A tonic coupling between the horizontal component of eye position and dorsal neck muscle activity has been demonstrated in animals and humans. In addition, a transient saccade related coupling has been found in animals. In order to investigate such a phasic component of the eye-head synergy in humans, we have recorded the activity of isolated motor units in the splenius muscle during large horizontal eye movements in head fixed subjects. Eye movement recording was achieved by conventional binocular electro-oculography and the activity of the right splenius muscle was recorded with Bronks coaxial electrodes inserted manually at the C4-C5 intervertebral level. We found two main types of motor unit discharge patterns in the splenius (SPMU), the first type (type A, 14 SPMUs) shows a phasic modulation of firing rate during saccades with a triphasic profile composed of a pre-saccadic suppression, a per-saccadic burst and a post saccadic tonic discharge proportional to eye position. The second type (type B, 6 SPMUs) exhibits little, if any, modulation of firing rate with either fixation or saccades. These results suggest that eye-head coupling is present not only during the fixation period but also during saccades and that a phasic activity or suppression related to saccadic eye velocity is present in dorsal neck muscle EMG.

A tonic coupling between the horizontal component of eye position and dorsal neck muscle activity has been demonstrated in the cat and monkey. In order to demonstrate this synergy in humans and study its characteristics, we have measured the relation between the firing rate of individual motor unit of the splenius muscle and voluntary horizontal shifts of gaze using 5 degrees steps, in head-fixed subjects. Eye movement recording was achieved by conventional binocular electro-oculography and the activity of the right splenius muscle was recorded with Bronks coaxial bipolar electrodes inserted manually at the C4-C5 intervertebral level. The activity of 51 motor units in 10 subjects has been recorded. For all subjects, motor units firing rate increased when the gaze shifted to the ipsilateral side, and both increase in firing rate and recruitment were observed. These results demonstrate that the tonic eye head synergy is also present in man.

A new approach to the study of peripheral vascular hemodynamics in microgravity was initiated by designing an alternative methodology: air plethysmography (AP). This is the only technique that enables the quantification of vascular hemodynamics from gross measurements performed on the limbs. This paper reports a description of the device and of the measurement protocol. A comparative study showed that AP yields results that are well correlated, although not in agreement, with those obtained by means of the reference method in the laboratory (mercury strain gauge plethysmography, MSGP; for venous capacity, correlation coefficient r = 0.8, P < 0.0001, limits of agreement--0.9 ml.100 ml-1 and 1.4 ml.100 ml-1; for arterial flow index, correlation coefficient r = 0.7, P < 0.0006, limits of agreements -20.4 ml.min-1 and 37.2 ml.min-1; for half-emptying time, correlation coefficient r = 0.9, P < 0.0001, limits of agreement -0.88 s and 0.77 s), and that are characterized by good reproducibility (coefficient of variation in general lower than 12%). Preliminary findings during spaceflight, on board the Mir Space Station, yielded data that is expected to improve our knowledge of vascular deconditioning in conditions of weightlessness.

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