Cortical response variation with different sound pressure levels: a combined event-related potentials and FMRI study.

INTRODUCTION: Simultaneous recording of electroencephalography (EEG) and functional magnetic resonance imaging (fMRI) provides high spatial and temporal resolution. In this study we combined EEG and fMRI to investigate the structures involved in the processing of different sound pressure levels (SPLs). METHODS: EEG data were recorded simultaneously with fMRI from 16 healthy volunteers using MR compatible devices at 3 T. Tones with different SPLs were delivered to the volunteers and the N1/P2 amplitudes were included as covariates in the fMRI data analysis in order to compare the structures activated with high and low SPLs. Analysis of variance (ANOVA) and ROI analysis were also performed. Additionally, source localisation analysis was performed on the EEG data. RESULTS: The integration of averaged ERP parameters into the fMRI analysis showed an extended map of areas exhibiting covariation with the BOLD signal related to the auditory stimuli. The ANOVA and ROI analyses also revealed additional brain areas other than the primary auditory cortex (PAC) which were active with the auditory stimulation at different SPLs. The source localisation analyses showed additional sources apart from the PAC which were active with the high SPLs. DISCUSSION: The PAC and the insula play an important role in the processing of different SPLs. In the fMRI analysis, additional activation was found in the anterior cingulate cortex, opercular and orbito-frontal cortices with high SPLs. A strong response of the visual cortex was also found with the high SPLs, suggesting the presence of cross-modal effects.

Determination of the loudness dependence of auditory evoked potentials: single-electrode estimation versus dipole source analysis.

OBJECTIVE: The loudness dependence of auditory evoked potentials (LDAEP) has been described as a measure of central serotonergic activity. Single-electrode estimation and dipole source analysis (DSA) are the most utilized methods for the estimation of LDAEP. To date, it is assumed that both methods are equally reliable. Nevertheless, according to our knowledge, the advantage of either method has not yet been shown directly. The aim of our study was to compare single-electrode estimation and dipole source analysis in the determination of the LDAEP. METHODS: Tones of five different intensities were presented binaurally to 10 healthy volunteers. Amplitudes of N1/P2 and LDAEP were determined at the central electrode site referenced to average and to linked mastoids and with DSA in the left and the right hemispheres. Scores were normalized (z-scores), compared, and correlated. RESULTS: Contrary to our expectations, we found a significant difference between scores obtained with single-electrode estimation and with DSA. CONCLUSION: The difference may be caused by confounding activation of a frontal source in the single-electrode method. The single-electrode approach cannot be equated with DSA in the determination of the LDAEP. This should be considered when comparing the results of different LDAEP studies using only one of these methods.