Postlesional plasticity in central auditory processing : MRI and FMRI image registration and statistical signal processing

The most general and striking evidence related with brain injury is that of the restoration of function. Recovery of motor and somatosensory functions has shown to commonly occur after stroke, but not all individuals show improvement. Clinical studies have shown the capacity of pharmacological and rehabilitative interventions to accelerate and/or augment recovery after stroke. However, the mechanisms underlying the post-stroke brain functional reorganization, i.e., the brain plasticity, are still not well established. Therefore, therapeutic trials of agents or rehabilitative procedures targeting stroke recovery may benefit from brain mapping studies that may aid to better understand this functional reorganization. Thus the research aimed at the identification of the mechanisms underlying functional recovery should be given high priority, particularly with regard to environmental enrichment, rehabilitation and pharmacological interventions. Prior to investigation of post-stroke functional reorganization, two important conditions have to be gathered: having knowledge of the brain function under normal conditions, i.e., in normal subjects without brain lesions, and having devices adapted to study brain region of interest. This thesis addresses the post-stroke functional reorganization in auditory processing with a cross-sectional study in patients with unilateral hemispheric lesions and a longitudinal study in a patient with a lesion of a right acoustic radiation. Before performing these two studies with patients, normal function of the auditory processing is assessed and appropriate tools are developed. Audition is the key to language processing, the most important communication system in man. Hearing impairments arising from pathology of the brain injury may have detrimental consequences on the quality of the patient life, restricting our ability to interact with others, causing misunderstandings and fatigue, heightening stress and filtering out the myriad of sound experiences that give pleasure and meaning to life. The perception of an auditory scene in everyday acoustic environments involves identifying the content ("what") and the location ("where") of sound. Evidence indicates that sound recognition and sound localization are processed by at least partially independent anatomically distinct networks. In humans, activation studies have suggested existence of a ventral, temporo-frontal, "what" and dorsal, parieto-prefrontal, "where" pathways on the convexities. However, no studies have been able to clearly demonstrate "what" and "where" specialization in early stage auditory areas. This is mainly due to high local interindividual sulcal variability. Indeed, the precise realignment of anatomical landmarks on the supratemporal plane could not be achieved with current registration methods, thus providing less detailed and accurate functional maps. After a short introduction on brain variability, a brief description of auditory cortex structure and