The answer is orientation, shape, and pitch audition. The transition of two closely proximate stimuli through a neuron is often interspersed by delays. This is because the neuron has to repolarize before it can carry another impulse. This also leads to the belief that the human operator also processes information intermittently (in a discontinuous servo). Therefore, according to the law of grouping, stimuli that happen so closely together of simultaneously are often merged into a single decision response by the central nervous system based on the principles of Proximity, Similarity, Continuity, Closure, and Connectedness.