Perception of speech reflects optimal use of probabilistic speech cues

Abstract

Listeners are exquisitely sensitive to fine-grained acoustic detail within phonetic categories for sounds and words. Here we show that this sensitivity is optimal given the probabilistic nature of speech cues. We manipulated the probability distribution of one probabilistic cue, voice onset time (VOT), which differentiates word initial labial stops in English (e.g., "beach" and "peach"). Participants categorized words from distributions of VOT with wide or narrow variances. Uncertainty about word identity was measured by four-alternative forced-choice judgments and by the probability of looks to pictures. Both measures closely reflected the posterior probability of the word given the likelihood distributions of VOT, suggesting that listeners are sensitive to these distributions.

Figure 1

[A] Probability distributions of tokens that listeners categorized in the narrow condition (dark lines) and wide condition (light lines). [B] Optimal response curves calculated from the probability distributions using Equation (1) for the narrow condition (dark lines) and wide condition (light lines).

Figure 2

Example display screen containing the items “beach”, “peach”, “lace” and “race”. Locations of items were randomized across trials. Actual displays were in color.

Figure 3

Fitted response curves for individual participants in [A] narrow condition and [B] wide condition. Optimal response curves (solid lines) and curves from average slope of individuals (dashed lines) for participants in [C] narrow condition and [D] wide condition.

Figure 4

Relationship between posterior probability and looks to the competitor object for each VOT. [A] Posterior probabilities of the competitor words calculated using Equation 1 for the narrow (dark lines) and wide (light lines) distributions. [B] Proportion of looks to the competitor object for the narrow group (shaded bars) and wide group (open bars) for all VOT values with sufficient trials to analyze. Error bars indicate SEM. * p <.05.