Two Mechanisms of Sensorimotor Set Adaptation to Inclined Stance.

Orientation of posture relative to the environment depends on the contributions from the somatosensory, vestibular, and visual systems mixed in varying proportions to produce a sensorimotor set. Here, we probed the sensorimotor set composition using a postural adaptation task in which healthy adults stood on an inclined surface for 3 min. Upon returning to a horizontal surface, participants displayed a range of postural orientations - from an aftereffect that consisted of a large forward postural lean to an upright stance with little or no aftereffect. It has been hypothesized that the post-incline postural change depends on each individual's sensorimotor set: whether the set was dominated by the somatosensory or vestibular system: Somatosensory dominance would cause the lean aftereffect whereas vestibular dominance should steer stance posture toward upright orientation. We investigated the individuals who displayed somatosensory dominance by manipulating their attention to spatial orientation. We introduced a distraction condition in which subjects concurrently performed a difficult arithmetic subtraction task. This manipulation altered the time course of their post-incline aftereffect. When not distracted, participants returned to upright stance within the 3-min period. However, they continued leaning forward when distracted. These results suggest that the mechanism of sensorimotor set adaptation to inclined stance comprises at least two components. The first component reflects the dominant contribution from the somatosensory system. Since the postural lean was observed among these subjects even when they were not distracted, it suggests that the aftereffect is difficult to overcome. The second component includes a covert attentional component which manifests as the dissipation of the aftereffect and the return of posture to upright orientation.

Hysteresis in Center of Mass Velocity Control during the Stance Phase of Treadmill Walking.

Achieving a soft landing during walking can be quantified by analyzing changes in the vertical velocity of the body center of mass (CoM) just prior to the landing of the swing limb. Previous research suggests that walking speed and step length may predictably influence the extent of this CoM control. Here we ask how stable this control is. We altered treadmill walking speed by systematically increasing or decreasing it at fixed intervals. We then reversed direction. We hypothesized that the control of the CoM vertical velocity during the late stance of the walking gait may serve as an order parameter which has an attribute of hysteresis. The presence of hysteresis implies that the CoM control is not based on simply knowing the current input conditions to predict the output response. Instead, there is also the influence of previous speed conditions on the ongoing responses. We found that the magnitudes of CoM control were different depending on whether the treadmill speed (as the control parameter) was ramped up or down. Changes in step length also influenced CoM control. A stronger effect was observed when the treadmill speed was speeded up compared to down. However, the effect of speed direction remained significant after controlling for step length. The hysteresis effect of CoM control as a function of speed history demonstrated in the current study suggests that the regulation of CoM vertical velocity during late stance is influenced by previous external conditions and constraints which combine to influence the desired behavioral outcome.

Specific interference between a cognitive task and sensory organization for stance balance control in healthy young adults: visuospatial effects.

We tested the hypothesis that a computational overload results when two activities, one motor and the other cognitive that draw on the same neural processing pathways, are performed concurrently. Healthy young adult subjects carried out two seemingly distinct tasks of maintaining standing balance control under conditions of low (eyes closed), normal (eyes open) or high (eyes open, sway-referenced surround) visuospatial processing load while concurrently performing a cognitive task of either subtracting backwards by seven or generating words of the same first letter. A decrease in the performance of the balance control task and a decrement in the speed and accuracy of responses were noted during the subtraction but not the word generation task. The interference in the subtraction task was isolated to the first trial of the high but not normal or low visuospatial conditions. Balance control improvements with repeated exposures were observed only in the low visuospatial conditions while performance in the other conditions remained compromised. These results suggest that sensory organization for balance control appear to draw on similar visuospatial computational resources needed for the subtraction but not the word generation task. In accordance with the theory of modularity in human performance, the contrast in results between the subtraction and word generation tasks suggests that the neural overload is related to competition for similar visuospatial processes rather than limited attentional resources.

A molecular analysis of European porcine reproductive and respiratory syndrome virus isolated in South Korea.

In a situation where European genotype of porcine reproductive and respiratory syndrome virus (PRRSV) has recently emerged in South Korea, this study aims to understand variations in and relatedness among 25 European (EU) genotype 1 PRRSV isolates obtained from Korean pig farms during the period ranging from 2006 to 2009 for their sequences of nonstructural protein 2 (NSP2), open reading frames (ORF) 5 and 7, which, in turn, were compared with those of published Korean type 1 PRRSV isolates (CP6874, IV3140 and KNU07) and other EU PRRSV strains. The sequence data revealed that all Korean type 1 isolates were found to possess notable 19 amino acid deletions within NSP2 between positions 748 and 766. Based on the complete ORF5 sequences, the results showed that the Korean isolates amounted to 82.0-99.5% in amino acid identity with one another, while sharing a lower level of amino acid identity ranging from 71.6% to 92.0% with EU genotype strains isolated in other geographic areas. According to an amino acid sequence comparison of ORF7, the level of identity among the Korean type 1 isolates was found to range from 86.7% to 100%. Phylogenetic analyses based on ORF5 and ORF7 sequences indicated that the Korean type 1 isolates belonged to the pan-European subtype 1; in ORF5 phylogeny, they form three distinct clusters from other EU genotype PRRSV strains. In conclusion, those findings suggest that the Korean type 1 PRRSV may have undergone a high degree of variations since EU genotype virus was first detected.

An instance of reduced center of mass displacement: the Ba Gua Zhang walking gait.

The Ba Gua Zhang walking gait was examined in an accomplished practitioner to test the hypothesis that reducing the vertical oscillation of the body center of mass (CoM) conserves energy. Compared to typical walking, center-of-mass displacement and peak vertical ground reaction force decreased during Ba Gua Zhang walking. Muscle activity in the lower extremities and lower back, however, increased. The results failed to support the concept of decreasing CoM oscillations to increase efficiency.