Figure 1.

Experimental timeline. (A) Prior to testing, all rats were required to pass a series of basic discrimination tasks. During the first 2 sessions, rats had to discriminate a complex mixture (coconut extract; vanilla extract) from an odorless control (diethyl phthalate). After reaching criterion performance (85%) on both sets, the reinforcement contingency was reversed for one of the odorants (vanilla extract). Under this condition, rats had to discriminate 2 complex odorants (orange extract vs. vanilla extract). Criterion remained unchanged. (B) Following discrimination training, threshold estimates were obtained for each rat on both odorants (citral and octanol). Order of testing was pseudorandomized across rats. Thresholds were recorded as the lowest concentration at which the rats received 85% or better accuracy on 3 consecutive trials. (C) Rats were then trained to discriminate both citral and octanol from odorless, diethyl phthalate. Relative concentrations for both odorants were based on individual threshold values for each rat. All subsequent conditions were also presented at individual thresholds estimates. Rats were required to consistently achieve 100% accuracy on full reinforcement before moving forward. (D) Before proceeding to nonreinforced probe trials, rats had to demonstrate identical criterion performance (100% accuracy) on partial reinforcement. (E) Finally rats were presented with nonreinforced, mixture probe trials (2 per block, 22 trials total). Target trials retained a partial reinforcement schedule to reduce potential reward cues.