Influence of reference electrodes, stimulation characteristics and task paradigms on auditory P50

To clarify the nature of auditory P50, middle latency auditory evoked potentials were recorded by using different condition of reference electrodes (linked earlobes, LE; balanced non-cephalic, BN), stimulation characteristics (tone burst, human voice) and tasks (counting, simple reaction) in 10 right- handed males (aged 21-36 years). EEG was recorded from Fz, Cz, Pz, C3, C4, T3 and T4 according to the 10-20 system. Two groups of electrode sites were made for the statistical analysis: a midline group. Fz, Cz and Pz; and a lateral group, T3, C3, Cz, C4 and T4. The results were that the P50 amplitudes with BN electrodes were significantly higher than those with LE in both groups (midline, P<0.01; lateral, P<0.01); the P50 amplitudes by voice stimulations were significantly higher than those by tone stimulation in the lateral group (P<0.05), and the P50 latencies under a simple reaction paradigm were significantly shorter than those under a counting task in both groups (midline, P<0.05; lateral, P<0.05). These results suggest that various factors including motor response affect the P50 amplitudes and latencies.