Attention modulates the use of spectral attributes in vowel discrimination: Behavioral and event-related potential evidence

Speech contains a variety of acoustic cues to auditory and phonetic contrasts that are exploited by the listener in decoding the acoustic signal. In three experiments, we tried to elucidate whether listeners rely on formant peak frequencies or whole spectrum attributes in vowel discrimination. We created two vowel continua in which the acoustic distance in formant frequencies was constant but the continua differed in spectral moments (i.e.; the whole spectrum modeled as a probability density function). In Experiment 1, we measured reaction times and response accuracy while listeners performed a go/no-go discrimination task. The results indicated that the performance of the listeners was based on the spectral moments (especially the first and second moments), and not on formant peaks. Behavioral results in Experiment 2 showed that, when the stimuli were presented in noise eliminating differences in spectral moments between the two continua, listeners employed formant peak frequencies. In Experiment 3, using the same listeners and stimuli as in Experiment 1, we measured an automatic brain potential, the mismatch negativity (MMN), when listeners did not attend to the auditory stimuli. Results showed that the MMN reflects sensitivity only to the formant structure of the vowels. We suggest that the auditory cortex automatically and pre-attentively encodes formant peak frequencies, whereas attention can be deployed for processing additional spectral information, such as spectral moments, to enhance vowel discrimination.