The perceptual chronometer is a really neat little piece of equipment. The basic concept employed is the flicker fusion threshold, meaning the frequency at which you perceive a blinking light to be solidly on. The perceptual chronometer operates by alternately flashing a number on a seven segment display, and then all of the segments that were not included in the number. If the number is flashed with a 50% duty cycle, then the number and its compliment are both illuminated for the same amount of time per cycle, and therefore, when above the flicker fusion threshold appear to be the same brightness. the display of a perceptual chronometer, therefore, looks like the number eight. Now, if the frequency is set just above the flicker fusion threshold, then in order to read the number, one would have to be processing at a faster rate than usual. What the graduated perceptual chronometer employs, however, is a tad neater: each of the six displays flashes a random number at a different frequency than the other displays. By gradually increasing the speed from one display to the next, the graduated perceptual chronometer shows you exactly how much faster your processing speed has moved from baseline. The guts of the device are six serial shift registers, coupled to the displays by being ex-or'd with NE555 generated 50% duty cycle square waves of variable frequency.