A listener's perception of a physically invariant sound often changes under prolonged stimulation, of the order of minutes. The listener may experience a decline in loudness, a loss of clarity and pitch strength, a change in locus (inside the head for earphone listening or externally for sounds presented from a loudspeaker). These changes may result from continued exposure to sound alone or from the presence of other sounds or from cognitive variables. The proposed research will continue to quantify the changes in loudness of an invariant sound and will extend the methods to quantify changes in clarity, pitch strength, and locus. Several explicit hypotheses about the source of loudness adaptation will be tested. This research is relevant not only to the development of our understanding of how auditory attributes change over time but also to understanding how they are coded in the nervous system. They have a bearing on individual differences in susceptibility to decrements in auditory function resulting from noise exposure, as suggested by strong correlation between temporary threshold shift and induced loudness adaptation. The results are relevant also to problems of adaptation in various types of auditory pathology and to the use of cochlear implants. More generally, we are also concerned with unraveling the bases for sound segregation by the auditory system. How do we manage to hear out particular sounds in a noisy environment? Many of our measurements will be with one sound presented in the presence of another. We also propose to extend our measurements to include cognitive effects such as selective attention, training, and perceived distance of a sound source. The methods to be used include direct estimation of subjective magnitude by the assignment of numbers to represent the subjective magnitude. We shall also use variants of the probe- signal method to explore the role of selective attention in detection and discrimination. (The probe-signal method has the listener attend primarily to one kind of sound--at a particular frequency or locus--while presenting on some trials sounds at other frequencies or loci.) These measurements will lay the foundation for measuring the effects of attention on changes in subjective magnitude.
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