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In the present paper, I selectively review a series of studies conducted in our laboratory over the last decade related to brain laterality, both experimentally and clinically. The studies reviewed have for most part utilized the dichotic listening and visual half-field techniques for lateralizing stimulus presentations. Three major conceptual distinctions in brain laterality are discussed in the paper: hemispheric specialization versus hemispheric activation, functional versus structural laterality, and hardwired versus dynamic modulatory laterality. A fourth distinction is between cognitive and physiological laterality. It is argued in the paper that left-right asymmetrical function is not only related to perceptual/cognitive functions subserved by the cortical hemispheres, but also involves subcortical structures and peripheral physiology, including the autonomic and endocrine systems. Thus, although research on laterality has been in the mainstream of general psychology and neuropsychology for the last 30-50 years, we have still probably only uncovered the tip of the iceberg with respect to the psychological processes and systems that are related to the asymmetrical structure and function of the brain.

The present paper is based on a talk on hemispheric asymmetry given by Kenneth Hugdahl at the Xth European Congress of Psychology, Praha July 2007. Here, we propose that hemispheric asymmetry evolved because of a left hemisphere speech processing specialization. The evolution of speech and the need for air-based communication necessitated division of labor between the hemispheres in order to avoid having duplicate copies in both hemispheres that would increase processing redundancy. It is argued that the neuronal basis of this labor division is the structural asymmetry observed in the peri-Sylvian region in the posterior part of the temporal lobe, with a left larger than right planum temporale area. This is the only example where a structural, or anatomical, asymmetry matches a corresponding functional asymmetry. The increase in gray matter volume in the left planum temporale area corresponds to a functional asymmetry of speech processing, as indexed from both behavioral, dichotic listening, and functional neuroimaging studies. The functional anatomy of the corpus callosum also supports such a view, with regional specificity of information transfer between the hemispheres.

In this study, the hypothesis that working memory capacity interacts with (foreign) language proficiency was tested on multilinguals, who were native (L1) Dutch speakers, were fluent in their second (L2) language, German, and had recently started the acquisition of their third (L3) language, Norwegian. So far, the results of second-language studies on simple and complex working-memory tasks are mixed. In previous second-language studies, however, languages that belong to different linguistic groups were used. The question arises whether the interaction between working memory capacity and language proficiency is language-specific. In our multilingualism study we, therefore, controlled for this. Both simple (digit-span) and complex working-memory tasks (reading-span task and letter-number ordering) were used. The general results show that differences in performance between L1, L2, and L3 can be found on both simple and complex working-memory tasks, supporting the working memory capacity interaction hypothesis.

Abstract
About 95% of right-handers and 70% of left-handers have a left-hemispheric specialization for language. Dichotic listening is often used as an indirect measure of this language asymmetry. However, while it reliably produces a right-ear advantage (REA), corresponding to the left-hemispheric specialization of language, it paradoxically often fails to obtain statistical evidence of mean differences between left- and right-handers. We hypothesized that non-normality of the underlying distributions might be in part responsible for the similarities in means. Here, we compare the mean ear advantage scores, and also contrast the distributions at multiple quantiles, in two large independent samples (Ns = 1,358 and 1,042) of right-handers and left-handers. Right-handers had an increased mean REA, and a larger proportion had an REA than in the left-handers. We also found that more left-handers are represented in the left-eared end of the distribution. These data suggest that subtle shifts in the distributions of DL scores for right- and left-handers may be at least partially responsible for the unreliability of significantly reduced mean REA in left-handers.

People's preferences for income distribution fundamentally affect their behavior and contribute to shaping important social and political institutions. The study of such preferences has become a major topic in behavioral research in social psychology and economics. Despite the large literature studying preferences for income distribution, there is no direct neuronal evidence of how the brain responds to income distributions when people have made different contributions in terms of work effort. The present paper reports from, to our knowledge, the first neuroimaging study designed to examine how the brain responds to the distribution of income in such situations. As such, to our knowledge it is also the first study to examine the neuronal basis for equity theory.