Adaptive plasticity in speech perception: Effects of external information and internal predictions

Abstract

When listeners encounter speech under adverse listening conditions, adaptive adjustments in perception can improve comprehension over time. In some cases, these adaptive changes require the presence of external information that disambiguates the distorted speech signals, whereas in other cases mere exposure is sufficient. Both external (e.g., written feedback) and internal (e.g., prior word knowledge) sources of information can be used to generate predictions about the correct mapping of a distorted speech signal. We hypothesize that these predictions provide a basis for determining the discrepancy between the expected and actual speech signal that can be used to guide adaptive changes in perception. This study provides the first empirical investigation that manipulates external and internal factors through (a) the availability of explicit external disambiguating information via the presence or absence of postresponse orthographic information paired with a repetition of the degraded stimulus, and (b) the accuracy of internally generated predictions; an acoustic distortion is introduced either abruptly or incrementally. The results demonstrate that the impact of external information on adaptive plasticity is contingent upon whether the intelligibility of the stimuli permits accurate internally generated predictions during exposure. External information sources enhance adaptive plasticity only when input signals are severely degraded and cannot reliably access internal predictions. This is consistent with a computational framework for adaptive plasticity in which error-driven supervised learning relies on the ability to compute sensory prediction error signals from both internal and external sources of information. (PsycINFO Database Record

Figure 1

Example of waveforms (Amplitude, top) and Spectrograms (Frequency, with amplitude in gray scale, bottom) at different levels of distortion for one of the words in the set, “zone.” From left to right: undistorted stimulus is on the left, stimulus at a mild distortion level of (9.25 mm), stimulus at the moderate distortion (13.25 mm), and stimulus at the severe distortion (15.25 mm).

Figure 2

Experiment 1 results showing percent word recognition at each distortion level. Error bars represent standard error of the mean over subjects.

Figure 3

The degree of adaptive plasticity across conditions, measured as the difference in the percent of correctly recognized severely-distorted words on the Post-test compared to the Pre-test. Error bars represent standard error of the mean over subjects.

Figure 4

Plots show correlations between adaptive plasticity (measured as the improvement in word recognition accuracy from Pre-test to Post-test, x-axis) and word recognition accuracy during training (as a measure of accurate lexical predictions, y-axis) for individual participants, for each condition. Top panel shows the results of Experiment 2. Bottom panel shows the results of Experiment 3.