A further parallel between selective adaptation and response contrast

It is generally believed that selective adaptation effects in speech perception are due to a reduction in sensitivity of auditory feature detectors. We have recently presented evidence that these effects may derive instead from response contrast. In a further test of the contrast hypothesis, we conducted both an adaptation and a contrast experiment with matching sets of stimuli. In the adaptation experiment, subjects identified two series of velar stimuli varying in VOT, [ga‐k^ha] and [gi‐k^hi], before and after adaptation with each of the following test stimuli: [ga], [k^ha], [gi], and [k^hi]. In the contrast experiment, subjects identified either of two ambiguous test items (drawn from near the phonetic boundaries of the [ga‐k^ha] and the [gi‐k^hi] series) following a single presentation of [ga], [k^ha], [gi], or [k^hi]. For both the adaptation and the contrast experiments, (1) the [Ca] test items were more greatly affected (in a contrast direction) by [Ca] than by the [Ci] adaptor/context stimuli, and (2) the [Ci] test items were not differentially affected by the [Ca] and [Ci] adaptor/context stimuli. These parallel results suggest that vowel contingent adaptation results may well be due to response contrast rather than sensory fatigue.