Potential cues for the "level discrimination" of a noise band in the presence of flanking bands

Neural Fluctuation Contrast as a Code for Complex Sounds: The Role and Control of Peripheral Nonlinearities

The nonlinearities of the inner ear are often considered to be obstacles that the central nervous system has to overcome to decode neural responses to sounds. This review describes how peripheral nonlinearities, such as saturation of the inner-hair-cell response and of the IHC-auditory-nerve synapse, are instead beneficial to the neural encoding of complex sounds such as speech. These nonlinearities set up contrast in the depth of neural-fluctuations in auditory-nerve responses along the tonotopic axis, referred to here as neural fluctuation contrast (NFC). Physiological support for the NFC coding hypothesis is reviewed, and predictions of several psychophysical phenomena, including masked detection and speech intelligibility, are presented. Lastly, a framework based on the NFC code for understanding how the medial olivocochlear (MOC) efferent system contributes to the coding of complex sounds is presented. By modulating cochlear gain control in response to both sound energy and fluctuations in neural responses, the MOC system is hypothesized to function not as a simple feedback gain-control device, but rather as a mechanism for enhancing NFC along the tonotopic axis, enabling robust encoding of complex sounds across a wide range of sound levels and in the presence of background noise. Effects of sensorineural hearing loss on the NFC code and on the MOC feedback system are presented and discussed.

The role of the medial olivocochlear reflex in psychophysical masking and intensity resolution in humans: a review

This review addresses the putative role of the medial olivocochlear (MOC) reflex in psychophysical masking and intensity resolution in humans. A framework for interpreting psychophysical results in terms of the expected influence of the MOC reflex is introduced. This framework is used to review the effects of a precursor or contralateral acoustic stimulation on 1) simultaneous masking of brief tones, 2) behavioral estimates of cochlear gain and frequency resolution in forward masking, 3) the buildup and decay of forward masking, and 4) measures of intensity resolution. Support, or lack thereof, for a role of the MOC reflex in psychophysical perception is discussed in terms of studies on estimates of MOC strength from otoacoustic emissions and the effects of resection of the olivocochlear bundle in patients with vestibular neurectomy. Novel, innovative approaches are needed to resolve the dissatisfying conclusion that current results are unable to definitively confirm or refute the role of the MOC reflex in masking and intensity resolution.

Masking of short tones in noise: Evidence for envelope-based, rather than energy-based detection

The "temporal effect" in simultaneous masking may be characterized by better probe detection thresholds for a short, tonal probe presented at the temporal center of a masker compared to at the onset of a masker. Energy-based models of masking have been used to interpret the temporal effect as evidence that the gain of the auditory system decreases during acoustic stimulation. This study shows that masking from temporal-envelope fluctuations of a precursor or from a temporal gap between stimuli violates the assumptions of energy-based models and complicates the interpretation of temporal effects in terms of a reduction in gain. Detection thresholds were measured for a 6-ms, 4000-Hz probe preceded by a narrowband precursor and presented 2-, 197-, or 392-ms after the onset of a narrowband masker. The delay between the precursor offset and masker onset ranged from -2 to 250 ms. Probe thresholds were elevated in the presence of precursors with fluctuating compared to flattened temporal envelopes and when a temporal gap was inserted between the precursor and masker. The results suggest that the interpretation and design of temporal-effect studies should consider the masking effects of temporal-envelope fluctuations. These findings are consistent with speech-perception experiments that show masking from temporal-envelope fluctuations.