Psychophysical evidence for adaptation of central auditory processors for interaural differences in time and level
Introduction
The sensory systems are normally ascribed a role in the continuous, moment-to-moment updating of a veridical mental representation of the stimulus as it unfolds in time. In the case of the auditory system, it is clear that the perceptual processing of temporally proximate events is not independent, and this results in modest inaccuracies in the perceptual representation of the stimulus. These inaccuracies or misperceptions are informative about the nature of the stimulus processing within the sensory modality. In some cases, the auditory system appears to integrate information over a finite temporal window so that the perceptual elaboration of one event incorporates information about temporally adjacent ones. An example of this is the auditory saltation illusion (Hari, 1995, Shore et al., 1998, Phillips and Hall, 2001). If a train of dichotic clicks is constructed so that the first half of the train is lateralized to one side, and the second half of the train is lateralized to the other, and if the inter-click intervals are both regular and short, then the clicks are perceived as coming not only from the anchor points, but also from points in between them. The spatial processing of early clicks in the train is contaminated by the processing of later ones, as if the processing aroused by any given event is updated en route to the “writing” of that event’s percept (after Dennett, 1991, Phillips and Hall, 2001).
In other cases, adaptive responses to maintained stimulation influence the perceptual response to later stimuli, as in forward masking (Moore, 2003). These are expressed, in part, as sensitivity losses for sounds with spectral content near that of the masker. In most such cases, the adaptive response is attributed to the auditory periphery, for the reason that cochlear neurons have been shown to have adaptive responses that often follow general rules similar to the perceptual ones (cf. Smith, 1977, Turner et al., 1994, Moore, 2003), although it is noteworthy that forward masking also occurs in cochlear implant users, in whom cochlear mechanisms of adaptation are moot (Chatterjee, 1999). Behavioral studies of adaptive responses in strictly central auditory processes have been less forthcoming. There has been a small number of reports of simple aftereffects ascribed to central adaptive processes, e.g., for coding auditory motion (Grantham and Wightman, 1979, Grantham, 1989, Dong et al., 2000), free-field sound location (Carlile et al., 2001) and interaural time (Thurlow and Jack, 1973) and intensity differences (Elfner and Perrott, 1966). Dong et al. (1999) described a contingent auditory aftereffect using second-order stimuli: direction of tone frequency change was coupled to direction of motion in azimuth in a selective adaptation paradigm lasting about 10 min, and the perceived spatial properties of a later, test frequency-modulated tone were found to depend on the direction of frequency change. There is, of course, a long history of study of contingent aftereffects in vision (Frisby, 1980).
The purpose of the present report is to describe two experiments in which we were able to generate contingent aftereffects in the auditory system, after only seconds of exposure to a first-order adapting stimulus. The auditory system executes much stimulus analysis on a frequency-specific basis, and this processing includes that of source azimuth and the interaural differences in stimulus phase and level which provide cues to azimuth (Jenkins and Merzenich, 1984, Phillips, 2001). For both interaural phase (Yin and Kuwada, 1983, McAlpine et al., 2001) and level (Phillips and Irvine, 1981, Phillips and Brugge, 1985), the neurophysiological evidence suggests that the auditory system contains hemifield-tuned channels: over the behaviorally relevant range of interaural phase and level disparities, neural response rates are usually a saturating function of disparity, with cells on each side of the brain responding maximally to disparities favoring the contralateral ear, and with the steep portion of the response function associated with disparities near zero. It follows that exposure to a tonal adapting stimulus with an interaural disparity strongly favoring one ear should selectively “fatigue” the circuits serving that laterality at the frequency of that adaptor tone. Moreover, since the processing of interaural disparities is executed independently at different frequencies, it should be possible to adapt the disparity coding mechanisms for two frequencies in opposite directions at the same time – by using adaptor tones at two frequencies, but with oppositely signed disparities.
In what follows, we describe two experiments. In the first experiment, listeners were exposed to alternating adaptor tones of 260 and 570 Hz, lateralized to opposite sides using an interaural time difference (ITD) equivalent to a quarter-period interaural phase disparity. Before and after adaptation, listeners were asked to judge whether test tones (of 260 or 570 Hz) were lateralized to the left or right of a brief train of clicks with zero interaural time difference, and a psychometric function was obtained for ITD separately for each test frequency. A comparison of the pre- and post-adaptation responses for each frequency provided an index of the effect of the adaptor tones. The second experiment was of comparable general design, except that the interaural cue employed in the adaptor tones was a level difference (ILD) of 12 dB, and the listeners were, in separate test sessions, studied with oppositely lateralized high- (1800, 3030 Hz) or low- (260, 570 Hz) frequency tones. In Experiment 2, listeners were also tested for the effect of the adaptor stimulus regimen on thresholds for test-frequency tone detection at the ear receiving the higher amplitude member of the ILD adaptor; this, to examine the possibility that the ILD adaptor exerted any effect through peripheral adaptation.
Section snippets
General discussion
There have been previous descriptions of aftereffects generated by selective adaptation of central auditory processors. Some of them have been “simple” aftereffects in the sense that the target feature has been unitary, e.g., a particular direction of stimulus motion (Grantham and Wightman, 1979, Dong et al., 2000) or a particular laterality of ITD (Thurlow and Jack, 1973) or a particular pole of speech sound parameter (e.g., Eimas and Corbit, 1973, Cooper, 1974, Sawusch and Jusczyk, 1981,
Acknowledgements
D.P.P. is a Faculty of Science Killam Professor in Psychology at Dalhousie University. This work was supported by grants from NSERC of Canada and CLLRNet to D.P.P.
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