Fig. 1.

Behavioral paradigm and stimulus sets used.a, Schematic diagram of the task. In each trial, the mechanical probe was lowered so that it touched one of the fingertips of the restrained hand (PD, probe down); the monkey reacted, placing its free hand on a lever within 1 sec after indentation (KD, key down); after a delay period (1.5–3 sec) the probe oscillated vertically, delivering a series of pulses at a base frequency; after an interstimulus interval (1–3 sec), a second set of pulses was delivered at a comparison frequency; after the end of the comparison stimulus, the monkey had to release the lever within 600 msec (KU, key up) and press one of two push-buttons (PB). One button indicated that the comparison frequency was higher than the base, and the other indicated that the comparison was lower than the base. b, c, Two stimulus sets frequently used in the experiments. The numbers inside the grid indicate the percentage of correct responses for each base-comparison combination.Set A had constant differences of 8 Hz between base and comparison. Percentages are based on the performance of three monkeys throughout 350 runs with this set. Set B was designed to vary the difficulty of the task in a more systematic manner. The percentages shown correspond to 42 runs from two monkeys.