Figure 2. Recording locations, techniques, task, and discrimination behavior. a, An illustration of the experimental setup for data gathering in the discrimination task. We gently restrained the resting arm with a plastic tube and cloth sling. We tracked a reflective IR bead taped on the middle digit of the unrestrained hand to mimic a touch screen and to provide an estimate of instantaneous arm position. We tracked eye position using an infrared reflective mirror placed in front of the monkey's nose. b, Timeline of the discrimination task, showing the prestimulus period (the 600 ms period before checkerboard onset, depicted as a blue rectangle), poststimulus period (the 300 ms period after checkerboard onset depicted in purple), and postmovement period (the 300 ms period after movement initiation at the RT shown in pink). c, Examples of different stimulus ambiguities used in the experiment parameterized by the color coherence of the checkerboard defined as follows: . The corresponding signed color coherence is defined as follows: . Positive values of signed color coherence indicate more red than green squares and vice versa. d–e, Average discrimination performance (d) and RT (e) over sessions of the 2 monkeys as a function of the signed color coherence of the checkerboard. RT plotted here includes both correct and incorrect trials for each session and then averaged across sessions. Gray markers represent measured data points along with 2x (SE) estimated over sessions, although variation is so small that they are difficult to see in d. The black line segments are drawn in between these measured data points to guide the eye. For most data points in d, the error bars lie within the markers. d, e, x axes represent the signed color coherence in %. y axes represent the percentage responded red in d and RT in e. Also shown in d are discrimination thresholds (mean ± SD over sessions) estimated from a Weibull fit to the overall percentage correct as a function of unsigned coherence. The discrimination threshold is the color coherence level at which the monkey made 81.6% correct choices. A total of 24 sessions for Monkey T (47,483 trials) and 44 sessions for Monkey O (70,250 trials) went into these averages. f, Box-and-whisker plot of RT as a function of unsigned checkerboard coherence. For each coherence, the central mark of the box indicates the median, the bottom and top edges of the box reflect the 25th and 75th percentiles, respectively, the maximum whisker length is specified as three times the interquartile range, and outliers are plotted as plus symbols. There is large variation of RTs both across and within coherences. g, Location of PMd along with an example recording from a 16-electrode, 150 μm spacing U probe. The neuronal waveforms are taken from the recordings, and properties of these neuronal responses were reported in a previous study.