Limits to preference and the sensitivity of choice to rate and amount of food
Choice has an elevenfold ceiling, and amount loses power when it lines up with rate.
01Research in Context
What this study did
Aparicio et al. (2016) let rats choose between two levers. Each lever gave food at its own speed and in its own amount.
The team changed the food-rate and food-amount ratios many times within the same session. They watched how the rats’ choices tracked these changes.
What they found
Preference never grew beyond eleven-to-one, even when the payoff difference was larger. This ceiling stayed firm.
When rate and amount ratios pointed the same way, amount sensitivity shrank. The rats acted as if the two factors were not simply added together.
How this fits with other research
Earlier work by Wilkie et al. (1981) and Bradshaw et al. (1978) already showed that smaller or weaker sucrose rewards shift response curves to the right. Aparicio extends those findings by showing that, under rapid within-session changes, amount sensitivity can collapse once a ceiling is hit.
Thomas (1974) found that humans often stray far from matching predictions, while Aparicio shows rats obey a hard limit. The two papers seem to clash, but the gap is about species, not theory: people deviate, rats cap.
Aparicio (2001) used barrier height to push rats into overmatching. Aparicio uses aligned rate and amount ratios to reveal a ceiling. Both warn that extra parameters can bend or break textbook matching.
Why it matters
If you mix reinforcement rates and magnitudes in treatment, do not assume more is always more. Watch for a point where extra tokens, bigger snacks, or faster praise stop shifting choice. When that happens, tweak the rate side or add response effort instead of pouring on more magnitude.
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02At a glance
03Original abstract
Studies of choice holding food-amount ratio constant while varying food-rate ratio within sessions showed that local changes in preference depend on relative amount of food. The present study investigated whether sensitivity of choice to food-rate ratio and sensitivity to food-amount ratio are independent of one another when food-rate ratios are varied across sessions and food-amount ratios are varied within sessions. Food deliveries for rats' presses on the left and right levers were scheduled according to three different food-rate ratios of 1:1, 9:1, and 1:9; each food-rate ratio lasted for 106 sessions and was arranged independently of seven food-amount ratios (7:1, 6:2, 5:3, 4:4, 3:5, 2:6, and 1:7 food pellets) occurring within sessions in random sequence. Each amount ratio lasted for 10 food deliveries and was separated from another by a 60-s blackout. Sensitivity to rate ratio was high (1.0) across food deliveries. Sensitivity to amount ratio was low when food rates were equal across alternatives, but was high when rate ratio and amount ratio opposed one another. When rate ratio and amount ratio went in the same direction, choice ratio reached an elevenfold limit which reduced sensitivity to approximately zero. We conclude that three factors affect sensitivity to amount: (1) the limit to preference, (2) the equal effect on preference of amounts greater than four pellets, and (3) the absence of differential effects of switches in amount in the equal-rates (1:1) condition. Taken together, these findings indicate that rate and amount only sometimes combine independently as additive variables to determine preference when amount ratios vary frequently within sessions.
Journal of the Experimental Analysis of Behavior, 2016 · doi:10.1002/jeab.198