We have studied habituation of the pattern-reversal visual evoked potential (VEP) in healthy volunteers (n = 16) and in patients suffering from migraine without (n = 27) or with aura (n = 9). Five blocks of 50 responses at a stimulation rate of 3.1 Hz were sequentially averaged and analyzed separately for latencies, peak-to-peak amplitudes of N1-P1 and P1-N2, and the area under the N2 component Latencies of N1, P1, or N2 components were not significantly different between the sequential trial blocks, or between groups. Mean amplitudes of N1-P1 and P1-N2 in the first and subsequent blocks of SO responses were not statistically different among groups. In healthy subjects, there was a decrement of N1-P1 and P1-N2 amplitudes and N2 area on successive averagings. This habituation was maximal in the third and fourth blocks, but tended to disappear in the fifth block. In marked contrast to healthy subjects, migraine patients were characterized by a transient amplitude increment (i.e. potentiation) of VEP components which reached its maximum in the second to fourth blocks. Amplitude changes in sequential blocks were not dependent on attention and differed significantly between healthy subjects and migraineurs, but not between migraine with and without aura. Taken together with previous studies showing deficient habituation of contingent negative variation in migraine, these results indicate a dysfunction of central information processing which might have behavioral and pathogenic correlates.

Early (ES1) and late (ES2) exteroceptive suppression periods elicited by electrical stimulation of the labial commissure during teeth-clenching were recorded over the temporalis muscle in 45 headache patients (25 tension headaches and 20 migraines) and 22 controls. Mean duration of ES2 for single shocks was significantly reduced in tension headache when compared with migraine or controls. At a stimulation rate of 2 Hz, ES2 was abolished in 40% of tension headache sufferers, but in none of the migraineurs. EMG analysis of temporalis late exteroceptive suppression might be a helpful diagnostic tool in functional headaches. Reduction of ES2 suggests that there is deficient activation or excessive inhibition of pontobulbar inhibitory interneurons which receive a strong input from limbic structures. ES2 might thus represent an interface between psychogenic and myogenic factors putatively involved in the pathogenesis of tension headache.

In order to determine the mono- or oligosynaptic character of connections between pyramidal axons and individual spinal motor neurons, we constructed peri-stimulus time histograms (PSTHs) of the firing probability of voluntarily activated single motor units (SMUs) of various upper and lower limb muscles upon slightly suprathreshold transcranial anodal electrical stimulations of the motor cortex in normal subjects. Weak anodal cortical stimuli are known to activate preferentially fast-conducting pyramidal axons directly, bypassing cell bodies and cortical interneurons. A narrow bin width (0.1 ms) was chosen to measure precisely the duration of the PSTH excitatory peak, which corresponds to the rise time of the underlying compound excitatory post-synaptic potentials (EPSP). A short duration PSTH peak indicates sharp-rising EPSPs, most commonly encountered in the case of monosynaptic connections. In flexor carpi radialis and soleus SMUs, the PSTHs of built-in responses to anodal cortical stimuli were compared with those produced by 1A afferent stimulation able to elicit a Hoffmann reflex, which is known to be largely monosynaptic. In all upper and lower limb muscles, excitable SMUs responded to anodal cortical stimuli with a highly synchronized peak of increased firing probability. In flexor carpi radialis and soleus SMUs, the mean duration of this peak was significantly narrower than that evoked by 1A afferent stimulation, indicating that monosynaptic corticomotor neuronal transmission dominates low-threshold motor units, even in proximal arm and leg muscles. In the various muscles studied, and particularly in forearm SMUs, we did not observe broad PSTH peaks against the activation of non-monosynaptic corticomotor neuronal pathways, even with near-threshold stimuli. In some triceps and forearm flexor SMUs, subthreshold anodal pulses caused significant inhibition of their voluntary firing, with a latency consistent with activation of 1A inhibitory interneurons by the descending volleys. Measurements of the maximal number of counts in the excitatory PSTH peak upon anodal cortical stimuli provide comparisons of the strength of monosynaptic inputs to various muscles which seems to be maximal for hand and finger extensor muscles, and also for deltoid.

Background and Purpose-Prevalence and characteristics of ipsilateral upper limb motor-evoked potentials (MEPs) elicited by focal transcranial magnetic stimulation (TMS) were compared in healthy subjects and patients with acute stroke. Methods-Sixteen healthy subjects and 25 patients with acute stroke underwent focal TMS at maximum stimulator output over motor and premotor cortices. If present, MEPs evoked in muscles ipsilateral to TMS were analyzed for latency, amplitude, shape, and center of gravity (ie, preferential coil location to elicit them). In stroke patients, possible relationships between early ipsilateral responses and functional outcome at 6 months were sought. Results-With relaxed or slightly contracting target muscle, maximal TMS over the motor cortex failed to elicit ipsilateral MEPs in the first dorsal interosseous (FDI) or biceps of any of 16 normal subjects. In 5 of 8 healthy subjects tested, ipsilateral MEPs with latencies longer than contralateral MEPs were evoked in FDI muscle (in biceps, 6 of 8 subjects) during strong (Ͼ50% maximum) contraction of the target muscle. In 15 of 25 stroke patients, ipsilateral MEPs in the unaffected relaxed FDI (in biceps, 6 of 25 stroke patients) were evoked by stimulation of premotor areas of the affected hemisphere. Their latencies were shorter than those that MEPs evoked in the same muscle by stimulation of the motor cortex of the contralateral unaffected hemisphere. Such responses were never obtained in normal subjects and were mostly observed in patients with subcortical infarcts. Patients harboring these responses had slightly better bimanual dexterity after 6 months. Conclusions-Ipsilateral