Auditory mechanisms involved in psychoacoustical intensity discrimination in quiet and in noise.

摘要

In order to represent the variety of sounds we encounter in our daily lives, it is critical that our auditory systems remain responsive to changes in sound level across a broad range of levels. In the peripheral auditory system, there are many mechanisms that may pose a limitation for intensity discrimination in quiet and background noise. These include limited dynamic ranges of individual and groups of auditory nerve fibers, basilar-membrane compression, and neural adaptation (for tone in the presence of noise). However, there are other mechanisms that can help overcome these limitations. These include spread of excitation cues for narrowband stimuli (in quiet), suppression (for a tone in noise), and the medial olivocochlear reflex (MOCR) (for a tone in a long noise). How all these mechanisms may contribute to intensity discrimination abilities in humans is not well understood.;Psychoacoustically, intensity discrimination abilities can be measured using a paradigm in which a tone called a pedestal is incremented in level. The smallest level increment a listener can detect is called an intensity discrimination limen or IDL. If IDLs are measured for short, high frequency tones, poorer IDLs are seen for mid-level tones than for low-level or high-level tones. There is evidence that this so-called "mid-level hump" reflects the limitation of basilar-membrane compression, which is overcome at high levels by the use of spread of excitation cues. However, some researchers propose instead that the mid-level hump originates more centrally in the auditory system. In Chapter 2, characteristics of the mid-level hump were compared to psychoacoustical estimates of basilar-membrane compression in the same listeners. Results supported the idea that the initial worsening in IDLs with increasing pedestal level reflects the decrease in basilar membrane input/output function slope. However, there were also differences across listeners consistent with central influences on intensity discrimination abilities.;Previous psychoacoustical studies have used notched noise (NN) to restrict the use of off-frequency listening. For tones at the mid-level hump, if the NN onset begins at least 50 ms prior to the onset of the pedestal, the IDL improves. This result may be consistent with activity of the MOCR, a sluggish, bilateral mechanism which can reduce effects of basilar-membrane compression and can reduce effects of neural adaptation. However, some researchers propose that a central mechanism---profile analysis---may be why the mid-level hump decreases with NN. In Chapter 3, IDLs at the mid-level hump were examined in forward, simultaneous, and backward NN of different durations and levels. These conditions were designed to separately test MOCR, suppression, and profile analysis mechanisms. Results showed improvements in IDLs with NN relative to quiet which were consistent with a suppression mechanism in some listeners and an MOCR mechanism in other listeners. No listeners showed results consistent with a benefit from profile analysis.;Another test of the MOCR is to measure IDLs with contralateral noise because the MOCR is a bilateral reflex. Previous physiological and modeling studies suggest that one role of the MOCR is to counteract the limiting effects of neural adaptation (brought about by the noise). However, the MOCR may also reduce the influence of compression for stimuli where basilar-membrane compression dominates. In Chapter 4, IDLs at the mid-level hump were measured in ipsilateral, contralateral, and bilateral broadband noise of different durations and levels. The results showed that in some listeners, ipsilateral noise led to improved IDLs (relative to quiet). Contralateral noise did not lead to improved IDLs for a tone in quiet. However, long contralateral noise led to improved IDLs for a tone in the presence of long ipsilateral noise. These results are consistent with MOCR activity which may reduce the limiting effects of neural adaptation in noise and can also reduce the limiting effects of compression relative to quiet.;Overall, these results provide perceptual evidence of the interplay of mechanisms that serve as limitations for discriminability over a wide range of levels and those that help overcome these limitations. Ultimately, mechanisms that aid in maintaining discriminability help the auditory system represent contrasts among stimuli such as speech in the presence of background noise.