An investigation of stimulus prevalence effects in rats
Rare targets hurt discrimination unless you pad them with added reinforcement or extra trials.
01Research in Context
What this study did
Brown et al. (2025) taught rats to tell two visual stimuli apart. One stimulus showed up often. The other appeared only once in a while.
The team then tracked how fast the rats learned and how many choices they got right.
What they found
Rats made more errors when the rare stimulus finally appeared. They also needed extra sessions to reach the same accuracy.
The low-prevalence cue slowed learning and cut correct responses.
How this fits with other research
Maddox et al. (2015) and Poling et al. (2011) showed the upside: pouched rats can learn to find rare land-mine odor or hidden people with over 80% success. Brown’s lab result sounds like the opposite, but the tasks differ. The pouched-rat studies used food and play for every hit, keeping motivation high even though targets were scarce.
Winett et al. (1972) saw the same drop with monkeys. When signals fell to only 4–7 per hour, detection crashed. Low rate, not low reward, drove the decline. Together the papers warn: if the target is rare, you must add extra reinforcement or extra training trials to protect accuracy.
Why it matters
If you teach a client to label uncommon items—like ‘tongs’ in a kitchen program—slip in bonus praise or tokens when it finally appears. Run a few mass-trial warm-ups before you return to the natural mix. These steps offset the prevalence effect Brown caught in rats and keep your learner’s accuracy high.
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02At a glance
03Original abstract
Antecedent stimulus prevalence can affect detectability. Two contrasting effects have been reported in humans. The low-prevalence effect is when participants are less likely to report the presence of the target stimulus when it occurs with low prevalence. Recently, an opposite effect has been discovered in which participants are more likely to report the presence of low-prevalence stimuli. There is little if any research on stimulus prevalence with nonhuman animals; therefore, the present study investigated prevalence effects in rats to extend species generality, determine which effect would occur, and identify controlling variables. Rats were trained to press left and right levers conditional on the flash rate of the sample stimulus (1 or 5 Hz). A between-group, within-subject comparison in which the two flash rates were not always equally prevalent was employed. Low-prevalence stimuli were underreported, systematically replicating the low-prevalence effect. Rats initially trained under the unequal-prevalence condition failed to acquire or took longer to acquire high accuracy with the low-prevalence stimulus but quickly achieved high accuracy with the high-prevalence stimulus. Subsequent training under equal-prevalence conditions remediated these effects, and prior training under equal prevalence seemed to offer a protective effect from later decreases in stimulus prevalence.
Journal of the Experimental Analysis of Behavior, 2025 · doi:10.1002/jeab.70033