Hemispheric lateralization of somatosensory processing

Processing of both painful and nonpainful somatosensory information is gene rally thought to be subserved by brain regions predominantly contralateral to the stimulated body region. However, lesions to right, but not left, pos terior parietal cortex have been reported to produce a unilateral tactile n eglect syndrome, suggesting that components of somatosensory information ar e preferentially processed in the right half of the brain. To better charac terize right hemispheric lateralization of somatosensory processing, (H2O)- O-15 positron emission tomography (PET) of cerebral blood flow was used to map brain activation produced by contact thermal stimulation of both the le ft and right arms of right-handed subjects. To allow direct assessment of t he lateralization of activation, left- and right-sided stimuli were deliver ed during separate PET scans. Both innocuous (35 degreesC) and painful (49 degreesC) stimuli were employed to determine whether lateralized processing occurred in a manner related to perceived pain intensity. Subjects were al so scanned during a nonstimulated rest condition to characterize activation that was not related to perceived pain intensity. Pain intensity-dependent and -independent changes in activation were identified in separate multipl e regression analyses. Regardless of the side of stimulation, pain intensit y-dependent activation was localized to contralateral regions of the primar y somatosensory cortex, secondary somatosensory cortex, insular cortex, and bilateral regions of the cerebellum, putamen, thalamus, anterior cingulate cortex, and frontal operculum. No hemispheric lateralization of pain inten sity-dependent processing was detected. In sharp contrast, portions of the thalamus, inferior parietal cortex (BA 40), dorsolateral prefrontal cortex (BA 9/46), and dorsal frontal cortex (BA 6) exhibited right lateralized act ivation during both innocuous and painful stimulation, regardless of the si de of stimulation. Thus components of information arising from the body sur face are processed, in part, by right lateralized systems analogous to thos e that process auditory and visual spatial information arising from extrape rsonal space. Such right lateralized processing can account for the left so matosensory neglect arising from injury to brain regions within the right c erebral hemisphere.