Potentials evoked by chirp-modulated tones: a new technique to evaluate oscillatory activity in the auditory pathway J. Artieda * , M. Valencia, M. Alegre, O. Olaziregi, E. Urrestarazu, J. Iriarte Neurophysiology Section, Department of Neurology, Clı ´nica Universitaria and Fundacio ´n para Investigacio ´n Me ´dica Aplicada, Universidad de Navarra, Avenida Pı ´o XII 36, 31008 Pamplona (Navarra), Spain Accepted 20 October 2003 Abstract Objective: Steady-state potentials are oscillatory responses generated by a rhythmic stimulation of a sensory pathway. The frequency of the response, which follows the frequency of stimulation, is maximal at a stimulus rate of 40 Hz for auditory stimuli. The exact cause of these maximal responses is not known, although some authors have suggested that they might be related to the ‘working frequency’ of the auditory cortex. Testing of the responses to different frequencies of stimulation may be lengthy if a single frequency is studied at a time. Our aim was to develop a fast technique to explore the oscillatory response to auditory stimuli, using a tone modulated in amplitude by a sinusoid whose frequency increases linearly in frequency (‘chirp’) from 1 to 120 Hz. Methods: Time-frequency transforms were used for the analysis of the evoked responses in 10 subjects. Also, we analyzed whether the peaks in these responses were due to increases of amplitude or to phase-locking phenomena, using single-sweep time-frequency transforms and inter-trial phase analysis. Results: The pattern observed in the time-frequency transform of the chirp-evoked potential was very similar in all subjects: a diagonal band of energy was observed, corresponding to the frequency of modulation at each time instant. Two components were present in the band, one around 45 Hz (30–60 Hz) and a smaller one between 80 and 120 Hz. Inter-trial phase analysis showed that these components were mainly due to phase locking phenomena. Conclusions: A simultaneous testing of the amplitude-modulation-following oscillatory responses to auditory stimulation is feasible using a tone modulated in amplitude at increasing frequencies. The maximal energies found at stimulation frequencies around 40 Hz are probably due to increased phase-locking of the individual responses. q 2004 International Federation of Clinical Neurophysiology. Published by Elsevier Ireland Ltd. All rights reserved. Keywords: Steady state evoked potential; Oscillatory activity; Auditory evoked potential; Human 1. Introduction Cortical oscillatory activity may be a key mechanism in perceptual binding (Singer, 1993). The role of synchronous oscillations in perception has been suggested by studies performed at very different levels, from single neuron activity to EEG and MEG (Engel et al., 1992; Tallon- Baudry et al., 1996; Rodriguez et al., 1999). These two latter techniques are limited by the spatial filter imposed by the head, although they have the invaluable advantage of being non-invasive. There are several approaches to the study of cortical oscillatory activity using EEG. An oscillatory response to a stimulus may have a constant phase relationship with it (phase-locked or ‘evoked’ response) or may consist of a change in the amplitude in the ongoing activity without a clear phase relationship with the stimulus (non phase-locked or ‘induced’ response) (Pfurtscheller and Lopes da Silva, 1999). The most typical examples of phase-locked responses are the evoked potentials. There are two possible mechanisms that can generate an evoked potential in an averaged response. It can result from the sum of low- amplitude potentials generated in each individual sweep; with the averaging process, the amplitude of the background EEG noise is reduced and these low-amplitude potentials come up and can be clearly observed. Alternatively, these responses might be due to synchronous phase-resetting in the ongoing activity caused by the stimulus, without any 1388-2457/$30.00 q 2004 International Federation of Clinical Neurophysiology. Published by Elsevier Ireland Ltd. All rights reserved. doi:10.1016/j.clinph.2003.10.021 Clinical Neurophysiology xx (2004) xxx–xxx www.elsevier.com/locate/clinph * Corresponding author. Tel.: þ 34-948-255400; fax: þ 34-948-296500. E-mail address: jartieda@unav.es (J. Artieda). ARTICLE IN PRESS