Responding by exclusion in Wistar rats in a simultaneous visual discrimination task

Responding by exclusion is to select a correct alternative by rejecting other potential alternatives. Studies describe this ability in some mammals and birds. However, this type of performance has not been reported in rodents. The aim of this study was to verify the occurrence of responding by exclusion in Wistar rats after a baseline of simple simultaneous visual discrimination. Six male Wistar learned nose-poking tunnels displaying visual stimuli (projected geometric shapes) in an operant chamber. After establishing the simultaneous discrimination baseline, three probe sessions were conducted. In each session, there was a novelty-control probe (a new stimulus was presented together with a stimulus trained as the S(+)) and an exclusion probe (a second new stimulus was presented simultaneously with a stimulus trained as the S(-)). Only one rat responded to the new stimulus in one of the three novelty probe trials. Four rats responded to the three new stimuli and one responded to the new stimulus in two of the three exclusion probes. One subject responded to the S(-) in all the exclusion probes. Five of the six subjects were therefore able to choose the new stimulus, rejecting stimuli trained as the S(-). This is the first experimental evidence for performance by exclusion in rats.