Enhancement of the acquisition speed of somatosensory evoked potentials by a response tuned wavelet filter

A pilot study was performed to see how much a Wavelet Transform (WT) based filter can reduce the recording time of intra-operative monitored somatosensory evoked potentials (SSEP) from contralateral median nerve stimulation. The designed filtering technique used a coefficient mask that was obtained from a WT of a completed SSEP response (n=500) that was overlaid on the time-scale coefficients of responses of individual subsums (n=25) before the signals underwent back transformation. The efficacy of the WT-filter was judged numerically by comparing the number of coefficients from the subsums with the number of coefficients of a completed response (threshold: 0.05 of the largest coefficient). Different Daubechies orthonormal wavelets (D4-D20) were examined. The completed SSEP response could be recognised in many of the WT-filtered subsums plots. The best results are obtained from wavelets between D12-D20. The reduction of the number of coefficients by the WT filter varied between 1/3 to 1/9 which predicts a reduction of the recording time by 30 to 70 percent, compared to conventional averaging.