Efferent modulation of pre-neural and neural distortion products.

Distortion product otoacoustic emissions (DPOAEs) and distortion product frequency following responses (DPFFRs) are respectively pre-neural and neural measurements associated with cochlear nonlinearity. Because cochlear nonlinearity is putatively linked to outer hair cell electromotility, DPOAEs and DPFFRs may provide complementary measurements of the human medial olivocochlear (MOC) reflex, which directly modulates outer hair cell function. In this study, we first quantified MOC reflex-induced DPOAE inhibition at spectral fine structure peaks in 22 young human adults with normal hearing. The f1 and f2 tone pairs producing the largest DPOAE fine structure peak for each subject were then used to evoke DPFFRs with and without MOC reflex activation to provide a related neural measure of efferent inhibition. We observed significant positive relationships between DPOAE fine structure peak inhibition and inhibition of DPFFR components representing neural phase locking to f2 and 2f1-f2, but not f1. These findings may support previous observations that the MOC reflex inhibits DPOAE sources differentially. That these effects are maintained and represented in the auditory brainstem suggests that the MOC reflex may exert a potent influence on subsequent subcortical neural representation of sound.

Auditory thalamic organization: cellular slabs, dendritic arbors and tectothalamic axons underlying the frequency map.

A model of auditory thalamic organization is presented incorporating cellular laminae, oriented dendritic arbors and tectothalamic axons as a basis for the tonotopic map at this level of the central auditory system. The heart of this model is the laminar organization of neuronal somata in the ventral division of the medial geniculate body (MGV) of the rabbit, visible in routine Nissl stains. Microelectrode studies have demonstrated a step-wise ascending progression of best frequencies perpendicular to the cell layers. The dendritic arbors of MGV neurons are aligned parallel to the cellular laminae and dendritic tree width along the frequency axis corresponds closely with the frequency steps seen in microelectrode studies. In the laminated subdivision, tectothalamic axons terminate in the form of bands closely aligned with the laminae and dendritic arbors of thalamic relay neurons. The bands of tectothalamic axons extend in the anterior-posterior (A-P) plane forming a dorsal-ventral series of stacked frequency slabs. In the pars ovoidea region, the homologous spiraling of somata, dendritic fields and tectothalamic axons appear to represent a low-frequency area in this species. At least two types of tectothalamic terminals were found within the bands: large boutons frequently arranged in a glomerular pattern and smaller boutons arising from fine caliber axons. We propose that the rabbit is an ideal model to investigate the structural-functional basis of functional maps in the mammalian auditory forebrain.

The effect of noise bandwidth on the contralateral suppression of transient evoked otoacoustic emissions.

The purpose of this study was to determine whether the bandwidth or loudness of a contralateral stimulus is the most important factor in evoking suppression of transient evoked otoacoustic emissions (TEOAEs). TEOAEs were measured in both ears of 10 women in quiet and in the presence of one of three contralateral noise bands; narrow band (NB), wide band (WB) and equalized (EQ), all centered at 2000 Hz. The NB (100 Hz bandwidth) and WB (2200 Hz bandwidth) noises were presented at 60 dB SPL. The SPL of the EQ (100 Hz bandwidth) noise was adjusted such that it was equal in loudness to the WB noise as determined using a psychoacoustic procedure. Only the WB noise was associated with a significant reduction of TEOAE levels. It is believed that this effect occurred because the WB noise has greater effective energy representation across frequency on the basilar membrane as it may receive more gain from the action of the cochlear amplifier. Results of the present study indicate that noise bandwidth is the most important factor in the contralateral suppression of TEOAEs.

Frequency organization and cellular lamination in the medial geniculate body of the rabbit.

Cellular laminae within the tonotopically organized ventral division of the medial geniculate body (MGV) of the cat have been proposed as the anatomical substrate for physiologically defined isofrequency contours. In most species, the laminae are not visible with routine Nissl stains, but are defined by the dendritic fields of principal cells and the terminal arbors of afferents arising from the inferior colliculus. In the present study, we have used the rabbit to directly examine the relationship between the laminar and tonotopic organization of the MGV. Best frequency maps of the MGV in anesthetized adult New Zealand white rabbits were generated from cluster responses recorded at 30-100 microm intervals to randomly presented tone bursts. Parallel vertical penetrations, roughly perpendicular to the laminae, revealed a low-to-high frequency gradient within the MGV. Non-laminated regions of the ventral division, generally found at the rostral or caudal poles, did not demonstrate a systematic frequency gradient. In contrast to a predicted smooth gradient, best frequencies shifted in discrete steps across the axis of the laminae. A similar step-wise frequency gradient has been shown in the central nucleus of the inferior colliculus of the cat. It is proposed that the central laminated core of the MGV represents an efficient architecture for creating narrow frequency filters involved in fine spectral analysis.