Human auditory fast and slow omitted stimulus potentials and steady-state responses.

Two kinds of omitted stimulus potentials (OSP) are called "fast" and 'slow." Fast OSPs, recently found with visual stimuli, are here extended to auditory; they occur after omissions or after the end of trains of 1 to > 20 Hz clicks. Slow OSPs, long known, follow trains of 0.3 to 4 Hz. Each has its constant peak latency after the due-time of the first missing stimulus, as though the system is expecting something quite accurately on schedule. They differ in dynamics and slow OSPs require the subject to attend; fast OSPs do not. Steady-state responses (SSR) at a critical click rate of 6-7 Hz sometimes appear to alternate between two forms and OSPs may depend on which they follow. Fast OSPs can occur to the first, second and even the third omissions after the end of a train. Short conditioning periods suffice. Irregular interstimulus intervals do not reduce fast OSPs but attenuate slow OSPs.