The stimulus was turned off once a saccade was detected. The monkey was rewarded with juice for choosing the correct choice target (congruent with the motion direction at nonzero coherence levels; randomly picked for 0%-coherence trials). Eye position was monitored using a video-based system (ASL) sampled at 240 Hz. Reaction time (RT) was measured as the time from stimulus onset to saccade onset, the latter identified offline with respect to

velocity (>40°/s) and acceleration (>8,000°/s2). At the beginning of a session, we identified a caudate find more site with single- or multiunit activity modulated on the dots task. Neural activity was recorded using glass-coated tungsten electrodes (Alpha-Omega) or polyamide-coated tungsten electrodes (FHC, Inc.). The motion direction that elicited Caspase inhibitor the largest responses was determined by online visual inspection and then used to define the axis of motion for the dots task used in the remainder of the experimental session (Table S1). Unlike cortical regions such as MT and LIP, the caudate is not topographically organized, and nearby neurons do not necessarily share the same response profiles (Ding and Gold, 2010; Hikosaka et al., 1989). We thus selected microstimulation sites based on only neural activity at those sites without considering nearby neural activity.

Electrical microstimulation was delivered at the same site during motion stimulus presentation (negative-leading bipolar current pulses, 300 Hz, 50–80 μA, 250 μs pulse duration). These parameters were chosen to maximize potential effect sizes while avoiding evoked saccades (Nakamura and Hikosaka, 2006a; Watanabe and Munoz, 2010, 2011). Because higher currents are needed to activate the thinner, sparsely myelinated projection axons in the caudate nucleus compared to the thicker, more myelin-dense projection axons of the cortex, the current intensity used is expected second to have similar effective current spread to that of comparable microstimulation studies in cortex (Adinolfi and Pappas, 1968; Blatt et al., 1990; Felleman and Van Essen, 1991; Spatz and Tigges, 1972; Tehovnik,

1996; Tomasi et al., 2012). Trials with and without microstimulation were equally divided and randomly interleaved in a session. The neural responses were sorted offline (Plexon, Inc.). Each neuron’s spatial selectivity was quantified as a receiver operating characteristic (ROC) index, which is the area under the ROC curve constructed using average spike rate during motion viewing (from 200 ms after stimulus onset to 100 ms before saccade onset, all coherence levels were included; also see Figure S3). Performance was quantified with psychometric and chronometric functions (Figure 2), which describe the relationship of motion strength (signed coherence, Coh, positive for toward T1, negative for toward T2) with choice and RT, respectively.