Perturbation-evoked cortical activity reflects both the context and consequence of postural instability.

The cerebral cortex may play a role in the control of compensatory balance reactions by optimizing these responses to suit the task conditions and/or to stimulus (i.e. perturbation) characteristics. These possible contributions appear to be reflected by pre-perturbation and post-perturbation cortical activity. While studies have explored the characteristics and possible meaning of these different events (pre- vs. post-) there is little insight into the possible association between them. The purpose of this study was to explore whether pre- and post-perturbation cortical events are associated or whether they reflect different control processes linked to the control of balance. Twelve participants were presented temporally-predictable postural perturbations under four test conditions. The Block/Random tasks were designed to assess modifiability in CNS gain prior to instability, while the Unconstrained/Constrained tasks assessed responsiveness to the magnitude of instability. Perturbations were evoked by releasing a cable which held the participant in a forward lean position. The magnitude of pre-perturbation cortical activity scaled to perturbation amplitude when the magnitude of the perturbation was predictable [F(3,11)=2.906, P<0.05]. The amplitude of pre-perturbation cortical activity was large when the size of the forthcoming perturbation was unknown (13.8 + or - 7.9, 11.4 + or - 9.9, 16.9 + or - 9.3, and 16.1 + or - 10.6 muV for the Block Unconstrained and Constrained and Random Unconstrained and Constrained, respectively). In addition, N1 amplitude scaled to perturbation amplitude regardless of whether the size of the forthcoming perturbation was known (30.1 + or - 17.7, 11.4 + or - 7.1, 30.9 + or - 18.4, 12.4 + or - 6.1 muV). This is the first work to examine modifiability in the pre-perturbation cortical activity related to postural set alterations. The cerebral cortex differentially processes independent components prior to and following postural instability to generate compensatory responses linked to the conditions under which instability is experienced.

Challenging the brain: Exploring the link between effort and cortical activation.

To better understand the contributions of effort on cortical activation associated with motor tasks, healthy participants with varying capacities for isolating the control of individual finger movements performed tasks consisting of a single concurrent abduction of all digits (Easy) and paired finger abduction with digits 2 and 3 abducted together concurrently with digits 4 and 5 (Hard). Brain activity was inferred from measurement using functional magnetic resonance imaging. Effort was measured physiologically using electrodermal responses (EDR) and subjectively using the Borg scale. On average, the Borg score for the Hard task was significantly higher (p=0.007) than for the Easy task (2.9+/-1.1 vs. 1.4+/-0.7, respectively). Similarly, the average normalized peak-to-peak amplitude of the EDR was significantly higher (p=0.002) for the Hard task than for the Easy task (20.4+/-6.5% vs. 12.1+/-4.9%, respectively). The Hard task produced increases in sensorimotor network activation, including supplementary motor area, premotor, sensorimotor and parietal cortices, cerebellum and thalamus. When the imaging data were subdivided based on Borg score, there was an increase in activation and involvement of additional areas, including extrastriate and prefrontal cortices. Subdividing the data based on EDR amplitude produced greater effects including activation of the premotor and parietal cortices. These results show that the effort required for task performance influences the interpretation of fMRI data. This work establishes understanding and methodology for advancing future studies of the link between effort and motor control, and may be clinically relevant to sensorimotor recovery from neurologic injury.

Perturbation-evoked electrodermal activity responds to instability, not just motor or sensory drives.

OBJECTIVE: To determine whether electrodermal responses (EDRs) evoked by postural perturbations were sensitive to the context of compensatory balance control, or simply reflected sensory or motor components of the reaction. METHODS: Thirteen participants were perturbed backwards in an upright chair and (1) performed compensatory reach-to-grasp movements to a handhold to recover balance (COMP); (2) received the perturbation only and the chair stopped via mechanical support (SENS); and (3) performed rapid self-initiated reach-to-grasp movements without perturbation (MOT). RESULTS: EDRs were most frequent and largest in the COMP task, observed in 100% of trials (1.42+/-0.16 microS), compared to 39% of SENS trials (0.31+/-0.12 microS, p<0.0001) and 85% of MOT trials (0.98+/-0.25 microS, p=0.073). EDRs in the MOT task followed two patterns across individuals, leading to post-hoc division of subjects into groups (smaller EDRs than COMP task, n=7, versus equivalent EDRs to COMP task, n=6). Motor patterns were equivalent in both groups, indicating that EDRs did not co vary with efferent drive. CONCLUSIONS: Perturbation-evoked EDRs are not a direct reflection of sensory input or motor drive. SIGNIFICANCE: These findings suggest that evoked autonomic activity may play a functional role in compensatory postural control.

Cortical responses associated with the preparation and reaction to full-body perturbations to upright stability.

OBJECTIVE: To examine the cortical activity associated with 'central set' preparations for induced whole-body instability. METHODS: Self-initiated and temporally unpredictable perturbations to standing balance were caused by the release of a load coupled to a cable affixed to a harness while participants stood on a force plate. Electroencephalographic and electromyographic signals were recorded. RESULTS: Peak activity was located at the Cz electrode. The predictable condition elicited a DC shift 950 ms prior to perturbation onset and was 18.0+/-10.5 micro V in magnitude. Pre-perturbation activity was not associated with the motor act of perturbation initiation and was dissociable from cortical activity related to anticipatory postural muscle activation. Following perturbation onset, N1 potentials were observed with a peak amplitude of 17.6+/-7.2 micro V and peak latency of 140.1+/-25.9 ms. In unpredictable trials, pre-perturbation activity was absent. The peak amplitude (32.0+/-14.8 micro V) and latency (156.5+/-11.8 ms) of the post-perturbation N1 potential were significantly larger (p=0.002) and later (p<0.001) than for predictable trials. CONCLUSIONS: Self-initiated postural instability evokes cortical activity prior to and following perturbation onset. Pre-perturbation cortical activity is associated with changing central set to modulate appropriate perturbation-evoked balance responses. SIGNIFICANCE: These findings establish a link between reactive balance control and cortical activity that precedes and follows perturbations to stability.

Factors affecting the common modulation of bilateral motor unit discharge in human soleus muscles.

The purpose of this study was to determine the factors that influence the co-modulation of motor unit discharge rate in soleus muscles of both legs during upright standing. Single motor units were recorded from the left and right soleus muscles under three experimental conditions: standing quietly with the eyes open and closed, standing with the eyes closed while vibration was applied to one Achilles tendon, and swaying voluntarily or producing variable low-force isometric contractions at a frequency of 0.05 Hz. Correlations in motor unit discharge rate between left and right soleus motor units were assessed using common drive analysis. The results showed that common drive to motoneurons of the two muscles did not differ between standing with the eyes open or closed, but there was an order effect with the second task having significantly lower common drive than the first. Common drive was also significantly lower when vibration was applied to one leg compared with when no vibration was applied. Common drive was higher as subjects swayed anteriorly as compared with when they swayed posteriorly. There were no significant differences in common drive across phases of the variable isometric force contraction. Common drive was higher during voluntary sway than during variable force production; both of these values were significantly lower than those derived from the quiet standing task. These results suggest that proprioceptive and sub-cortical inputs contribute to the co-modulation of the firing rate of soleus motor unit pairs of the left and right leg during standing posture.

Low-frequency common modulation of soleus motor unit discharge is enhanced during postural control in humans.

The maintenance of quiet stance requires the activation of muscles bilaterally. The soleus muscles in each leg share a common function in standing; that is, each muscle acts to control antero-posterior (AP) sway on its own side. We sought to determine the extent to which oscillations in motor unit discharge were related in motor unit pairs of the soleus muscles during postural and voluntary isometric tasks, both within and between legs. Subjects stood quietly for 5 min or performed a voluntary isometric plantarflexion contraction in a seated position. During the postural tasks, the excursions of AP sway between legs were highly correlated (rho = 0.86 +/- 0.06). The strength of common modulation of motor unit discharge rates was assessed using time- and frequency-domain analyses. The time-domain common drive analysis revealed that the strongest correlation in motor unit discharge modulation occurred in the postural task with unilateral pairs (rho = 0.71 +/- 0.13) being more strongly correlated than bilateral pairs (rho = 0.50 +/- 0.16). Common modulation of motor unit discharge was lowest for the voluntary tasks, with rho = 0.38 +/- 0.11 and 0.16 +/- 0.08 for unilateral and bilateral pairs, respectively. Similarly, the frequency-domain coherence analysis demonstrated an identical ordering effect, with the largest maximum pooled coherence occurring during standing posture in unilateral (0.070 at 1.6 Hz) and bilateral (0.055 at 1.6 Hz) recordings, whereas minimal coherence was observed in the voluntary task in both unilateral and bilateral recordings within the 0-5 Hz range. These results indicate that in the soleus muscle, common modulation of motor unit discharge is greater during postural tasks than during voluntary isometric tasks and can be observed in both bilateral and unilateral motor unit pairs. Differences in the extent of co-modulation of motor unit discharge between tasks may be attributed to either differences in the descending control or differences in the proprioceptive input between postural and isometric tasks.

Synchronization of motor units in human soleus muscle during standing postural tasks.

During standing posture, the soleus muscles acts to control sway in the anteroposterior (AP) direction. The soleus muscles bilaterally share a common function during standing tasks. We sought to determine whether common descending inputs, as evidenced by the synchronization of bilateral motor unit pairs, were employed as a strategy to control this common function. Single motor units were recorded from the soleus muscles in subjects who stood on adjacent force platforms for 5 min with their eyes open or closed. While standing with the eyes open, only 4/39 bilateral motor unit pairs showed significant synchronization. Similarly, only 3/36 motor unit pairs were significantly synchronized during the eyes closed task. The low incidence of synchronization was observed despite a high correlation in the amount of sway in the AP direction between legs in both the eyes open and eyes closed tasks (rho = 0.80 and rho = 0.83, respectively). When the extent of synchronization was assessed between pairs of motor units within the same leg with the eyes open, 10/12 pairs were synchronized. Furthermore, when pairs of soleus motor units were recorded both bilaterally and unilaterally during voluntary isometric ankle plantarflexion, only 4/30 bilateral pairs showed significant synchronization, whereas 19/24 unilateral pairs had significant synchronization. In this study, there was little evidence of the existence of synchronization between bilateral soleus motor unit pairs in either postural tasks or voluntary isometric contractions. In cases in which bilateral synchronization was observed, it was considerably weaker than the synchronization of motor units within a single soleus muscle. The results of this study reveal that it is rather uncommon for bilateral soleus motoneurons to receive common descending synaptic inputs, whereas two motoneurons within a single soleus muscle do.

Postural muscle activity during bilateral and unilateral arm movements at different speeds.

The muscle activation patterns in anterior and posterior leg muscles were investigated with two types of perturbations to standing balance. Subjects stood with each foot on adjacent force platforms and performed arm flexion movements to shoulder height. Nine subjects performed ten repetitions unilaterally and bilaterally at 100, 75, 50, 25, and 12.5% of maximal acceleration as measured by an accelerometer placed on the dominant hand. Four subjects also performed the fastest movements while leaning forwards and backwards. The area and latency of the EMG activity from the quadriceps (QUAD), hamstrings (BF), soleus (SOL), and tibialis anterior (TA) were measured bilaterally, along with the excursions of the center of pressure (COP) during each movement. In both unilateral and bilateral tasks, subjects showed a scaling of EMG area and COP excursion with the acceleration of the arm movement. Prior to movement onset, significant scaling of EMG area with movement speed occurred in both unilateral and bilateral tasks in most muscles. Following movement onset, EMG areas scaled significantly to movement speed in only the anterior musculature, with the exception of the left BF. The latency of BF was consistent for the four fastest movements. Only the slowest movements resulted in a significant rightward shift of the BF EMG latency. During the unilateral task, the ipsilateral hamstrings were activated significantly earlier than in the bilateral task and the contralateral hamstrings were activated significantly later. It was also observed that subjects utilized one of two different strategies to maintain balance. Five individuals displayed simultaneous anterior/posterior muscle activation while the other four displayed a reciprocal pattern of activation. Regardless of the initial standing position (leaning forwards or backwards), subjects used the same simultaneous or reciprocal activation strategy. The results indicate that muscle activation patterns change with different tasks, but remain the same during variations of the same task.

Perfluorocarbon-mediated aeration applied to recombinant protein production by virus-infected insect cells.

Perfluorocarbon (PFC) was used as an oxygen carrier in the cultures of insect cells and virus-infected insect cells. The cell suspensions were placed on a planar layer of PFC, which was re-oxygenated in an outer aeration unit and continuously recirculated, and were agitated by two sets of impeller blades, lower one of which was set in such a way that the ridge of the blade touched the PFC layer. The maximum cell density attained in the PFC-mediated aeration culture was higher than that in surface aeration culture. On viral infection, a recombinant protein yield was significantly high in the PFC-mediated aeration culture as compared with that in the surface aeration culture, though the production was largely decreased by setting apart the lower set of the blade from the PFC-medium interface. These results showed that the PFC-mediated aeration would be a useful technique for insect cell/baculovirus expression system. Overall mass-transfer coefficient K(L) for oxygen was examined in both the PFC-mediated aeration and surface aeration systems, by using a flask whose dimensions were identical to those of spinner flasks used for the cultures. The K(L) value in the PFC-mediated system was 2.60x10(-3)cms(-1), 1.6 times higher than that in the surface aeration system, when impeller blades were positioned at PFC-medium and medium-air interfaces, respectively. However, the K(L) values in both the PFC-mediated and surface aeration systems were decreased and their differences were brought so close, as the blade was set apart from the interfaces. DO behavior in the cultures was well explained by the model calculation using the determined K(L) values and oxygen-consumption rates of viable cells. This calculation further suggested that crucial DO, under which recombinant protein productions were unsuccessful, was 0.24-0.5ppm (3-7%) in the insect cell/baculovirus expression system.