An On-line Spike Form Discriminator for Extracellular Recordings Based on an Analog Correlation Technique

The discrimination of single unit activity in extracellular recordings presents a serious problem when the signal-to-noise ratio is low or when the amplitudes of interspersed spikes are similar. By exploiting spike form, the system described here performs discrimination using on-line hardware template matching. Using analog delay lines, the combined deviation of 8 input signal values from 8 stored template values is calculated simultaneously. The 8 template values are selected by adjusting 8 cursors to the desired spike trace on a CRT; the spike form discriminator (SPIFODIS) then generates a deviation function which steeply drops to zero whenever form similarity occurs, allowing for easy triggering. The performance of SPIFODIS was compared quantitatively with that of a conventional amplitude trigger in two cases: when detecting a single unit with varied signal-to-noise ratios and when separating double units of equal amplitude. At signal-to-noise ratios between 2 and 1 the error rate for SPIFODIS was only 15-50% of that of an amplitude trigger. In double-unit recordings showing only form differences, spikes are discriminated with very low error rate, while an amplitude trigger fails completely.