, 2006). This manipulation increased the OSD and MT (Figure 2Aiii; Table 1) but again failed to induce an increase in accuracy (Figure 2Aii; Table 1). Therefore, we next tested the effects of increasing the incentive to obtain correct responses by eliminating water outside the task, increasing task difficulty and decreasing the number of available trials (see Experimental Procedures for details). Although this manipulation produced a drop in body weight of test subjects compared to controls (Figure 2Bi) demonstrating its effectiveness, there was no difference in accuracy,

OSD or MT between test and control MLN0128 molecular weight groups (Figures 2Bii and 2Biii; Table 1). To directly assess the impact of differential reward AT13387 ic50 expectation on measures of response time, we trained another set of rats on a one-direction-rewarded (1DR) version of the two-alternative choice task. In this task

version, only responses to one choice direction were rewarded (when correct) and this rewarded direction changed across blocks within a session. As expected, animals were biased to choose the rewarded side (Figure 2Ci) and performance increased for the rewarded side for the difficult odor mixtures (Figure 2Cii). We found that OSD for nonrewarded choices was slower than for the rewarded ones (Figure 2Ciii). Moreover the effect of stimulus difficulty on OSD was diminished for the nonrewarded choices (Figure 2Ciii), those choices whose difficulty no longer predicted the likelihood of reward availability. These results suggest that the effect of difficulty on OSD arises not only from varying perceptual uncertainty but also reflects the effect of difficulty on reward Adenylyl cyclase expectation and hence response speed. Having seen that response times are sensitive to reward and punishment but that changes in OSD did not produce significant changes in accuracy, we next sought to test the possible effect of larger changes in stimulus sampling time by manipulating the OSD more directly. To do so, following

a previous study (Rinberg et al., 2006), we introduced an auditory go signal to cue the timing of the response while the odor stimulus continued to cue the correct choice direction (Figure 3A). Responses initiated prior to the go signal were not rewarded regardless of choice. We first trained subjects to wait for the go signal (see Experimental Procedures). After training, we used fixed go-signal delays of 0, 0.2, 0.4, and 0.8 s, each repeated for 3–6 sessions before switching (Figure 3B). Within each session odor mixtures of the same difficulties as the RT task were randomly interleaved from trial-to-trial. Subjects obeyed the go signal, resulting in much longer OSDs than those seen in the original RT task (Figure 3C; Table 1). However, despite the substantial increase in odor sampling durations, we observed no change in accuracy (Figure 3D; Table 1; Figure S3).