Dichotic listening of auditory stimuli is a method of assessing brain lateralisation. Different stimuli are presented simultaneously to the left and right ears, with the listener reporting which stimulus is perceived most clearly. To date, several studies that have examined the effects of aging on dichotic listening performance have indicated a pronounced right ear advantage (REA) with increasing age, but few studies have considered the effects of sex, and findings to date have been inconclusive. The aim of this research was to investigate whether the effects of age and sex resulted in a difference in the magnitude of the REA in both undirected and directed attentional tasks. Forty sex-matched, right-handed subjects with normal hearing or symmetrical bilateral sensorineural hearing loss participated in a series of directed and undirected dichotic listening tasks using consonant-vowel (CV) stimuli. The interaural intensity difference (IID) was modulated randomly during the undirected attention task. Results indicated that all groups (age & sex) showed a REA for both undirected and directed dichotic listening tasks. No age or sex-related differences were found. The findings were suggestive of a task-linked effect for dichotic listening performance. The use of CV stimuli, in combination with detailed testing via manipulation of the IID, appeared to minimize any possible age or sex-related differences. These findings have implications for theories on laterality and hemispheric asymmetry for older adults.

Studying language development through event-related potentials provides specific information regarding how the brain processes specific aspects of language over time. In this study, the P600 component, a positive wave occurring approximately 600 ms post-stimulus and known for detecting syntactic errors, was specifically analyzed. Thirty children between the ages of 5 and 12 years listened to linguistically correct, syntactically incorrect, and semantically incorrect sentences in three ear conditions: monaurally to the right ear, monaurally to the left ear, and binaurally. The participants were instructed to judge the sentences to be correct or incorrect. Comparisons were then made of the latency and amplitude of the P600 between the age groups, sentence types, and ear conditions. The results of this study indicate that younger children exhibit later latencies and higher amplitudes than do adults. The study also suggests that syntactic processing becomes fully established around the age of 8 to 9 years. In reference to ear condition, this study found that ear condition may be a factor in a child's ability to recognize syntax. This was the first study that investigated developmental ERPs and ear condition. Therefore, this finding is a result of interest that needs to be further explored in future studies. The current study also suggests that the right ear advantage (REA) phenomenon may exist neurologically in older ages with monotic sentences. This is another area that would benefit from additional research as this phenomenon has not been previously described.

This study investigated the neurophysiology of semantic language processing in children, ages 5 to 12 years. A well-established marker of semantic processing, the N400 event related potential (ERP), was analyzed within and across child age groups. Child N400s were recorded in response to correct sentences, semantically incorrect sentences, and syntactically incorrect sentences. N400s were also recorded across ear condition to examine potential processing differences. Children across all age groups consistently demonstrated N400s in the semantic error condition. N400s were also regularly observed in the syntactic error condition; especially, for younger children. Younger children also demonstrated N400s even in response to correct sentence types. Interestingly, clear N400 effects (i.e. N400 amplitude differences between correct and semantically incorrect sentences) were only observed for one age group. While these findings indicate that children across all age groups detect semantic errors, the ability to consistently parse error types develops later.

The examination of Event Related Potentials during language processing tasks provides valuable information of how the brain processes language over time. In the current study, the development of the early left anterior negativity (ELAN) was analyzed in young children. Previous research has described the ELAN as a negative waveform elicited during syntactic processing between 200 and 500 ms post linguistic stimuli. Thirty children from 5 to 12 years of age listened to sentences that were linguistically correct, syntactically incorrect, or semantically incorrect. Sentences were presented for right monaural, left monaural, and binaural ear conditions to determine possible differences related to right ear advantage (REA). An ELAN-like component in regards to latency and amplitude was observed in children 8 years of age and older; however, comparison between linguistic conditions suggest that the ability to differentiate between linguistically correct, syntactically incorrect, and semantically incorrect stimuli is not established until 12 years of age. Results suggest that adult-like syntactic processing of morphosyntactic errors is not established until after 12 years of age. Comparison between ear conditions suggests that the REA effect may exist in older children, a finding that has not been reflected in previous behavioral research.

The right ear advantage (REA) phenomenon has been utilized in clinical and research settings to study auditory processing disorders and linguistic lateralization. Previous research has established that the REA is not reliable in its measures within or between individuals. This is likely due to the influence of other variables, such as neuromaturation and attention. One variable that has not been studied in depth in this context is linguistic complexity. It was hypothesized that stimulus conditions with levels of linguistic complexity would elicit corresponding levels of temporal lobe activity. Understanding and controlling the variables that affect the REA will increase the reliability of the measure. Twenty right handed, neurotypical individuals aged 18-29 participated in a functional magnetic resonance imaging (fMRI) study that identified the regions and the extent of activation involved in listening to dichotic syllables, words, and sentences. Three durations of speech babble corresponding to the mean duration of the syllables, words, and sentences were used as control stimuli. Participants listened to dichotic stimuli and reported the stimulus they heard best during an fMRI scan. Reaction time (RT), ear preference, and fMRI data were recorded simultaneously and analyzed post hoc. Behavioral results showed that words had the shortest RTs and the greatest REA; syllables and sentences were similar to each other for both measures. Significant main effects were found in brain regions known to be involved in cognitive control of attention and linguistic processing. Words were associated with significant activation differences for ear preferences and minimal frontal lobe involvement for right ear preference. Syllables caused the least activity in the frontal lobe regions and less voxel activity in the temporal lobes than syllable-length babble. Sentences had the greatest voxel activity in the frontal and temporal lobe regions. It was concluded that words would best reflect the REA in clinical and experimental designs. Words had minimal involvement of frontal lobe regions indicating minimal cognitive control of attention and the largest discrepancies in activation patterns between right and left ear preferences that showed less cognitive power to process right ear stimuli in a dichotic listening situation.

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Un biais fonctionnel entre les oreilles droites et gauches a été largement documenté, avec un avantage de l'oreille droite (AOD) dans les tâches perceptives, en particulier celles utilisant le traitement verbal. Les sons complexes tels que la parole sont perçus plus facilement dans l'oreille droite (OD) que dans l'oreille gauche (OG) pour les tâches d’écoute dichotique (ED) ainsi que pour les tâches de perception de la parole dans le bruit (PDB). Des études sur les tâches d’ED ont démontré que le vieillissement est associé à une réduction globale des performances des deux oreilles, avec un déficit de performance plus important pour les stimuli présentés à l'OG par rapport à ceux présentés à l'OD entraînant une augmentation de l’AOD. Ce déficit à l'OG pourrait être le résultat d'une baisse de l'efficacité du transfert inter hémisphérique d'informations auditives via le corps calleux due au vieillissement ou de changements liés au vieillissement dans les fonctions cognitives. Cependant, des preuves chez les jeunes adultes ayant une audition normale suggèrent que les différences interaurales (DI) dans le traitement sensoriel au niveau des parties inférieures du système auditif pourraient également expliquer l'ampleur de l’AOD pour les tâches d’ED. L'objectif principal de cette thèse est d'étudier si les DI dans le traitement sensoriel aux niveaux inférieurs du système auditif sont associés à l'ampleur de l’AOD chez les personnes âgées. De plus, ce projet vise également à déterminer si les DI dans le traitement sensoriel au niveau des parties inférieures du système auditif sont associées aux processus de perception de la PDB chez les personnes âgées. Pour ce faire, 70 personnes âgées ont participé à cette étude. Tous les participants ont été évalué avec une batterie complète de tests auditifs examinant les seuils de sons purs, les émissions otoacoustiques évoquées transitoires avec et sans stimulation acoustique controlatérale, la réponse auditive du tronc cérébral et la performance sur les tâches d'ED et de perception de la PDB. Afin de contrôler la fonction cognitive, les habiletés cognitives telles que la vitesse de traitement, la flexibilité cognitive et la mémoire de travail ont également été évaluées. Les DI dans le traitement sensoriel aux niveaux inférieurs du système auditif ont été dérivées en calculant la différence entre l’OD et l’OG pour chacune des mesures auditives. Des modèles de régression bivariées et multivariées ont été réalisés en intégrant les variables d’habiletés cognitives dans les modèles de régression. Les résultats ont révélé que la DI pour les seuils de sons purs et la DI pour les émissions otoacoustiques évoquées transitoires avec suppression étaient associés significativement à l'ampleur de l’AOD chez les personnes âgées. De même, la DI pour les émissions otoacoustiques évoquées transitoires et la DI pour la réponse auditive du tronc cérébral expliquent la performance de la perception de la PDB chez les personnes âgées. De plus, les résultats ont révélé que les habiletés cognitives contribuent de manière significative à l'ampleur de l’AOD ainsi que pour la performance de la perception de la PDB chez les personnes âgées. Dans l'ensemble, la présente thèse fournit des données qui suggèrent que les DI dans le traitement sensoriel au niveau des parties inférieures du système auditif explique en partie l'ampleur de l’AOD pour les tâches d’ED ainsi que les problèmes de perception de la PDB chez les personnes âgées. / A functional bias between the right and left ears has been widely documented, with a right-ear advantage (REA) in perceptual tasks, particularly those employing verbal processing. Complex sounds such as speech are more accurately perceived in the right ear (RE) compared to the left ear (LE) for dichotic listening (DL) and speech perception in noise (SIN) tasks. Although previous studies have shown that aging is associated with an overall decline in DL performance in both ears among older adults, the left ear (LE) performance often decreases more dramatically relative to the right ear (RE) performance, causing an increased REA for speech stimuli. This greater LE deficit may be the result of age-related changes in (A) cognitive functions; (B) functions of the right hemisphere (RH); and /or (C) the efficiency of interhemispheric transfer of auditory information via the corpus callosum. However, evidence in normal hearing young adults suggest that interaural difference (ID) in sensory processing at lower portions of the auditory system might also explain the magnitude of the REA for DL. The main aim of this thesis is to investigate whether ID in sensory processing at lower levels of the auditory system are associated with the magnitude of the REA for DL among older adults. In addition, this project aimed to investigate whether ID in sensory processing at lower levels of the auditory system is associated with SIN performance in older adults. A total of 70 older adults participated in this study. To assess sensory processing at lower levels of the auditory system, hearing thresholds, transient evoked otoacoustic emission (TEOAE), contralateral suppression of TEOAE, a proxy measure of medial olivocochlear (MOC) activation and click-and-speech auditory brainstem response to speech stimuli were measured in both ears separately. The ID in sensory processing was derived by calculating the difference between the right and left ears for each auditory measure. The mobile device app version for iPhone of the Bergen Dichotic Listening test, the iDichotic, was used to evaluate DL. In addition, SIN performance was evaluated with the hearing-in-noise test (HINT). With the aim to control for cognitive function, cognitive abilities such as speed of processing, cognitive flexibility and working memory were also evaluated. Bivariate and multivariate regression models were performed, and cognitive measures were accounted for in the regression models. Results revealed that ID in pure-tone thresholds and ID in MOC-induced TEOAE suppression were significantly associated with the magnitude of the REA for DL among older adults. Similarly, ID in TEOAE and ID in speech-ABR measures explained SIN performance in older adults. In addition, results revealed that cognitive measures significantly contributed to the magnitude of the REA for DL and SIN performance in older adults. The present thesis provides evidence indicating that ID in sensory processing at lower levels of the auditory system partially explain the magnitude of the REA for DL as well as SIN performance among older adults.

Nach dem Prinzip synchronisierter Reimwörter wurde ein deutscher dichotischer Hörtest entwickelt und erprobt (Fused Words, FW10b). Die Reliabilitätskennwerte lagen für die innere Konsistenz bei alpha=0,89 und für die Re-Test-Reliabilität im Bereich von r-tt=0,67 bis r-tt=0,87. Der dichotische Test korrelierte hoch mit einem Paralleltest (r=.73, FW12k). In mehreren Untersuchungen fand sich keine Assoziation des dichotischen Lateralisationsergebnisses mit dem Hormonzyklus bei Frauen. Zwischen der Klassifikation im Wada-Test und der Klassifikation im dichotischen Test wurde eine hohe prozentuale Übereinstimmung beobachtet (91%). Auch in einer externen Studie wurde eine hohe Konkordanz mit einem fMRT Paradigma gefunden (97%, Hund-Georgiadis et al 2002). Unter Rechtshändern hatten 70% einen Rechts-Ohr-Vorteil, 23% einen Links-Ohr-Vorteil; bei 7% wurde kein Ohr-Vorteil beobachtet. Es bestand eine hohe Assoziation zwischen dem Lateralitätsquotient der Händigkeit und dem Lateralitätsindex Lambda aus dem dichotischen Test (R=0,76, r=0,72). Bei epilepsiechirurgischen Kandidaten mit links- oder rechtsseitigen Temporallappenläsionen, die alle durch einen Wada-Test als unilateral links-hemisphärisch sprachdominant kategorisiert worden waren, wurden Läsionseffekte analysiert (Hippocampus-Sklerose vs. extra-hippocampale Läsionen). Wie erwartet ergaben sich massive Läsionseffekte für die Patienten mit den Läsionen in der linken sprachdominanten Hemisphäre. In der linken extra-hippocampalen Untergruppe kam es zu einer signifikanten Abnahme der Rechts-Ohr-Punkte und gleichzeitig zu einem signifikanten Anstieg der Links-Ohr-Punkte. Die Einbeziehung von neokortikalen und anderen extra-hippocampalen Strukturen in die temporalen Läsionen war von entscheidender Bedeutung für die Unterdrückung der Antworten vom kontralateralen (rechtsseitigen) Ohr. Die Zunahme der Links-Ohr-Punkte in der Gruppe mit linksseitigen extra-hippocampalen Läsionen deutet eine kompensatorische Verlagerung von Sprachfunktionen auf die rechte Hemisphäre an, die durch den Wada Test nicht erfasst wurde. / Following the principle of synchronized rhyming words a German dichotic listening test was developed and tested (fused words, FW10b). Reliability coefficients for the inner consistency were at alpha=0,89 and for the re-test-reliability between r-tt=0,67 and r-tt=0,87. The dichotic test FW10b was highly correlated with a parallel test (r=.73, FW12k). None of several studies did show any association between the dichotic language lateralization and the hormonal cycle in women. A high percentage of agreement was observed between the Wada classification and the classification by the dichotic test (91%). Furthermore Hund-Georgiadis et al. (2002) found a high percentage of concordance with the lateralization by a fMRI paradigm in an external study (97%). Among right-handers 70% showed a right ear advantage, 23% had a left ear advantage, and 7% had no ear advantage. There was a high association between the laterality quotient of the handedness inventory and the laterality index Lambda of the dichotic listening test FW10b (R=0.76, r=0.72). Lesion effects were analyzed in epilepsy surgical candidates with left and right temporal lobe lesions (hippocampus sclerosis vs. extra-hippocampal lesions), who were all categorized as left-hemispheric language dominant by a Wada-test. As expected, massive lesion effects were apparent in those patients who had their lesions in the left language dominant hemisphere. In the subgroup with left extra-hippocampal lesions there was a significant decrease of right-ear-points and at the same time a significant increase of left-ear-points. The involvement of neocortex and other extra-hippocampal structures in temporal lobe lesions were of crucial importance for the suppression of contralateral (right) ear responses. The elevation of left-ear-points in the left-lateral group indicated a partial compensational shift of language functions to the right hemisphere, which was not detected by the Wada procedure.