Temporal analysis in normal and impaired hearing.

The ear contains an array of filters that separate the components of a complex signal into "channels" tuned to different center frequencies. Temporal analysis can be considered as two processes: analysis of the time pattern occurring within each channel, and comparison of the time patterns across channels. Within-channel acuity can be characterized by tasks such as gap detection, or by the ability to detect amplitude modulation as a function of modulation rate. The smallest detectable gap duration for a white noise stimulus is 2-3 ms. The results can be modeled by an array of filters, with each filter followed by a nonlinearity and a (central) sliding temporal integrator. Hearing impairment of cochlear origin can have adverse effects on temporal resolution because it often reduces the audible bandwidth of the stimuli, and because it results in a reduced sensation level of the stimuli. The sliding temporal integrator appears to be unaffected by hearing loss, although the nonlinearity preceding the integrator may be abnormal, and this can lead to reduced temporal resolution for sounds with slowly fluctuating envelopes. Hearing impairment of more central origin may also adversely affect temporal resolution, but the mechanisms responsible for this are not known. The acuity of across-channel temporal analysis depends on whether the task is one of discrimination or of identification of temporal order. The finest acuity (1-2 ms) occurs for discrimination tasks. Identification of temporal order is an order of magnitude worse. When the elements of a sequence of sounds are perceived as more than one source (more than one perceptual stream), the ability to judge the order of the elements can be very poor. Perceptual grouping processes can also have dramatic effects on the perceived temporal structure of sound. Conversely, temporal structure can have a powerful influence on perceptual grouping.