We tested the hypothesis that a computational overload results when two activities, one motor and the other cognitive that draw on the same neural processing pathways, are performed concurrently. Healthy young adult subjects carried out two seemingly distinct tasks of maintaining standing balance control under conditions of low (eyes closed), normal (eyes open) or high (eyes open, sway-referenced surround) visuospatial processing load while concurrently performing a cognitive task of either subtracting backwards by seven or generating words of the same first letter. A decrease in the performance of the balance control task and a decrement in the speed and accuracy of responses were noted during the subtraction but not the word generation task. The interference in the subtraction task was isolated to the first trial of the high but not normal or low visuospatial conditions. Balance control improvements with repeated exposures were observed only in the low visuospatial conditions while performance in the other conditions remained compromised. These results suggest that sensory organization for balance control appear to draw on similar visuospatial computational resources needed for the subtraction but not the word generation task. In accordance with the theory of modularity in human performance, the contrast in results between the subtraction and word generation tasks suggests that the neural overload is related to competition for similar visuospatial processes rather than limited attentional resources. (Contains 2 tables and 5 figures.)