Stepping to recover balance in complex environments: Is online visual control of the foot motion necessary or sufficient?

Rapid step reactions evoked by balance perturbation must accommodate constraints on limb motion imposed by obstacles and other environmental features. Recent results suggest that the required visuospatial information (VSI) is acquired and stored “proactively”, prior to perturbation onset (PO); however, the extent to which “online” (post-PO) visual feedback can contribute is not known. To study this, we used large unpredictable platform perturbations to evoke rapid step reactions, while subjects wore liquid crystal goggles that occluded vision: (1) prior to PO (forcing use of online-VSI), (2) after PO (forcing use of stored-VSI), or (3) not at all (normal-VSI). Subjects stood behind a barrier in which the location of a narrow slot, through which the foot had to be moved during forward step reactions, was varied unpredictably between trials. Within subjects who were able to do the task (6 of 8 young adults tested), responses in stored-VSI and normal-VSI trials were very similar. However, in online-VSI trials, the foot-off time for the step through the slot was delayed (by ∼50 ms, on average). Presumably, this delay allowed more time to acquire and process online-VSI regarding the required foot trajectory, yet subjects were still more likely to select the “wrong” foot (contralateral to the slot location) and to contact the barrier while moving the foot through the slot, in online-VSI trials. These results suggest a critical role for stored-VSI during the earliest phase of the step, in selecting the step limb and planning the initial trajectory. Online acquisition and processing of the required VSI may be too slow to allow effective control of this early phase, particularly in situations where the demands for accurate foot motion are high.     

Redirection of gaze and switching of attention during rapid stepping reactions evoked by unpredictable postural perturbation

In many situations successful execution of a balance-recovery reaction requires visual information about the environment. In particular, reactions that involve rapid limb movements, such as stepping, must be controlled to avoid obstacles and accommodate other constraints on limb trajectory. However, it is unknown whether the central nervous system can acquire the necessary visuospatial information prior to perturbation onset or must, instead, redirect gaze at the floor during the execution of the stepping reaction. To study this we examined gaze behaviour, during rapid forward-directed stepping reactions triggered by unpredictable platform perturbation, in 12 healthy young adults. We also monitored switching of attention, as inferred from onset of significant error in performing a concurrent visuomotor tracking task. Obstacles and/or step targets were used as constraints, to increase demands for accurate foot movement. Downward gaze shifts towards the floor almost never occurred during stepping reactions when foot motion was unconstrained but did occur more frequently as the demands for accurate foot movement increased. Nonetheless, even in the most challenging condition (target plus obstacle), downward redirection of gaze occurred in less than 40% of the trials, and subjects were commonly well able to clear the obstacle and land the foot on the target without redirecting their gaze towards the floor. An apparent switching of attention, subsequent to perturbation onset, occurred frequently (>80% of trials) in all task conditions, independent of the gaze shifts. The findings indicate that visual fixation of the foot or floor was not essential for accurate control of the foot movement, nor was the apparent switching of attention that followed perturbation onset linked, in any consistent way, to overt changes in visual fixation. Spatial features of the support surface were apparently remembered prior to perturbation onset, thereby allowing both vision and attention to be directed to other demands during the execution of the balance reaction.Presented (in part) at the 16th International Symposium on Posture and Gait (Sydney, Australia; March 2003), the 13th Biennial Conference of the Canadian Society for Biomechanics (Halifax, Canada; August 2004), and the 34th Annual Meeting of the Society for Neuroscience (San Diego, California; October 2004).     

Effects of stimulus-induced saccades on manual response times in healthy elderly and in patients with right-parietal lesions

Response delays of hemi-neglect patients to invalidly cued left targets in Posner's visual cueing task have been interpreted as reflecting disorders of covert attention, although shifts of overt attention have so far not been systematically measured in patients performing this task. Therefore, we measured saccades (i.e., fast goal-directed eye movements) induced by cues and targets in this task in ten patients with lesions of the right posterior parietal cortex and in age-matched healthy subjects. Participants were not aware that their saccades were studied. Of greatest interest was whether the presence or absence of saccades would modify the patients' response delay with invalidly cued left targets. In both groups, saccades occurred in many trials, key-press responses were slower in saccade trials than in no-saccade trials, and the delay by invalid cues was larger in saccade trials. The patients' responses were in particular delayed if preceded by saccades to the left. Their delay to invalidly cued left targets almost doubled (144 ms vs 76 ms) when only saccade trials were analyzed compared to when only no-saccade trials were analyzed. Thus, overt shifts (saccades) have a similar but larger effect on manual responses than covert shifts of attention. In particular, overt shifts make a considerable contribution to the patients' pathological delay of responses to invalidly cued left targets.     

Simultaneous determination of directional tuning of complex cells in cat striate cortex for bar and for texture motion

Summary A method is described for the simultaneous determination of directional tuning of cat striate cortical complex cells for two dissimilar stimuli. Reliable pairs of tuning curves for typically four round-the-clock sequences of the two interleaved stimuli (direction stepped by 10 °) can be derived in approximately 10 min, including on-line plotting of data. Large numbers of comparisons are thus possible, including the evaluation of long-term fluctuations in cortical responsiveness.Assessment of over 300 pairs of directional tuning curves for motion of bar and textured stimuli extends and entirely confirms earlier data.     

Effects of anticipatory anxiety and visual input on postural sway in an aversive situation

We have previously reported that state anxiety scores were positively correlated with postural sway while standing upright and gazing at a visual target (Ohno et al., 2004 [16]). The present study examines the effect of anticipatory anxiety and visual input on postural control in healthy individuals. An unpredictable aversive sound (100 dB SPL) was delivered in order to induce anticipatory anxiety. Participants were asked to stand upright on a force plate with their eyes open and closed, and their center of pressure (COP) was measured. Analysis of the postural parameters revealed that the path lengths of the COP and the enveloped areas were greater in the anticipatory situation with the aversive sound than in the silent situation. Fast Fourier transform analysis showed that the frequency component related to vestibular inputs (0.1–1.0 Hz) was increased during the anticipatory situation. The lower frequency (<0.1 Hz) component was decreased in the medio-lateral axis during anticipation with the eyes closed due to shifting mean power frequencies to high frequency. The results suggest that anticipatory anxiety in healthy participants amplified the sway regardless of whether the eyes were open or closed, and that the vestibular inputs greatly influenced the amplification of postural sway.     

Transcranial direct current stimulation for balance rehabilitation in neurological disorders: A systematic review and meta-analysis

Postural instability is common in neurological diseases. Although transcranial direct current stimulation (tDCS) seems to be a promising complementary therapy, emerging evidence indicates mixed results and protocols’ characteristics. We conducted a systematic review and meta-analysis on PubMed, EMBASE, Scopus, and Web of Science to synthesize key findings of the effectiveness of single and multiple sessions of tDCS alone and combined with other interventions on balance in adults with neurological disorders. Thirty-seven studies were included in the systematic review and 33 in the meta-analysis. The reviewed studies did not personalize the stimulation protocol to individual needs/characteristics. A random-effects meta-analysis indicated that tDCS alone (SMD = −0.44; 95%CI = −0.69/−0.19; p < 0.001) and combined with another intervention (SMD = −0.31; 95%CI = −0.51/−0.11; p = 0.002) improved balance in adults with neurological disorders (small to moderate effect sizes). Balance improvements were evidenced regardless of the number of sessions and targeted area. In summary, tDCS is a promising therapy for balance rehabilitation in adults with neurological disorders. However, further clinical trials should identify factors that influence responsiveness to tDCS for a more tailored approach, which may optimize the clinical use of tDCS.     

Changes in motor activity and biomechanics during balance recovery following cutaneous and muscular deafferentation

The effects of lower limb deafferentation were examined during execution of a balance recovery step following a forward fall induced by release of an initial inclined posture. The subjects were healthy control subjects and patients with a unilateral loss of the Achilles tendon reflex following S1 radiculitis. Deafferentation of healthy subjects was obtained by unilateral leg ischemia (four subjects) and by foot anesthesia (five subjects). The balance recovery step was characterized by the surface electromyographic (EMG) activity of right and left soleus and tibialis anterior muscles and the kinetics of the center of gravity and center of foot pressure. Experimentally induced and pathological deafferentation decreased the EMG activity of the ipsilateral soleus and lowered the vertical ground reaction force. The lower limb motor activity was more affected by loss of muscle proprioceptive afferents than by loss of plantar cutaneous afferents. Patients showed early and bilateral changes in soleus and tibialis activities, whichever side was affected. The step length of patients was also shorter than that of controls, but it remained similar before and after deafferentation in the healthy subjects. The results are discussed in terms of ipsilateral and crossed pathway connections and functional adaptive strategies.     

Tibolone low dose (1.25 mg/d) therapy and postural balance in elderly women

Most hip fractures occur in subjects without osteoporosis and are associated with a fall. Conventional menopausal hormone therapy (HT) improves postural balance, which might explain the rapid reduction in hip fracture risk. It is unclear whether tibolone improves postural balance, which might determine its effects on peripheral fracture risk.     

Diversity of complex cell responses to even- and odd-symmetric luminance profiles in the visual cortex of the cat

Summary We have tested the hypothesis that complex cell receptive fields are made up of subfields which, for a given cell, have either exclusively even or exclusively odd symmetry. To do this we have measured the response of complex cells in the visual cortex of the cat to members of pairs of spatially limited even-symmetric stimuli (single light and dark bars) and pairs of odd-symmetric stimuli (light-dark and dark-light double bars) successively drifting across their receptive fields. The strength of a cell's response was estimated by measuring the sum of all spikes produced by a stimulus. Some complex cells respond about equally to single light and dark bars; others respond appreciably more to either the light or dark bar. The central tendency of average response histograms was estimated by measuring the mean with respect to position across the width of the receptive field. Many complex cells show distinct spatial offsets between the mean for narrow single light and that for dark bars as well as between means to double bars of opposite phase. Combined offset plots were constructed with the spatial offsets between means for single light bars and single dark bars along the x axis and the offsets between means to double bars of opposite phase along the y axis. There is significant scatter in the combined offset points; some falling at the origin, some at significant distances from the origin along the axes, and others well within each of the four quadrants. These diverse localizations in the offset plots rule out the simple models of complex cell spatial substructure described above and, therefore, imply considerable heterogeneity within the population of complex cells.     

White matter hyperintensities and impaired choice stepping reaction time in older people

White matter hyperintensities (WMHs) are associated with impaired mobility in older people, but no studies have identified neuropsychological and sensorimotor mediating factors for this association. Our objective was to determine whether neuropsychological and sensorimotor functions mediate the association of WMHs and choice stepping reaction time (CSRT) under standard and dual-task conditions using structural equation modeling. Two hundred fifty-four older community dwellers (77.8 ± 4.5 years) underwent structural magnetic resonance imaging, CSRT tests, neuropsychological and sensorimotor assessments. WMH volumes were quantified using an automated method. WMH volumes were significantly associated with neuropsychological tests and dual task CSRT performance. All neuropsychological and sensorimotor variables were also significantly associated with standard and dual task CSRT. The structural equation modeling revealed that impaired sensorimotor function was the only factor influencing impaired stepping performances in the standard condition. In the dual task condition, the association between WMHs and CSRT was mediated through slowed cognitive processing and not through reduced sensorimotor functioning. The conclusion was that WMHs are associated with slowed performance on a dual task CSRT task and this relationship is explained primarily by underlying neuropsychological impairments.     

A comparison of the effects of milk and a carbohydrate-electrolyte drink on the restoration of fluid balance and exercise capacity in a hot,humid environment

Following a 2.0 ± 0.1% body mass loss induced by intermittent exercise in the heat, seven male volunteers ingested either a carbohydrate–electrolyte solution (CE) or skimmed milk (M) in a volume equal to 150% of body mass loss. At the end of the 3 h recovery period, subjects were essentially in positive fluid balance on trial M (191 ± 162 mL), and euhydrated on trial CE (−135 ± 392 mL) despite being in negative sodium balance on both trials and negative potassium balance on trial CE. This difference of 326 ± 354 mL or 0.4% body mass approached significance (P = 0.051). Subjects ingested 137 ± 15 and 113 ± 12 g of CHO during the CE and M trials, respectively, as well as 75 ± 8 g of protein during the M trial. At the end of the 3 h recovery period, an exercise capacity test was completed at 61% VO_2peak in warm (35.3 ± 0.5°C), humid (63 ± 2%) conditions. HR (P = 0.020) and rectal temperature (P = 0.045) were higher on trial M, but no difference in exercise time to exhaustion was observed between trials (39.6 ± 7.3 min vs. 39.7 ± 8.1 min on trials CE and M, respectively). The results of the present study suggest that milk can be an effective post-exercise rehydration drink, with subjects remaining in net positive fluid balance throughout the recovery period. Despite the effect on fluid retention, exercise capacity was not different between skimmed milk and a commercially available carbohydrate–electrolyte drink 4 h following exercise/heat-induced body mass loss.     

Clemastine promotes recovery of neural function and suppresses neuronal apoptosis by restoring balance of pro-inflammatory mediators in an experimental model of intracerebral hemorrhage

Intracerebral hemorrhage (ICH) represents a common acute cerebrovascular event that imparts high rates of disability. The microglia-mediated inflammatory response is a critical factor in determining cerebral damage post-ICH. Clemastine (CLM) is a histamine receptor H1 (HRH1) antagonist that has been shown to modulate the inflammatory response. However, the effects of CLM on ICH and the underlying mechanism remain to be determined. This investigation reveals that CLM resulted in reduction of cerebral hematoma volume, decreased cerebral edema and lower rates of neuronal apoptosis as well as improved behavioral scores in an acute ICH murine model. CLM treatment was noted to decrease pro-inflammatory effectors and increased anti-inflammatory effectors post-ICH. In addition, CLM reduced the deleterious effects of activated microglia on neurons in a transwell co-culture system. Our findings show that CLM likely mediates its therapeutic effect through inhibition of microglia-induced inflammatory response and apoptosis, thereby enhancing restoration of neuronal function.     

An enriched environment restores normal behavior while providing cytoskeletal restoration and synaptic changes in the hippocampus of rats exposed to an experimental model of depression

The exposure of rats to an enriched environment (EE) has several effects in common with the administration of antidepressants. However, there is still little information about the molecular underpinnings of these effects on rats subjected to experimental models of depression. The aim of this research was to evaluate the effects of EE on rats exposed to the learned helplessness paradigm (LH), a well-known model of the disease. We found that a 21 day exposure to EE reverts helplessness behavior to normal in LH animals. Inmunohistochemical labeling showed that this effect was accompanied by normalization of two structural proteins of hippocampal neurons to control values: the light neurofilament subunit (NFL) and the postsynaptic density 95 (PSD-95) protein, which were decreased in LH animals housed in standard cages. The decrease in the presynaptic protein synaptophysin (SYN) observed in LH animals remained unchanged after exposure to EE. There was no increase in neurogenesis as measured by quantification of double-labeled cells with 5-bromo-2′-deoxyuridine (BrdU) and the neuronal marker β-tubulin class III. These results show that EE may have behavioral and synaptic effects on animals exposed to an experimental model of depression, and that such actions seem to be independent from neurogenesis.