Power spectrum and optimal filtering for visual evoked potentials to pattern reversal.

The optimal bandwidth for recording the visual evoked potential (VEP) to pattern reversal was investigated in 8 normal subjects by re-analyzing off-line data recorded on tape using an open bandwidth of 0.1 Hz-3 kHz. Power spectral analysis of the VEP revealed little energy above 50 Hz. With digital filtering, the amplitude N70-P100 was significantly attenuated only when the low-pass filter was reduced to 50 Hz or when the high-pass filter was raised to 8 Hz. With analogue filtering, there was significant prolongation of latency of P100 when the low-pass filter (12 dB/octave) was reduced to 250 Hz and a significant decrease in latency when the high-pass filter (6 dB/octave) exceeded 3 Hz. However, the effects of analogue filtering were not uniform across subjects: in 2 subjects the latency of P100 was prolonged using a low-pass filter of 600 Hz and in 2 other subjects the latency was shortened when the 1 Hz high-pass filter was introduced. If a restricted bandwidth is used, non-uniform distortion of latency could make a significant contribution to the variability in latency of P100. The optimal bandwidth is one which minimises this contribution to the variability, 0.3 Hz to greater than 600 Hz.