ATTENTION MODULATES BOTH PRIMARY AND 2ND SOMATOSENSORY CORTICAL ACTIVITIES IN HUMANS - A MAGNETOENCEPHALOGRAPHIC STUDY

To clarify the role of primary and second somatosensory cortex (SI and SII) in somatosensory discrimination, we recorded somatosensory evoke d magnetic fields during a stimulus strength discrimination task. The temporal pattern of cortical activation was analyzed by dipole source model coregistered with magnetic resonance image. Stimulus intensity w as represented in SI as early as 20 ms after the stimulus presentation . The later components of SI response (latency 37.7 and 67.9 ms) were enhanced by rarely presented stimuli (stimulus deviancy) during passiv e and active attention. This supports an early haptic memory mechanism in human primary sensory cortex. Contra- and ipsilateral SII response s followed the SI responses (latency 124.6 and 138.3 ms, respectively) and were enhanced by attention more prominently than the SI responses . Active attention increased SII but not SI activity. These results ar e consistent with the concept of ventral somatosensory pathway that SI and SII are hierarchically organized for passive and active detection of discrete stimuli.