MICROSTRUCTURE AUDIOGRAMS AND PSYCHOPHYSICAL TUNING CURVES FROM SUBJECTS WITH SPONTANEOUS OTOACOUSTIC EMISSIONS (HEARING, ACOUSTICS).

Abstract

Spontaneous otoacoustic emissions (SOAEs) are narrowband continuous signals that can be recorded from the ear canals of humans in the absence of external stimulation. SOAEs have been detected in 30 to 50% of individuals whose hearing is within normal limits. They are believed to originate in the cochlea but the mechanism by which they are produced remains unknown. It has not been determined if SOAEs are an indication of subtle abnormality in the cochlea or if they are the result of an auditory system that is functioning normally. Most investigators agree, however, that an active source of energy within the cochlea is required for the generation of SOAEs. The objective of this study was to describe the effects of SOAEs on behavioral sensitivity and frequency selectivity. Eight subjects with SOAEs were evaluated. Sensitivity was measured using a Bekesy tracking method to obtain thresholds at 30-Hz intervals. Microstructure audiograms were obtained in this manner from the frequency region surrounding the SOAE and from the same frequency region in the other ear where no SOAE was detected. Frequency selectivity was assessed using a Bekesy tracking method to obtain psychophysical tuning curves (PTCs) from frequency regions with and without SOAEs. SOAEs were found to be associated consistently with threshold minima (regions of increased sensitivity) on the microstructure audiograms, although not all threshold minima corresponded to SOAE frequencies. PTCs centered at SOAE frequencies were characterized by higher Q10 values than PTCs from the same frequency region in the other ear or from a higher frequency region in the same ear. The high and low frequency slopes were not found to differ significantly when PTCs from frequency regions with and without SOAEs were compared. These results suggest that the differences between PTCs obtained from frequency regions with and without SOAES are restricted to the tips of the PTCs. Complex interactions among the SOAE, the probe tone, and the masker were thought to be responsible for the enhanced frequency selectivity observed near SOAE frequencies and manifested by higher Q10 values.