Neurophysiological Evaluation of Right-Ear Advantage During Dichotic Listening

Right-ear advantage refers to the observation that when two different speech stimuli are simultaneously presented to both ears, listeners report stimuli more correctly from the right ear than the left. It is assumed to result from prominent projection along the auditory pathways to the contralateral hemisphere and the dominance of the left auditory cortex for the perception of speech elements. Our study aimed to investigate the role of attention in the right-ear advantage. We recorded magnetoencephalography data while participants listened to pairs of Japanese two-syllable words (namely, “/ta/ /ko/” or “/i/ /ka/”). The amplitudes of the stimuli were modulated at 35 Hz in one ear and 45 Hz in the other. Such frequency-tagging allowed the selective quantification of left and right auditory cortex responses to left and right ear stimuli. Behavioral tests confirmed the right-ear advantage, with higher accuracy for stimuli presented to the right ear than to the left. The amplitude of the auditory steady-state response was larger when attending to the stimuli compared to passive listening. We detected a correlation between the attention-related increase in the amplitude of the auditory steady-state response and the laterality index of behavioral accuracy. The right-ear advantage in the free-response dichotic listening was also found in neural activities in the left auditory cortex, suggesting that it was related to the allocation of attention to both ears.

Influence of Alignment Strategies and Consonant Acoustic Features on Laterality Index in Dichotic Testing in Indian Languages

Background The right ear advantage (REA) observed for dichotic CV (DCV) testing in Indian languages is seen to be smaller compared with the observations in several studies in English and other languages. Purpose The present study aims to explore whether the differences in the temporal alignment of consonant–vowel (CV) stimuli used for the dichotic task is a contributing factor that can explain the smaller REAs observed in Indian languages. Research Design The study compared the laterality index (LI) values between DCV test results obtained using CV stimuli that were temporally aligned at the acoustic onset versus the burst onset, with 0 milliseconds lag between the right and left ears. Study Sample A total of 50 right-handed individuals (native speakers of Kannada language) in the age range of 17 to 30 years with normal hearing sensitivity participated in the study. Data Collection and Analysis The participants' responses to the two sets of DCV stimuli were recorded and compared. LI was calculated for the overall scores as well as for specific voiced and unvoiced segment combinations. To test the reliability of the findings, 10 of the participants were re-tested using the same stimuli and the data were checked for reliability. Results The overall LI was 15.1% for the stimuli temporally aligned at the burst onset, whereas it was 5.7% for the stimuli temporally aligned at the acoustic onset. The difference in LI was not significant between the alignment conditions for unvoiced–unvoiced and voiced–voiced CV combinations, but this difference was significant for the unvoiced–voiced and voiced–unvoiced CV combinations. Comparing test and re-test scores showed good reliability. Conclusion The results support our hypothesis that smaller REA observed in DCV task in Indian languages was due to the temporal alignment of CV stimuli. In the acoustic-onset-aligned condition, presenting pairs contrasting in the voicing feature tends to cancel out a relative ear advantage.

Relations between hemispheric asymmetries of grey matter and auditory processing of spoken syllables in 281 healthy adults

Most people have a right-ear advantage for the perception of spoken syllables, consistent with left hemisphere dominance for speech processing. However, there is considerable variation, with some people showing left-ear advantage. The extent to which this variation is reflected in brain structure remains unclear. We tested for relations between hemispheric asymmetries of auditory processing and of grey matter in 281 adults, using dichotic listening and voxel-based morphometry. This was the largest study of this issue to date. Per-voxel asymmetry indexes were derived for each participant following registration of brain magnetic resonance images to a template that was symmetrized. The asymmetry index derived from dichotic listening was related to grey matter asymmetry in clusters of voxels corresponding to the amygdala and cerebellum lobule VI. There was also a smaller, non-significant cluster in the posterior superior temporal gyrus, a region of auditory cortex. These findings contribute to the mapping of asymmetrical structure–function links in the human brain and suggest that subcortical structures should be investigated in relation to hemispheric dominance for speech processing, in addition to auditory cortex.

“The Dichotic Digit Test” as an Index Indicator for Hearing Problem in Dementia: Systematic Review and Meta-Analysis

Background Patients with dementia commonly have problems processing speech in the presence of competing background speech or noise. This difficulty can be present from the very early stages of dementia, and may be a preclinical feature of Alzheimer's disease. Purpose This study investigates whether people with dementia perform worse on the dichotic digit test (DDT), an experimental probe of speech processing in the presence of competing speech, and whether test performance may predict dementia onset. Research Design Systematic review and meta-analysis. Data Collection and Analysis A literature search was conducted in Medline, Embase, Scopus, and Psycinfo. We included (1) studies that included people with a diagnosis of dementia and a healthy control group with no cognitive impairment; (2) studies that reported results from a DDT in a free-recall response task; and (3) studies that had the dichotic digit mean correct percentage score or right-ear advantage, as outcome measurements. Results People with dementia had a lower DDT total score, with a pooled mean difference of 18.6% (95% confidence interval [CI]: 21.2–15.9). Patients with dementia had an increased right-ear advantage relative to controls with a pooled difference of 24.4% (95% CI: 21.8–27.0). Conclusion The DDT total scores are lower and the right-ear advantage increased in cognitively impaired versus normal control participants. The findings also suggest that the reduction of dichotic digit total score and increase of right-ear advantage progress as cognitive impairment increases. Whether abnormalities in dichotic digit scores could predict subsequent dementia onset should be examined in further longitudinal studies.

Relations between hemispheric asymmetries of grey matter and auditory processing of spoken syllables in 281 healthy adults

Most people have a right-ear advantage for the perception of spoken syllables, consistent with left hemisphere dominance for speech processing. However, there is considerable variation, with some people showing left-ear advantage. The extent to which this variation is reflected in brain structure remains unclear. We tested for relations between hemispheric asymmetries of auditory processing and of grey matter in 281 adults, using dichotic listening and voxel-based morphometry. This was the largest study of this issue to date. Per-voxel asymmetry indexes were derived for each participant following registration of brain magnetic resonance images to a template that was symmetrized. The asymmetry index derived from dichotic listening was related to grey matter asymmetry in clusters of voxels corresponding to the amygdala and cerebellum lobule VI. There was also a smaller, non-significant cluster in the posterior superior temporal gyrus, a region of auditory cortex. These findings contribute to the mapping of asymmetrical structure-function links in the human brain, and suggest that subcortical structures should be investigated in relation to hemispheric dominance for speech processing, in addition to auditory cortex.

Polygenic risk scores implicate genetic pathways involved in neurodevelopmental disorders in hearing thresholds and hearing asymmetry in children

An efficient auditory system contributes to cognitive and psychosocial development. A right ear advantage in hearing thresholds (HT) has been described in adults and atypical patterns of left/right hearing threshold asymmetry (HTA) have been described for psychiatric and neurodevelopmental conditions. Previous genome-wide association studies (GWAS) on HT have mainly been conducted in elderly participants whose hearing is more likely to be affected by environmental effects. We analysed HT and HTA in a children population cohort (ALSPAC, n = 6,743, 7.6 years). Better hearing was associated with more advanced cognitive skills and higher socioeconomic status (SES). Mean HTA was negative (−0.28 dB), suggesting a left ear advantage in children but mainly driven by females (−0.48 dB in females v -0.09 dB in males). We performed the first GWAS on HT in children and the very first GWAS on HTA (n = 5,344). Single marker trait association analysis did not yield significant hits. Polygenic risk score (PRS) analysis revealed associations of PRS for schizophrenia with HT, which remained significant after controlling for SES and cognitive skills, and of PRS for autism spectrum disorders (ASD) with HTA. Gene-based analysis for HTA reached genome-wide significance for MCM5, which is implicated in axon morphogenesis. This analysis also highlighted other genes associated with contralateral axon crossing. Some of these genes have previously been reported for ASD. These results further support the hypothesis that pathways distinguishing the left/right axis of the brain (i.e. commissural crossing) contribute to both different types of asymmetries (i.e. HTA) and neurodevelopmental disorders.

Lateralization of Auditory Processing of Silbo Gomero

Left-hemispheric language dominance is a well-known characteristic of the human language system. However, it has been shown that leftward language lateralization decreases dramatically when people communicate using whistles. Whistled languages present a transformation of a spoken language into whistles, facilitating communication over great distances. In order to investigate the laterality of Silbo Gomero, a form of whistled Spanish, we used a vocal and a whistled dichotic listening task in a sample of 75 healthy Spanish speakers. Both individuals that were able to whistle and to understand Silbo Gomero and a non-whistling control group showed a clear right-ear advantage for vocal dichotic listening. For whistled dichotic listening, the control group did not show any hemispheric asymmetries. In contrast, the whistlers’ group showed a right-ear advantage for whistled stimuli. This right-ear advantage was, however, smaller compared to the right-ear advantage found for vocal dichotic listening. In line with a previous study on language lateralization of whistled Turkish, these findings suggest that whistled language processing is associated with a decrease in left and a relative increase in right hemispheric processing. This shows that bihemispheric processing of whistled language stimuli occurs independent of language.