The use of psychophysical tuning curves to explore dead regions in the cochlea

Objective. "Dead regions" are regions in the cochlea with no functioning in ner hair cells (IHCs) and/or neurons. Amplification (using a hearing aid) o ver a frequency range corresponding to a dead region may not be beneficial and may even impair speech intelligibility. The objective of this article i s to illustrate the use of psychophysical tuning curves (PTCs) as a tool fo r investigating dead regions and to illustrate the variety of audiogram con figurations that can be associated with dead regions. We explore the influe nce of signal level and signal frequency to test the hypothesis that the fr equency at the tip of the tuning curve defines the boundary of the dead reg ion. Design: PTCs were measured for five subjects with sensorineural hearing los s who were suspected of having dead regions. One had a relatively "flat" lo ss, one had a mild mid-frequency loss and three had high-frequency losses, varying in severity from 70 dB to more than 120 dB. For each PTC, the level and frequency of the sinusoidal signal were fixed, and the level of a narr owband noise masker needed just to mask the signal was determined as a func tion of the masker frequency. When the signal falls in a frequency region t hat is not "dead," the signal is detected via IHCs with characteristic freq uencies (CFs) at or close to the signal frequency. In such a case, the tip of the PTC (the masker frequency at which the masker level is lowest) lies at or close to the signal frequency. When a dead region is present, the sig nal is detected via IHCs with CFs different from that of the signal frequen cy. In such a case, the tip of the PTC is shifted away from the signal freq uency. Results. PTCs with frequency-shifted tips (indicative of dead regions) were found for all subjects. The frequencies at the tips sometimes decreased sl ightly with increasing signal level. For the subject with a relatively flat loss, PTCs with tips close to 3000 Hz were obtained for signal frequencies of 400, 1000 and 1500 Hz. A PTC with a tip at 5000 Hz was found for a sign al frequency of 6000 Hz. These results suggest that this subject had an "is land" of surviving IHCs and neurons with CFs ranging from 3000 to 5000 Hz, with extensive dead regions on either side. For the subject with a mid-freq uency loss, the pattern of results suggested a mid-frequency dead region. F or the subjects with high-frequency loss, the results suggested the presenc e of high-frequency dead regions, in one case starting at a frequency where absolute thresholds were only slightly higher than normal. Conclusions: PTCs can be used to detect and delimit dead regions. Often, th e frequency at the tip of the PTC can be used to define approximately one b oundary of the dead region. However, the detection of beats can affect the shape of the PTC around the tip when the signal frequency lies just inside the dead region. The level of the signal can also have some effect on the f requency at the tip of the PTC. Very low signal levels can lead to variable results. Dead regions can start at frequencies where absolute thresholds a re near normal.