PhD thesis - The impact of auditory presentation procedures on behavioural measures of emotion lateralisation

Hansen, Louise (2017) The impact of auditory presentation procedures on behavioural measures of emotion lateralisation. PhD thesis, James Cook University.

https://researchonline.jcu.edu.au/51732/

Abstract
Given the importance of human emotion for survival, emotion is a fundamental topic within neuroscience. Two major theories of emotional processing forwarded over the last century are the right hemisphere hypothesis and the valence effect. The former is that all emotions are processed in the right hemisphere of the human brain while the latter is that positive emotions are processed in the left hemisphere and negative emotions in the right. A contemporary account of human emotion reveals complex and bilateral processing. Despite this, distinct effects supporting both hypotheses are robustly observed. While divided visual field research is consistent with both hypotheses, dichotic presentation is almost always consistent with the right hemisphere hypothesis. In both visual and auditory studies, when individuals process one discrete emotion per trial, evidence is consistent with the valence effect while processing two competing emotions elicits results consistent with the right hemisphere hypothesis. Overall, auditory studies have employed monaural, distractor noise, and dichotic presentation without considering whether these procedures adequately measure emotional processing. This makes it unclear whether the right hemisphere superiority reported from dichotic presentation reflects a true emotion effect or competing stimuli masking contributions from the left hemisphere. In the experiments reported in this thesis, participants classified the emotional aspect of speech and music during monaural, distractor noise, and dichotic presentation. All competing stimuli were neutral in emotional valence to ensure participants only processed one target emotion per trial. A right ear effect occurred in the time it took participants to classify each aspect of words from dichotic presentation: nonemotional, emotional content, and emotional prosody. These ear effects were attributed to left hemisphere superiority in language processing. When participants classified the non-emotional or emotional content of words with monaural presentation, unpleasant words were classified least correctly when presented to the right ear. When participants classified music, ear advantages only occurred from dichotic presentation and depended on the duration of the melodies and behaviour measured. A left ear effect emerged in response times to emotional classifications of longer duration melodies, and no ear difference occurred in the control task confirming that this left ear effect was consistent with the right hemisphere hypothesis. However, with affective classifications of brief versions of the same melodies, sensitivity to pleasantness elicited a right ear effect consistent with the valence effect, while sensitivity to a non-emotional classification revealed a left ear advantage consistent with the right hemisphere's role in processing music. Response bias also showed a bias to respond "pleasant" with brief melodies presented to the right ear consistent with the valence effect, and no ear effect occurred in the control condition. In the experiments reported in this thesis, only dichotic presentation consistently produced ear effects associated with language and music processing and the right hemisphere hypothesis and the valence effect. The valence effect can occur with dichotic presentation with melodies when the duration of the melodies is brief and sensitivity and response bias is measured, a finding consistent with divided visual field research. However, the validity of using any visual field or ear advantage to explore which emotion laterality theory best explains emotional processing is questioned. This thesis forms the basis from which a more systematic study of the behavioural consequences of emotional processing with auditory information might proceed.