Auditory brain stem of the ferret: some effects of rearing with a unilateral ear plug on the cochlea, cochlear nucleus, and projections to the inferior colliculus

Abstract

To examine the influence of acoustic experience on the development of the mammalian auditory brain stem, darkly pigmented ferrets were reared with a plug inserted in the right outer ear. The plugs were first inserted on postnatal day 23-34 and produced a variable, frequency-dependent attenuation of up to 60 dB. Between 3-15 months after the ear plug was begun, animals were prepared for physiological recording and injection of wheat germ agglutinin-HRP (WGA-HRP) in the left inferior colliculus (IC). The plug was removed and the condition of the right ear was assessed by pure-tone stimulation and recordings from neurons in the left IC. Neural audiograms for each animal showed a residual deficit in most cases. Following 24-60 hr survival, the animals were perfused and the right ear was examined. Brain-stem sections were reacted with tetramethylbenzidine. Outer and/or middle ear pathology was present in over half of the animals. However, the cochleas appeared to be normal and the spiral ganglion cells were normal by several quantitative criteria: number, area, and nucleolar eccentricity. The volume of each division of the cochlear nuclei (CN) and the areas of individual neurons in the anteroventral CN were the same on the right and left sides. The number of CN neurons retrogradely labeled from the left IC injection of WGA-HRP was found to be significantly increased in the left CN, relative to normal animals, when expressed as a ratio of the number labeled in the right CN. We conclude that the residual hearing loss in the previously plugged ears was predominantly or exclusively conductive. Neonatal, unilateral conductive hearing loss in the ferret does not lead to degeneration of the CN on the side of the loss, but it does lead to at least one rearrangement of auditory brain-stem connectivity. We suggest that the extent to which the brain stem is modified by early auditory deprivation is dependent on the type, degree, and symmetry of the hearing loss.

Lesions of the Auditory Cortex Impair Azimuthal Sound Localization and Its Recalibration in Ferrets

“…The concha of the external ear was filled with Otoform-K 2 silicone impression material (Dreve Otoplastik, Unna, Germany). The sound attenuation produced by these earplugs has previously been estimated, using auditory brain stem response audiometry, to be ϳ40 dB across a broad range of frequencies (Moore et al 1989). Acoustical measurements from a number of different model ferret ears created from Otoform imprints of variously sized real ears confirmed that the earplugs produced 40 -50 dB of attenuation at frequencies of Ͼ3.5 kHz, which gradually rolled off at lower frequencies.…”

“…The remaining three ferrets provided control data for the earplugging experiment. The methods used to train the ferrets have been described in detail in previous reports (Kacelnik et al 2006;Nodal et al 2008) and are outlined briefly below.…”

“…We have previously characterized sound localization behavior in ferrets by measuring both the head orienting and approach-to-target responses made by the animals following presentation of broadband sounds from loudspeakers covering the full 360°of sound azimuth (Kacelnik et al 2006;Nodal et al 2008;. In this study, we explore the neural circuitry responsible for translating auditory signals into motor commands by investigating the effects on the accuracy of both the initial head orienting movement and the subsequent locomotor response of bilaterally aspirating either the primary auditory cortex (A1) alone or larger areas of the cortex that also encompass nonprimary fields.…”

Impaired Binaural Hearing in Children Produced by a Threshold Level of Middle Ear Disease

“…More recent research provides further support for changes in auditory brain function (Moore et al 1999;King et al 2000;Tucci et al 2001). However, while the effects of visual form deprivation and interocular misalignment in infancy are profound and relatively well understood, the early, mild, asymmetric conductive hearing loss produced by OME (Kokko 1974;Gravel and Wallace 2000; or, experimentally, by occlusion of the ear canal (Knudsen et al 1984;Moore et al 1989Moore et al , 1999King et al 2000) or malleus removal (Cook et al 2002) seems to exert a more subtle influence on central auditory processing.…”

“…1A) and incidence (Hogan et al 1997(Hogan et al , 1998Paradise et al l997;Hogan 1999) of otitis media with effusion (OME, also known as ''glue ear'') in early childhood has generated much speculation concerning its effect on the development of the human auditory system and on language acquisition and perception. Concern about the effects of OME as a form of auditory deprivation was further heightened by research from animal studies, first in the visual system (Wiesel and Hubel 1965;Sherman and Spear 1982) and later in the auditory system (Knudsen et al 1984;Moore et al 1989), showing that abnormal sensory experience could produce long-term changes in the structure and function of the primary brain circuits responsible for vision and hearing. More recent research provides further support for changes in auditory brain function (Moore et al 1999;King et al 2000;Tucci et al 2001).…”