Middle latency auditory-evoked fields reflect psychoacoustic gap detection thresholds in human listeners.

The resolution of the temporal processing in the primary auditory cortex (PAC) was studied in human listeners by using temporal gaps of 3, 6, 10, and 30 ms inserted in 100-ms noise bursts. Middle latency auditory-evoked fields (MAEFs) were recorded and evaluated by spatio-temporal source analysis. The dependency of the neurophysiological activation at about 37 ms (P37m) on the temporal position of the gap was investigated by inserting silent periods 5, 20, and 50 ms after noise burst onset. The morphology of the waveforms evoked by the gap showed that the MAEFs were largely determined by the on-response to the noise burst following the gap. The comparison of the source waveforms revealed two major effects: 1) the amplitudes of the MAEFs increased with longer gap durations and 2) the amplitudes increased with the length of the leading noise burst. When the gap was inserted after 50 ms, a significant deflection of the collapsed left and right hemisphere data was observed for all gap durations. The P37m amplitude failed to reach significance for the shortest gap duration of 3 ms when the gap occurred after 20 and 5 ms. These neuromagnetically derived minimum detectable gap responses closely resembled psychoacoustic thresholds obtained from the same subjects (leading noise burst, 50 ms: 2.4 ms; 20 ms: 3.2; and 5 ms: 5.3 ms). The correspondence between psychoacoustic thresholds and the cortical activation indicates that the recording of MAEFs provides an objective and noninvasive tool to assess cortical temporal acuity.