The experiments presented in this thesis were designed to examine a number of issues raised by previous experiments employing the dual task paradigm to investigate cerebral organization. Whereas experiment one failed to support previous findings, experiments two and three did reveal that concurrent verbal tasks interfered more with right-hand tapping than with left-hand tapping in right-handed subjects. Secondly left-handed subjects did not show the same pattern of interference as right-handed subjects and, thirdly, visuo-spatial tasks did not interfere in the same way with single finger pressing as verbal ones. Close examination of both previous and present results revealed two important issues that warranted further examination. Firstly the role task difficulty plays in determining the amount and kind of interference observed in motor performance and, secondly the extent to which the locus of interference is motor-motor in nature. The results of experiments four, five, six and seven indicated that the level of task demands may play only a minor role in determining the pattern of interference generated. Furthermore, although interference does not appear to arise solely during responding processes, overt speech is more interfering than covert speech. Experiments eight and nine failed to show that this difference stemmed from the necessity to access the phonological properties of the words. A general discussion of the results concluded that proper evaluation of the theoretical explanations of interference and facilitation effects is not possible until the mechanism of such effects is understood. Furthermore, methodological problems and the assumptions underlying the use of the dual-task paradigm were examined, and it was concluded that it is extremely difficult to relate the results of combined motor and cognitive performance to cerebral organization.