In this short review, we outline the challenges presented by the hostile environment for auditory fMRI. There are two main elements to this: the high level magnetic fields and the intense sound generated by the scanner operation. We then describe a series of techniques that have been employed to minimize the effects of the scanner noise on the subject and upon the subject’s evoked responses to sound stimuli. These include scanner designs, passive attenuation, use of specifically designed scanning sequences, specially designed sound delivery systems and active noise control. Finally, we briefly describe the use of some of these in combination to allow measurement of sound activation of different brain areas non-invasively using fMRI.
When implementing out-of-head sound localization with headphones, it is well known that using head-related transfer functions (HRTFs) other than those of a listener degrades sound image localization, and enhances localization error and front-back confusion. Some studies have indicated that moving a sound image eases these problems. We focus on moving sound images to achieve highly accurate localization, and propose a swing sound image method that enables sound image swing between two locations on a horizontal plane. Listening tests reveal that the proposed method greatly reduces the front-back confusion.
In this study, we investigated the effects of a previous sound on loudness by performing paired comparison experiments. Two pure tones measuring 74 to 86 dB SPL at 500 Hz were presented monaurally at a certain interstimulus interval (ISI), which was set between 0.5 and 8 s. One of the pure tones was presented in one ear, and then the other was presented in the contralateral or ipsilateral ear. The subject compared the loudness of two pure tones and responded which sound was felt louder. The effect of presentation order in the paired comparison was calculated from the experimental results, and then the previous sound effect was obtained from the presentation order effect. As results, the sound presented second in the ipsilateral ear was perceived to be about 1 dB softer than the first sound at an ISI of 0.5 s even when both sounds had the same SPL. On the contrary, the second sound in the contralateral ear was perceived to be about 1 dB louder than the first sound significantly. This effect is referred to as “auditory reinforcement.” The effect level of auditory reinforcement decreased as ISI increased.
The present study is intended as an investigation of speech dynamics, particularly the unsteady motion of glottal flow in the larynx. In order to focus on only fluid motion, the vocal cords are assumed to be non-vibrating rigid bodies, although the glottal sound source is described as the interaction between the flow and the vibrating vocal cords. The glottal flow based on the two-dimensional rigid body model is simulated by solving basic equations in a compressible viscous fluid that is subject to appropriate initial and boundary conditions. The obtained results demonstrate that the initial glottal flow was a simple symmetric jet and that the flow became an unsteady complicated flow with vortices distributed in two-dimensional space. Furthermore, the structure of the complicated flow changed with time. These results indicate that simple assumptions, such as linearization of the fluid equations or one-dimensional models, are inappropriate for analysis of the speech production process.
In this paper, the instantaneous frequency is defined as the one obtained by converting a real time signal to a complex analytic signal and by differentiating the time-dependent phase with respect to time. Theoretical expressions of instantaneous frequencies for signals given as combinations of sinusoidal components are presented. Those are compared to the results obtained by numerical methods using the discrete Fourier transform in order to confirm the validity of the expressions and to check accuracies of the numerical methods. A reason for the existence of negative instantaneous frequencies is given by a vector representation of signal components. The instantaneous frequencies of frequency- and amplitude-modulated signals are also discussed. For a periodic sinusoidal burst signal, it was found that the instantaneous frequency stays zero during the period when the signal is zero and takes a value equal to twice that of the sinusoid at the onset of the signal.