Bilateral processing of numbers: parallel processing versus functional unity.

Thirty-six normal subjects had to add two numbers ranging from 3 to 9 which were presented tachistoscopically. The two numbers were displayed either unilaterally to the left or right visual field, or bilaterally with one number to the left and one to the right visual field. They were represented as two word names, two dot clusters, or two bargraphs, or the two numbers were represented inconsistently in two different of these representations. The results showed that bilateral processing of a word and a bargraph was performed without visible interactions, and the hemispheres performed independently. When the display included a dot cluster, however, its processing (sequential vs. holistic) appeared to depend on the type of stimulus processed by the contralateral hemisphere, indicating strong interhemispheric interactions.