The auditory P300 at or near threshold.

Psychophysical and P300 (P3) thresholds and suprathreshold measures were obtained in 16 normal-hearing subjects. Subjects followed a classic oddball paradigm using 1000 (frequent) and 2000 Hz (rare) tones as stimuli. The P3 was obtained for all 16 subjects at or 5 dB above their behavioral threshold. The P3 was obtained more often at behavioral threshold and 5 dB SL than N1 and P2 late potentials. The P3 was larger in amplitude than either the N1 or the P2 at threshold and for 75 dB SPL stimuli. In comparing P3s obtained at threshold and for the 75 dB stimuli, significant effects were noted in latency and amplitude reflecting exogenous aspects to this endogenous potential. Differences in latency and amplitude were also noted in N1 and P2 waveforms obtained from the rare versus frequent stimuli. Clinical implications of these results are discussed.