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An Optimal Dichotic-Listening Paradigm for the Assessment of Hemispheric

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An Optimal Dichotic-Listening Paradigm for the Assessment of Hemispheric 1 Research article 2 An optimal dichotic-listening paradigm for the assessment of hemispheric 3 dominance for speech processing 4 5 René Westerhausen (1,2)* & Fredrik Samuelsen (2) 6 7 (1) Department of Psychology, University of Oslo, Norway 8 (2) Department of Biological and Medical Psychology, University of Bergen, Norway 9 10 Running title: Optimal dichotic-listening paradigm 11 12 13 *Corresponding author 14 René Westerhausen 15 Department of Psychology, University of Oslo, 16 POB 1094 Blindern, 0317 Oslo, Norway 17 email: [email protected], Phone: (+47) 228 45230 18 1 19 Abstract 20 Dichotic-listening paradigms are widely accepted as non-invasive tests of hemispheric 21 dominance for language processing and represent a standard diagnostic tool for the 22 assessment of developmental auditory and language disorders. Despite its popularity in 23 research and clinical settings, dichotic paradigms show comparatively low reliability, 24 significantly threatening the validity of conclusions drawn from the results. Thus, the aim of 25 the present work was to design and evaluate a novel, highly reliable dichotic-listening 26 paradigm for the assessment of hemispheric differences. Based on an extensive literature 27 review, the paradigm was optimized to account for the main experimental variables which are 28 known to systematically bias task performance or affect random error variance. The main 29 design principle was to minimize the relevance of higher cognitive functions on task 30 performance in order to obtain stimulus-driven laterality estimates. To this end, the key 31 design features of the paradigm were the use of stop-consonant vowel (CV) syllables as 32 stimulus material, a single stimulus pair per trial presentation mode, and a free recall (single) 33 response instruction. Evaluating a verbal and manual response-format version of the 34 paradigm in a sample of N = 50 healthy participants, we yielded test-retest intra-class 35 correlations of rICC = .91 and .93 for the two response format versions. These excellent 36 reliability estimates suggest that the optimal paradigm may offer an effective and efficient 37 alternative to currently used paradigms both in research and diagnostic. 38 39 Keywords: dichotic listening test; behavioral laterality; reliability; hemispheric 40 specialisation; speech perception; neuropsychology; 41 42 2 43 Introduction 44 Verbal dichotic-listening paradigms offer well-established behavioral tests for the assessment 45 of latent hemispheric differences for language processing (1, 2) and are integral part of test 46 procedures for the diagnosis of auditory processing disorders (3-5). A significant advantage 47 of dichotic compared with alternative paradigms (e.g., visual-half field techniques or 48 neuroimaging approaches) is the simplicity of the testing procedure which can be easily 49 understood and performed also by young children (e.g., 6, 7), elderly individuals (e.g., 8, 9), 50 or patients with cognitive disabilities (e.g., 10, 11). That is, in its basic form, pairs of verbal 51 stimuli (e.g., words or syllables) are presented via headphones, with one of the stimuli 52 presented to the left ear and the other one, simultaneously, to the right ear (12). Instructed to 53 report the one of the two stimuli which was heard best, participants typically report the right- 54 ear stimulus more frequently, more accurately, and more rapidly than the left-ear stimulus. 55 This right-ear advantage is widely accepted to indicate left hemispheric dominance for speech 56 processing (2, 13, 14) and differences in the magnitude of this right-ear preferences have 57 been related to interhemispheric auditory integration (15-17). 58 However, many different versions of dichotic-listening paradigms have been 59 suggested (12, 18) and the test-retest reliability of most dichotic paradigms – even of those 60 used for diagnostic purposes – are far from optimal (19, 20). This shortcoming severely 61 threatens the inferences that can be made using dichotic-listening measures, as the reliability 62 of a test also sets the upper limit of its validity (21). At the same time, however, substantial 63 differences in reliability estimates between paradigms (19) suggest that certain design 64 features systematically affect the consistency of an individual’s dichotic listening task 65 performance. In fact, since the early conceptualization of dichotic listening more than six 66 decades ago (22, 23), a plethora of studies has accumulated a significant amount of evidence 67 on how features of the experimental set-up (e.g., stimulus order, stimulus material) and task 3 68 instructions lead to systematic response biases (for review see ref. 18). For example, it has 69 been demonstrated that paradigms instructing participants to selectively attend to only one ear 70 at a time are more difficult for participants to perform than paradigms allowing for a free 71 selection (24). Also, presenting multiple stimulus pairs per trial increases the working- 72 memory load of the task by requiring the participant to keep the representation of multiple 73 stimuli activated simultaneously, leading to a decreased right-ear advantage (25). 74 Ignoring these design variables or leaving them uncontrolled introduces error variance 75 to the obtained measures, affecting both the reliability of the obtained laterality measures and 76 the efficiency of the paradigm. In turn, however, considering these variables when designing 77 a dichotic-listening task makes it possible to tailor a paradigm which is optimized for a given 78 purpose. In the present paper, the intention is to design a dichotic listening paradigm 79 optimized for the assessment of hemispheric dominance for speech and language processing. 80 As theoretical framework, we assume that dichotic-listening performance can be best 81 explained using a two-stage model (26-28): an initial stage leading to a perceptual 82 representation of the two competing stimuli in verbal short‐term memory (29, 30), and a 83 second stage characterized by cognitive-control processes which may modulate the initial 84 representation in line with task requirements, resulting in a response selection (31, 32). It is 85 further assumed that a “true” underlying right-ear preference exists as consequence of left- 86 hemispheric specialization for speech and language processing (33, 34). That is, in the initial 87 stage, the right-ear stimulus can be expected to be more salient than the left-ear stimulus. An 88 optimal paradigm to assess this underlying perceptual, “built-in” laterality, consequently 89 needs to assure both an unbiased initial stimulus representation and a cognitively unaltered 90 response selection during second stage processing (27). Based on recent literature review 91 (18), we here aim to create and evaluate such an optimal paradigm by following the eight 92 design features listed in Table 1. 4 93 However, while many design features of dichotic paradigms have been studied 94 systematically, it appears somewhat surprising that little is known about the effect of the 95 response format on task performance (18). When using a verbal-response format, the 96 participant is typically instructed to repeat orally the stimulus she/he perceives after each 97 trial, and the response is either ad-hoc scored by an experimenter or recorded for later 98 decoding (25, 35). Manual responses are typically collected via button press on a keyboard, 99 special response box, or computer mouse (28, 36). Both alternatives come with theoretical 100 advantages and disadvantages (18). For example, the verbal-response format carries the risk 101 of increasing error variance in the recorded data as the response has to be decoded by the 102 experimenter. As compared to a verbal version, using a manual response format changes the 103 cognitive demands of the paradigm as it demands additional response mapping and selection 104 processes including visual-motor coordination (37). However, an empirical comparison of the 105 two response formats is missing from the literature and it is to date unknown if the reliability 106 of a dichotic paradigm is affected by the response format utilized. 107 In summary, the aim of the present study was to develop and test a new dichotic- 108 listening paradigm for the assessment of hemispheric dominance which considers the main 109 design feature known to affect task performance and yields high test-retest reliability. To 110 evaluate the retest reliability of this novel paradigm, participants had to complete the full 111 paradigm four times: each two times using verbal and manual response format. This setup 112 also allows evaluating possible effects of the response format on laterality estimates. 113 Table 1. Design features of the optimal dichotic-listening paradigm and arguments for their implementation # Design feature Argumentation 1 Stop consonant-vowel (CV) syllables as Proven to be valid test material (38). CV show stimulus material higher reliability than numeric or non-numeric words as stimulus material (19) 5 2 Pair only CV stimuli from the same Same-voicing category pairs are more likely to fuse voicing category into one percept than mixed pairs; reduces relevance of attention and cognitive-control processes (28) 3 Includes binaural (diotic) trials Allows to demonstrate stimulus appropriateness, i.e. whether participant is able to identify the used stimulus material (18) 4 Alternating trials of voiced and unvoiced Limits negative-priming effects (39) by preventing
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