The auditory evoked middle-latency responses (MLRs): their normative variation and generators.

Auditory evoked middle-latency responses (MLRs) to clicks were recorded in 68 normal adults (ages from 21 to 59 year-old) at 16 scalp locations, all referred to a balanced non-cephalic reference. Both Na and Pa components were clearly demonstrated in all the subjects studied through the digital high-pass filtering. Both Na and Pa components were distributed dominantly over the frontal area in all subjects. The amplitudes of Na and Pa were significantly higher with binaural stimulation than with a monaural stimulation, and the existence of binaural interaction was also suggested. Binaural effect was more prominent in Pa than in Na component. When the data recorded from F3 and F4 electrodes were compared, most of F3/F4 amplitude ratios of Na and Pa were included between 0.7 and 1.5. The F3/F4 ratio should be useful indices for the diagnosis of excessive laterality. We also demonstrated gradual phase shifts of Na and Pa between the frontal and temporo-occipital area in about one-third of the records. This phenomenon strongly suggests that both Na and Pa components are derived from complex generators.