Time-frequency analysis of chemosensory event-related potentials

Huart, Caroline [UCL] Rombaux, Philippe [UCL] Mouraux, André [UCL]

Introduction and aim Chemosensory event-related potentials (CSERP) are an objective technique for the evaluation of chemo- sensory function. Classically, they are revealed using across-trial time-domain averaging of electro- encephalographic (EEG) signals. However, this approach cancels out all changes in electric potential that are not strictly time-locked and phase-locked to the stimulus onset, and is thus blind to a significant fraction of the elicited cortical activity (e.g. event-related synchronization and desynchronization, ERS and ERD). Here, we used a novel approach based on the wavelet transform to perform a time-frequency decomposition of EEG epochs and, thereby, disclose the non phase-locked EEG responses that can be elicited by a chemosensory stimulus. Material and methods A total of 15 normosmic subjects took part in the study. After a psychophysical evaluation of olfactory function (Sniffin’ Sticks), CSERPs were recorded using 64 EEG channels. Phenylethyl alcohol was used for olfactory stimulation and CO2 was used for trigeminal stimulation (rise-time: 20 ms; 60 trials/condition; ISI = 15 s). Time-frequency decomposition of EEG signals was performed using the continuous Morlet transform. Results Concomitant to the phase-locked CSERPs, chemosensory stimulation elicits a brief non phased-locked increase of oscillation amplitude (ERS) followed by a widespread and long-lasting decrease (ERD), maximal in the alpha-band of the EEG (8-12 Hz). Conclusion Time-frequency analysis of CSERPs can be used to disclose an important fraction of the stimulus-induced cortical activity that is lost by conventional time-domain averaging. Because these responses are robust, they could improve the reliability of CSERPs recorded for the evaluation of patients suffering from olfactory disorders.