Neural measures of acoustic sensitivity

A central question in understanding speech perception is whether listeners discard acoustic information early in perception or maintain it for longer periods of time. Using event-related potentials (ERPs), we are able to study how acoustic variability affects speech perception by examining differences in electrophysiological components. ERPs are a measure of brain activity that can be obtained using non-invasive electrodes attached to the head. Electrical activity produced by the brain can be detected at the scalp and recorded by these electrodes in real-time. This methodolgy gives us a measure of neural activity that has a high temporal precision, allowing us to study how listeners process variations in acoustic information as it unfolds over millisecond time scales.

We are using ERP techniques to look at how listeners use fine-grained acoustic differences in speech. Subjects are presented with a series of sounds that vary from one phonetic category to another. The sound clip below varies in voice-onset time (VOT) along a continuum from the word dart to the word tart. When you play it you will hear the words varying in nine VOT steps from 0 ms (a good dart) to 40 ms (a good tart).

In this experiment, subjects were presented with stimuli like the ones in the sound clip and asked whether or not the word they heard was the same as a target word. We found that the size of the P3 component, which corresponds to categorizaton in this task, decreased as the distance from the target endpoint increased. The figure on the right shows the size of the P3 component as a function of distance from the subject's category boundary.

This result suggests that listeners are able to detect small changes in acoustic information. In addition, it tells us that this information persists to the point at which they are identifying words rather than being discarded early in perception.

Future work on this project will be looking at similar effects in other ERP components, as well as examining other sets of acoustic cues and phonetic distinctions.

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