Topographic distribution of the tibial somatosensory evoked potential using coherence

Abstract

The objective of the present study was to determine the adequate cortical regions based on the signal-to-noise ratio (SNR) for somatosensory evoked potential (SEP) recording. This investigation was carried out using magnitude-squared coherence (MSC), a frequency domain objective response detection technique. Electroencephalographic signals were collected (International 10-20 System) from 38 volunteers, without history of neurological pathology, during somatosensory stimulation. Stimuli were applied to the right posterior tibial nerve at the rate of 5 Hz and intensity slightly above the motor threshold. Response detection was based on rejecting the null hypothesis of response absence (significance level α = 0.05 and M = 500 epochs). The best detection rates (maximum percentage of volunteers for whom the response was detected for the frequencies between 4.8 and 72 Hz) were obtained for the parietal and central leads mid-sagittal and ipsilateral to the stimulated leg: C4 (87%), P4 (82%), Cz (89%), and Pz (89%). The P37-N45 time-components of the SEP can also be observed in these leads. The other leads, including the central and parietal contralateral and the frontal and fronto-polar leads, presented low detection capacity. If only contralateral leads were considered, the centro-parietal region (C3 and P3) was among the best regions for response detection, presenting a correspondent well-defined N37; however, this was not observed in some volunteers. The results of the present study showed that the central and parietal regions, especially sagittal and ipsilateral to the stimuli, presented the best SNR in the gamma range. Furthermore, these findings suggest that the MSC can be a useful tool for monitoring purposes.

Rice Detector: Proposal of a Novel Objective Response Detection Technique

“…For the case in which the stimulation is periodic and the EEG signal is discrete-time, finite-duration and windowed, the MSC can be estimated as (Melges et al, 2008): where "^" denotes estimation, X i ( f ) is the Fourier Transform of the i-th EEG epoch and M is the number of epochs employed in the calculation. The analytical critical values for the MSC can be obtained as described in (Dobie and Wilson, 1993;Miranda de Sá and Infantosi, 2007):…”

“…Such techniques are based on statistical tests and, therefore, allow inferring about the sensorial response occurrence with a previously established significance level (false positives). One of the univariate frequency-domain ORD techniques with most promising results in the somatosensory response detection is the MagnitudeSquared Coherence (MSC) (Melges et al, 2008;2011). The univariate techniques are, in general, easier to apply for both clinical and surgical monitoring, since they allow a more compact acquisition apparatus, by recording only one EEG derivation.…”

Frequency-Domain Objective Response Detection Techniques Applied to Evoked Potentials: A Review

“…Later, Melges et al (2008a) investigated the topographic distribution of the tibial nerve somatosensory evoked potential (SEP) using the MSC and verified that the best regions for SEP recording, in an ORD approach, includes the central and parietal leads at the midline and parasagittal line ipsilateral to the stimulated limb. Two years later, Farina et al (2010) proposed a novel ORD technique based on the Rice distribution, obtaining the analytical critical values and using simulated signals to calculate the probability of detection for different values of signal-to-noise ratio.…”

Using Objective Response Detection techniques for detecting the tibial somatosensory evoked response with different stimulation rates

“…(1)-(4), respectively, with˛= 5% and M = 500 and 100 epochs. Only the derivations Cz, Pz, C4 and P4 were evaluated, since they were considered the best sites for tibial SEP recording, as reported in a previous work (Melges et al, 2008).…”

“…This subjectivity can be reduced by the application of the Objective Response Detection (ORD) techniques, which are methods based on statistical tests and have been presenting encouraging results for detecting visual (Miranda de Sá et al, 2002;Miranda de Sá and Felix, 2003), auditory (Cagy and Infantosi, 2007) and somatosensory Melges et al, 2008) cortical responses. These techniques allow detection with a maximum false-positive rate defined a priori.…”