MEG tomography of human cortex and brainstem activity in waking and REM sleep saccades.

We recorded the magnetoencephalographic (MEG) signal from three subjects before, during and after eye movements cued to a tone, self-paced, awake and during rapid eye movement (REM) sleep. During sleep we recorded the MEG signal throughout the night together with electroencephalographic (EEG) and electromyographic (EMG) channels to construct a hypnogram. While awake, just prior to and during eye movements, the expected well time-locked physiological activations were imaged in pontine regions, with early 3 s priming. Activity in the frontal eye fields (FEF) was identified in the 300 ms before the saccade onset. Visual cortex activation occurred 200 ms after saccades. During REM, compared to the eyes closed awake condition, activity was higher in supplementary motor area (SMA) and lower in inferior parietal and precuneus cortex. Electro-occulographic (EOG) activity just prior to REM saccades correlated with bilateral pontine and FEF activity some 250-400 ms before REM saccade onset, which in turn was preceded 200 ms earlier by reciprocal activation of the pons and FEF. An orbitofrontal-amygdalo-parahippocampal-pontine sequence, possibly related to emotional activation during REM sleep, was identified in the last 100 ms leading to the REM saccade, but not linked to saccade initiation.

Brain activation sequences following electrical limb stimulation of normal and paraplegic subjects.

In current clinical practice the degree of paraplegia or quadriplegia is objectively determined with transcranial magnetic stimulation (TMS) and somatosensory-evoked potentials (SSEP). We measured the MEG signal following electrical stimulation of upper and lower limbs in two normal and three clinically complete paraplegic subjects. From the MEG signal we computed distributed estimates of brain activity and identified foci just behind the central sulcus consistent in location with primary somatosensory (SI) for arm and foot and secondary somatosensory (SII) areas. Activation curves were computed from regions of interest defined around these areas. Activation of the SI foot area was observed in normal and paraplegic subjects when the upper limb was stimulated. Surprisingly, for each paraplegic subject, stimulation below the lesion was followed by cortical activations. These activations were weak, only loosely time-locked to the stimulus and were seen intermittently behind the central sulcus and nearby cortical areas. Statistical analysis of tomographic solutions and activation curves showed consistent responses following foot stimulation in one paraplegic (PS1) and intermittently in another paraplegic subject. We repeated the same experiment for PS1 in a different laboratory and the results from the analysis of foot stimulation from both laboratories revealed statistically significant focal cortical response only in the contralateral SI foot area.

Timing and connectivity in the human somatosensory cortex from single trial mass electrical activity.

Parallel-distributed processing is ubiquitous in the brain but often ignored by experimental designs and methods of analysis, which presuppose sequential and stereotypical brain activations. We introduce here a methodology that can effectively deal with sequential and distributed activity. Regional brain activations elicited by electrical median nerve stimulation are identified in tomographic estimates extracted from single trial magnetoencephalographic signals. Habituation is identified in both primary somatosensory cortex (SI) and secondary somatosensory cortex (SII), often interrupted by resurgence of strong activations. Pattern analysis is used to identify single trials with homogeneous regional brain activations. Common activity patterns with well-defined connectivity are identified within each homogeneous group of single trials across the subjects studied. On the contralateral side one encounters distinct sets of single trials following identical stimuli. We observe in one set of trials sequential activation from SI to SII and insula with onset of SII at 60 msec, whereas in the other set simultaneous early co-activations of the same two areas.