Effects of Signal Level and Background Noise on Spectral Representations in the Auditory Nerve of the Domestic Cat

Abstract

Background noise poses a significant obstacle for auditory perception, especially among individuals with hearing loss. To better understand the physiological basis of this perceptual impediment, the present study evaluated the effects of background noise on the auditory nerve representation of headrelated transfer functions (HRTFs). These complex spectral shapes describe the directional filtering effects of the head and torso. When a broadband sound passes through the outer ear en route to the tympanic membrane, the HRTF alters its spectrum in a manner that establishes the perceived location of the sound source. HRTF-shaped noise shares many of the acoustic features of human speech, while communicating biologically relevant localization cues that are generalized across mammalian species. Previous studies have used parametric manipulations of random spectral shapes to elucidate HRTF coding principles at various stages of the cat's auditory system. This study extended that body of work by examining the effects of sound level and background noise on the quality of spectral coding in the auditory nerve. When fibers were classified by their spontaneous rates, the coding properties of the more numerous low-threshold, high-spontaneous rate fibers were found to degrade at high presentation levels and in low signal-to-noise ratios. Because cats are known to maintain accurate directional

The neurogram matching similarity index (NMSI) for the assessment of similarities among neurograms

“…Deviating from the physiological composition of the auditory nerve, where HSR fibers are the dominating population [13], we consider a composition of ANFs as 10 % for HSR, 20 % for MSR and 70 % for LSR fibers. This is consistent with the theory that neural coding may be disproportionately based on the enhanced dynamic range of LSR fibers in noisy environments as suggested in [14].…”

Vertical-plane sound localization with distorted spectral cues

“…This has been observed for vowels (Sachs and Young, 1979) and for DTF-filtered noise bursts (e.g. Reiss et al, 2011). Consistent with this, Alves-Pinto and Lopez-Poveda (2005) have reported that listeners’ ability to detect notches in wideband noise spectra declines with increasing stimulus intensity.…”

“…Reiss et al (2011) have, however, identified a population of low-spontaneous-rate (LSR), high-threshold fibers in the cat which have extremely large dynamic ranges, and for which spectral encoding remains robust at higher intensities. They propose that since cats’ vertical-plane localization performance does not decline at high intensities (for relatively long noise-burst stimuli), these fibers are the ones subserving vertical-plane localization.…”

“…The DTF manipulations we employed, based on single notional auditory-nerve rate-level functions, are undoubtedly simplistic models of whatever distortions of stimulus spectrum representation occur at low and high levels. DTF spectral profiles are more likely encoded at different stimulus intensities by multiple populations of auditory nerve fibers with a diversity of thresholds and dynamic ranges (Reiss et al, 2011). Our manipulations also do not take into account the normal compressive action of the outer hair cells at intermediate intensity levels and its absence at very low and high levels.…”

Auditory Efferents Facilitate Sound Localization in Noise in Humans: Figure 1.

“…Judgments in the up/down and front/back dimensions are highly sensitive to signal-to-noise ratio (SNR) degradation (Good and Gilkey, 1996; Best et al, 2005). It has been suggested that the MOC system can help to counteract detrimental effects of background noise on sound localization by “un-masking” spectral cues (Reiss et al, 2011). Behavioral studies, which have found poorer sound-localization performance in cats with a lesioned OC system than in controls, have provided some support for this hypothesis (May et al, 2004).…”