Identification of Everyday Sounds Perceived as Noise by Migraine Patients.

Objective Sound hypersensitivity is highly comorbid with migraine headaches. To elucidate the pathogenic mechanism of migraine attacks, we must first identify the types of everyday environmental sounds they perceive as unpleasant and clarify the acoustic properties of such sounds. This study aimed to clarify the true nature of "noise," i.e. everyday sounds perceived as unpleasant by migraineurs, by evaluating their subjective comfort/discomfort in response to several sounds commonly heard in everyday life. Methods Participants were presented with 20 environmental sounds they would likely hear daily. Subjects rated the pleasantness/unpleasantness of each stimulus using a nine-step scale. Patients We recruited 50 adults with migraine headaches (46 women, 4 men) and 50 healthy controls (35 women, 15 men). Results Migraineurs provided statistically significantly lower (more unpleasant) ratings to ambulance sirens, police car sirens, and railroad crossing bells than did controls. Our analysis also investigated the acoustic characteristics associated with higher rating gaps between the two groups. Greater divergence in ratings for the same stimulus was associated with less power (smaller amplitude envelope) and slower temporal variation in signals in the 400-Hz band. Conclusion We identified specific signal components associated with different subjective (un) pleasantness scores between migraineurs and healthy adults, which may lead to the elucidation of the pathogenic mechanism underlying migraine attacks triggered by sound.

A photosensitive polyimide based method for an easy fabrication of multichannel neural electrodes.

To develop practical and inexpensive multichannel neural electrodes, we designed and fabricated photosensitive polyimide based flexible multichannel neural electrodes using MEMS technologies. Compared to a conventional micromachining with non-photosensitive materials, the advantage of applying photosensitive polyimide is the elimination of the dry etching, which involves the complex multilevel processes for controlling the etching conditions to define the outline of the neural electrodes and expose the microelectrodes. Thus, photosensitive polyimide allows more options in optimization of the configuration and size of neural electrodes depending on experimental purposes, and enables fabrication at a lower cost with improvement of process yields.

Sensitivity of the auditory middle latency response of the guinea pig to interaural level and time differences.

This study examines the extent to which the auditory middle latency response (MLR) of the guinea pig is sensitive to sound localization cues such as interaural level and time differences (ILD and ITD, respectively). The MLR was recorded with an epidural electrode placed over the auditory cortex of an anesthetized guinea pig. Click stimuli were presented monaurally or binaurally with various ILDs and ITDs. The MLR was much larger for contralateral stimulation than for ipsilateral stimulation, and its amplitude was intermediate for diotic stimulation. The MLR amplitude was sensitive to both ILD and ITD: it decreased as the ipsilateral stimulus increased in level or advanced in time relative to the contralateral stimulus. The steep slope of the amplitude-versus-ITD function fell within an ITD range of +/-330 micros, namely the guinea pig's physiological ITD range. The response reduction that resulted from increasing the relative level of the ipsilateral level could be cancelled out by advancing the contralateral onset time relative to the ipsilateral onset time. This parallels the "time-intensity trading" in sound lateralization exhibited in human psychophysics. The results imply that the binaural interaction in the guinea pig MLR reflects aspects of neural processes that are involved in sound localization.

Reducing individual differences in the external-ear transfer functions of the Mongolian gerbil.

This study examines individual differences in the directional transfer functions (DTFs), the directional components of head-related transfer functions of gerbils, and seeks a method for reducing these differences. The difference between the DTFs of a given animal pair was quantified by the intersubject spectral difference (ISSD), which is the variance in the difference spectra of DTFs for frequencies between 5 and 45 kHz and for 361 source directions. An attempt was made to reduce the ISSD by scaling the DTFs of one animal in frequency and/or rotating the DTFs along the source coordinate sphere. The ISSD was reduced by a median of 12% after optimal frequency scaling alone, by a median of 19% after optimal spatial rotation alone, and by a median of 36% after simultaneous frequency scaling and spatial rotation. The optimal scaling factor (OSF) and the optimal coordinate rotation (OCR) correlated strongly with differences in head width and pinna angles (i.e., pinna inclination around the vertical and front-back axes), respectively. Thus, linear equations were derived to estimate the OSF and OCR from these anatomical measurements. The ISSD could be reduced by a median of 22% based on the estimated OSF and OCR.

Acoustical cues for sound localization by the Mongolian gerbil, Meriones unguiculatus.

The present study measured the head-related transfer functions (HRTFs) of the Mongolian gerbil for various sound-source directions, and explored acoustical cues for sound localization that could be available to the animals. The HRTF exhibited spectral notches for frequencies above 25 kHz. The notch frequency varied systematically with source direction, and thereby characterized the source directions well. The frequency dependence of the acoustical axis, the direction for which the HRTF amplitude was maximal, was relatively irregular and inconsistent between ears and animals. The frequency-by-frequency plot of the interaural level difference (ILD) exhibited positive and negative peaks, with maximum values of 30 dB at around 30 kHz. The ILD peak frequency had a relatively irregular spatial distribution, implying a poor sound localization cue. The binaural acoustical axis (the direction with the maximum ILD magnitude) showed relatively orderly clustering around certain frequencies, the pattern being fairly consistent among animals. The interaural time differences (ITDs) were also measured and fell in a +/- 120 micros range. When two different animal postures were compared (i.e., the animal was standing on its hind legs and prone), small but consistent differences were found for the lower rear directions on the HRTF amplitudes, the ILDs, and the ITDs.

Dependency of the interaural phase difference sensitivities of inferior collicular neurons on a preceding tone and its implications in neural population coding.

This study examined the sensitivities of the neuronal responses in the inferior colliculus (IC) to the interaural phase difference (IPD) of a preceding tone, and explored its implications in the neural-population representation of the IPD. Single-unit responses were recorded from the IC of anesthetized gerbils. The stimulus was a dichotic tone sequence with a common frequency (typically the unit's best frequency) and level (10-20 dB relative to the threshold), consisting of a conditioner (200 ms) followed by a probe (50 ms) with a silent gap (5-100 ms) between them. The IPDs of the 2 tones were varied independently. The presence of a conditioner generally suppressed the probe-driven responses; the effect size increased as the conditioner IPD approached the unit's most responsive IPD. The units' preferred IPDs were relatively invariant with the conditioner IPD. Two types of models were used to evaluate the effects of a conditioner on the IPD representation at the level of IC neural population. One was a version of the population-vector model. The other was the hemispheric-channel model, which assumed that the stimulus IPD is represented by the activities of 2 broadly tuned hemispheric channels. Both models predicted that, in the presence of a conditioner, the IPD representation would shift in a direction away from the conditioner IPD. This appears to emphasize the difference between the conditioner and the probe IPDs. The results indicate that in the IC, neural processes for IPD adapt to the stimulus history to enhance the representational contrast between successive sounds.

Time-frequency distribution of neuronal activity in the gerbil inferior colliculus responding to auditory stimuli.

We classified the firing pattern of neurons in the central nucleus of the inferior colliculus (ICc) in a time-frequency coordinate, except for simple phasic onset responses, into four groups using tone bursts with a wide frequency range. The ICc is the major nucleus in the auditory midbrain. Single-unit recordings were made from ICc contralateral to the monaurally stimulated ear in anesthetized gerbils. Neurons of three out of the four groups (53.8%) demonstrated that the firing distribution changed depending on time and frequency, which was not shown previously. Neurons of one group (37.6%) exhibited a frequency response range that changed little with time. The remaining neurons (8.6%) belonged to none of the above classifications. The time-frequency-sensitive neurons in ICc may be good candidates for coding communication sounds, which comprise complex temporal changes in frequency.