Hearing with One Ear: Consequences and Treatments for Profound Unilateral Hearing Loss

Abstract

There is an increasing global recognition of the negative impact of hearing loss, and its association to many chronic health conditions. The deficits and disabilities associated with profound unilateral hearing loss, however, continue to be under-recognized and lack public awareness. Profound unilateral hearing loss significantly impairs spatial hearing abilities, which is reliant on the complex interaction of monaural and binaural hearing cues. Unilaterally deafened listeners lose access to critical binaural hearing cues. Consequently, this leads to a reduced ability to understand speech in competing noise and to localize sounds. The functional deficits of profound unilateral hearing loss have a substantial impact on socialization, learning and work productivity. In recognition of this, rehabilitative solutions such as the rerouting of signal and hearing implants are on the rise. This review focuses on the latest insights into the deficits of profound unilateral hearing impairment, and current treatment approaches.

Keywords: cochlear implant; monaural; single-sided deafness; spatial hearing; unilateral hearing loss.

Figure 1

(A) An illustration of the acoustic head-shadow and the principle of time-delay between the two ears, dependent on the sound source. (B) Right and left temporal signals arriving from a right-leading sound source location. The ILD and ITD are illustrated on the amplitude and temporal domains, respectively. (C) The frequency and azimuth dependencies of the head and pinnae filter are shown. As the sound source moves from the hearing side (positive azimuth angles) to the deaf side (negative azimuth angles), the acoustic barrier created by the head attenuates high frequency signals contralateral to the source. Pinna gains are also observed for some frequencies at the ear ipsilateral to the source. (D) An illustration of the monaural spectral-pinnae cues as a function of the vertical position (elevation) of the sound source. The black arrows indicate the position-dependent frequency notch.

Figure 2

Comparison of the head-shadow and pinnae filtering for the words (A) ”Hat” and (B) “Sat” when presented at the deaf side. Spectrograms and sound waveforms (in gray) of the speech stimuli are shown for five corresponding azimuth positions. High frequency speech components are attenuated on the hearing side, erasing the unique informational marks that differentiate the words from each other.

Figure 3

An illustration of spatial hearing abilities for (A) normal hearing and (B) single-sided deaf listeners. Normal hearing listeners localize target sounds accurately and precisely along the horizontal (azimuth) and vertical (elevation) planes. Monaural hearing listeners, instead, can localize target sounds mainly in the hemifield of the hearing ear, with impaired performance on the contralateral deaf side.

Figure 4

Actual treatment solutions for profound unilateral hearing loss. Rerouting devices transmit the captured sound on the deaf side to the hearing ear (monaural input). In contralateral routing of signal (CROS) hearing aids, the transmitter wirelessly communicates to the contralateral hearing aid. Bone conduction devices transfer the sounds via transcranial skull vibrations towards the functional cochlea. The cochlear implant provides electrical stimulation directly to the deaf ear, preserving the contralateral acoustic hearing (bilateral input).