Executive Laterality: Evidence from Hemisometry, Hemisity, and MRI

The bilateral human brain includes structures producing a preconscious executive element, the source of the final decision to act. This executive element appears to be located unilaterally in the brain on the larger side of the anterior cingulate cortex. In a given individual, its side of residence is genetically determined in idiosyncratic manner by a phenomenon called hemisity. Using flash hemisometers, a single 0.5ms flash was directed to both cerebral hemispheres of the subject simultaneously. This was done either by stimulating nasal retinal surfaces or temporal retinal surfaces of the subject's eyes. Of 91 subjects, 87 (96%) reported seeing two flashes; the first appearing on one idiosyncratic side, closely followed by another on the same side. Yet, only single flashes were reported when nasal and temporal retinae were simultaneously stimulated. This non-intuitive two flash result would occur if on one side of the brain the flash went directly to a unilateral executive observer to be seen first. Because the flash going to the other, non-executive side of the brain would not be perceived there, it would have to cross the corpus callosum to reach the executive observer as a delayed second flash. Here, the individual brain side of the executive observer determined by this method was highly correlated with each subject's larger anterior cingulate and MRI-calibrated hemisity. These results provide confirmatory evidence for the existence of executive laterality.