Functional Synergy Between Postural and Visual Behaviors When Performing a Difficult Precise Visual Task in Upright Stance

Virtual Reality-Induced Modification of Vestibulo-Ocular Reflex Gain in Posturography Tests

Background: The aim of the study was to demonstrate the influence of virtual reality (VR) exposure on postural stability and determine the mechanism of this influence. Methods: Twenty-six male participants aged 21-23 years were included, who underwent postural stability assessment twice before and after a few minute of single VR exposure. The VR projection was a computer-generated simulation of the surrounding scenery. Postural stability was assessed using the Sensory Organization Test (SOT), using Computerized Dynamic Posturography (CDP). Results: The findings indicated that VR exposure affects the visual and vestibular systems. Significant differences (p < 0.05) in results before and after VR exposure were observed in tests on an unstable surface. It was confirmed that VR exposure has a positive influence on postural stability, attributed to an increase in the sensory weight of the vestibular system. Partial evidence suggested that the reduction in vestibulo-ocular reflex (VOR) reinforcement may result in an adaptive shift to the optokinetic reflex (OKR). Conclusions: By modifying the process of environmental perception through artificial sensory simulation, the influence of VR on postural stability has been demonstrated. The validity of this type of research is determined by the effectiveness of VR techniques in the field of vestibular rehabilitation.

Diabetic older women without peripheral neuropathy amplify body sway but are capable of improving postural stability during a saccadic gaze task

BACKGROUND

Diabetic older people tend to present deteriorated performance in balance and locomotion activities, even those without peripheral neuropathy. There is evidence that saccadic eye movements are used to reduce body sway in young and older healthy adults, but it has not been shown that diabetic older people preserve this visuomotor adaptation capacity.

RESEARCH QUESTION

Are diabetic older women without peripheral neuropathy capable of improving postural stability during a saccadic gaze task?

METHODS

Seventeen type 2 diabetic older women (68.2 ± 10.7 years old) and seventeen healthy women, age-matched controls (66.0 ± 8.4 years old) voluntarily participated in the study. All participants were instructed to stand upright, barefoot, as stable as possible, for 30 s. Participants maintained their feet parallel to each other, at standard and narrow bases of support, while either fixating on a stationary target (fixation condition) or performing horizontal saccadic eye movements to follow a target (eccentricity of 11° of visual angle), which continuously disappeared and reappeared immediately on the opposite side (saccade 0.5 Hz and saccade 1.1 Hz conditions).

RESULTS

Results indicated that the diabetic group clearly had deteriorated postural control, as shown by increased values of mean sway amplitude and mean sway velocity. However, diabetic and control groups were similarly capable of using saccadic eye movements to improve their postural stability, reducing their sway velocity compared to a gaze fixation condition.

SIGNIFICANCE

Diabetes per se (without peripheral neuropathy) amplifies postural sway of older women as compared to their healthy age-matched controls. However, diabetic older women without peripheral neuropathy are capable of improving postural stability during a saccadic gaze task.

The impact of eye movement on postural control depends on the type of oculomotor behavior and the visual task

Recent evidence suggests that performing a task inducing saccades will improve stability when compared to static fixation. However, they assume the linearity of postural control by only interpreting the area of displacement and/or the velocity of sway. Conversely, non-linear measures could bring a complementary understanding of postural control. The aim is to examine the effect of eye movements on different linear and non-linear measures of stability. 21 healthy adults (24.0 ± 3.3 years) were asked to stand on a force platform with their feet together and look at the monitor in front of them. Five conditions were tested: eyes closed, random looking, fixed static point, saccade, and visual pursuit (gaze shift angle of 15°). Five 60-second trials per condition were performed. An ANOVA with repeated measures was completed for each postural control variables in each direction: antero-posterior (AP) and medio-lateral (ML). The absence of vision had a negative impact on sway, as seen by an increase in sway area and variability as well as reduced contributions from the ultra-low band. The saccade led to greater stability than the random looking, as evidenced by a smaller area. However, the visual pursuit led to decreased stability compared to random looking, as evidenced by a larger area, as well as increased variability. Of note, the energy contained in the very-low band, which indicates the contribution of the vestibular system, was highest in the visual pursuit compared to the fixed static point. The findings support that the visual system is an important, but complex contributor to stability as different eye movements result in distinct postural responses with saccade and visual pursuit causing a decrease and an increase in sway, respectively.

Preventing falls: the use of machine learning for the prediction of future falls in individuals without history of fall

Nowadays, it becomes of paramount societal importance to support many frail-prone groups in our society (elderly, patients with neurodegenerative diseases, etc.) to remain socially and physically active, maintain their quality of life, and avoid their loss of autonomy. Once older people enter the prefrail stage, they are already likely to experience falls whose consequences may accelerate the deterioration of their quality of life (injuries, fear of falling, reduction of physical activity). In that context, detecting frailty and high risk of fall at an early stage is the first line of defense against the detrimental consequences of fall. The second line of defense would be to develop original protocols to detect future fallers before any fall occur. This paper briefly summarizes the current advancements and perspectives that may arise from the combination of affordable and easy-to-use non-wearable systems (force platforms, 3D tracking motion systems), wearable systems (accelerometers, gyroscopes, inertial measurement units-IMUs) with appropriate machine learning analytics, as well as the efforts to address these challenges.

Benefits associated with the standing position during visual search tasks

The literature on postural control highlights that task performance should be worse in challenging dual tasks than in a single task, because the brain has limited attentional resources. Instead, in the context of visual tasks, we assumed that (i) performance in a visual search task should be better when standing than when sitting and (ii) when standing, postural control should be better when searching than performing the control task. 32 and 16 young adults participated in studies 1 and 2, respectively. They performed three visual tasks (searching to locate targets, free-viewing and fixating a stationary cross) displayed in small images (visual angle: 22°) either when standing or when sitting. Task performance, eye, head, upper back, lower back and center of pressure displacements were recorded. In both studies, task performance in searching was as good (and clearly not worse) when standing as when sitting. Sway magnitude was smaller during the search task (vs. other tasks) when standing but not when sitting. Hence, only when standing, postural control was adapted to perform the challenging search task. When exploring images, and especially so in the search task, participants rotated their head instead of their eyes as if they used an eye-centered strategy. Remarkably in Study 2, head rotation was greater when sitting than when standing. Overall, we consider that variability in postural control was not detrimental but instead useful to facilitate visual task performance. When sitting, this variability may be lacking, thus requiring compensatory movements.

Deterioration of postural control due to the increase of similarity between center of pressure and smooth-pursuit eye movements during standing on one leg

Upright postural control is regulated by afferent and efferent/reafferent visual mechanisms. There are two types of efferent and conjugate eye movements: saccades and smooth pursuits. Although postural control is improved by saccades, the effects of smooth pursuits on postural control are still debated, because the difficulties of postural and visual tasks differ in the previous research. Additionally, the mechanisms that interfere with postural control and smooth pursuit are not fully understood. To address these issues, we examined the effects of different patterns of smooth-pursuit eye movement on the path length of the center of pressure (COP) displacement under bipedal and unipedal standing conditions. The relative frequency and amplitude of the COP displacement were remarkably increased when uniform linear visual targets were presented during unipedal standing. In addition, dynamic time warping analysis demonstrated that the similarity between the displacement of the COP and eye movements was increased by the presentation of uniform linear visual targets with orientation selectivity during unipedal standing but not during bipedal standing. In contrast, the attenuation of similarity between the displacement of the COP and eye movements significantly decreased the path length, relative frequency, and amplitude of the COP displacement. Our results indicate that postural stability is deteriorated by the increase of similarity between the displacement of the COP and smooth-pursuit eye movements under unstable conditions.

Balance Adaptation While Standing on a Compliant Base Depends on the Current Sensory Condition in Healthy Young Adults

Background

Several investigations have addressed the process of balance adaptation to external perturbations. The adaptation during unperturbed stance has received little attention. Further, whether the current sensory conditions affect the adaptation rate has not been established. We have addressed the role of vision and haptic feedback on adaptation while standing on foam.

Methods

In 22 young subjects, the analysis of geometric (path length and sway area) and spectral variables (median frequency and mean level of both total spectrum and selected frequency windows) of the oscillation of the centre of feet pressure (CoP) identified the effects of vision, light-touch (LT) or both in the anteroposterior (AP) and mediolateral (ML) direction over 8 consecutive 90 s standing trials.

Results

Adaptation was obvious without vision (eyes closed; EC) and tenuous with vision (eyes open; EO). With trial repetition, path length and median frequency diminished with EC (p < 0.001) while sway area and mean level of the spectrum increased (p < 0.001). The low- and high-frequency range of the spectrum increased and decreased in AP and ML directions, respectively. Touch compared to no-touch enhanced the rate of increase of the low-frequency power (p < 0.05). Spectral differences in distinct sensory conditions persisted after adaptation.

Conclusion

Balance adaptation occurs during standing on foam. Adaptation leads to a progressive increase in the amplitude of the lowest frequencies of the spectrum and a concurrent decrease in the high-frequency range. Within this common behaviour, touch adds to its stabilising action a modest effect on the adaptation rate. Stabilisation is improved by favouring slow oscillations at the expense of sway minimisation. These findings are preliminary to investigations of balance problems in persons with sensory deficits, ageing, and peripheral or central nervous lesion.

Behavioral synergic relations between eye and postural movements in young adults searching to locate objects in room inside houses

During precise gaze shifts, eye, head, and body movements exhibit synergic relations. In the present study, we tested the existence of behavioural synergic relations between eye and postural movements in a goal-directed, precise, visual search task (locate target objects in large images). More precisely, we tested if postural control could be adjusted specifically to facilitate precise gaze shifts. Participants also performed a free-viewing task (gaze images with no goal) and a fixation task. In both search and free-viewing tasks, young participants (n = 20; mean age = 22 years) were free to move their eyes, head, and body segments as they pleased to self-explore the images with no external perturbation. We measured eye and postural kinematic movements. The results showed significant negative correlations between eye and postural (head and upper back) movements in the precise task, but not in the free-viewing task. The negative correlations were considered to be stabilizing and synergic. Indeed, the further the eyes moved, the more postural variables were adjusted to reduce postural sway. These results suggest that postural control was adjusted to succeed in subtle and active self-induced precise gaze shifts. Furthermore, partial correlations showed significant relations between (1) task performance to find target objects and (2) synergic relations between eye and postural movements. These later results tend to show that synergic eye-postural relations were performed to improve the task performance in the precise visual task.

Variability of visually-induced center of pressure displacements is reduced while young adults perform unpredictable saccadic eye movements inside a moving room

This study investigated the modulation of center of pressure (CoP) displacements of young adults as they performed predictable and unpredictable saccadic eye movements in stationary and moving visual scenarios. We also examined whether the relationship between CoP displacements and visual stimulus, provided by the moving scenario, and gaze control itself, are affected by the demands of the saccadic tasks. Fifteen young adults (20.53 ± 2.1 years old) stood upright on a force plate, inside a moving room, wearing an eye tracker while performing three tasks: gaze fixation (fixating on a target in the center of the screen), predictable task (saccades while following a target which continuously appeared and disappeared on the right and left sides), and unpredictable task (similar to the previous task, but the participants did not know which side the target would appear on). For saccadic tasks, the target appeared at a frequency of 1.1 Hz and with eccentricity of 11.5 degrees of visual angle. Two blocks of six trials were performed: in the first block, the room remained stationary and in the second, it oscillated (0.6 cm amplitude; 0.2 Hz frequency). Mean amplitude of CoP displacements was lower in the saccadic tasks compared to the gaze fixation, in both conditions; and higher in the moving scenario than in the stationary condition. Variability of CoP displacements was lower in the unpredictable saccades than gaze fixation task. Saccade reaction time was longer in the unpredictable than predictable task. We conclude that CoP displacements are reduced to facilitate performance of the saccadic tasks regardless of conditions and task complexity. Furthermore, lower variability suggests modulation of CoP displacements to deal with the increased attentional demands associated with the performance of the unpredictable saccades, indicating the important role of visual task constraints in postural control.

Optimization of postural control in precise gaze shifts and laser pointing

Young adults are known to reduce their postural sway to perform precise visual search and laser pointing tasks. We tested if young adults could reduce even more postural and/or center of pressure sway to succeed in both tasks simultaneously. The methodology is novel because published pointing tasks usually require continuously looking at the pointed target and not exploring an image while pointing elsewhere at the same time. Twenty-five healthy young adults (23.2 ± 2.5 years) performed six visual tasks. In the free-viewing task, participants randomly explored images with no goal. In two visual search tasks, participants searched to locate objects (easy search task) or graphical details (hard search task). Participants additionally pointed a laser beam into a central circle (2°) or pointed the laser turned off. Postural sway and center of pressure sway were reduced complementarily - in various variables - to perform the visual search and pointing tasks. Unexpectedly, the pointing task influenced more strongly postural sway and center of pressure sway than the search tasks. Overall, the participants adopted a functional strategy in stabilizing their posture to succeed in the pointing task and also to fully explore images. Therefore, it is possible to inverse the strength of effects found in the literature (usually stronger for the search task) in modulating the experimental methodology. In search tasks more than in free-viewing tasks, participants mostly rotated their eyes and head, and not their full body, to stabilize their posture. These results could have implications for shooting activities, video console games and rehabilitation most particularly.

Parkinson's disease-related changes in the behavioural synergy between eye movements and postural movements

Patients with Parkinson's disease (PD patients) have been shown to exhibit abnormally low levels of synergy in their posture control. The goal of this study was to determine how synergic interactions between vision and posture are affected in PD patients. These synergic interactions were expected to be impaired because PD affects the basal ganglia, which are involved in the modulation of both types of movement. Twenty patients (mean age: 60) on levodopa and 20 age-matched-controls (mean age: 61) performed a precise visual task (searching for targets in an image) and an unprecise control task (randomly looking at an image) in which images were projected onto a large panoramic display. Lower back, upper back, head and eye movements were recorded simultaneously. To test behavioural synergies, Pearson correlations between eye and postural movements were analysed. The relationships between eye movements and upper and lower back movements were impaired in the patients. The age-matched controls did not show any significant correlations between eye and postural movements. Overall, our results showed that the PD patients failed to adjust and control their postural stability for success in the visual task. The impaired synergy between eye and postural movements was not related to clinical variables-probably because our patients had early-stage PD. Our results showed that impairments in synergy can occur very early in PD. Hence, the analysis of this synergy might provide a better understanding of postural instability, visual task performance in the upright stance, and perhaps the risk of falls in PD patients.

In the upright stance, posture is better controlled to perform precise visual tasks than laser pointing tasks

PURPOSE

In the upright stance, young adults better stabilize their posture when they perform precise visual or pointing movements than when they stand quietly. We tested if postural stability could be improved further if precise and pointing tasks were combined.

METHOD

Twenty-four healthy young adults (22 ± 12 years) performed six tasks combining three visual tasks (precise search, unprecise free-viewing and fixation tasks) and two pointing tasks (pointing-on and pointing-off tasks with laser beam on and off, respectively). In the visual tasks, participants either searched to locate targets within an image (precise task), looked at the image with no goal (unprecise task) or fixated on a cross (fixation task). In the pointing-on tasks, participants pointed a laser beam onto a small circle (2°) located in the middle of a larger circle (21°) containing the image.

RESULT

As expected, postural sway was reduced in the precise tasks in contrast to the fixation tasks. Contrary to expectations, both precise and pointing-on tasks did not add their stabilizing effects. Furthermore, the pointing-on task almost did not influence body movements. The participants rotated their eyes and head more and their upper back less in the precise visual tasks than in the unprecise visual tasks.

CONCLUSION

The participants used a stabilizing coordination to fully explore images with eye and head rotations while stabilizing their body to perform precise gaze shifts. Our findings suggest that posture stabilization is performed to facilitate success in precise visual tasks more so than to perform pointing-on tasks.

Investigation of the effects of cognitive tasks on balance performance in young adults

Introduction

Individuals routinely perform cognitive tasks concurrent to balance functions. The attention is one of the most important cognitive functions and it has effects on vestibular system. This study aims to investigate the connection between balance and cognitive tasks under different conditions.

Methods

Visual attention tasks (VAT) and auditory attention tasks (AAT) were given to 30 healthy adults (40.42 ± 11.22 years) during functional balance tasks. Sensory Organization Test (SOT) and Adaptation Test (ADT) were used for the evaluation of postural stability and adaptation. The sounds were presented from the computer speakers in AATs, and VATs were presented in the virtual reality (VR) environment.

Results

The first SOT condition had a statistically significant difference between all dual VAT (DT-VAT) and single task (ST) (p < 0.001), but there was no statistically significant difference between ST and DT-AAT (p = 1.00). In the fourth SOT condition, there was a statistically significant difference between all DT-VATs versus ST and DT-VA (p < 0.001); there was no statistically significant difference between ST and DT-AA scores (p = 0.80). While there was a significant difference between DT-VA and ST and DT-AA (p < 0.001), no statistically significant difference was observed between ST and DT-AA in the ADT (p = 0.321).

Conclusions

Balance performance gets worse with VAT in a VR environment. VR technology can be used to effectively evaluate balance and cognitive performance. The use of experimental environments in standard postural evaluations increases the efficiency of the postural stability tests.

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Postural control decreases during visual tasks.

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3D visual stimuli have the same effect with inhibition of the visual sensation.

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Additional auditory attention tasks did not change the balance performance.

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VR is useful for cognitive tasks if it is presented under ideal conditions.

Effects on Postural Kinematics of Performing a Cognitive Task During Upright Standing

The execution of cognitive tasks is known to alter postural sway during standing, but the underlying mechanisms are still debated. This study investigated how performing a mental task modified balance control during standing. We required 15 healthy adult males to maintain an upright stance under conditions of simply relaxing and maintaining normal quiet standing (control condition) or while performing a secondary cognitive task (mental arithmetic). Under each condition, we measured the participants' center of pressure and used kinematic measurements for a quantitative evaluation of postural control modulation. We calculated the standard deviation of the joint angles (ankle, knee, and hip) and the estimated joint stiffness to measure joint mobility changes in postural control. To estimate the kinematic pattern of covariation among these joints, we used uncontrolled manifold analysis, an assessment of the strength of multijoint coordination. Compared to normal standing, executing the cognitive task while standing led to reduced movements of the ankle and hip joints. There were no significant differences in ankle stiffness or uncontrolled manifold ratios between the conditions. Our results suggest that when performing a secondary cognitive task during standing, neither changes in the modification of stiffness nor the strength of multijoint coordination (both of which preserve the center of mass position) explains changes in postural sway.

Positive Relations Between Vision and Posture in the Fixation Task Performed Upright

In an upright stance, individuals sway in unpredictable ways. Their eyes also move in unpredictable ways in fixation tasks. The objective of this study was to analyze visual functions, postural control, and cognitive involvement in stationary gaze. A total of 14 healthy young adults performed a fixation task and a free-viewing task (three trials per task, 45 s per trial). As expected, the results showed many (n = 32) significant positive Pearson correlation coefficients between the eye and center of pressure/body (head, neck, and lower back) movements in the fixation task. In the free-viewing task, the correlations were nonsignificant. Only 3 of the 32 significant correlations (9.4%) were significantly related to cognitive involvement (measured with a subjective questionnaire). These results indirectly strengthened the validity of the synergistic model of postural control.

Cognitive engagement in emotional text reading: concurrent recordings of eye movements and head motion

The present study examined the effects of emotions on eye movements, head motion, and iPad motion during reading. Thirty-one participants read neutral, emotionally negative texts and emotionally positive texts on a digital tablet and both participants' eye movements and body movements were recorded using respectively eye-tracking glasses and a motion capture system. The results showed that emotionally positive texts were read faster than neutral texts, and that readers' movements decreased when reading emotional texts regardless of valence polarity. Recent studies suggested that postural movements may reflect cognitive engagement and especially the engagement in the task to be done. Our findings seem to validate this hypothesis of a bodily engagement in reading emotional contents. The present results suggest that the novel methodology of eye and postural movement recordings is informative in studying the readers' embodied engagement during reading emotional materials.

Balance Impairment in Radiation Induced Leukoencephalopathy Patients Is Coupled With Altered Visual Attention in Natural Tasks

Background: Recent studies have shown that alterations in executive function and attention lead to balance control disturbances. One way of exploring the allocation of attention is to record eye movements. Most experimental data come from a free viewing of static scenes but additional information can be leveraged by recording eye movements during natural tasks. Here, we aimed to provide evidence of a correlation between impaired visual alteration in natural tasks and postural control in patients suffering from Radiation-Induced Leukoencephalopathy (RIL).

Methods: The study subjects were nine healthy controls and 10 patients who were diagnosed with RIL at an early stage, with isolated dysexecutive syndrome without clinically detectable gait or posture impairment. We performed a balance evaluation and eye movement recording during an ecological task (reading a recipe while cooking). We calculated a postural score and oculomotor parameters already proposed in the literature. We performed a variable selection using an out-of-bag random permutation and a random forest regression algorithm to find: (i) if visual parameters can predict postural deficit and, (ii) which are the most important of them in this prediction. Results were validated using the leave-one-out cross-validation procedure.

Results: Postural scores indeed were found significantly lower in patients with RIL than in healthy controls. Visual parameters were found able to predict the postural score of RIL patients with normalized root mean square error (RMSE) of 0.16. The present analysis showed that horizontal and vertical eye movements, as well as the average duration of the saccades and fixations influenced significantly the prediction of the postural score in RIL patients. While two patients with very low MATTIS-Attention sub score showed the lowest postural scores, no statistically significant relationship was found between the two outcomes.

Conclusion: These results highlight the significant relationship between the severity of balance deficits and the visual characteristics in RIL patients. It seems that increased balance impairment is coupled with a reduced focusing capacity in ecological tasks. Balance and eye movement recordings during a natural task could be a useful aspect of multidimensional scoring of the dysexecutive syndrome.

Referential processing during reading: concurrent recordings of eye movements and head motion

The present study utilized a new experimental set-up synchronizing eye movements and head motion for investigating referential change occurring in a reading task. We examined the effects of a change in narrative perspective during reading on eye movements and head motion. Forty-four participants read texts on a digital tablet, and both participants' eye movements and head movements were recorded using eye tracker and motion capture. The results showed longer eye fixation duration, longer reading time and decreasing head motions when perspective changed. Recent studies have supported the dynamic engagement hypothesis suggesting that there is a fluctuation in cognitive engagement reflected by postural movements. Our findings on head movements seem to validate this hypothesis of a bodily engagement in reading. The results provided by our study showed that the novel methodology of eye and head movement recordings we used was proven to be informative in studying the reader's embodiment during reading.

Gaze position interferes in body sway in young adults

Postural control is influenced by eye movements. Gaze fixation, which comprises a component of ocular vergence, is important in the acquisition of highly specific task information, but its relation to postural control is little investigated. The aim of the study was to investigate the effects of gaze fixation position (central and lateral fixations) on postural sway in young adults. Forty young adults with ages ranging from 20 to 35 years were invited to participate in the study. Postural sway was measured in quiet stance in bipedal support in three 60-s trials under the following conditions: gaze fixation on a target positioned in front of participant, gaze fixation on a target positioned on right side of participant, and gaze fixation on a target positioned on left side of participant. The following center of pressure parameters (COP) in the anteroposterior (AP) and mediolateral directions (ML) were analyzed for each of the trials: body sway displacement, mean velocity of sway, root mean square (RMS) of sway, and median frequency. In addition, detrended fluctuation analysis (DFA) exponent, in anteroposterior and medio-lateral directions, was calculated. The COP presented greater AP and ML displacement (p<0.03, effect size=1.37; and p<0.03, effect size=1.64, respectively) and RMS AP and ML (p<0.04, effect size=1.66; and p<0.02, effect size=2.50, respectively) for lateral gaze fixation compared to central gaze fixation. These results suggest that gaze fixation on a laterally positioned target increases body sway in anteroposterior and mediolateral directions.

Active vision task and postural control in healthy, young adults: Synergy and probably not duality

In upright stance, individuals sway continuously and the sway pattern in dual tasks (e.g., a cognitive task performed in upright stance) differs significantly from that observed during the control quiet stance task. The cognitive approach has generated models (limited attentional resources, U-shaped nonlinear interaction) to explain such patterns based on competitive sharing of attentional resources. The objective of the current manuscript was to review these cognitive models in the specific context of visual tasks involving gaze shifts toward precise targets (here called active vision tasks). The selection excluded the effects of early and late stages of life or disease, external perturbations, active vision tasks requiring head and body motions and the combination of two tasks performed together (e.g., a visual task in addition to a computation in one's head). The selection included studies performed by healthy, young adults with control and active - difficult - vision tasks. Over 174 studies found in Pubmed and Mendeley databases, nine were selected. In these studies, young adults exhibited significantly lower amplitude of body displacement (center of pressure and/or body marker) under active vision tasks than under the control task. Furthermore, the more difficult the active vision tasks were, the better the postural control was. This underscores that postural control during active vision tasks may rely on synergistic relations between the postural and visual systems rather than on competitive or dual relations. In contrast, in the control task, there would not be any synergistic or competitive relations.