An event-related fMRI investigation of voice-onset time discrimination

Abstract

The discrimination of voice-onset time, an acoustic-phonetic cue to voicing in stop consonants, was investigated to explore the neural systems underlying the perception of a rapid temporal speech parameter. Pairs of synthetic stimuli taken from a [da] to [ta] continuum varying in voice-onset time (VOT) were presented for discrimination judgments. Participants exhibited categorical perception, discriminating 15-ms and 30-ms between-category comparisons and failing to discriminate 15-ms within-category comparisons. Contrastive analysis with a tone discrimination task demonstrated left superior temporal gyrus activation in all three VOT conditions with recruitment of additional regions, particularly the right inferior frontal gyrus and middle frontal gyrus for the 15-ms between-category stimuli. Hemispheric differences using anatomically defined regions of interest showed two distinct patterns with anterior regions showing more activation in the right hemisphere relative to the left hemisphere and temporal regions demonstrating greater activation in the left hemisphere relative to the right hemisphere. Activation in the temporal regions appears to reflect initial acoustic-perceptual analysis of VOT. Greater activation in the right hemisphere anterior regions may reflect increased processing demands, suggesting involvement of the right hemisphere when the acoustic distance between the stimuli are reduced and when the discrimination judgment becomes more difficult.

Figure I

Spectrograms of the six synthetic syllable stimuli, ranging from a VOT of 0ms (left) to 40ms (right). VOT values used were 0ms, 10ms, 15ms, 25ms, 30ms and 40ms.

Figure II

The top panel shows the mean % correct (‘different’) responses on the three syllable conditions, 30 ms Between Phonetic Category, 15 ms Between Phonetic Category, and 15 ms Within Phonetic Category, for all 14 participants. Error bars represent standard error. The bottom panel shows the mean RT (in ms) for correct different responses for all 14 participants. Error bars represent standard error.

Figure II

The top panel shows the mean % correct (‘different’) responses on the three syllable conditions, 30 ms Between Phonetic Category, 15 ms Between Phonetic Category, and 15 ms Within Phonetic Category, for all 14 participants. Error bars represent standard error. The bottom panel shows the mean RT (in ms) for correct different responses for all 14 participants. Error bars represent standard error.

Figure III

A and B. Regions showing significant differences in activation level at a voxel level threshold of p<0.01 and a corrected threshold of p<0.01 (44 contiguous voxels) for the three VOT discrimination conditions compared to the tone different condition. Axial views at z=60, z=69, z=107 and z=114 (from left to right) are shown. The top panel shows the 30ms Between-Category vs. Tone Different, the middle panel the 15ms Between-Category vs. Tone Different and the bottom panel the 15ms Within-Category vs. Tone Different. Images are scaled from a percent change range of -0.35 to 0.35.

Figure IV

Mean percent signal change for the three syllable different conditions is displayed for anatomically defined ROIs in the left and right hemisphere where significant differences emerged. The anatomically defined ROI from AFNI’s implementation of the Talairach and Tournoux atlas is displayed in the upper right hand corner of each graph.