Time flies when you're having fun: Cognitive load makes speech sound fast

Speech perception in spontaneous conversation typically involves the execution of several concurrent tasks, such as driving a car or searching a menu. This simultaneous attentional and mnemonic processing taxes the cognitive system since it recruits limited central processing resources. How this cognitive load influences speech perception is debated. One account states that cognitive load has detrimental effects on speech perception by disrupting the sublexical (phonetic) encoding of the speech signal. This leads to an ‘impoverished encoding’ (Mattys & Wiget, 2011) of the phonetic cues in the signal, possibly induced by impaired perceptual acuity at the auditory periphery. Another account suggests that cognitive load affects the temporal computation of sensory input. People reliably underestimate durations of sensory input received under cognitive load, including speech (‘shrinking of time’; Block, Hancock, & Zakay, 2010), making spoken segments sound shorter (Casini, Burle, & Nguyen, 2009). This study tested the two accounts of the effects of cognitive load on speech perception (‘impoverished encoding’ and ‘shrinking of time’) by investigating acoustic context effects. The temporal and spectral context in which a particular word occurs influences that word’s perception. For instance, the perception of an ambiguous Dutch vowel midway between /ɑ/ (short duration, low F2) and /a:/ (long duration, high F2) may be biased towards /a:/ by presenting it in a fast context (rate normalization) or a context with a relatively low F2 (spectral normalization; Reinisch & Sjerps, 2013). The ‘impoverished encoding’ account hypothesizes that, when context sentences are presented under cognitive load, the phonetic encoding of the context sentence would be disrupted. As such, the temporal and spectral characteristics of that context sentence should have a reduced influence on the perception of a subsequent target word (cognitive load modulating context effects). Alternatively, the ‘shrinking of time’ account holds that cognitive load leads to an underestimation of the duration of the context sentence, inducing a perceptually faster speech rate. This account would therefore not predict a modulation of context effects under cognitive load but rather an independent effect of this perceived increase in speech rate of the context sentence on target perception (higher proportion of /a:/ responses). In two experiments, participants were presented with context sentences followed by target words containing vowels ambiguous between Dutch /ɑ/ and /a:/. In Experiment 1, the context varied in speech rate (slow or fast); in Experiment 2, the context varied in average F2 (high or low). Crucially, during the presentation of the context sentence (not during target presentation), a concurrent easy or difficult visual search task was administered (low vs. high cognitive load). We found reliable acoustic context effects: contexts with a higher speech rate (Experiment 1) or a lower average F2 (Experiment 2) biased target perception towards /a:/. Moreover, cognitive load did not modulate these temporal or spectral context effects. Rather, a consistent main effect of cognitive load was found: higher cognitive load biased perception towards /a:/. This suggests a perceptual increase in the context’s speech rate under increased cognitive load, providing support for the ‘shrinking of time’ account