Figure 1.

Task and performance, (a–c) Sequence of events on 2- and 4-choice direction discrimination tasks. The monkey fixates a central point until the random dot motion appears and is then permitted to indicate its decision by making a saccadic eye movement to a choice target. The motion is in one of 2 or 4 directions (trials randomly interleaved in a 1:2 ratio). A liquid reward is given for choosing the target along the axis of random dot motion, or it is given with probability ½ or ¼ when the motion strength is zero. Random intervals (truncated exponential distributions) separate fixation, appearance of choice targets and motion onset. The random-dot motion is extinguished when the monkey initiates a saccade to one of the choice targets. This interval, from motion onset to saccade initiation, is the RT. One of the choice targets is in the RF of an LIP neuron recorded during the task (shading). The shading is only meant as a guide: actual RF size varied considerably, (a) 2-choice task. The directions are 180° apart. One direction is toward the target in the neuron’s RF (T_in), (b) 4-choice task. The directions are 90° apart, (c) 90° control task, (d–g) Speed and accuracy of decisions. Smooth curves in all panels are fits to the bounded diffusion model described at the end of Results. The fits were performed separately for panels (d,f) and for panels (e,g). (d) Psychometric functions. The probability of a correct choice is plotted as a function of motion strength. All experiments contribute to these graphs. At 0% motion strength, choices are rewarded randomly (open symbols), (f) Psychometric functions for the 29 experiments that included the 90° control, (e) Chronometric functions. Mean RT for correct trials is plotted as a function of motion strength. Each point reflects correct responses from all experiments. Error bars are s.e.m.; some are smaller than the symbols, (g) Chronometric functions for the 29 experiments that included the 90° control.