(A) An illustration of the motion boundary (MB)-orientation-discrimination task. A fixation point appeared for 500 ms, then a MB stimulus appeared for 750 ms. The fixation point maintained for additional 200 ms before it was replaced by two saccade targets. During this period, the monkey was required to maintain fixation at the fixation point. The monkey needed to make an eye saccade choice within 1000 ms, according to the tilt of the MB line. For example, for MB stimulus tilted to the right of vertical (as shown in the top right), the monkey should saccade to the right target. (B) The psychometric function of monkey S in the MB-orientation-discrimination task. (C) The response peri-stimulus time histograms (PSTHs) of an example MB neuron to MB stimuli presented different levels of motion coherence (four out of seven were shown), and the red and black curves are for the preferred and null MB orientations, respectively. (D) The receiver operating characteristic (ROC) curves obtained from the response PSTHs in (C), and horizontal and vertical coordinates are false alarm rate and hit rates, respectively. (E) The neurometric function obtained from the ROC curves in (D). (F) The neurometric functions (gray) of the 15 MB neurons recorded with the coherence stimuli. The red dots represent the average of the 15 neurometric functions, and the black line is its fitted curve. The green dots represent the average psychometric function measured during the recordings of the 15 neurons, and the black dashed line is its fitted curve. (G) Comparison of the neuronal thresholds and behavioral thresholds. The inset plot shows the distribution of ratios of the neural and behavioral thresholds. (H) The distribution of the choice probability (CP) values for the 15 MB neurons tested with the coherence stimuli, in which 12 neurons had CPs significantly larger than 0.5 (filled bars). The mean CP value (0.59) is also larger than 0.5 (t-test, p<0.001). The inset plot shows the distribution of CPs for all of the 32 neurons, including 15 tested with coherence stimuli (shown in the main panel) and 17 tested with brightness stimuli (also see Figure 2—figure supplement 1B–E).