Thus, the impact of correlated noise on population coding depends on (1) the structure of noise

correlations and their dependence on signal correlation, and (2) the composition of neuronal pools upon PS-341 chemical structure which decoding is based. We conclude that the effects of training on heading discrimination are not likely to be driven by the reduction in correlated noise that we have observed in area MSTd. Combined with previous observations that perceptual learning has little or no effect on basic tuning properties of single neurons in visual cortex (Chowdhury and DeAngelis, 2008, Crist et al., 2001, Ghose et al., 2002, Law and Gold, 2008, Raiguel et al., 2006, Schoups et al., 2001, Yang and Maunsell, 2004 and Zohary et al., 1994a), our results suggest that changes in sensory representations are not necessarily involved in accounting for the improvements in behavioral sensitivity that accompany perceptual learning (at least for some sensory systems and tasks; see also Bejjanki et al., 2011). Rather, our findings support the idea that perceptual learning may primarily alter the routing and/or weighting of sensory inputs to decision circuitry, an idea that has recently received experimental support (Chowdhury

and DeAngelis, 2008, Law and Gold, 2008 and Law and Gold, 2009). Physiological experiments were performed in 8 male rhesus monkeys (Macaca mulatta) weighing 4–8 kg. Animals were chronically implanted with a plastic MTMR9 head-restraint ring that was firmly anchored to the apparatus to minimize head movement. All monkeys were implanted with scleral coils for measuring eye movements in a magnetic field (Robinson, 1963). Animals were trained using standard BAY 73-4506 nmr operant conditioning to fixate visual targets for fluid reward. All animal surgeries and experimental procedures were approved by the Institutional Animal Care and Use Committee at Washington University and were in accordance with NIH guidelines. Neurons

were tested with two types of motion stimuli using a custom-built virtual reality system (Gu et al., 2006, Gu et al., 2007 and Gu et al., 2008b). In the “vestibular” stimulus condition, monkeys were passively translated by a motion platform (Moog 6DOF2000E; East Aurora, NY) along a smooth trajectory (Gaussian velocity profile with peak-acceleration of ∼1 m/s2 and duration of 2 s, Figure 1A). In the “visual” stimulus condition, optic flow was provided by rear-projecting images onto a tangent screen in front of the monkey using a 3-chip DLP projector (Christie Digital Mirage 2000) that was mounted on the motion platform. Visual stimuli (90 × 90°) depicted movement through a 3D cloud of stars that occupied a virtual space 100 cm wide, 100 cm tall, and 50 cm deep. The stimulus contained multiple depth cues, including horizontal disparity, motion parallax, and size information. Animals were trained to maintain visual fixation on a head-fixed target at the center of the screen.