An investigation of stimulus prevalence effects in rats

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.