Speech processing asymmetry revealed by dichotic listening and functional brain imaging

Abstract

In this article, we review research in our laboratory from the last 25 to 30 years on the neuronal basis for laterality of speech perception focusing on the upper, posterior parts of the temporal lobes, and its functional and structural connections to other brain regions. We review both behavioral and brain imaging data, with a focus on dichotic listening experiments, and using a variety of imaging modalities. The data have come in most parts from healthy individuals and from studies on normally functioning brain, although we also review a few selected clinical examples. We first review and discuss the structural model for the explanation of the right-ear advantage (REA) and left hemisphere asymmetry for auditory language processing. A common theme across many studies have been our interest in the interaction between bottom-up, stimulus-driven, and top-down, instruction-driven, aspects of hemispheric asymmetry, and how perceptual factors interact with cognitive factors to shape asymmetry of auditory language information processing. In summary, our research have shown laterality for the initial processing of consonant–vowel syllables, first observed as a behavioral REA when subjects are required to report which syllable of a dichotic syllable-pair they perceive. In subsequent work we have corroborated the REA with brain imaging, and have shown that the REA is modulated through both bottom-up manipulations of stimulus properties, like sound intensity, and top-down manipulations of cognitive properties, like attention focus.

Introduction

In this invited article we review studies on auditory laterality from our laboratories at the University of Bergen, Norway, and Uppsala University, Sweden, which were conducted over the last 25–30 years on auditory laterality. The reviewed studies and articles have in common the use of the dichotic listening paradigm. For most parts, these studies have been conducted in our laboratory at the University of Bergen, Norway, or by our collaborators. We also review, when relevant, other studies in order to emphasize a point, or clarify an issue. We have, moreover, and somewhat untraditionally, divided the paper according to separate studies. We believe that the overall pattern of findings from these studies reveals a coherent story on the laterality of auditory language processing. The review particularly focuses on studies using functional neuroimaging, conceptualized both as hemodynamic imaging (Hugdahl et al., 1999; van den Noort et al., 2008; Westerhausen et al., 2010; Kompus et al., 2012; Westerhausen et al., 2014) as well as electrophysiological and magnetoencephalographic measures (Alho et al., 2012, Davidson and Hugdahl, 1996, Eichele et al., 2005, Passow et al., 2013). These findings are further contrasted with observations in clinical populations (Carlsson et al., 1994; Pollmann et al., 2002, Pollmann et al., 2004; Hugdahl et al., 1997; Kompus et al., 2011). A focus of and core interest in our research on hemispheric asymmetry has been to reveal how bottom-up and top-down factors interact to program conscious perception, whether reflected as behavioral response or as change in brain activity patterns.