Three models of perceptual adaptation to speech

Abstract

Experiments were conducted to test the merits of three competing models of speech adaptation. Two models were sensory based and concerned (a) a shift and/or decline in the sensitivity peak of a speech analyzer following repeated listening to an adapting stimulus or (b) a narrowing of the stimulus range to which an analyzer is sensitive following repeated listening. A third model was response based, representing a bias effect of contrast. Three selective adaptation experiments were conducted using series of test syllables ranging from [ǰa] to [da], produced by variations in the duration of frication. In Experiments I and II, listeners were asked to identify by number each of seven test stimuli before and after adaptation with the [ǰa] endpoint member of the test series or after adaptation with either [ǰi] or [di]. A signal detectability analysis revealed highly systematic shifts in the peaks of sensitivity, as measured by values of d′ for adjacent stimuli. No systematic effects of response bias were obtained. In Experiment III, listeners were asked to identify nine test stimuli by consonant category [ǰ] or [d] before adaptation and after adaptation with either a single adapter (as in previous experiments) or with an adapting sequence that included the first six members of the test series. The average duration of frication of the six adapters was equal to the frication duration of the single adapter. The two adaptation tests produced nearly identical effects, suggesting that repeated listening to the single adapter did not reduce an analyzer's sensitivity range. The results provided support for a model in which the sensitivity peak of a speech analyzer is shifted by adaptation.