Accounting for rate-dependent category boundary shifts in speech perception

Abstract

The perception of temporal contrasts in speech is known to be influenced by the speech rate in the surrounding context. This rate-dependent perception is suggested to involve general auditory processes because it is also elicited by nonspeech contexts, such as pure tone sequences. Two general auditory mechanisms have been proposed to underlie rate-dependent perception: durational contrast and neural entrainment. This study compares the predictions of these two accounts of rate-dependent speech perception by means of four experiments, in which participants heard tone sequences followed by Dutch target words ambiguous between /ɑs/ "ash" and /a:s/ "bait". Tone sequences varied in the duration of tones (short vs. long) and in the presentation rate of the tones (fast vs. slow). Results show that the duration of preceding tones did not influence target perception in any of the experiments, thus challenging durational contrast as explanatory mechanism behind rate-dependent perception. Instead, the presentation rate consistently elicited a category boundary shift, with faster presentation rates inducing more /a:s/ responses, but only if the tone sequence was isochronous. Therefore, this study proposes an alternative, neurobiologically plausible account of rate-dependent perception involving neural entrainment of endogenous oscillations to the rate of a rhythmic stimulus.

Keywords: Durational contrast; Neural entrainment; Rate normalization; Rate-dependent perception; Speech rate.

Fig. 1

Examples of the precursor conditions used the different experiments. The top panel (Conditions A, B, C, and D) shows the isochronous precursor conditions used in Experiments 1–3; the bottom panel shows the anisochronous precursor conditions used in Experiment 4. Each plot shows the final second of a tone sequence (total duration = 4 s), followed by a target word, with the precursor condition given to the left

Fig. 2

Average categorization data (in % /a:/ responses) for Experiment 1, split by four different precursor conditions

Fig. 3

Average categorization data (in % /a:/ responses) for Experiment 2, split by four different precursor conditions

Fig. 4

Results of Experiment 3. Each panel displays % /a:/ responses to the two-dimensional vowel continuum (F2 on x-axis, duration on y-axis) for one of four different precursor conditions (slow long, slow short, fast long, fast short). For each combination of duration and F2 value (each tile), the tint reflects the % /a:/ responses indicated in the tile (the darker the tint, the higher the % /a:/ responses)

Fig. 5

Average categorization data (in % /a:/ responses) for Experiment 4, split by four different precursor conditions