Temporal organization of cerebral events: neuromagnetic studies of the sensorimotor system

Somatosensory and motor processes are closely linked to each other; smooth voluntary movements require continuous interaction of sensory and motor cortices. Sensorimotor cortical processes are readily studied with magnetoencephalography (MEG) by recording evoked responses to external stimuli or spontaneous brain oscillations. With whole-scalp coverage activation of several cortical source areas can be detected even when they are temporally overlapping. For example, electric median nerve stimuli has been shown to activate at least five different widely distributed cortical areas. With MEG recordings, temporal order of activation of different areas can be monitored to reveal functional organization of the somatosensory cortical network. Temporal resolution in millisecond scale is needed also in studies of spontaneous brain rhythms. Somatomotor mu-rhythm, with its characteristic 10 and 20Hz peaks, is typically observed over bilateral sensorimotor cortex. Mu rhythm is dampened during tactile stimulation, movement or even during action observation. Reactivity of the cortical rhythm can be quantified by temporal spectral evolution (TSE) analyses; changes in reactivity of rhythm may reveal modifications in exitatory/inhibitory balance of the sensorimotor cortex. Many neurological diseases, such as stroke and cortical myoclonus, distort activation of sensorimotor cortical network. Identification of modified activation sequences and their comparison with patients' clinical signs and symptoms may reveal pathophysiological mechanisms underlying the diseases.