Multisensory activation of the intraparietal area when classifying grating orientation: a functional magnetic resonance imaging study.

Humans can judge grating orientation by touch. Previous studies indicate that the extrastriate cortex is involved in tactile orientation judgments, suggesting that this area is related to visual imagery. However, it has been unclear which neural mechanisms are crucial for the tactile processing of orientation, because visual imagery is not always required for tactile spatial tasks. We expect that such neural mechanisms involve multisensory areas, because our perception of space is highly integrated across modalities. The current study uses functional magnetic resonance imaging during the classification of grating orientations to evaluate the neural substrates responsible for the multisensory spatial processing of orientation. We hypothesized that a region within the intraparietal sulcus (IPS) would be engaged in orientation processing, regardless of the sensory modality. Sixteen human subjects classified the orientations of passively touched gratings and performed two control tasks with both the right and left hands. Tactile orientation classification activated regions around the right postcentral sulcus and IPS, regardless of the hand used, when contrasted with roughness classification of the same stimuli. Right-lateralized activation was confirmed in these regions by evaluating the hemispheric effects of tactile spatial processing with both hands. In contrast, visual orientation classification activated the left middle occipital gyrus when contrasted with color classification of the same stimuli. Furthermore, visual orientation classification activated a part of the right IPS that was also activated by the tactile orientation task. Thus, we suggest that a part of the right IPS is engaged in the multisensory spatial processing of grating orientation.