SUMMARY1. Single motor units were recorded with highly selective electrodes from intact tibialis anterior muscle in the adult man. A detailed parametric analysis was made of the discharge patterns during voluntary isometric contractions of different peak forces carried out at various rates of force development.2. During the smooth tracking of a ramp force, the different motor units recorded from a given muscle site were recruited in a consistent order, each unit becoming active when the muscle developed a certain level of force. The threshold of some of the units in such slow ramp contractions exceeded 8 kg. By contrast, in brisk ballistic contractions reaching a peak force of 12 kg in less than 0 15 sec, the same motor units discharged in a transient burst which largely preceded the muscle force production.3. In slow tracking ramp contractions, the instantaneous frequency of single motor units was initially rather low (5-15/sec) and it increased as the ramp force augmented. By contrast, in (strong) ballistic contractions, the same units discharged at an unusually high instantaneous frequency (60-120/sec) early in the burst and the firing frequency decreased thereafter. Such hitherto unknown pattern appears characteristic of ballistic contractions and it was not found in even fast tracking ramp contractions achieving 12 kg in only 0 4 sec. 4. The potentials of the different motor units activated are rather crowded at intervals of a few msec in the early burst of a strong ballistic contraction and observations on the rank activation of the different motor units do not provide reliable data for the analysis of the recruitment order of units in ballistic contractions.5. A new method is described for estimating ballistic force threshold of single motor units. When a large series of brisk ballistic contractions with peak forces ranging from 005 to 12 kg was carried out any given motor J. E. DESMEDT AND E. GODA UX unit only became active when the ballistic peak force exceeded a certain reproducible value. A detailed analysis of the recruitment order based on these ballistic force thresholds showed it to be virtually identical to the recruitment order of the same units in slow tracking ramp contractions (correlation = 0.95).6. Ballistic contractions are graded in force both by the recruitment of additional motor units in stronger contractions, and by an increase in their rate of firing. These gradation mechanisms are discussed.

Detailed clinical sensory and motor signs were correlated case by case with somatosensory evoked potentials (SEP) in 22 selected patients with a single circumscribed hemisphere lesion. The lesions collectively mapped out a variety of cerebral sites from the anterior frontal to the posterior parietal regions. SEPs were averaged from 8 standard scalp sites with an earlobe reference electrode, so that parietal N20-P27-P45 were differentiated from prerolandic P22-N30 SEP components. SEP wave forms to stimulation on the unaffected side served as the patient's own control. A complete parietal lesion produced contralateral hemianaesthesia without upper motor neuron signs and eliminated the parietal N20-P27-P45 while the prerolandic P22-N30 persisted at usual latencies. The neural generators for the N20 and the P22 components are thus distinct. It is also proposed that direct, short latency pathways convey somatosensory inputs to the motor cortex, independently of connections via parietal areas 2 and 5. Enhancement of P22-N30 after chronic parietal lesions suggests collateral reinnervation by residual inputs after partial deafferentiation of prerolandic cortex. Small postcentral lesions produced astereognosis (with preserved tactile and deep sensation) and reduced or eliminated the N20 and P27 SEP components, but did not affect the P22-N30 components. Precentral lesions with severe hemiplegia (but not prefrontal lesions) eliminated the prerolandic P22-N30 SEP components and did not alter the parietal N20-P27-P45 components. The data are pertinent to the understanding of the pathophysiology of somatosensory deficits and for the diagnostic use of SEPs in cerebral lesions.

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SUMMARY1. Cerebral potentials evoked by random sequences of electrical stimuli to four fingers were recorded in intact man performing selective attention tasks. Eye movements and other artifacts were excluded from the averaged traces. Different finger stimuli were designated as targets to be mentally counted in alternate runs of each experiment. The high mean random rate of stimuli (Il10/min) fully involved the processing capacities of the subject. Vigilance changes or differential expectancy effects were excluded by the reciprocal random design with four different sensory channels. Task 4. Enhancements of N140 were not observed in similar random fourfinger experiments carried out at a 4 times slower mean rate, but they occurred in a bisensory paradigm with finger shocks and acoustic clicks at that slower rate. 5. A large positive P400 component was only elicited by target stimuli. Its voltage was maximum over the parietal region and was equal on both sides.

Prerolandic and parietal SEPs to electrical stimulation of fingers or median nerve were studied with non-cephalic reference in 40 normal young adults and in 35 healthy octogenarians. Limb temperatures were 36-37 degrees C. Intersubject variations of SEP components were analysed. A new widespread component N18 was identified and shown to be generated below the cortex. This N18 is about the only early component recorded at the parietal ipsilateral region after the positive far-field potentials P9, P11 and P13-P14. Transit times along the central somatosensory pathway were replicated and discussed as well as other evidence about the sequential activation of the various neural structures involved. The N20 potential representing the earliest cortical response is recorded from the contralateral parietal region, but is absent ipsilaterally. The prerolandic potential is related to distinct generators and is elicited by a separate thalamocortical pathway rather than by corticocortical connections from areas 2 and 5 in parietal cortex. The changes associated with normal aging have been confirmed and extended.