The relevance of task-irrelevant sounds: hemispheric lateralization and interactions with task-relevant streams

Ana A Amaral,Dave R M Langers

doi:10.3389/fnins.2013.00264

Abstract

The effect of unattended task-irrelevant auditory stimuli in the context of an auditory task is not well understood. Using human functional magnetic resonance imaging (fMRI) we compared blood oxygenation level dependent (BOLD) signal changes resulting from monotic task-irrelevant stimulation, monotic task-relevant stimulation and dichotic stimulation with an attended task-relevant stream to one ear and an unattended task-irrelevant stream to the other ear simultaneously. We found strong bilateral BOLD signal changes in the auditory cortex (AC) resulting from monotic stimulation in a passive listening condition. Consistent with previous work, these responses were largest on the side contralateral to stimulation. AC responses to the unattended (task-irrelevant) sounds were preferentially contralateral and strongest for the most difficult condition. Stronger bilateral AC responses occurred during monotic passive-listening than to an unattended stream presented in a dichotic condition, with attention focused on one ear. Additionally, the visual cortex showed negative responses compared to the baseline in all stimulus conditions including passive listening. Our results suggest that during dichotic listening, with attention focused on one ear, (1) the contralateral and the ipsilateral auditory pathways are suppressively interacting; and (2) cross-modal inhibition occurs during purely acoustic stimulation. These findings support the existence of response suppressions within and between modalities in the presence of competing interfering stimuli.

Highlights

The auditory system relies on various clues to segregate concurrent sound streams
Since the interaction between both factors was not significant (p = 0.93), results were averaged over the non-significant factor ear and a paired Wilcoxon signed-rank test was calculated for all pairs of conditions, and False Discovery Rate (FDR) corrected for multiple comparisons
This study provides a comparison between functional magnetic resonance imaging (fMRI) blood oxygenation level dependent (BOLD) signal changes resulting from monotic unattended stimulation (No-Task), monotic attended stimulation (No-Distractor), and dichotic stimulation with one attended and one unattended ear simultaneously (Concordant, Discordant, Non-Letter)

Summary

Introduction

The auditory system relies on various clues to segregate concurrent sound streams. These among others include clues related to sound source location, derived from head-related transfer functions, binaural interaural time differences, and interaural level differences, for instance (Ehret and Romand, 1997; Moore et al, 2010). Brain responses resulting from monotic stimulation are bilateral and stronger in the hemisphere contralateral to stimulus presentation (Rosenzweig, 1951; Reite et al, 1981; Pantev et al, 1986; Scheffler et al, 1998; Alho et al, 1999; Woldorff et al, 1999; Langers et al, 2005; Della Penna et al, 2007) It has been shown using functional magnetic resonance imaging (fMRI) (Scheffler et al, 1998; Jäncke et al, 2002; Krumbholz et al, 2005) and magnetoencephalography (MEG) (Pantev et al, 1986; Fujiki et al, 2002; Kaneko et al, 2003) that the responses are sub-additive, that is, the sum of brain responses to left and right monotic stimulation exceeds the response to dichotic stimulation, a phenomenon known as ’binaural interaction’. It has been suggested that a competition arises between the two pathways causing the stronger contralateral pathway to suppress the ipsilateral one, decreasing the overall brain responses (Fujiki et al, 2002; Kaneko et al, 2003; Brancucci et al, 2004; Della Penna et al, 2007)

Methods

Results

Conclusion