Figure 1. Discrimination task and neuronal ensemble activity. A, Timeline of the discrimination task. The mechanical probe is lowered, indenting the glabrous skin of one digit (PD); the monkey places its free hand on an immovable key (KD). The probe oscillates vertically, at the base frequency (f1); after a fixed delay (3 s), a second mechanical vibration is delivered at the comparison frequency (f2); after another fixed delay (3 s) between the end of f2 and probe up (PU), the monkey releases the key (KU) and presses either the lateral or medial push button (PB) to indicate whether the comparison frequency was higher or lower than the base, respectively. B, Stimulus set during recordings. Each trial type is defined by a base/comparison frequency stimulus pair. The number inside the box indicates overall percentage of correct trials for each trial type, except for the ambiguous trial type, where f1 = f2 = 22 Hz, for which we plotted the percentage of trials the animal pressed the lateral push button. C, Psychophysical performance when f1 was maintained fixed at 22 Hz and f2 was variable. The black/gray points indicate the percentage of trials in which monkey 1/2 estimated f2 higher than f1. Error bars indicate SE. D, Reaction time distributions for monkey 1/2 (black/gray traces). E, Recorded cortical areas: S1, Primary somatosensory cortex; S2, secondary somatosensory cortex; DPC, dorsal premotor cortex; MPC, medial premotor cortex; M1, primary motor cortex (notations used in all following figures). F, The series of black ticks represent the spike trains of 8 M1 neurons recorded simultaneously. The NE activity, during the period between t(f2) − 0.5 s and PU is plotted for two trials of the same trial type (f1 = 18 Hz and f2 = 26 Hz). The gray area corresponds to the presentation of f2.