Listening under cognitive load makes speech sound fast

Abstract

Listeners interpret local temporal cues (e.g., vowel durations) relative to the surrounding speech rate. For instance, an ambiguous Dutch vowel midway between short /ɑ/ and long /a:/ may be perceived as long /a:/ when presented in a fast context, but as short /ɑ/ in a slow context [1]. It is widely assumed that this process known as rate normalization is an early general auditory process [1, 2], and as such would operate independent from other higher level influences such as attention. However, when the perceptual system is taxed by the concurrent execution of another task, the encoding of the incoming speech signal is known to be negatively affected [3]. Therefore, listening to, for example, fast speech under cognitive load may result in impoverished encoding of the fast speech rate, reducing the effect that a fast context may have on the perception of subsequent speech (i.e., a reduction of the rate effect; cf. [4]). Alternatively, an increase in cognitive load has been shown to speed up time perception (the “shrinking of time”, [5]), potentially increasing the perceived rate of concurrent speech. This argument has, for instance, been used to explain why foreign-accented speech sounds faster than native speech [6]. Here we attempt to distinguish between these alternatives by testing Dutch /ɑ/-/a:/ categorization as a function of (1) the rate of the preceding carrier sentence and (2) the difficulty of a dual task (visual search) performed during carrier presentation.