The effects of experimentally induced conductive hearing loss on spectral and temporal aspects of sound transmission through the ear

Abstract

Conductive hearing loss (CHL) is known to produce hearing deficits, including deficits in sound localization ability. The differences in sound intensities and timing experienced between the two tympanic membranes are important cues to sound localization (ILD and ITD, respectively). Although much is known about the effect of CHL on hearing levels, little investigation has been conducted into the actual impact of CHL on sound location cues. This study investigated effects of CHL induced by earplugs on cochlear microphonic (CM) amplitude and timing and their corresponding effect on the ILD and ITD location cues. Acoustic and CM measurements were made in 5 chinchillas before and after earplug insertion, and again after earplug removal using pure tones (500 Hz to 24 kHz). ILDs in the unoccluded condition demonstrated position and frequency dependence where peak far-lateral ILDs approached 30 dB for high frequencies. Unoccluded ear ITD cues demonstrated positional and frequency dependence with increased ITD cue for both decreasing frequency (± 420 µs at 500 Hz, ± 310 µs for 1–4 kHz ) and increasingly lateral sound source locations. Occlusion of the ear canal with foam plugs resulted in a mild, frequency-dependent conductive hearing loss of 10–38 dB (mean 31 ± 3.9 dB) leading to a concomitant frequency dependent increase in ILDs at all source locations. The effective ITDs increased in a frequency dependent manner with ear occlusion as a direct result of the acoustic properties of the plugging material, the latter confirmed via acoustical measurements using a model ear canal with varying volumes of acoustic foam. Upon ear plugging with acoustic foam, a mild CHL is induced. Furthermore, the CHL induced by acoustic foam results in substantial changes in the magnitudes of both the ITD and ILD cues to sound location.

The acoustical cues to sound location in the guinea pig (Cavia porcellus)

“…Maximum mean ITD observed was 324µs, remarkably similar to the 330 µs observed by Sterbing et al (2003). As predicted by Kuhn (1977), and consistent with the data from cats (Roth et al, 1980) and chinchilla (Koka et al, 2011; Lupo et al, 2011) the low frequency ITDs in the guinea pig were ~50% larger than the high frequency ITDs.…”

“…Previous results suggest that ITDs are larger for low- than high-frequency sounds in humans (Kuhn 1977) and animals (chinchilla: Koka et al 2011; Lupo et al 2011; cat: Roth et al 1980; guinea pig: Sterbing et al 2003). The analysis of ITD as in Section 3.2.3 obscures the possible frequency dependence of ITD.…”

“…Frequency dependent ITDs have been observed in human (Kuhn 1977), cat (Roth et al 1980; Tollin and Koka 2009a), gerbil (Maki and Furukawa 2005), chinchilla (Koka et al 2011; Lupo et al 2011) and guinea pig (Sterbing et al 2003). The frequency dependence of ITD observed in the present study was very similar to that reported by Sterbing et al (2003).…”

Development of the head, pinnae, and acoustical cues to sound location in a precocial species, the guinea pig ( Cavia porcellus )

“…The resulting acoustic waveforms were simultaneously recorded in the left and right ear canals using two probe tube microphones, amplified, collected using two analog to digital converter channels, and stored for off-line analysis. Measurements made previously in the lab (e.g., Lupo et al 2011) demonstrated that the chamber is effectively anechoic for frequencies down to 250 Hz. An acoustic calibration measurement was also made in the absence of the animal to capture the spectral characteristics of the loudspeakers and microphones.…”

The Physiological Basis and Clinical Use of the Binaural Interaction Component of the Auditory Brainstem Response

“…It is clear that peripheral hearing loss affects the BIC via amplitude reduction or peak latency delay. Laska et al (1992) noted that in the adult animals that experienced a 4-week monaural or binaural temporary conductive hearing loss, the conduction delay caused by the earplug (see also Lupo et al, 2011) resulted in an immediate temporary latency increase that resolved upon occlusion removal. In one subject showing a combined low-frequency and high-frequency hearing loss, Wrege and Starr (1981) similarly found a longer BIC latency compared to no latency effect for high-frequency hearing loss alone.…”