The effects of low pass filtering on central somatosensory conduction time

Central Somatosensory conduction time (CSCT) in the rat was measured by simultaneously recording the cortical somatosensory evoked potential (SEP) and cervical SEP and then subtracting the peak latency of the cervical response from that of the primary cortical response. The low frequency (high pass) filter of the recording system was kept fixed at 3.2 Hz but the high frequency cut-off was progressively raised from 32 to 16 kHz to examine the effects of low pass filtering on the two waveforms from which CSCT is derived. With a bandpass of 3.2-32 Hz, no activity could be reliably identified in either the cortical or cervical traces. With subsequent rises in the high frequency filter, the amplitude of both potentials increased with a concomitant decrease in their latencies. Stable values were obtained with a bandpass of 3.2-3.2 kHz and there was little additional change in waveform configuration. In contrast to the waveforms, low pass filtering had only minimal effects on CSCT and near constant values (in the range 3.6-4.1 ms) were obtained regardless of the setting of the low pass filter. I concluded that low pass filtering has a largely uniform effect on somatosensory activity generated in both primary cortical and cervico-medullary locations.