The role of spectral and durational properties on hemispheric asymmetries in vowel perception.

The aim of the current study is to investigate potential hemispheric asymmetries in the perception of vowels and the influence of different time scales on such asymmetries. Activation patterns for naturally produced vowels were examined at three durations encompassing a short (75 ms), medium (150 ms), and long (300 ms) integration time window in a discrimination task. A set of 5 corresponding non-speech sine wave tones were created with frequencies matching the second formant of each vowel. Consistent with earlier hypotheses, there was a right hemisphere preference in the superior temporal gyrus for the processing of spectral information for both vowel and tone stimuli. However, observed laterality differences for vowels and tones were a function of heightened right hemisphere sensitivity to long integration windows, whereas the left hemisphere showed sensitivity to both long and short integration windows. Although there were a number of similarities in the processing of vowels and tones, differences also emerged suggesting that even fairly early in the processing stream at the level of the STG, different mechanisms are recruited for processing vowels and tones.