Interfield Differences in Reaction Times to Lateralised Visual Stimuli in Normal Subjects

Abstract

When a visual stimulus is projected to the right of a fixation point the response with the right hand to that stimulus is faster than the response with the left hand, and similarly, when a stimulus is projected to the left of fixation the response with the left hand is faster than that with the right hand. This advantage of ipsilateral over contralateral responses was first described by Poffenberger at the beginning of the century in experiments in which he measured reaction times to simple light stimuli. Poffenberger (1912) attributed this advantage (which was of about 4 ms in his experiments) to the different lengths of the nervous circuits responsible for the two responses. Thus ispsilateral responses can be executed using central nervous circuits located entirely in one hemisphere, whereas the contralateral responses need pathways which cross the interhemispheric commissures. Since the pathway in this last condition is longer and most likely includes one additional synapse, a response mediated by it requires more time. Poffenberger also calculated, using the few data available on the conduction velocity of nervous fibres, how long the delay should be between ipsilateral and contralateral responses provided his interpretation was correct. He wrote: ‘If the speed of transmission of the nerve impulse be taken as 30 m/per second, then in one sigma it would travel 3 cm. Considering the length of these fibers (corpus callosum) to be 6 cm the reaction time would be 2 sigma, which would be constant entity.’ (Poffenberger, 1912, p. 68).