In modern neuropsychology attention is subdivided into different domains (Fernandez-Duque & Posner, 2001; Posner & Boies, 1971; Posner & Rafal, 1987; Sturm, 2005; van Zomeren & Brouwer, 1994). One of these domains is divided attention, which describes the ability to process two or more different tasks at the same time. According to Wickens´ (1984) multiple resources theory, processing two tasks simultaneously presented in the same sensory modality (within-modal) is more difficult than processing two tasks presented in two different sensory modalities (cross-modal). Furthermore, many experimental studies demonstrated age-related differences in different divided attention tasks (Brouwer et al., 1991; Levitt et al., 2006; Ponds et al., 1988; McDowd & Craik, 1988; Somberg & Salthouse, 1982; Coburn et al., 2006). Compared with young adults, older adults showed a significantly decreased ability to divide attention. Modern functional imaging techniques such as functional magnetic resonance imaging (fMRI) provide the opportunity to identify brain structures which are linked to known psychological processes and effects. This thesis describes two different fMRI-experiments dealing with the execution of divided attention tasks. In the first experiment, we were interested in distinguishing the cortical representations of within-modal and cross-modal divided attention tasks. Sixteen healthy male subjects aged between 21 and 30 years underwent two within-modal (auditory/auditory, visual/visual) and one cross-modal (auditory/visual) divided attention task, as well as related selective attention control conditions. After subtraction of the corresponding control tasks the three divided attention tasks, irrespective of sensory modality, revealed significant activation in a right hemisphere network involving the prefrontal cortex, the inferior parietal cortex and the claustrum. Under the cross-modal condition, however, the frontal and parietal activation was more extended and more bilateral and there was also stronger right hemisphere activation of the anterior cingulate gyrus and the thalamus. In comparison to the within-modal conditions additional bilateral frontal and left inferior parietal activation was found for the cross-modal one. The supplementary fronto-parietal, anterior cingulate gyrus and thalamus activation in the auditory/visual condition could be argued to reflect an additional demand for coordination of two ongoing cross-modal cognitive processes. In the second fMRI-experiment, we were interested in detecting aging effects possibly causing age-related differences in cortical representations of within-modal and cross-modal divided attention tasks. A young group (aged 21 to 39 years) and an old group (aged 51 to 74 years) each comprising sixteen healthy male subjects underwent one cross-modal (auditory/visual) and two within-modal (auditory/auditory; visual/visual) divided attention tasks like in the first experiment. During the cross-modal task, the old group revealed stronger divided attention specific bilateral activation in inferior and middle frontal areas, in the cingulate gyrus and in the inferior parietal lobule as well as in the left claustrum than the young group. In the old group the right inferior parietal lobule was also stronger activated while performing the two within-modal tasks, indicating that in older people this specific brain structure is highly involved in divided attention irrespective of sensory modality. We assume that the described additional activation in the old group reflects compensatory effects caused by the greater demand for attentional resources in order to regulate the system. On the other hand, the young group only showed additional activation in the thalamus during the cross-modal task. This might represent a more bottom-up organized processing of divided attention tasks.
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