Audio-vocal responses to repetitive pitch-shift stimulation during a sustained vocalization: improvements in methodology for the pitch-shifting technique

Abstract

The pitch-shift reflex is a sophisticated system that produces a "compensatory" response in voice F0 that is opposite in direction to a change in voice pitch feedback (pitch-shift stimulus), thus correcting for the discrepancy between the intended voice F0 and the feedback pitch. In order to more fully exploit the pitch-shift reflex as a tool for studying the influence of sensory feedback mechanisms underlying voice control, the optimal characteristics of the pitch-shift stimulus must be understood. The present study was undertaken to assess the effects of altering the duration of the interstimulus interval (ISI) and the number of trials comprising an average on measures of the pitch-shift reflex. Pitch-shift stimuli were presented to vocalizing subjects with ISI of 5.0, 2.5, 1.0, and 0.5 s to determine if an increase in ISI altered response properties. With each ISI, measures of event-related averages of the first 10, 15, 20, or 30 pitch-shift reflex responses were compared to see if increases in the number of responses comprising an event-related average altered response properties. Measures of response latency, peak time, magnitude, and prevalence were obtained for all ISI and average conditions. While quantitative measures were similar across ISI and averaging conditions, we observed more instances of "non-responses" with averages of ten trials as well as at an ISI of 0.5 s. These findings suggest an ISI of 1.0 s and an average consisting of at least 15 trials produce optimal results. Future studies using these stimulus parameters may produce more reliable data due to the fivefold decrease in subject participation time and a concomitant decrease in fatigue, boredom, and inattention.

FIG. 1

Event-related averages of voice F₀ for a representative subject tested with ISI of 2.5, 1.0, and 0.5 s. Under each rate, averages of 10, 15, 20, and 30 responses were calculated (vertically stacked traces). Black lines represent the overall average response. Gray bars represent standard error of the mean for all responses comprising an average on a point-by-point basis. Shaded boxes represent pitch-shift stimulus onset and duration (0.1 s) beginning at time 0 s. Vertical line scales=20 cents.

FIG. 2

(a) Median box-plots of response latency (s) by ISI and number of averages (N=31). (b) Median box-plots of responses peak time (s). (c) Median box-plots of response magnitude (cents). Box-plot definitions: The horizontal line through a box is the median. The shaded region surrounding the median is the 95% confidence interval. The upper and lower limits of the box represent the 75th and 25th percentiles, respectively. Whiskers extend to upper and lower limits of the main body of data. Points depicted by a circle are considered to be extreme data values, while very extreme values are plotted as asterisks.

FIG. 3

Event-related averages of voice F₀ for a representative subject tested with ISI of 5.0, 2.5, 1.0, and 0.5 s. Under each rate, averages of 10, 15, 20, and 30 responses were calculated (vertically stacked traces). Black lines represent the overall average response. Gray bars represent standard error of the mean for all responses comprising an average on a point-by-point basis. Shaded boxes represent pitch-shift stimulus onset and duration (0.1 s) beginning at time 0 s. Vertical line scales=20 cents.

FIG. 4

Event-related average for a representative subject displaying an overlap of successive pitch-shift reflex responses due to the short ISI at 0.5 s. Black lines represent the overall average voice F₀ and arrows denote response onset. Duration of the post-stimulus averaging window was extended to 1.0 s. Shaded boxes represent pitch-shift stimulus onset and duration (0.1 s) beginning at time 0.0 and 0.6 s.