It is well known that language processing depends on specialized areas in the left side of the brain in the majority of the population. A popular view is that developmental language disorders result from a poorly lateralized brain, but evidence in support of this has been weak and inconsistent. In this thesis, I investigated language-related asymmetries in brain structure and function, and their behavioural relevance in both individuals with specific language impairment (SLI) and typically developing adults. Combining different brain imaging techniques, I looked at group-level as well as individual estimates of language laterality and its relationship to language abilities. The aim of my first two studies was to investigate the neural underpinnings of SLI in terms of white matter microstructure and functional organization associated with auditory processing. For this, diffusion and functional MRI data was obtained in a small number of families with a history of SLI and in control families. Compared with neurotypical controls, children with SLI had lower white matter integrity in the corpus callosum, and in white matter areas corresponding to the dorsal and ventral language pathways. The expected functional lateralization for auditory processing was not observed in either group. In the second half of my thesis, I assessed language laterality in 215 neurotypical adults. I demonstrated that functional transcranial Doppler (FTCD) ultrasonography could reliably assess functional lateralization across different language processes. From this large group, I identified 16 individuals with atypical language lateralization and compared them to a group of 16 typically lateralized individuals using a combination of FTCD, MRI and behavioural measures of language laterality and language abilities. The two groups differed significantly in terms of lateralization assessed by functional MRI and diffusion imaging. The atypical group had lower left and greater right hemisphere activation compared with the typical group, and lacked the leftwards asymmetry in the ventral language tract seen in the typical group. The groups did not differ in terms of cognitive measures. Different functional laterality assessments were concordant in the typically lateralized individuals but were inconsistent in the individuals assessed as atypical by FTCD. In brief, my findings suggest that for some individuals language lateralization may be unstable and varies depending on task or other factors. Even so, such differences do not appear to have consequences for language or other cognitive development.

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.