Probing the independence of formant control using altered auditory feedback

Abstract

Two auditory feedback perturbation experiments were conducted to examine the nature of control of the first two formants in vowels. In the first experiment, talkers heard their auditory feedback with either F1 or F2 shifted in frequency. Talkers altered production of the perturbed formant by changing its frequency in the opposite direction to the perturbation but did not produce a correlated alteration of the unperturbed formant. Thus, the motor control system is capable of fine-grained independent control of F1 and F2. In the second experiment, a large meta-analysis was conducted on data from talkers who received feedback where both F1 and F2 had been perturbed. A moderate correlation was found between individual compensations in F1 and F2 suggesting that the control of F1 and F2 is processed in a common manner at some level. While a wide range of individual compensation magnitudes were observed, no significant correlations were found between individuals' compensations and vowel space differences. Similarly, no significant correlations were found between individuals' compensations and variability in normal vowel production. Further, when receiving normal auditory feedback, most of the population exhibited no significant correlation between the natural variation in production of F1 and F2.

Figure 1

(Color online) Schematic of perturbations used in experiment 1 in a vowel space context. Each pair of concentric ellipses indicates the distribution of an average talker’s production of ∕ɪ∕, ∕ɛ∕, and ∕æ∕ in an ∕hVd∕ context. The center of each pair of ellipses indicates the mean production of an average individual and the solid and dashed ellipses indicate one and two standard deviations, respectively. The four arrows indicate the auditory feedback perturbations the four groups received.

Figure 2

(Color online) Average normalized formant frequency for (a) F1 and (b) F2 over the course of the experiment. The two vertical dashed lines indicate when the perturbed feedback was introduced and when auditory feedback was returned to normal.

Figure 3

(Color online) Histograms of individuals’ compensation in (a) F1 and (b) F2. For each experimental condition, two intervals were defined. The baseline interval was defined as the last 15 utterances spoken with normal feedback before a perturbation was introduced. The shift interval was defined as the last 15 utterances spoken with altered feedback. For each individual, the magnitude of the compensation was calculated from the difference in average formant frequency between the two intervals. The sign of the compensation was defined as positive when the change in production is opposed to that of the formant shift and negative when it follows that of the formant shift.

Figure 4

(Color online) Scatter plot of individuals’ compensations in F1 and F2. The solid line indicates the linear regression on the data.

Figure 5

(Color online) Histogram of individuals’ correlation coefficients between F1 and F2 for normal production of ∕ɛ∕ in the word “head.”