Development of sound measurement systems for auditory functional magnetic resonance imaging

Activation of auditory white matter tracts as revealed by functional magnetic resonance imaging

INTRODUCTION

The ability of functional magnetic resonance imaging (fMRI) to detect activation in brain white matter (WM) is controversial. In particular, studies on the functional activation of WM tracts in the central auditory system are scarce. We utilized fMRI to assess and characterize the entire auditory WM pathway under robust experimental conditions involving the acquisition of a large number of functional volumes, the application of broadband auditory stimuli of high intensity, and the use of sparse temporal sampling to avoid scanner noise effects and increase signal-to-noise ratio.

METHODS

Nineteen healthy volunteers were subjected to broadband white noise in a block paradigm; each run had four sound-on/off alternations and was repeated nine times for each subject. Sparse sampling (TR=8 s) was used.

RESULTS

In addition to traditional gray matter (GM) auditory center activation, WM activation was detected in the isthmus and midbody of the corpus callosum (CC), tapetum, auditory radiation, lateral lemniscus, and decussation of the superior cerebellar peduncles. At the individual level, 13 of 19 subjects (68 %) had CC activation. Callosal WM exhibited a temporal delay of approximately 8 s in response to the stimulation compared with GM.

CONCLUSIONS

These findings suggest that direct evaluation of the entire functional network of the central auditory system may be possible using fMRI, which may aid in understanding the neurophysiological basis of the central auditory system and in developing treatment strategies for various central auditory disorders.

Spatiotemporal Segregation of Neural Response to Auditory Stimulation: An fMRI Study Using Independent Component Analysis and Frequency-Domain Analysis

Although auditory processing has been widely studied with conventional parametric methods, there have been a limited number of independent component analysis (ICA) applications in this area. The purpose of this study was to examine spatiotemporal behavior of brain networks in response to passive auditory stimulation using ICA. Continuous broadband noise was presented binaurally to 19 subjects with normal hearing. ICA was performed to segregate spatial networks, which were subsequently classified according to their temporal relation to the stimulus using power spectrum analysis. Classification of separated networks resulted in 3 stimulus-activated, 9 stimulus-deactivated, 2 stimulus-neutral (stimulus-dependent but not correlated with the stimulation timing), and 2 stimulus-unrelated (fluctuations that did not follow the stimulus cycles) components. As a result of such classification, spatiotemporal subdivisions were observed in a number of cortical structures, namely auditory, cingulate, and sensorimotor cortices, where parts of the same cortical network responded to the stimulus with different temporal patterns. The majority of the classified networks seemed to comprise subparts of the known resting-state networks (RSNs); however, they displayed different temporal behavior in response to the auditory stimulus, indicating stimulus-dependent temporal segregation of RSNs. Only one of nine deactivated networks coincided with the “classic” default-mode network, suggesting the existence of a stimulus-dependent default-mode network, different from that commonly accepted.