extend a recent analysis of the variable-interval response function by Dougan, Kuh, and Vink (1993) by identifying food density as an important factor determining the form of the function. The present results also help clarify the controversy over the correct empirical form of the variable-interval response function by further defining the variables responsible for differences in the form of that function.
Journal of the experimental analysis of behavior, 1995 · doi:10.1901/jeab.1995.64-95