Tonic and phasic alertness - One or two right hemispheric cortico-subcortical networks?

The functional role of the right hemisphere for sustained attention and vig ilance is well-known. This investigation focusses on the localisation of di fferent components of sustained attention. We analysed visual simple reacti on time with and without acoustic warning for 63 patients with infarcts in the right hemisphere and compared the results with a control group of 30 pa tients with lesions in the cerebellum. Moreover we calculated a value which indicated how much the patients profited by the warning tone. We argue tha t this value is a good indicator for sustained attention. The involved brai n areas were transferred into a standardized brain atlas blinded for the re sults of the neuropsychological investigation. The neuropsychological resul ts of the patients with right hemisphere brain damage, as a whole, were the n compared with the control group. We compiled the mean reaction time for o verlapping lesion areas and after sub-grouping the 63 right hemisphere pati ents according to different infarct patterns, we tested the statistical dif ferences between the lesion locations. As a result we found a general slowi ng in right hemisphere patients. This was most pronounced for lesions invol ving the right frontal and the posterior parietal areas. But only patients with lesions involving the white matter of the corona radiata (including th e corpus of the caudatum) and patients with lesions involving the insular c ortex were impaired in sustained attention, as operationalized by our calcu lation. The results of the patients with fronto-parietal lesions fit recent PET studies which give evidence to the meaning of these areas for simple r eaction tasks. On the other hand. there may be a second network responsible for sustaining attention in the absence of external arousal signals.