Evidence for a reappraisal of the psychophysical selective adaptation paradigm.

The human psychophysical adaptation literature infers the existence of channels in the auditory system sensitive to frequency modulation (FM) from selective increases in FM detection thresholds following adaptation with FM stimuli. Using this psychophysical paradigm to characterize the attributes of feature-sensitive channels requires knowledge of the phenomenon's stability over repeated testing. In this study FM detection thresholds were measured in human subjects with continued testing over numerous sessions. During adapting intervals within the sessions either FM upsweeps or silence was presented. Exposure to FM upsweeps initially resulted in an increase in FM detection thresholds by a factor of 2 to 3 relative to those measured following silence. These initial threshold elevations decreased markedly with repeated testing (more than five 30-min experimental sessions). Final threshold differences between adapted and nonadapted conditions approached zero. In one subject, such asymptotic threshold values were regained in a single session, after a 4-month hiatus. The findings suggest multiple determinants of the selective adaptation function, as well as a reevaluation of inferred mechanisms.