Selective modulation of interhemispheric connectivity by transcranial alternating current stimulation influences binaural integration.

Brain connectivity plays a major role in the encoding, transfer, and integration of sensory information. Interregional synchronization of neural oscillations in the γ-frequency band has been suggested as a key mechanism underlying perceptual integration. In a recent study, we found evidence for this hypothesis showing that the modulation of interhemispheric oscillatory synchrony by means of bihemispheric high-density transcranial alternating current stimulation (HD-TACS) affects binaural integration of dichotic acoustic features. Here, we aimed to establish a direct link between oscillatory synchrony, effective brain connectivity, and binaural integration. We experimentally manipulated oscillatory synchrony (using bihemispheric γ-TACS with different interhemispheric phase lags) and assessed the effect on effective brain connectivity and binaural integration (as measured with functional MRI and a dichotic listening task, respectively). We found that TACS reduced intrahemispheric connectivity within the auditory cortices and antiphase (interhemispheric phase lag 180°) TACS modulated connectivity between the two auditory cortices. Importantly, the changes in intra- and interhemispheric connectivity induced by TACS were correlated with changes in perceptual integration. Our results indicate that γ-band synchronization between the two auditory cortices plays a functional role in binaural integration, supporting the proposed role of interregional oscillatory synchrony in perceptual integration.

A comment on the order of report in two-pair dichotic digit testing in children assessed for auditory processing.

OBJECTIVE: This clinical note aimed to 1) describe the order-of-report (OoR) strategies used by a large sample of children who completed 2pDD testing in the free-recall condition as part of an audiological assessment of auditory processing (AP), and 2) determine if use of OoR correlated with 2pDD test performance. DESIGN: A retrospective (case-review), single observation design. STUDY SAMPLE: A convenient sample of 77 children (50 males and 27 females aged 7.0 to 17.4 years [median = 8.8 years]) referred to audiology for an AP assessment. RESULTS: The participating children used temporal more than ear or sequential OoR and showed decreased test scores with increased use of ear OoR. CONCLUSIONS: These findings suggest children referred for an AP assessment could favour a temporal OoR when completing 2pDD testing for its opportunity to rehearse the first digit pair (possibly strengthening its memory trace) prior to processing the second digit pair, while their use of an ear OoR could come at the cost detrimental ear dominance effects that reduce their test performance. Future research could consider if similar results are observed in other child populations and if preferred dichotic processing strategy could inform auditory training for children with dichotic listening deficits.

Opposite perceptual biases in analogous auditory and visual tasks are unique to consonant-vowel strings and are unlikely a consequence of repetition.

Despite wide reporting of a right ear (RE) advantage on dichotic listening tasks and a right visual field (RVF) advantage on visual half-field tasks, we know very little about the relationship between these perceptual biases. Previous studies that have investigated perceptual asymmetries for analogous auditory and visual consonant-vowel tasks have indicated a serendipitous finding: a RE advantage and a left visual field (LVF) advantage with poor cross-modal correlations. In this study, we examined the possibility that this LVF advantage for visual processing of consonant-vowel strings may be a consequence of repetition by examining perceptual biases in analogous auditory and visual tasks for both consonant-vowel strings and words. We replicated opposite perceptual biases for consonant-vowel strings (RE and LVF advantages). This did not extend to word stimuli where we found RE and RVF advantages. Furthermore, these perceptual biases did not differ across the three experimental blocks. Thus, we can firmly conclude that this LVF advantage is unique to consonant-vowel strings and is not a consequence of the repetition of a relatively limited number of stimuli. Finally, a test of covariances indicated no cross-modal relationships between laterality indices suggesting that perceptual biases are dissociable within individuals and cluster on mode of presentation.

Neural coding of dichotic pitches in auditory midbrain.

Dichotic pitches such as the Huggins pitch (HP) and the binaural edge pitch (BEP) are perceptual illusions whereby binaural noise that exhibits abrupt changes in interaural phase differences (IPDs) across frequency creates a tonelike pitch percept when presented to both ears, even though it does not produce a pitch when presented monaurally. At the perceptual and cortical levels, dichotic pitches behave as if an actual tone had been presented to the ears, yet investigations of neural correlates of dichotic pitch in single-unit responses at subcortical levels are lacking. We tested for cues to HP and BEP in the responses of binaural neurons in the auditory midbrain of anesthetized cats by varying the expected pitch frequency around each neuron's best frequency (BF). Neuronal firing rates showed specific features (peaks, troughs, or edges) when the pitch frequency crossed the BF, and the type of feature was consistent with a well-established model of binaural processing comprising frequency tuning, internal delays, and firing rates sensitive to interaural correlation. A Jeffress-like neural population model in which the behavior of individual neurons was governed by the cross-correlation model and the neurons were independently distributed along BF and best IPD predicted trends in human psychophysical HP detection but only when the model incorporated physiological BF and best IPD distributions. These results demonstrate the existence of a rate-place code for HP and BEP in the auditory midbrain and provide a firm physiological basis for models of dichotic pitches.NEW & NOTEWORTHY Dichotic pitches are perceptual illusions created centrally through binaural interactions that offer an opportunity to test theories of pitch and binaural hearing. Here we show that binaural neurons in auditory midbrain encode the frequency of two salient types of dichotic pitches via specific features in the pattern of firing rates along the tonotopic axis. This is the first combined single-unit and modeling study of responses of auditory neurons to stimuli evoking a dichotic pitch.

Auditory Attention Ability under Dichotic Dual-Task Situation in Adults with Listening Difficulties.

INTRODUCTION: Recent studies have reported poor cognition, such as attention and working memory, in adults with listening difficulties (LiD). However, they do not adequately describe the actual state of poor attention ability in adults with LiD. We examined the state of auditory attention in adults with and without LiD in tasks requiring multiple attention controls. METHODS: Twenty-one adults who had normal hearing but complained about LiD encountered during everyday life and 22 healthy controls were included. We presented a target detection task using an odd-ball format for one ear and a sentence repetition task for the other ear. In the target detection task, participants listened to the 1,000-Hz tone served as the standard stimulus, while they had to accept a 2,000-Hz tone presented as the deviant stimulus. In the sentence repetition task, short sentences were presented. The stimuli presented to them were played on a personal computer at the most comfortable level. The participants heard these stimuli through headphones. They were required to press a key for standard stimuli in the target detection task and repeat what they heard immediately in the repetition task. We compared the response accuracy for each ear task between adults with and without LiD. RESULTS: Our results showed that there were significant differences between the participant groups in the auditory dual-task under the dichotic listening situation. When examined individually, four adults with LiD had decreased scores in both the sentence repetition and target detection task, while the other nine participants showed a bias toward either task. Furthermore, the analysis of reaction time for pressing button revealed that the standard deviation of reaction time was extended in participants who scored poorly in either of the ear tasks. On the other hand, all adults without LiD were able to conduct the auditory dual-task exactly and promptly. CONCLUSION: The results suggest that adults with LiD have difficulties in appropriately allocating various cognitive abilities required for each task. We concluded that auditory attention is an important ability to conduct the auditory dual-task, and this is applicable for adults with LiD. Therefore, we believe that it is necessary to use auditory tests that require complex attentional abilities in listening, such as those required in daily life, to assess adults with LiD.

Improving non-native duration contrast with dichotic training in dyslexic and non-dyslexic individuals.

Perceiving and producing English phonemic vowel length contrasts is challenging for non-native speakers. According to multi-time resolution models, endogenous slow/fast rhythms contribute, respectively, in the right/left hemispheres, to long/short acoustic cue processing. This study introduced a perceptual training method implementing dichotic stimulation to improve /i:/-/ɪ/ processing by promoting hemispheric complementarity. Twenty non-dyslexic and 20 dyslexic French adults received 1 hr-training over 3 days. Productions were evaluated with pre-/post-tests. Training enhanced vowel duration contrast in word production by /i:/ lengthening and /ɪ/ shortening in both groups. Adults with dyslexia compensated fewer /i:/ lengthening by /ɪ/ shortening than did non-dyslexic adults. Transfer from perceptual training to production seems possible for foreign-language learning even in dyslexic adults. The extent to which dichotic presentation contributed to training effectiveness cannot be evaluated here, but the triggering of lengthening and shortening mechanisms suggests that lateralized complementary skills have been enhanced by dichotic stimulation.

The use of the Feather Squadron to identify auditory processing disorders in South African children: a comparative study.

OBJECTIVE: Development of the Feather Squadron (FS) has created a cost-effective, accessible form of assessment through tele-audiology. As accessibility to reliable, cost-effective assessment of auditory processing (AP) skills is limited in South Africa (SA), this study aims to establish whether there is correlation between the FS and traditional tests of AP viz. Frequency Pattern and Dichotic Digits Test, and to establish its relevance to the SA population. DESIGN: Retrospective, within-subject, comparative design used to compare performance on the FS and the traditional tests. Data were considered in two ways: Firstly, using the traditional -2SD cut-off on some subtests of the FS compared to the same traditional tests, and secondly, by comparing the Z-scores obtained on the FS compared to Z-scores on the same traditional tests obtained by applying a double arcsine transformation. STUDY SAMPLE: Sixty-six subjects (35 males, 31 females) aged between 8.08 and 9.75 years (mean = 8.73; SD = 0.51) seen for AP assessments between 2016 and 2019. RESULTS: -2SD criteria yielded no significant association (p > 0.05). Z-score comparison suggested a statistically significant correlation (p < 0.05). CONCLUSION: A statistically significant correlation between certain subtests of FS with traditional tests of AP to merit its use in the SA population is suggested.

Speech sound categorization: The contribution of non-auditory and auditory cortical regions.

Which processes in the human brain lead to the categorical perception of speech sounds? Investigation of this question is hampered by the fact that categorical speech perception is normally confounded by acoustic differences in the stimulus. By using ambiguous sounds, however, it is possible to dissociate acoustic from perceptual stimulus representations. Twenty-seven normally hearing individuals took part in an fMRI study in which they were presented with an ambiguous syllable (intermediate between /da/ and /ga/) in one ear and with disambiguating acoustic feature (third formant, F3) in the other ear. Multi-voxel pattern searchlight analysis was used to identify brain areas that consistently differentiated between response patterns associated with different syllable reports. By comparing responses to different stimuli with identical syllable reports and identical stimuli with different syllable reports, we disambiguated whether these regions primarily differentiated the acoustics of the stimuli or the syllable report. We found that BOLD activity patterns in left perisylvian regions (STG, SMG), left inferior frontal regions (vMC, IFG, AI), left supplementary motor cortex (SMA/pre-SMA), and right motor and somatosensory regions (M1/S1) represent listeners' syllable report irrespective of stimulus acoustics. Most of these regions are outside of what is traditionally regarded as auditory or phonological processing areas. Our results indicate that the process of speech sound categorization implicates decision-making mechanisms and auditory-motor transformations.

Separate neural subsystems support goal-directed speech listening.

How do humans excel at tracking the narrative of a particular speaker with a distracting noisy background? This feat places great demands on the collaboration between speech processing and goal-related regulatory functions. Here, we propose that separate subsystems with different cross-task dynamic activity properties and distinct functional purposes support goal-directed speech listening. We adopted a naturalistic dichotic speech listening paradigm in which listeners were instructed to attend to only one narrative from two competing inputs. Using functional magnetic resonance imaging with inter- and intra-subject correlation techniques, we discovered a dissociation in response consistency in temporal, parietal and frontal brain areas as the task demand varied. Specifically, some areas in the bilateral temporal cortex (SomMotB_Aud and TempPar) and lateral prefrontal cortex (DefaultB_PFCl and ContA_PFCl) always showed consistent activation across subjects and across scan runs, regardless of the task demand. In contrast, some areas in the parietal cortex (DefaultA_pCunPCC and ContC_pCun) responded reliably only when the task goal remained the same. These results suggested two dissociated functional neural networks that were independently validated by performing a data-driven clustering analysis of voxelwise functional connectivity patterns. A subsequent meta-analysis revealed distinct functional profiles for these two brain correlation maps. The different-task correlation map was strongly associated with language-related processes (e.g., listening, speech and sentences), whereas the same-task versus different-task correlation map was linked to self-referencing functions (e.g., default mode, theory of mind and autobiographical topics). Altogether, the three-pronged findings revealed two anatomically and functionally dissociated subsystems supporting goal-directed speech listening.

Binaural Integration of Spectrally Degraded Speech in Adult Cochlear Implant Recipients with Single-Sided Deafness.

INTRODUCTION: Our purpose was to investigate binaural integration for spectrally degraded speech in normal-hearing (NH) subjects, single-sided deafness (SSD) cochlear implant (CI) recipients, and bilateral deaf bilateral CI recipients. METHODS: We tested ten adult subjects in each group with a modified version of the binaural fusion test according to Matzker. Speech recognition was assessed for monotic listening with the better-hearing ear or CI, monotic listening with the poorer-hearing ear or CI, and dichotic listening. We employed two presentation modes: (1) low pass (LP)-filtered speech to the better ear or CI and high pass (HP)-filtered speech to the poorer ear or CI, and (2) LP-filtered speech to the poorer ear or CI and HP-filtered speech to the better ear or CI. Five magnitudes of LP and HP filtering, i.e., spectral degradation, for each presentation mode were applied yielding two spectrally overlapping and three nonoverlapping presentation conditions. Sentences from the Oldenburg Children's sentence test were applied to assess speech recognition. RESULTS: NH subjects, SSD CI recipients, and bilateral CI recipients were able to understand spectrally degraded speech under both monotic and dichotic listening conditions for both the presentation modes. Speech performance decreased with the increasing loss of spectral information in all the three subject groups. In the NH subjects, speech recognition scores significantly improved for dichotic compared with each monotic listening type under two spectrally nonoverlapping conditions. The SSD CI subjects showed a significant improvement in speech scores for dichotic listening compared with monotic listening with the NH ear under one nonoverlapping condition, i.e., a dichotic benefit in speech recognition with CI. We saw a dichotic benefit in the bilateral CI recipients who achieved significantly better speech scores for dichotic compared with monotic listening with the better CI in three nonoverlapping conditions. CONCLUSIONS: All the three groups (NH subjects, SSD CI recipients, and BiCI recipients) showed binaural integration for the side-separated presentation of spectrally degraded speech. Use of strictly side-separated dichotic stimulus presentation avoided the possible occurrence of physical effects such as the head shadow effect and therefore, confirmed the binaural benefit attributable to central binaural processing.

Dichotic listening abilities among liturgical teachers of the Ethiopian Orthodox Church.

This study examines patterns of ear advantage and attentional capacity among religious teachers of the Ethiopian Orthodox Church versus adults who are not religious teachers. Religious education, rooted mainly in Christianity and Islam, has a long history in Ethiopia. Most of such education has been practised through recitations and oral presentations, which demand perceptual vigour particularly on the part of the teachers. The present study employed a dichotic listening paradigm, using monosyllabic word and CV-syllables listening tasks, administered in three attentional conditions: non-forced (NF), forced-right (FR) and forced-left (FL). 54 right-handed male adults (27 teachers and 27 non-teachers) served as participants. Percentages of correct responses per ear, as well as Laterality Index (LI) were calculated and analysed using a repeated measures analysis of variance (ANOVA) statistical procedure. Results showed that, on both listening tasks, the religious teachers demonstrated a higher ear advantage in all conditions, indicative of stronger language asymmetry (in non-forced condition) and better ability to focus on one specific ear (in forced conditions). The findings of the present study are very much in support of the theory of neuroplasticity in human cognition (Konorski, 1948).

Binaural responses in the auditory midbrain of chicken (Gallus gallus).

The auditory midbrain is the location in which neurons represent binaural acoustic information necessary for sound localization. The external nucleus of the midbrain inferior colliculus (IC) of the barn owl is a classic example of an auditory space map, but it is unknown to what extent the principles underlying its formation generalize to other, less specialized animals. We characterized the spiking responses of 139 auditory neurons in the IC of the chicken (Gallus gallus) in vivo, focusing on their sensitivities to the binaural localization cues of interaural time (ITD) and level (ILD) differences. Most units were frequency-selective, with best frequencies distributed unevenly into low-frequency and high-frequency (> 2 kHz) clusters. Many units showed sensitivity to either ITD (65%) or ILD (66%) and nearly half to both (47%). ITD selectivity was disproportionately more common among low-frequency units, while ILD-only selective units were predominantly tuned to high frequencies. ILD sensitivities were diverse, and we thus developed a decision tree defining five types. One rare type with a bell-like ILD tuning was also selective for ITD but typically not frequency-selective, and thus matched the characteristics of neurons in the auditory space map of the barn owl. Our results suggest that generalist birds such as the chicken show a prominent representation of ITD and ILD cues in the IC, providing complementary information for sound localization, according to the duplex theory. A broadband response type narrowly selective for both ITD and ILD may form the basis for a representation of auditory space.

Prognostic Value of a Test of Central Auditory Function in Conversion from Mild Cognitive Impairment to Dementia.

BACKGROUND/OBJECTIVE: It has been suggested that central auditory processing dysfunction might precede the development of cognitive decline and Alzheimer's disease (AD). The Dichotic Digits Test (DDT) has been proposed as a test of central auditory function. Our objective was to evaluate the predictive capacity of the DDT in conversion from mild cognitive impairment (MCI) to dementia. METHODS: A total of 57 participants (26 females) with MCI were tested at baseline with pure tone audiometry, speech in quiet and in noise, and the DDT. The cognitive outcome was retrieved from medical files after 5 years. Groupwise comparisons of the baseline DDT scores were performed and the relative risk was calculated. RESULTS: Altogether 22 subjects developed any kind of dementia. Of the original 57 individuals within the MCI group, 15 developed AD and 7 developed other types of dementia. There was no significant difference in baseline DDT scores between the participants who converted to AD and those who did not. However, the group who developed other types of dementia (especially frontotemporal dementia) had lower DDT scores in the left ear than those participants who did not develop dementia. With a baseline DDT score below 50% correct responses, the participants diagnosed with MCI had a 2.49-times-higher risk of developing dementia than those with scores of 50% or better. CONCLUSION: The DDT as a central auditory test may be suitable when evaluating cognitive decline.

Effect of different blood groups on tympanometric findings and acoustic reflex thresholds.

PURPOSE: The blood group can have an effect on the auditory system, and it is suggested that it could be an indicator of noise-induced hearing loss. There could be changes in the immittance findings, too, in adults having different blood groups. The present study attempted to determine if there are any differences in tympanometric results (admittance, peak pressure, gradient, resonance frequency, and ear canal volume) and acoustic reflex thresholds (ART) at 500, 1000, 2000 and 4000 Hz between individuals with different blood groups (A positive, B positive, O positive and AB positive). METHODS: Eighty normal hearing adults between the age of 18 and 27 years were considered for the study. They were divided into 20 participants, each with blood groups A, B, AB, and O. The immittance findings were recorded from all the participants of the study. RESULTS: The results showed that the resonance frequency was slightly higher in blood group O compared to other blood groups. Also, the acoustic reflex thresholds were slightly elevated at all frequencies (ipsilateral and contralateral) for individuals with blood group O. CONCLUSIONS: The results of the study suggest possible reduced outer hair cells in persons with blood group O. This could have resulted in elevated acoustic reflex thresholds.

Dichotic listening using Mandarin CV-words of six plosives and vowel /a/.

OBJECTIVE: Available data on dichotic listening using tonal Chinese words have been limited, and conflicting results on a right-ear advantage (REA) have been reported. The current study developed a set of Mandarin CV-words based on the Bergen dichotic listening paradigm. DESIGN: The test materials consisted of two sets of stimuli. The English set, derived from the iDichotic application, consisted of six nonsense syllables with plosives conjugated to /a/. The Mandarin set had these 6 CV-syllables spoken as real words according to the Mandarin phonology. Study sample: Forty young, native Mandarin speakers were tested dichotically with both sets of test materials. Twenty participants were right-handed, and the other twenty were left-handed. Fourteen participants were randomly selected to repeat the test with the Mandarin set. RESULTS: The mean difference of correct recognition responses between two ears indicated a REA with both sets of stimuli for the right-handers, but not for left-handers. For the right-handers, performance with the Mandarin set was significantly better than performance with the English set. Dichotic listening with the Mandarin set had strong correlation between two time points, r = 0.8. CONCLUSIONS: Performance using the Mandarin set is reliable in dichotic listening; a REA is observed for the right-handers.

Transcranial Static Magnetic Field Stimulation Over the Temporal Cortex Modulating the Right Ear Advantage in Dichotic Listening.

OBJECTIVE: Transcranial static magnetic field stimulation (tSMS) has proposed a new, promising, and simple non-invasive brain stimulation method. While several studies gained certain evidence about tSMS effects in the motor, somatosensory, and visual domains, there is still a controversial debate about its general effectiveness. In the present study, we investigated potential tSMS effects on auditory speech processing as measured by a dichotic listening (DL) task. MATERIALS AND METHODS: Fifteen healthy participants received in randomized order on three different days one session of either sham, tSMS over the left, or tSMS over the right auditory cortex (AC). Under stimulation, participants performed a standard DL task with consonant-vowel syllables. Simultaneously, we recorded electroencephalogram from central sites (Fz, Cz, Pz). RESULTS: TSMS over the left AC changed the behavioral performance and modulated auditory evoked potentials. Stimulation of the left AC significantly reduced the right ear advantage during the DL task and the N1 component of auditory evoked potentials in response to these syllables. CONCLUSIONS: The preliminary results of the present exploratory study demonstrate the ability of tSMS to modulate human brain activity on a behavioral as well as physiologic level. Furthermore, tSMS effects on acoustic processing may have clinical implications by fostering potential approaches for a treatment of speech-related pathologies associated with hyperexcitability in the AC.

Estimates of functional cerebral hemispheric differences in monolingual and bilingual people who stutter: dichotic listening paradigm.

Recent studies indicate functional cerebral hemispheric processing differences between monolinguals and bilinguals who stutter, as well as monolinguals and bilinguals who do not stutter. Eighty native German speakers, half of whom were also proficient speakers of English as a second language (L2), were assessed on a dichotic listening paradigm using CV syllables as stimuli. The participants were organised into four different groups according to speech status and language ability: 20 monolinguals who stutter, 20 bilinguals who stutter, 20 monolinguals who do not stutter, and 20 bilinguals who do not stutter. A right ear advantage (REA) was observed across all groups with no significant group differences in regard to hemispheric asymmetry. Although MWS (18 dB) and BWS (16 dB) crossed over to an LEA at an earlier point compared to the MWNS (5 dB) and BWNS (2 dB), the difference between groups was minor and not significant. Thus, a significant difference in REA resistance, as proposed by other researchers, was not reflected in the current study neither for people who stutter nor for bilinguals. In addition, no meaningful relationship was found between dichotic listening and stuttering severity, as well as the four language modalities (listening, speaking, reading, writing). Thus, we contend that neither stuttering nor bilingualism has any non-trivial effect on functional cerebral hemispheric differences in language processing in dichotic listening.

Auditory semantic priming and the dichotic right ear advantage.

The current study presents two experiments that aimed to explore the effects of auditory semantic priming on the dichotic right ear advantage. In Experiment 1, a classic fused dichotic words task was modified with the addition of auditory associative primes with three levels of relatedness (right, left, or neither ear). In Experiment 2, a new dichotic listening task was developed based on a binaural task used in a published auditory priming study. In both experiments, we expected that priming would produce a large right ear advantage when related to the right ear target but that the magnitude of this advantage would decrease for left ear related targets. Although evidence of priming (faster responses for related than unrelated primes) was found in both experiments, only Experiment 2 confirmed our prediction of an ear by prime relatedness interaction. Results are interpreted in the context of models concerned with the role of each cerebral hemisphere in semantic processing as well as models of perceptual asymmetries.

Factors affecting the Mandarin dichotic digits test.

Objective: Data with regard to the Mandarin dichotic digits test (DDT) are limited, with conflicting results reported between the Mandarin and English DDTs. The current study examined factors that might affect the performance in the Mandarin DDT. Design: The digits were arranged in 3 sets of 20 dichotic pairs; each set comprised 2, 3 or 4 digits in a pair. Study sample: Forty-one young, native Mandarin speakers with normal hearing were tested; 30 of them were right-handed and 11 left-handed. Six participants repeated the test. Results: The prevalence of ceiling effect in recognition score decreased systematically as the digit-pair length increased. At digit sets of 3-pair and 4-pair, the right-handed group showed a significant higher recognition score at right ear than left ear, while the left-handed group reversed the performance. The score difference between ears is significantly different between the right- and left-handed groups. The retest of the Mandarin DDT was reliable. Conclusions: For the right-handed group, a right-ear advantage can exist in the Mandarin DDT at a longer digit-pair length, similar to what was reported in the English DDT. Moreover, for the left-handed group, the attended right hemisphere in processing the tonal digits can result in a left-ear advantage.

A primer on dichotic listening as a paradigm for the assessment of hemispheric asymmetry.

Dichotic listening is a well-established method to non-invasively assess hemispheric specialization for processing of speech and other auditory stimuli. However, almost six decades of research also have revealed a series of experimental variables with systematic modulatory effects on task performance. These variables are a source of systematic error variance in the data and, when uncontrolled, affect the reliability and validity of the obtained laterality measures. The present review provides a comprehensive overview of these modulatory variables and offers both guiding principles as well as concrete suggestions on how to account for possible confounding effects and avoid common pitfalls. The review additionally provides guidance for the evaluation of past studies and help for resolving inconsistencies in the available literature.