Dependence of the negative BOLD response on somatosensory stimulus intensity.

The primary somatosensory cortex (SI) has been shown to encode the intensity of a stimulus applied to the contralateral side of the body. Recent studies have demonstrated that ipsilateral SI is also involved in the processing of somatosensory information. In this study, we investigated the dependence of the negative BOLD response in ipsilateral SI on the intensity of somatosensory stimulation. Functional MRI was performed in 12 healthy subjects during electrical median nerve stimulation at four different intensities. A monotonic relationship between stimulus intensity and the strength of the negative BOLD response in ipsilateral SI was found. Additionally, a psychophysiological experiment revealed tight coupling between the stimulus intensity applied to one hand and increased perceptual threshold of the other hand. These findings indicate a stimulus intensity-dependent inhibition of ipsilateral SI.