In functional MRI experiments on the central auditory system, activation caused by acoustic scanner noise is a dominating factor that partially masks the hemodynamic response signals to sound stimuli of interest. In this study, the nonlinear interaction between auditory responses to single scans and those to tone stimuli was investigated. By using irregular acquisition repetition times and quasi-random stimulus timings, the brain responses to pure tone stimuli were analyzed, as well as their interaction with scanner noise. The tone frequencies were chosen to match either the fundamental frequency of the scanner noise (730 Hz) or a region with little spectral power (4.70 kHz). The hemodynamic responses could be characterized by amplitudes of 1.3% and a time-to-peak of 4.0-4.5 sec in the absence of scanner noise. Interaction effects due to a single previous scan typically decreased the response magnitudes to 0.9%. The functional shape of the interaction was analyzed and could be described by a highly separable, dominantly symmetric interaction function that fairly agreed with a low-order Volterra expansion of a simple nonlinear model. Interactions were stronger and more complex in shape when the spectral content of the tone stimulus and the scanner noise were more similar.
Copyright 2004 Wiley-Liss, Inc.