Hemispheric Specialization in Normal Man Studied by Bilateral Measurements of the Regional Cerebral Blood Flow

Changes of regional cerebral blood flow (rCBF) during mental activation have so far only been studied unilaterally by means of the intracarotid 133Xe injection technique. In the present study bilateral rCBF measurements were made by the 133Xe inhalation technique during mental activities supposed to engage the hemispheres selectively. The study was carried out on 24 normal right-handed male volunteers by the short 133Xe inhalation method. Fourteen scintilation detectors were placed in parallel overcorresponding regions of the two hemispheres. Four rCBF measurements were made on each subject during three randomized conditions: (1) Rest (two studies); (2) Verbal test (Miller Analogy Test), and (3) Spatial test (Street). The recordings were separated by at least one hour to avoid contamination from previous measurement. The answers to the problems were given either by raising the fingers of the left hand (Verbal test) or after the study (Spatial test). In order to investigate the effects of increased motivation 12 of the subjects were promised an extra money reward in addition to the basic payment, if they performed above a certain level on the tests. Small but statistically significant differences between the average hemisphere flows were recorded during the two tests. In the highest motivated (paid) group the verbal test increased the flow in the left hemisphere by 16 per cent while the right-sided increase was significantly smaller, 13 per cent (P less than 0-001). The corresponding figures for the spatial test were: left 7 per cent, right 10 per cent (P less than 0-01). The same tendencies were seen in the less motivated group although the hemisphere differences were smaller and statistically not significant. Analysis of the regional results showed that the largest interhemispheric differences (about 5 per cent) were seen in occipital and parietal regions during the verbal test and in frontal and parietal regions during the spatial test. It is concluded that bilateral measurements of rCBF can be used with advantage in the study of lateralization of mental functions in the human brain.