Stephan Salenius

Brain Research Unit
Low Temperature Laboratory

A dissertation for the degree of Doctor in Medicine was presented at the Helsinki University Central Hospital on June 13, 1997
Opponent was Professor Fernando Lopez da Silva, Amsterdam

Pictures from the occasion


Functions of the primary somatosensory and motor cortices as well as cortical mechanisms underlying mental imagery were studied by recording evoked and spontaneous neuromagnetic signals from 21 right-handed volunteers in association with several sensorimotor and mental imagery tasks. The study was performed in the Brain Research Unit at the Low Temperature Laboratory of Helsinki University of Technology. Recordings were made in a magnetically shielded room with a whole-scalp, helmet-shaped 122-channel first-order planar SQUID neuromagnetometer.

Somatosensory evoked fields were recorded to right and left median nerve stimulation during rest and simultaneous tactile stimulation. Early responses from contralateral SI were attenuated when tactile and electrical stimulation was applied to the same hand. In contrast, when tactile and electrical stimulation was applied to opposite hands, the SI responses were enhanced, suggesting excitatory transfer of tactile information to the ipsilateral hemisphere, possibly via the corpus callosum, which may reflect somatosensory fusion across the midline for distal limb-parts.

Oscillatory signals were monitored in association with median nerve stimulation. All subjects displayed bilateral rhythmic bursts in the rolandic areas following MN stimuli, with contralateral and left hemispheric dominance. The bursts, which were dominated by 10- and especially 20-Hz activity, were attenuated by simultaneous sensorimotor tasks, particularly by exploratory movements, indicating stronger bilateral activation of primary sensorimotor areas during exploration than during simple finger movements or tactile stimulation.

Motor imagery significantly attenuated the rhythmic 20-Hz bursts evoked by median nerve stimulation. The attenuation was stronger than during weak isometric contraction but weaker than during actual movements. Attempted movement during temporary deafferentation and paralysis of the limb, induced by tourniquet ischemia, also attenuated the 20-Hz activity. SEFs were affected only by manipulatory movements. Thus the contralateral MI appears to be activated during motor imagery, i.e., during mere internal representation, without execution, of movements.

The effect of visual imagery on the parieto-occipital alpha rhythm was examined. The generation of a visual image and particularly the inspection of this image, but not a nonvisual control task demanding similar effort, caused a dampening of alpha activity. Both response times and alpha suppression were found to be similar for small and large mental images. In all subjects, magnetic alpha arising close to the parieto-occipital sulcus was dampened during the imagery task. In one subject alpha activity was also dampened close to the calcarine fissure. Visual areas around the parieto-occipital sulcus, which may subserve vi-quo-spatial workingmemory, were thus systematically activated during visual imagery. The activation increased when the visual images had to be attended and evaluated.

Movement-associated rhythmic activity was studied in a subject, who had previously been reported to show prominent 40-Hz activity around onset of finger movements. Activity close to 40 Hz increased in the contralateral hemisphere before and particularly during slow self-paced finger lifts. A small ipsilateral 40-Hz enhancement was also found, but the 10- and 20-Hz rhythms were bilaterally attenuated in association with the movements. The 40-Hz activity originated in the motor cortex, in the precentral gyrus, and was coherent with the simultaneously recorded EMG. The observed 40-Hz activity could contribute to motor binding or sensorimotor integration or reflect feedback from, e.g., muscle spindles.

To further investigate coherence between EMG and MEG, cortical rhythms and muscle activity were recorded during isometric contraction of three upper limb and one foot muscle. For all muscles and in all subjects, EMG and MEG were found to be coherent at 15-33 Hz. The corresponding MEG signals originated in the precentral gyrus in the hand or foot areas, without evidence for within-limb somatotopy. The time-lags between cortical and muscular activity increased systematically with increasing cortico-muscular distance. The observed systematic MEG-EMG coherence indicates that motor cortex oscillations influence the timing of efferent commands.

This study generated new information about sensorimotor and mental imagery functions of the human brain by quantifying task-related reactivity of cortical rhythms. Particularly, it demonstrated a systematic relation between motor cortex activity and firing of the spinal motoneuronal pool, raising several questions about the functional significance of sensorimotor rhythms that remain to be answered by subsequent studies.