We investigated the effects of increase of attentional dispersion on anticipatory postural control during unilateral arm abduction. Fifteen healthy young adults performed right arm abduction for a target imperative stimulus under the three types of attentional dispersion conditions. The imperative stimulus was presented at one of two, four or six positions, following a cue signal. By the number of the cue signal, subjects either focused their attention on the position or divided attention into two, four or six positions. The reaction time of the middle deltoid (MD) and onset time of erector spinae (ES) and gluteus medius (GM) were measured. MD reaction time was significantly longer with attentional dispersion than with attentional focusing in all position conditions (p < 0.05), and longer in four- or six-position conditions than in two-position condition with both attentional state (p < 0.001). Onset time of ES and GM showed no significant difference between attentional focusing and dispersion in two-position condition, however was significantly later with attentional dispersion than with attentional focusing in four- and six-position conditions (p < 0.05). Furthermore, onset time of GM with attentional dispersion was significantly later in four- or six-position conditions than in two-position condition (p < 0.05). These results suggested that the effects of attentional dispersion would become greater as increase of the number of positions where attention was directed and reach plateau until four positions. In relation to these changes, onset time of postural muscles became clearly later with attentional dispersion to four positions and over.
Reaction time for memory-guided saccade shortens during maintenance of neck flexion. The present study applied transcranial magnetic stimulation (TMS) to the frontal oculomotor field, and was investigated the effect of maintaining neck flexion on information processing time in the neural pathway of memory-guided saccade before the frontal oculomotor field. The reaction time was measured with the chin resting on a stand (chin-on condition) and with voluntary maintenance of neck flexion (chin-off condition) at 80 % maximal neck flexion angle, with and without TMS. TMS timing producing the longest prolongation of the reaction time was first roughly identified for 10 ms intervals from 0 to 180 ms after the lights-out of central fixation point. Thereafter, TMS timing was set finely at 2 ms intervals from －20 to ＋20 ms of the 10 ms interval producing the longest prolongation. The reaction time without TMS was significantly shorter (25.6 ms) for the chin-off condition (231.6 ± 44.6 ms) than the chin-on condition (256.6 ± 45.9 ms). TMS timing producing maximal prolongation of the reaction time was significantly earlier (23.3 ms) for the chin-off condition (102.6 ± 16.8 ms) than the chin-on condition (125.8 ± 13.6 ms). The ratio of the forward shift in TMS timing relative to the shortening of reaction time was 93.2 %. We confirmed that information processing time in the neural pathway of the memory-guided saccade before the frontal oculomotor field shortened with maintaining neck flexion, and that the reduction accounted for approximately 93 % of the shortening of reaction time.
We investigated regional activity of the cerebral cortex during postural adaptation to a periodic floor oscillation. Subjects (N = 11) maintained a standing posture on a force platform attached to a table that oscillated in the anteroposterior direction with 0.5-Hz frequency and 2.5-cm amplitude. A one-minute trial was repeated until the decrease in the velocity of the center of foot pressure reached a plateau, which is an index of adaptation of postural control. Changes in the concentration of oxygenated hemoglobin (oxy-Hb) were measured as an index of activation of brain areas along the longitudinal fissure of the cerebrum (prefrontal cortex (PFC), supplementary motor and primary sensory-motor areas (SM1) and the posterior part of the parietal association cortex (pPAC)), using the near infrared spectroscopy. In the 1st trial, oxy-Hb in all measurement areas increased quickly about 10 s after oscillation onset. The increase became significantly smaller with trial repetition. The decrease in oxy-Hb in SM1 with trial repetition was highly correlated with the decrease in lower-leg muscle activity. Though the postural task was performed with eyes closed, oxy-Hb in pPAC increased until the end of the 1st trial. The time course of changes in oxy-Hb in the PFC showed marked individual differences in the latter half of the 1st trial. In addition, after adaptation, oxy-Hb was significantly lower than before the onset of oscillation. The results showed that there is a particular changing pattern of blood flow in specific brain regions, implying particular activation patterns of these brain regions.