304

RT’s were analyzed by means of an ANOVA and post hoc T-test. Exp1: IT

for phosphenes induced by O-TMS (36.5 ms) was about ten times slower

than that for visually presented stimuli (4.96 ms). Exp2: IT for phosphenes

induced by P-TMS and that obtained with visually presented stimuli were

similar (4.95 ms)

In sum, the present results show that phosphenes obtained by V1 TMS

transfer via sparse callosal connections while those generated by parietal

TMS transfer via widespread callosal connections. Interestingly, the latter

transfer is similar to that of visually presented stimuli.

Movement DisordersPoster Only

203 Motor imagery in focal dystonia

Tyc F, Boyadjian A, Brasil-Neto JP, Allam N, Laboratoire plasticite &

physio-patholog (Marseille Cedex, FR)

Focal task-specific dystonias, such as writer’s cramp, are motor disorders

with abnormalities of motor control mechanisms which impair fine

coordination between muscles involved in specific tasks, often character-

ized by excessive involuntary muscular activation.

We have used transcranial magnetic stimulation (TMS) to compare the

effects of motor imagery (MI) on corticospinal excitability during coor-

dination between proximal and distal muscles in 10 patients with focal

hand dystonia and in 6 healthy controls.

We applied single pulse TMS on the extensor carpi radialis (ECR) hot spot

and recorded motor evoked potentials (MEPs) from contralateral upper

limb muscles (ECR, medial deltoid MD, first dorsal interosseus FDI) in

several conditions while subjects maintained a position of the arm and

while the subjects imagined the movement. For the ‘‘overt movement’’ the

subject had to maintain the position of the wrist by a tonic activation of the

ECR muscle: 1) alone and 2) with the co-activation of the MD muscle. In

the motor imagery, three conditions were proposed: 1) imagine the

movement with the whole upper limb, 2) with the forearm only and 3)

imagine the movement while the MD was contracted.

Seven increasing intensities were applied in each condition. For each

intensity, four stimulations were done and the MEPs were averaged to

determine the excitability curves for the three recorded muscles.

During motor imagery, the excitability curves of the controls showed lower

plateau and smaller slope in the condition where MD was co-activated

compared to the curves obtained during whole movement imagery. This was

not the case in the dystonic patients where the MD activation facilitated the

MEP in the ECR during MI. Furthermore, in patients but not in controls, the

MEP amplitude in FDI increased when MD was activated during MI.

Motor imagery seems to facilitate the three muscles MEPs in dystonic

patients compared to control. This excessive facilitation extending to

several muscles that are not facilitated in control seems to modify the

coordination between muscles and seems to reveal an activation of the

three muscles as a block. The global activation observed during overt

movement and MI in dystonics may be interpreted as a dysfunction of the

motor networks with an excess of excitability. Modifications in the neural

networks implicated in coordination between the shoulder, the elbow and

the wrist could be at the origin of an excess of activity observed in dystonic

muscles.

TMSPoster Only

204 Time series analysis of ensemble motor unit discharge reveals

I-wave periodicity (600 Hz) during skilled voluntary activity

Stewart M, Cracco R, Maccabee P, Amassian V, SUNY Downstate

Medical Center (Brooklyn, US)

Single TMS pulses over parietal cortex generate a series of I-waves with

periodicity w1.5 ms which reflect activation of corticospinal tract fibers by

a vertically-organized neural network (Amassian & Stewart, Clin Neuro-

physiol 56 [Suppl]:119-142, 2003). Does this excitatory cortical network

also function during voluntary activity?

Searching for such activity was facilitated by recording from muscles with

very short peripheral conduction times (w2 ms in extrinsic laryngeal

muscles), and taking recordings during precisely timed voluntary activity

(e.g. silently making plosive consonants like ‘‘T,’’ ‘‘P,’’ ‘‘priest’’). Surface

EMG recordings of multiple single unit action potentials were made for

50-200 ms after their onset. Recordings were half-wave rectified and each

negative deflection triggered a standard 100 or 200 ıs pulse with

subsequent 4 ms dead time. Multiple sweeps were accumulated to generate

a histogram of motor unit ensemble activity, which permitted intervals

shorter than 4 ms to occur.

First, we searched for firing tendencies at different intervals using a

variation of the expectation density (ED) function that was applied to the

sum of the discriminated EMG sweeps. Secondly, a more precise measure

of periodic tendencies was sought by computing the cross-channel ED

(XED) between the EMG histogram and (a) a fixed period spike train, with

periods adjusted from 1.0-2.0 ms (in 0.1 ms steps), or (b) random spike

trains from radioactive decay. The ‘‘iterative ED’’ and XED functions

revealed periodicity at 1.3.-1.6 ms and their harmonics. The periodic

content of the XED was assessed by cross-correlating the XED with a sine

wave having the same period as the fixed spike train. The magnitude of this

correlation (maximum for all phases) was plotted as a function of period to

detect and quantify tendencies in the EMG for particular frequencies.

Significant tendencies for periodicity were identified by calculating the

mean and 95 or 99% confidence intervals (two-tailed) at each frequency

from randomly shuffled versions of the data (N 5 100-500 shuffled ver-

sions at each frequency).

Summarizing, typical I-wave periodicities normally related to sTMS are

now identified in skilled voluntary activity of motor neurons. Thus, the I-

wave periodicity in not merely an artifact of powerful synchronous

stimulation but reflects also a clock-like function favoring excitation

within the cortex at coincidences occurring during computation.

Abstracts

tDCSPoster Only

205 Non-invasive modulation of spinal cord function with

transcutaneous direct current (dc) stimulation

Vergari M, Cogiamanian F, Pulecchi F, Marceglia S, Tadini L,

Ferrucci R, Priori A, Fondazione IRCCS Ospedale Maggiore

Policlinico,Dip. di Scienze Neurologiche, Universita di Milano

(Milano, IT)

Objective: To evaluate whether transcutaneous direct current (DC) stim-

ulation can modulate the function of the human spinal cord, we studied the

after-effects of anodal, cathodal and sham transcutaneous spinal DC

stimulation (tsDCS) delivered over the human thoracic spinal cord on the

somatosensory evoked potentials (SEPs).

Methods: tsDCS was delivered over the thoracic spinal cord (2.5 mA, 15

min.) in eleven healthy subjects. SEPs evoked by posterior tibial and by

median nerve stimulation were recorded, before tsDCS and after tsDCS

offset.

Results: anodal tsDCS stimulation decreased significantly (44,3 6 8,7%;

p 5 0,005) the amplitude of the cervico-medullary component of posterior

tibial nerve SEPs (P30). Anodal tsDCS left median nerve SEPs unchanged.

Sham and cathodal tsDCS failed to change the SEPs.

Conclusions: tsDCS induces prolonged and focal changes of conduction

along the human lemniscal pathway at spinal cord level. Our finding

prompts to assess whether tsDCS can be an alternative to invasive spinal

cord stimulation in patients.