Tibial nerve somatosensory evoked response detection using uni and multivariate coherence

This work aims at comparing the capability of two Objective Response Detection techniques, the Magnitude-Squared Coherence (MSC or Ordinary Coherence) and its multivariate extension, the Multiple Coherence (MC), of detecting the somatosensory evoked response. Electroencephalographic (EEG) signals were collected during somatosensory stimulation from forty adult volunteers without history of neurological disease and with normal somatosensory evoked potential (SEP), using the 10-20 International System. All leads were referenced to the earlobe average. Current pulses with 200 μs of duration were applied to the right posterior tibial nerve at the motor threshold intensity level (the lowest intensity able to produce hallux oscillations) at the rate of 5 Hz. The MSC was applied to the derivations Cz], Fz], C3] and C4] – commonly used for tibial nerve SEP recordings with bipolar derivations – and the MC was applied to the pairs Cz]Fz] and C3]C4]. Both estimates (MC and MSC) were calculated with M = 100 and 500 epochs and the response detection was based on rejecting the null hypothesis of response absence, which is achieved when the estimates exceed the critical value (detection threshold) calculated for a given significance level (α = 0.05). The results showed that if two leads are available, the application of the MC is better than the MSC applied to each lead individually.