Author(s): René Westerhausen 1,2,*, Fredrik Samuelsen 2
Verbal dichotic-listening paradigms offer well-established behavioral tests for the assessment of latent hemispheric differences for language processing [1,2] and are integral part of test procedures for the diagnosis of auditory processing disorders [3-5]. A significant advantage of dichotic compared with alternative paradigms (e.g., visual-half field techniques or neuroimaging approaches) is the simplicity of the testing procedure which can be easily understood and performed also by young children [e.g., 6,7], elderly individuals [e.g., 8,9], or patients with cognitive disabilities [e.g., 10,11]. That is, in its basic form, pairs of verbal stimuli (e.g., words or syllables) are presented via headphones, with one of the stimuli presented to the left ear and the other one, simultaneously, to the right ear . Instructed to report the one of the two stimuli which was heard best, participants typically report the right-ear stimulus more frequently, more accurately, and more rapidly than the left-ear stimulus. This right-ear advantage is widely accepted to indicate left hemispheric dominance for speech processing [2,13,14] and differences in the magnitude of this right-ear preferences have been related to interhemispheric auditory integration [15-17].
However, many different versions of dichotic-listening paradigms have been suggested [12,18] and the test-retest reliability of most dichotic paradigms-even of those used for diagnostic purposes-are far from optimal [19,20]. This shortcoming severely threatens the inferences that can be made using dichotic-listening measures, as the reliability of a test also sets the upper limit of its validity . At the same time, however, substantial differences in reliability estimates between paradigms  suggest that certain design features systematically affect the consistency of an individual's dichotic listening task performance. In fact, since the early conceptualization of dichotic listening more than six decades ago [22,23], a plethora of studies has accumulated a significant amount of evidence on how features of the experimental set-up (e.g., stimulus order, stimulus material) and task instructions lead to systematic response biases [for review see ref. 18]. For example, it has been demonstrated that paradigms instructing participants to selectively attend to only one ear at a time are more difficult for participants to perform than paradigms allowing for a free selection . Also, presenting multiple stimulus pairs per trial increases the working-memory load of the task by requiring the participant to keep the representation of multiple stimuli activated simultaneously, leading to a decreased right-ear advantage .
Ignoring these design variables or leaving them uncontrolled introduces error variance to the obtained measures, affecting both the reliability of the obtained laterality measures and the efficiency of the paradigm. In turn, however, considering these variables when designing a dichotic-listening task makes it possible to tailor a paradigm which is optimized for a given purpose. In the present paper, the intention is to design a dichotic listening paradigm optimized for the assessment of hemispheric dominance for speech and language processing. As theoretical framework, we assume that dichotic-listening performance can be best explained using a two-stage model [26-28]: an initial stage leading to a perceptual representation of...