Preattentive processing of horizontal motion, radial motion, and intensity changes of sounds.

Localization of sound sources is critical for an appropriate behavioral response. This is not only true for localization in the horizontal plane but also for localization in depth. Depth ranging of sound sources implicates various distance cues, among others sound intensity. In this study, we measured human electroencephalography and compared mismatch negativity (MMN) amplitudes and latencies for horizontal motion, radial motion, and pure intensity changes in the free field. We observed similar MMN latencies for horizontal and radial motion, whereas MMN responses to pure intensity changes were comparably delayed. MMN amplitudes and latencies were not different for approaching and receding sounds. Our data suggest similar fast processing for horizontal and radial motion, whereas pure intensity changes are possibly processed with less priority.

Effect of amplitude modulation of background noise on auditory-evoked magnetic fields.

Although it is known that amplitude modulation of a background masker improves signal detection, its effects on supra-threshold sounds are still unclear. In this study, the effects of amplitude modulation of a background masker on supra-threshold sounds were evaluated using a whole-head magnetoencephalography system. We compared the auditory-evoked magnetic fields recorded with a 500-Hz tone-burst of supra-threshold level with either random speech-spectrum noise (Unmodulated noise) or amplitude-modulated noise (Modulated noise) in 10 young normal-hearing subjects. The sounds were presented binauraly. The strengths of the N100m in the two noise conditions were examined. The masking effect of Modulated noise on supra-threshold sound was stronger than that of Unmodulated noise, a finding opposite to that on peri-threshold sound. The growth rate of the N100m strengths in the Modulated noise condition was less than that in the Unmodulated noise condition in the left hemisphere, whereas they were not different in the right hemisphere, suggesting that the left hemisphere was more susceptible to the envelope fluctuation.

Clinical characteristics of delayed endolymphatic hydrops: long-term results of hearing and efficacy of hyperbaric oxygenation therapy.

CONCLUSION: Diuretics significantly improved hearing in patients with contralateral-type DEH, whereas they did not inpatients with ipsilateral-type DEH. OBJECTIVE: We report a review of 26 cases of DEH treated in recent 6 years. PATIENTS AND METHODS: The study group comprised 22 contralateral and 4 ipsilateral types of DEH. The efficacy of diuretics on the improvement in hearing was examined quantitatively. The efficacy of hyperbaric oxygenation therapy (HBO) on hearing was examined in six patients. RESULTS: The general properties of these cases were similar to those reported previously, except for the high proportion of patients with contralateral-type DEH. Diuretics significantly improved the hearing of patients with contralateral-type DEH. In contrast, no significant improvement by diuretics was noted in ipsilateral-type DEH.

Effects of continuous masking noise on tone-evoked magnetic fields in humans.

Two different types of steep loudness growth have been reported in detail in psychoacoustical studies but have rarely been evaluated by objective methods in humans. One occurs in inner-ear hearing-impaired patients and is known as loudness recruitment. Another similar phenomenon is observed in healthy subjects with concurrent presence of background noise. Concerning the first type, our previous study using magnetoencephalography (MEG) showed that enhancement of the dipole moment of N100m with increase in stimulus intensity was greater in patients than in normal individuals. However, it is unclear whether the enhancement of activity in auditory cortex will also be detected with background noise in healthy subjects. To elucidate the effects of continuous background noise on tone-evoked cortical activity, we measured auditory-evoked magnetic fields (AEFs) from 7 normal-hearing subjects in two different conditions, with and without 55 dB SPL continuous masking white noise (noise/quiet conditions). The stimuli were 200 ms 1-kHz tones delivered monaurally and randomly at 4 different intensities (40-70 dB SPL) with constant 1-s interstimulus intervals. The N100m increased in amplitude and decreased in latency as a function of stimulus intensity in both noise and quiet conditions. The dipole moment of N100m was significantly smaller in the noise than in the quiet condition, showing that continuous background noise suppresses the strength of tone-evoked cortical responses. The mechanisms underlying these two psychoacoustically similar phenomena of rapid loudness growth thus differ.

Right hemispheric predominance in the segregation of mistuned partials.

To elucidate the central mechanisms of sound segregation, we compared responses to a harmonic sound and a mistuned sound using a whole-head magnetoencephalography system. The harmonic sound was composed of a 200-Hz tone and its 2nd to 12th harmonics. The mistuned sound had, instead of the 600-Hz harmonic, a 696-Hz tone. In the right hemisphere, the amplitude of N100m responses evoked by the mistuned sound was significantly larger and the peak latency significantly longer than that evoked by the harmonic sound, suggesting that the right hemisphere plays a more important role than the left in detecting mistuned partials.