Dissertation committee:
Κωνσταντίνος Πόταγας, Αναπληρωτής Καθηγητής, Ιατρική Σχολή, ΕΚΠΑ
Μιχάλης Πετρίδης, Καθηγητής, Department of Psychology, McGill University
Ιωάννης Ζαλώνης, Αναπληρωτής Καθηγητής, Ιατρική Σχολή, ΕΚΠΑ
Σοφία Βασιλοπούλου, Επίκουρη Καθηγήτρια, Ιατρική Σχολή, ΕΚΠΑ
Διονύσιος Γούτσος, Καθηγητής, Φιλοσοφική Σχολή, ΕΚΠΑ
Ιωάννης Ευδοκιμίδης, Ομότιμος Καθηγητής, Ιατρική Σχολή, ΕΚΠΑ
Παναγιώτης Σίμος, Καθηγητής, Τμήμα Ψυχολογίας, Πανεπιστήμιο Κρήτης
Summary:
Broca’s area, an area located in the third frontal convolution of the inferior frontal gyrus, composed of pars triangularis (Brodmann's area 45) and pars opercularis (Brodmann's area 44), plays a significant role in language production. The study of Broca’s area connectivity may add to the understanding of the contribution of both the connections as well as the connected cortical areas, including those in the right hemisphere, to language functions. The present thesis investigated three issues: two of them concern the participation of right hemisphere structures in the language network a. through the documentation of the connections between the left and right language circuitry, and particularly, via Broca’s area and b. through the documentation of the structural changes that occur within the right network, after a lesion in the left language system. The third issue focuses on further clinical documentation of the different functions of the double route for language and the separate contribution of Broca’s area sections to the two routes. Thus, aim of the current thesis was:
a) the investigation of the interhemispheric connections of Broca’s area to its’ right homologue.
b) the investigation of the connections between the right Broca’s area homologue and other areas of the right hemisphere
c) the clinical documentation of the participation of Broca’s area to different language functions, via its’ ventral and dorsal connections which constitute the double route for language, through the study of rare clinical cases. Methods: 25 aphasic individuals with a single left cerebrovascular accident and 24 healthy participants, matched for age, gender and education were included in the study. All participants underwent magnetic resonance imaging, with high resolution sequence acquisition, in order to investigate grey (HR_3DT1w) and white matter (30dir-DTI). With the use of previously described, but also original protocols of diffusion tensor tractography, seven white matter tracts of both the left and right hemisphere, as well as the callosal fibers that connect Broca΄s area with its’ right homologue were reconstructed. Moreover, aphasic individuals were examined for language and other cognitive functions. In this context, a rare case of a patient with a lesion which selectively affected the ventral pathway was examined in comparison with two patients, who had a lesion that selectively affected the dorsal pathway. Results: 1. In 9/10 healthy participants, no callosal fibers connecting the left and right inferior frontal gyri were reconstructed with the use of a widely known and used algorithm (Philips FiberTrak). On the contrary, callosal fiber reconstruction was achieved in all 10 participants with the use of a robust deterministic algorithm (Brainance MD). To our knowledge this is the first in vivo documentation of the connection between Broca’s area and its’ right homologue. 2. Significant structural differences were found in all tracts of the intact right hemisphere of chronic aphasic individuals, which presented with significantly increased axonal diffusivity (AD). Moreover, statistically significant correlations were observed between radial diffusivity (RD) of the right hemisphere tracts and language performance of the aphasics. In one aphasic subgroup, in which a large left hemisphere lesion affected all the tracts of the double route for language, only the right ventral temporo-frontal extreme capsule fasciculus showed significant correlations to language performance. 3. In a patient with a selective left ventral route lesion, which affected the temporo-frontal extreme capsule fasciculus, regions of the anterior temporal gyrus and BA45 of Broca’s area – sparing BA44 – decreased performance was observed only in tests that included selective and strategic retrieval of lexicosemantic information, such as the Peabody picture vocabulary test- Revised (PPVT-R), while speech, repetition, and comprehension were intact. This came in sharp contrast with the performance of the two patients with selective lesion of the dorsal route, who presented with phonemic and semantic errors during speech and comprehension, repetition and naming deficits, but normal performance in the PPVT-R test. In conclusion: the present thesis aimed to contribute to the documentation of the network approach of the brain function regarding speech and other language functions, in order to provide further information that contributes to the distancing of this hypothesis from the influence of phrenology. In this context, a specific symptom-lesion matching is no longer sought, but rather, a sum of symptoms expressing the alterations that the entire network and its’ structures may undergo. The proposal suggesting that, after a focal lesion, changes in the function of the entire network as well as the participating structures occur, does not exclude the fact that each of the structures and fasciculi that comprises it have specific roles and participate in specific functions. Therefore, we sought for evidence regarding the modification of the function of structures irrelevant to the focal lesion, and also regarding the specific function of parts of the network, in an attempt to move away from a rigid conception of the function of the brain structures. In our first study, restrictions of DTI methodology in the reconstruction of fibers passing through voxels where massive tracts of different directions meet, are outlined. In the future, new white matter reconstruction methodologies will possibly be able to shed light to the separate contribution of specific callosal fibers to language functions and the recovery after an acquired left hemispheric lesion. In the second study, right white matter tracts (corresponding to the left double route for language) were found to play a role in language processing, in a functional analogy with the tracts of the left hemisphere, after a left hemisphere lesion. More particularly, the significance of the temporo-frontal extreme capsule fasciculus in language functions, such as verbal fluency and reading, is highlighted, when the left hemisphere lesion is large enough to affect the sum of the fasciculi that are important for language. Finally, in our third study, through the examination of rare cases who suffered selective lesions to either the ventral or the dorsal pathway, Broca’s area was found to participate in different ways to language functions: area BA44, via its’ connections to posterior temporal and parietal regions (dorsal route), participates in phonemic and semantic processing, sentence comprehension and repetition, while area BA45, via its’ connections to anterior temporal regions (ventral route), is involved in strategic selective retrieval of lexicosemantic information.
