Monday 31 December 2012

Dichotic listening for vowel-consonant syllables


This project stands out from my main projects, and aims to explore the generalizability of a popular experimental approach, rather than theoretical development. The design is simple and the paper is short with unsurprising results, although I think it provides necessary clarification to otherwise unanswered questions.

In our lab (and I think also a lot of other labs) we use a number of different variations of the dichotic listening (DL) task, but we tend to use a standard set of stimuli, namely the nonsense syllables constructed by the six stop-consonants followed by the vowel "a", constituting the consonant vowel, or CV syllables: BA / DA / GA / PA / TA / KA. When presenting one of these CV syllables in one ear simultaneously as a different CV syllable in the other ear, participants tend to report the CV syllable presented in the right ear, an effect referred to as the right ear advantage (REA). This effect is explained as left cortical hemisphere having an advantage in language processing and sensory fibres going from each ear to the hemisphere on the opposite side. Studies using DL with CV syllables have been used to explore both attentional effects and clinical groups. However, DL is often used in arguments concerning language in general, although the task only uses a small subset of the language phonemes. In this project, I wanted to test whether the REA could also be found when using different kinds of syllables. I also wanted to see how different types of syllables are selected between when presented simultaneously.

I constructed new stimuli, some with the same CV structure as in earlier experiments, while others used the same consonants and vowels, but in reverse order, i.e. vowel-consonant (VC) syllables. Both types of syllables were paired up in all four left-right combinations: CV-CV, CV-VC, VC-CV and VC-VC. Participants were asked to report which consonant they heard. Results showed that the REA was found both for CV and for VC dichotic pairs, thus showing that that the typical assumptions behind DL can be extended to a wider array of phonemes than just the CV syllables. Further, when CV is presented in one ear and VC in the other, CV has a perceptual advantage. In the paper's discussion, I go through some of the research from the sixties and seventies when different types of stimuli were used in DL, and see to which extent they showed a REA.

The right ear advantage revisited: Speech lateralisation in DL using CV and VC syllables - Laterality