Similar scaling of contralateral and ipsilateral cortical responses during graded unimanual force generation.

Hemibody movements are strongly considered as being under the control of the contralateral hemisphere of the cerebral cortex. However, some neuroimaging studies have found a bilateral activation of either the primary sensori-motor (SM1) areas or the rostral prefrontal cortex (PFC), during unimanual tasks. More than just bilateral, the activation of these areas was found to be symmetrical in some studies. However, the symmetrical response remains strongly controversial notably for handgrip force generations. We therefore aimed to examine the bilateral SM1 and rostral PFC area activations in response to graded submaximal force generation during a unilateral handgrip task. Fifteen healthy subjects performed 6 levels of force (ranging from 5 to 50% of MVC) during a handgrip task. We concomitantly measured the activation of bilateral SM1 and rostral PFC areas through near-infrared spectroscopy (NIRS) and the electromyographic (EMG) activity of the bilateral flexor digitorum superficialis (FDS) muscles. Symmetrical activation was found over the SM1 areas for all the investigated levels of force. At the highest level of force (i.e., 50% of MVC), the EMG of the passive FDS increased significantly and the ipsilateral rostral PFC activation was found more intense than the corresponding contralateral rostral PFC activation. We suggest that the visuo-guided control of force levels during a handgrip task requires the cross-talk from ipsi- to contralateral SM1 to cope for the relative complexity of the task, similar to that which occurs during complex sequential finger movement. We also propose alternative explanations for the observed symmetrical SM1 activation including (i) the ipsilateral corticospinal tract and (ii) interhemispheric inhibition (IHI) mechanism. The increase in EMG activity over the passive FDS could be associated with a release of IHI at 50% of MVC. Finally, our results suggest that the greater ipsilateral (right) rostral PFC activation may reflect the greater demand of attention required to control the motor output at high levels of force.

Cardiovascular responses during hypoventilation at exercise.

This study aimed to determine the cardiovascular responses during a prolonged exercise with voluntary hypoventilation (VH). 7 men performed 3 series of 5-min exercise at 65% of normoxic maximal O (2) uptake under 3 conditions: (1) normal breathing (NB) in normoxia (NB (0.21)), (2) VH in normoxia (VH (0.21)), (3) NB in hypoxia (NB (0.157), inspired oxygen fraction=0.157). In both VH (0.21) and NB (0.157), there was a similar drop in arterial oxygen saturation and arterial O (2) content (CaO (2)) which were lower than in NB (0.21). Heart rate (HR), stroke volume, and cardiac output (-) were higher in VH (0.21) than in NB (0.21) during most parts of exercise whereas there was no difference between NB (0.157) and VH (0.21) or NB (0.21). HR variability analysis suggested an increased sympathetic modulation in VH (0.21) only. O (2) transport and oxygen uptake were generally not different between interventions. Mixed venous O (2) content (C-O (2)) was lower in NB (0.157) than in both VH (0.21) and NB (0.21) and not different between the latter. CaO (2)-C-O (2) was not different between NB (0.157) and NB (0.21) but lower in VH (0.21). This study shows that a prolonged exercise with VH leads to a greater cardiac activity, independent from the hypoxic effect. The greater - in VH compared to normal breathing seems to be the main factor for compensating the drop of arterial oxygen content.