The effects of background noise on asymmetrical speech perception

The present thesis aimed at examining the effects of environmental background noise on asymmetrical speech perception. To assess asymmetrical speech perception, the Bergen Dichotic Listening (DL) task with consonant-vowel (CV) syllables was performed. In Report I, two background noises (traffic and ‘babble’) were presented while healthy subjects performed the DL task. In Report II, the intensity of the two background noises varied to additionally study the effects of intensity level on DL performance. In Report III, the neuronal mechanisms behind the effect of background noise on DL performance were studied by measuring hemodynamic brain responses using functional magnetic resonance imaging (fMRI). The behavioral data of the three Reports point in the same direction: They revealed a modulation of the typical right-ear advantage (REA) in DL to CV-syllables, especially in traffic background noise, which was due to a decrease in right ear correct reports while left ear correct reports increased. Intensity level was observed to play an additional modulating role that intensified the noise effect with increasing intensity level. The behavioral noise effect was even shown when fMRI was done simultaneously in Report III. The behavioral results are discussed in terms of alertness and attentional mechanisms. fMRI data did not reveal an one-to-one mapping of the behavioral data, however showed different activation patterns especially in the peri-Sylvian region, superior temporal gyrus (STG)/superior temporal sulcus (STS) and STG/supramarginal gyrus (SMG). Pre-activation mechanisms due to noise are proposed to influence subsequent processing of CV-syllables resulting in the observed modulation of behavioral REA. The thesis provides new knowledge to laterality research, especially research in DL. Furthermore, by using day-to-day noises, the thesis brings forward noise research, and stimulates future research in those areas