Morphed probe trials. After the acquisition of the discrimination problem, performance was evaluated by using feature-ambiguous probe trials. These probe trials increased the difficulty of the discrimination task by increasing the similarity of the S+ and S−. Probe trials were created by morphing the S+ and S− into one another in 14 steps (Morpheus Photo Animator; ACD Systems, Saanichton, Canada). Thus, one stimulus was gradually morphed into the other, physically changing each stimulus from one step to

the next ( Figure 2). This morphing procedure is similar to procedures used in previous work with monkeys ( Bussey et al., 2003) and humans ( Lee et al., Selleckchem MDV3100 2005 and Shrager et al., 2006). Note that one stimulus was not blended into the other. Rather, the entire stimuli were gradually altered so that they became more alike. Probe level 1 consisted of the least amount of feature overlap (i.e., the two stimuli were quite distinct and most similar to the training stimuli). At level 14 the two stimuli contained substantial feature overlap and appeared quite similar ( Figure 2). During this phase of testing, DNA Damage inhibitor 80% of the trials were standard trials (training stimuli). The remaining 20% of the trials

were rewarded morphed probe trials. The order of the probe trials (levels 1–14) was pseudorandom with the constraint that each of the 14 difficulty levels had to be presented once before any one difficulty level could be repeated. This procedure ensured that data for probe trials accrued at the same rate for every difficulty level. This phase of testing continued until 150 probe trials were completed at each difficulty level. Thus, across this phase of testing each animal received 2,100

probe trials across the 14 different difficulty levels (150 × 14) and an additional 10,500 trials with the training stimuli. Surgery. Animals were assigned to a perirhinal lesion group or a normal control group based upon their trials-to-criterion score for the discrimination task (to create two equal groups). The intention was to and remove the entire perirhinal cortex bilaterally. For surgery, the rat was placed in a Kopf stereotaxic instrument and the incisor bar was adjusted until bregma was level with lambda. Bilateral excitotoxic perirhinal lesions were produced by local microinjections of ibotenate acid (IBO; Biosearch Technologies, San Rafael, CA). IBO was dissolved in 0.1 M phosphate-buffered saline to provide a solution with a concentration of 10 mg/ml, pH 7.4. IBO was injected at a rate of 0.1 μl/min with a 10 μl Hamilton syringe mounted on a stereotaxic frame and held with a Kopf microinjector (model 5000). The syringe needle was lowered to the target coordinate and left in place for 1 min before beginning the injection. After the injection, the syringe needle was left in place for a further 5 min to reduce the spread of IBO up the needle tract. A total of 0.