Keywords

1 Introduction

The maintenance of balance and body orientation in static and dynamic situations is a core element for the performance of daily life, work, exercise, and sport activities [1]. Postural control requires maintaining position of body segments with respect to both other segments and environment (orientation) and controlling internal forces to compensate external forces acting on the body (balance) [2]. To achieve these behavioral goals, the postural control system needs sensory information regarding relative position of body parts and forces acting on it to, then, trigger appropriate muscular contractions in order to maintain or change a given postural position. The sensory bases of this perception–action linkage are complex as information arriving from visual, vestibular, and somatosensory channels must be integrated by the superior levels of central nervous system [3]. On the other hand, any motor action related to the maintenance or achievement of a given body position requires that the effector system works with a multi-segment body, applying specific strategies and muscle activities to constrain many organism degrees of freedom and have them acting as a functional unit [2].

Analyzing the so-called quiet stance , when individuals standing upright are looking straight ahead, is a standard method in posturography that offers the opportunity of better understanding the perceptual–motor complexity of posture [4]. However, exploring situations in which the postural system is challenged by subtle stimuli or dynamical actions may reveal a more comprehensive perspective on postural adaptability and allow novel manipulations of visual information. Thus, the purpose of the present chapter was to characterize how the postural system of healthy individuals is capable of adaption when facing distinct levels of challenge, all planned to vary the visual context involved. A series of postural studies is summarized where the effects of vision were tested in tasks such as moving their line-of-gaze in space (during saccadic and smooth pursuit eye movements), grasping an object (under different optic flow conditions), three-ball juggling (under narrow and wide stances), performing a ballet skill (with eyes opened and blindfolded), and driving a car (under simulated conditions), as shown in the following sections. In addition, a section on a particular type of postural adaptation due to the aging process is presented to describe the effects of progressive perceptual, motor, and cognitive challenges faced by older adults, which is followed by final considerations.

2 Static Posture and Eye Movements

Specific characteristics define different gaze behaviors such as fixation, saccades, and smooth pursuit eye movements. In fixation, the eyes remain stationary focusing at a given point in the environment so as to stabilize the visual image on the fovea region allowing the extraction and processing of visual information in detail. Saccade s are fast eye movements used to bring a new part of the visual field to the fovea region [5]. Due to this high velocity, the processing of visual information during the saccade is extremely poor, a phenomenon known as saccadic suppression [6]. Smooth pursuit is the visual tracking of a moving target in which a continuous movement rotates the eyes to compensate for the motion of the visual object, minimizing blur and therefore occurring within a nonstationary frame of reference [7].

The influence of each one of these behaviors on the control of static posture has been the object of innumerous investigations [8, 9]. Besides their great importance on daily life activities, the fact that eye movements do not generate great biomechanical constraints on posture and large center of mass displacements has made the study of their effects on balance even more attractive. In this section, we explore these effects as well as their interaction with the complexity of the postural and visual tasks.

2.1 Effects of Distance to Fixed Object

Lee and Lishman [10] argued that the magnitude of the optic flow resulting from movements of the head is a function of the linear distance between visual object and observer. Consequently, bigger changes in optic flow will be produced while fixating an object nearby compared to a distant one when the observer produces a head movement of the same amplitude. Evidence for this interpretation has been provided by the authors by showing that body sway decreases when subjects are fixating nearby targets (0.4 m) compared to distant ones (3 m). Since greater changes in optic flow will be produced for head displacements when the observer is fixating nearby targets, body sway becomes more “detectable” and would be minimized more precisely to stabilize the fixed object in the visual field. On the other hand, for distant targets greater magnitude of body oscillation can be allowed without having a large effect on the stabilization of the visual image [10].

Stoffregen et al. [11] extended this finding demonstrating that when the subject is fixating a distant object but the object nearby is still present in the visual field, body sway is still greater than when the nearby object is being fixated . The authors argue that even though visual information on the nearby object is still available in the optic array, body sway occurred as a function of the task at hand. This means that oscillation was still greater because the subject was fixating a distant target, regardless on the available visual information that might be “automatically” detected by the postural control system but was not part of the suprapostural task [11].

This study has been replicated in further investigations, which moved on to show how these effects interact with more complex postures [12]. Rodrigues et al. [12] explored the effects of object distance on body sway during the maintenance of the kiba-dachi posture, a karate stance which challenges balance specially in the anterior–posterior direction since it requires the person to maintain their legs very distant from each other (larger than shoulder width) with knees flexed and trunk aligned with the feet. Results revealed decreased body sway while fixating a nearby object as compared to a distant one, showing that the effect of distance is also present during the control of such complex posture [12].

2.2 Effects of Saccades on Posture

Although a few studies suggested that eye movements might actually increase body sway, this finding was only present in very specific contexts, such as adding cognitive tasks while performing eye movements [13] and exploring these effects in patients with spontaneous nystagmus as opposed to planned voluntary saccades [8, 14]. Overall, the literature provides solid evidence that saccades decrease body sway in neurologically healthy individuals [15]. This is exemplified by experiments showing reduced body oscillation during saccades performed both horizontally and vertically [16] and at near and far distances between observer and visual object [17]. Several studies confirm this pattern of results [18, 19]. Interestingly, eye movements performed with eyes closed do not reduce body sway [19].

Stoffregen et al. [9] argue that the reason why body oscillation is reduced during eye movements is to achieve the goal of stabilizing the visual scene, allowing eye movements to be performed more accurately. Two mechanisms are thought to be responsible for visually detecting body sway and stabilizing posture, which are referred to as afferent and efferent motion perception, or ocular and extraocular mechanisms, respectively [20]. The first, afferent motion perception is a feedback based mechanism which uses information contained in the optic flow to minimize retinal slip and therefore stabilize the distance between the eye and the visual environment. The second, efferent motion perception is based on either the copy of the motor command, named efference copy—which is a feedforward mechanism that predicts the sensory consequences of the movement produced —or the extraocular muscle afferents, named re-afferences, that are consecutive to eye movements [20].

Rodrigues et al. [18] investigated not only the effects of saccades on postural stabilization but also their interaction with the complexity of the postural and eye-movements tasks. While performing horizontal saccades at different frequencies (0.5 and 1.1 Hz), young adults controlled their balance by standing either with feet apart at shoulder width or pressed together reducing stability especially in medial–lateral axis. Results indicated in general reduced body sway during saccades compared to fixation and in narrow compared to wide stance, as expected, but also that specifically in the anterior–posterior axis body sway was greater for narrow compared to wide stance only in the condition of high frequency (1.1 Hz) saccades. This shows that the reduction in body sway which takes place during eye movements was actually stronger with the combination of the most complex postural (narrow stance) and eye-movements (1.1Hz saccades) tasks. Therefore, the reduced time for the planning of the next saccade which occurs for the high frequency saccades compared to the low frequency ones might have required greater stabilization of posture in the non-stable narrow stance condition in order to accurately fulfill the demands of the eye-movements task being performed. The authors argue that the fact that the frequency of saccades could affect posture differently according to the stance condition is in line with an adaptive resource-sharing model, proposed by Mitra and Fraizer [21–23], which postulates that postural and suprapostural tasks compete for the same limited resources which will be dedicated to either one of these tasks according to the demands of each one [18].

2.3 Effects of Smooth Pursui t on Posture

Although saccades clearly stabilize posture, contradictory results have been presented on whether performing smooth pursuit eye movements decreases or increases body oscillation. Visual tracking of a moving target has been shown to cause larger body sway, especially when the target appeared in a dark background [8, 24]. The lack of spatial reference of a dark background together with additional manipulations on stance conditions in these studies—using complex tasks such as heel-to-toe position and standing on a foam cushion—makes these results able to be interpreted only in very specific contexts.

Recent evidence has been provided, however, indicating that smooth pursuit eye movements performed in a regular spatial frame of reference—i.e., not in darkness—which is the most common situation observed in daily life activities, actually decrease body sway, stabilizing posture similarly to saccades [25]. In this investigation, Rodrigues et al. [25] varied the velocity of saccades and smooth pursuit eye movements and demonstrated that both types of eye movements decreased body oscillation compared to fixation, regardless of their velocities .

There are important differences between saccades and smooth pursuit eye movements. These include saccadic suppression—which drastically reduces the capacity for processing of visual information during saccades but does not exist for smooth pursuit, the nonstationary spatial frame of reference that occurs during smooth pursuit but not during saccades, and perhaps more importantly, the fact that while smooth pursuit are movements entirely based on feedback to keep the target at the fovea region continuously, saccades need to be planned beforehand due to their high velocity, leaving no space for corrections during their execution. Nevertheless, even with these different aspects, both eye-movements tasks seem to reduce body sway, leaving authors to conclude that the mechanisms involved in the execution of saccades and smooth pursuit eye movements share similarities in postural control [25], possibly the fact that greater postural stabilization is required to perform both tasks accurately [9].

3 Posture, Optic Flow, and Arm Movements

3.1 Effects of Grasping Movements Under Different Optic Flow Conditions

Many environmental aspects such as the optical flow and one’s own movement can affect posture stabilization. However, little is known about how these two features can be combined as participants maintain upright stance during grasping actions. Rodrigues et al. [25] examined the effects of discrete movements of a moving room (approaching to or moving back from participants) combined with grasping an object on postural control stability. Twelve participants stood upright inside a moving room, moved discretely approaching or moving away from the participants, as they had to remain still or grasping a stationary or an approaching ball, totalizing nine experimental conditions (Table 15.1).

Table 15.1 Rodrigues et al.’s [25] experimental conditions combining manipulations of moving room, ball, and catching action
Full size table

A pulley system was connected to a servomechanism in order to move simultaneously the room and a shaft, which had a ball (6 cm of diameter) fixed in its end. Forward and backward movements of the room were associated with clockwise and anticlockwise rotations of the pulley system, moving the shaft in speed and amplitude proportional to the movement of the room in the same or opposite directions. Participants were instructed to stay in the starting position as stable as possible and keep it through the end of the experimental trials with no grasping (conditions 1, 4, and 5), grasping the ball after the light being turned on with no movement of the room and of the ball (condition 2), and grasping the ball after the light being turned on simultaneously to the room and shaft movements initiation (conditions 3, 6, 7, 8, and 9). The discrete movement of the room had approximate duration of two seconds, with peak velocity of 2.6 cm/s and amplitude of 2.5 cm [26]; the shaft approached the participant, being moved approximately during two seconds , with peak velocity of 74.8 cm/s, and amplitude of 53 cm. During all conditions, participants were required to stand upright for 18 s, maintaining fixation with their eyes on the ball. Each trial was divided into four time windows: prior to room movement initiation (four seconds), during room movement (two seconds), after room movement—first and second periods (four seconds each), totalizing 14 s; the remaining time was excluded from analysis. The variables body displacement and correlation coefficient were obtained during the room movement; the variable mean sway amplitude was used to infer stability prior to and after movement of the room.

Rodrigues et al. [25] found that grasping leads to increased effect of moving room on induced body sway, as body displacement is larger during grasping. However, body sway is more related to room movement when the room is moved back compared to the approaching movement. Despite inducing larger body sway during the movement of the moving room, grasping disruption of postural equilibrium is even larger when there is no moving room visual flow. These results also showed that grasping a stationary ball in a moving scenario is worse, leading to larger body sway induced by the moving room and such effect is even larger in the moving back movement of the room. Such postural equilibrium disruption is still observed after the grasping has ended (following the four seconds).

It was surprising that grasping during room discrete movement leads to a larger sway; literature had pointed out only correspondent body sway in children [27], adults [26], and even in elderly [28]. This new finding can be explained by the fact that when adults have to gather information about the ball position, they are more influenced by the peripheral flow. In the context of a moving room with no arm movement involved, it is known that flow with large lamellar (peripheral) structure generates stronger sway response than more radially (centrally) structured flow [29]. In line with this reasoning is that although vision of the hand in Rodrigues et al.’s [25] experiment is peripherally available during its initial movement, the use of central vision during the later stages of grasping is advantageous for controlling movement amplitude as the limb decelerates [30].

The correlation coefficient in the moving back condition, which indicates the relationship between moving room and body sway, was dependent on direction, being stronger in the condition of backward optical flow, with the room moving away from the participant. It is well known that infant rhesus monkeys [31] and infant humans [32] show defensive reactions to approaching (looming) patterns of optic flow; these responses were in clear contrast to those related to objects approaching on a non-collision path or a shrinking (as opposed to a looming) pattern [33]. In the moving room paradigm, the room approaches the observers who respond to the optical flow (looming) as if it arose from their own postural sway [34], attempting to keep a constant relation with the room’s walls. As in Rodrigues et al.’s [25] experiment, a grasping action was required, and the postural adjustment in response to the approaching movement of the room would result in increase of hand–ball distance; instead, a sway stabilization took place to allow a proper grasping movement. Differently, the room moving back from participants seemed to have facilitated grasping as they both were in the same direction.

In short, Rodrigues et al. [25] showed that grasping leads to increased effect of moving room on induced body sway. In this context, posture was more affected when the room was moved back compared to the approaching movement. Based upon these results, authors suggest that postural control takes advantage of the structured optical flow available due to room movement, even when posture might be destabilized due to one’s own movement, to maintain postural equilibrium .

3.2 Effects of Juggling Movements Under Different Bases of Support Conditions

Cascade ball juggling is a complex perceptual–motor skill, which requires an efficient postural stabilization. Experience in the juggling task may play a role in the linkage between manual and postural tasks. To clarify how the postural system works in this context, Rodrigues et al. [35] investigated effects of experience (expert and intermediate groups) and feet distance (wide and narrow stances) on body sway of jugglers during three-ball cascade juggling. Participants stood barefoot on the force plate (some participants wore a gaze tracking system), with feet maintained in wide and narrow conditions and performed three 40-s trials of the three-ball juggling task.

The main finding of Rodrigues et al. [35] was that only intermediate jugglers showed smaller oscillation in the narrow as compared to wide stance. Expert jugglers showed a superior postural stabilization in both stances, with more consistent patterns of center of pressure displacements characterized by measures of mean velocity and amplitude in both AP and ML axes, corroborating the notion according to which juggling skill level is associated with decreased body sway. Previous studies have shown that expertise similarly affect other motor skills, such as rifle shooting, manual rhythmic movements, circus activities, and gymnastics [36–40]. In a study with jugglers, Leroy et al. [41] found that although experts and intermediate participants showed similar body sway amplitude (measured through sacrum lateral displacement), the skilled group was able to perform anticipatory postural adjustments differently from novice jugglers.

Interestingly, gaze behavior seems to participate in controlling more complex postural tasks. The increased body sway of intermediate jugglers could be explained by the larger area of gaze displacement observed throughout each trial. A more stable, spatially reduced gaze pattern, named gaze-through strategy [42], may improve movement planning via the attentional system [43, 44]. During a gaze fixation, balls move in the retinal periphery, which most likely involves a stage operating in gaze-centered coordinates which is more accurate [42, 45]. With a more stable gaze, movement planning is improved because the entire visual background is stable since allocentric and gaze-centered representations become aligned [42, 46]. Although visual information acquisition through fixation patterns (number of fixations, mean duration of fixations and its variability) was not affected by group or condition in Rodrigues et al.’s study [35], the significant base of support by group interaction found for the area of gaze displacement seems to corroborate the experts’ gaze-through strategy, suggesting an attentional linkage between postural (center of pressure) and visual (gaze) stability. Although acknowledging the small sample size of gaze data, authors interpreted these results as a confirmation that experts were less dependent on foveal vision and more capable of decoupling the control of posture and bimanual rhythmic arm movements [36, 46].

Rodrigues et al. [35] found a surprising significant effect of base of support; greater postural instability was expected during more challenging tasks, but the opposite was observed. Particularly, intermediate jugglers increased the sway laterally more than experts to facilitate their arms movement pattern, as shown by the significant main effect of group on the ML sway amplitude. Additionally, intermediate jugglers showed significantly smaller oscillation in the narrow stance, indicating that experts were less affected by the basis of support reduction than intermediate jugglers. Expert jugglers showed differences in body oscillation between narrow and wide stances, which were not significant while the intermediate group was clearly affected by the smaller basis of support and had to reduce more drastically their body sway to keep balance and the juggling task ongoing. Authors interpreted their results as corroborating previous studies which showed effects of the experience and motor specialization on the improvement of postural control [41, 47–49].

In sum, Rodrigues et al. [35] concluded that experts’ body sway was characterized by lower velocity and smaller amplitude as compared to intermediate group. Interestingly, authors found that the more challenging (narrow) basis of support caused significant attenuation in body sway only for the intermediate group, suggesting that expertise in cascade juggling was associated with refined postural control .

4 Eye–Head Stabilization in Complex Contexts

The role of visual information on the control of movement has been investigated in a variety of complex contexts such as everyday life and sport activities [50–53]. The eyes move around the environment to align fovea region to a specific portion of visual scene for processing of visual information in detail [51]. Acquisition of useful visual information in dynamical postures involves controlled coordination between body, head, and eye movements. Eye and head stabilization during the execution of motor actions is an important mechanism to keep stationary the image on the retina in order to ensure a clearer visual processing during the execution of motor actions [52, 53].

Two kinds of compensatory mechanisms participate in the stabilization of eyes and head during dynamical postures, making the eyes to move in line with head rotations. In the vestibulo-ocular reflex, head rotation velocity is detected by semicircular channels (vestibular system) that provide sensory signals to the oculomotor system to move the eyes in the opposite direction. The second mechanism, the optokinetic reflex, is engaged in adjusting the actual velocity of the retinal image with the velocity of eyes rotation in the same direction [51]. In this section, it will be discussed the role of eye and head stabilization in several everyday life and sport activities that directly affect visual information acquisition and the maintenance of dynamical stability.

4.1 Stabilizing Posture During Dynamical Skills : Ballet and Table Tennis

Visual information during the execution of a ballet pirouette is the crucial sensory input used to maintain postural stability. However, visual processing is impaired during the rotation of the trunk, head, and eyes that can affect dynamical posture. Denardi et al. [53] investigated the role of visual information on postural control of expert ballet dancers with analyses of trunk, head, and eyes movements during the execution of a pirouette with (opened eyes) and without (blindfolded eyes) vision available. Gaze stability for as long as possible during performance is an important strategy to minimize rotational flow on the retina generated by the movement of whole body [52–54]. In Denardi et al.’s [53] experiment, gaze stability strategy was checked with analyses of the quiet eye period. The concept of quiet eye proposed by Vickers [54, 55] suggested that a longer final fixation before the initiation of movement is used to extract the maximum of the critical information from environment to define details of motor performance. Denardi et al. [53] observed a longer quiet eye period in the beginning of the pirouette turns; however, there was absence of correlation between quiet eye duration and postural stability. In contrast with previous studies [52, 54, 56], a longer period of quiet eye did not cause changes on pirouette performance. However, the authors assumed that visual information acquired during gaze stability period was sufficient for expert ballet dancers to achieve dynamical postural requirements of the task [53]. In addition, a clear sequence of trunk, head, and eyes rotations in the execution of the pirouette with opened eyes was observed. In the initiation of movement, trunk rotation starts to turn following with the rotation of the head and finishing with the delayed eyes rotation. At the end, eyes rotation starts following to the head and trunk rotations sequentially. Interestingly, in the experimental conditions without visual information (blindfolded eyes) the rotations of the head and trunk were coupled [53]. These findings showed evidence that when visual information was available, head movements were delayed to maintain the gaze stability as long as possible in order to ensure a useful extraction of information during quiet eye period. Eye and head stabilization during quiet eye period facilitated visual information acquisition to perform the task [53].

Rodrigues, Vickers, and Williams [52] investigated the coordination of head, eye, and arm movements of skilled and less skilled table tennis players; although they did not focus directly on postural control, head contribution to visual stability may help in understanding the role played by vision in posture. Their experimental task consisted in a forehand drive shot to one of the two cued target areas, which was illuminated in order to generate distinct temporal conditions: pre-cue, before serve; early cue, during the initial portion of ball flight; and late-cue, during the final portion of ball flight. The duration of quiet eye was reduced according to the increase in complexity of the task (from pre- to early cue and from early to late-cue conditions). Eye–head stabilization was defined as the period in which head and eye were aligned. During the most difficult condition (late-cue), eye–head stabilization was less frequent suggesting that task complexity disrupted optimal conditions for information acquisition via eye–head stability, which occurred along with a reduction in the quiet eye duration. These changes in eye–head stabilization and quiet eye period during more complex task induced a lower accuracy performance for all players (51, 52).

4.2 Eye and Head Stabilization During Driving

Driving a car is a particularly complex type of locomotion supported by visual information available in the surrounding environment. Eye and head movements have been studied during driving situations to determine drivers’ visual search strategies [50, 51, 57]. Land and Lee [58] investigated the steering control on a road with steeper bends, recording steering wheel angle and drivers’ gaze direction. Authors found that before the car entering on the bend, drivers spent much of time fixating the tangent point—defined as the most salient point inside of the bend—and also observed a clear correlation between gaze and steering wheel angle, with gaze movements preceding the rotation of the steering wheel by about 0.8 s. The authors suggested that gaze angle directed at the tangent point can provide the signal needed for the arms to control the steering wheel [58]. In another study, Land and Tatler [57] examined the eye and head movements of a Formula 3 racing driver in a winding road. The racing driver fixated most of the time close, but not exactly, at the tangent point before starting to turn the steering wheel [57]. Interestingly, head movements from central line of the lane to the tangent point begun 1.5 s before starting cornering, and the correlation between driver’s head direction and the rotation of the vehicle was stronger (r = 0.96) than correlation between gaze and head direction (r = 0.3). Authors suggested that even though the racing driver did not displace his gaze exactly to the tangent point location, driver’s head direction provided sufficient information for the steering wheel control [57].

Carizio [50] investigated the effects of cognitive workload (use of cell phone) on gaze and head stability during simulated driving. The use of a cell phone in speakerphone mode or holding the device in the driver’s hand can interfere in the attentional mechanism of drivers and tend to disrupt perceptual and motor performance. Drivers had eyes and head movements recorded while driving in a driver simulator under three experimental conditions: control (no talking, no cell phone), speakerphone mode (talking, with cell phone in speakerphone mode), and holding in hand (talking, with cell phone held in the hand). Results revealed that drivers performed a higher number of fixations per time unit and a higher spread of fixation locations in the horizontal and vertical axes during both cell phone conditions as compared to the control condition. Similar to gaze behavior, variance of head position and orientation also increased during both speakerphone mode and holding in hand conditions of mobile phone use. These findings suggest that increased cognitive workload/task complexity provoked a decrease in eye and head stabilization [50], corroborating previous studies on eye–head stabilization in sports [52].

In natural contexts as those referred above [50, 52, 53, 57], eye and head movements seem to work together to extract useful visual information from a relevant location at a proper time. Particularly, head control plays a critical role in maintaining gaze stability in such complex contexts which is in line with previous evidence of linkage between eye movements and postural control discussed above [50, 52, 53]. The activity of eyes, head, trunk, and whole body emphasizes the organism collective effort towards an optimal acquisition of useful visual information; the observed coupling between gaze and motor behavior evidenced that visual information is essential for planning motor actions and maintaining dynamical posture in complex contexts [50–54].

5 Effects of Aging on Eye Movements and Postural Control

In addition to increased complexity of sensory stimuli and motor actions involved in natural situations, the aging process represents a progressive challenge and requires important adaptation of older adults. Aging is known to affect both the motor and sensory systems which are intimately related in the control of static posture [59]. Older adults show poorer performance compared to young adults in both postural control [59–61] and eye-movements tasks [62–64]. Specifically, postural control has been shown to be affected by aging in both simple [65] and complex postural tasks [66]. For this reason, the risk of falls in older adults has received great attention from researchers and authorities representing an important public health issue [67]. Mechanisms contributing to poorer postural control in older adults include motor deficits such as muscle weakness [68], sensory deficits in visual [69], vestibular [70], and somatosensory systems [71], and limitations in attentional capabilities [72]. Importantly, besides causing specific deficits in both motor and sensory systems contributing to postural control, aging has been shown to affect also sensorimotor integration in the control of posture [26, 66].

Deficits caused by aging specifically in the visual system such as a reduced visual acuity [73] and a poorer contrast sensibility [74] provoke an impaired visual perception to older adults. Furthermore, the effect of aging on performance of eye-movements tasks has also been well documented. Older adults perform saccades less accurately [63], with slower reaction times [75] and with reduced amplitude, velocity, and frequency [76] compared to young adults. The gain [77] and accuracy [64] of smooth pursuit eye movements are also affected by aging. A comparison between the performance of eye movements in real-world and laboratory situations has shown that in real world performance of saccades is greatly deteriorated by aging while performance on tracking movements can remain unaffected by aging probably due to the use of additional sensory cues compensating for aging deficits [76]. Interestingly, the effects of aging on eye-movements performance have been shown also for common tasks such as reading [78] and have been even proposed as an indicator on driver’s ability in older adult drivers [79] and as a tool for detecting mild cognitive impairments [80].

Similar to results shown previously from young adults [18], Aguiar et al. [59] also explored the effects of saccades at different frequencies and different stance conditions in postural control but in a population of older adults. Discussions around the adaptive resource-sharing model would be enriched with this investigation since the sensory and motor deficits presented by older adults would potentially alter these limited resources available for postural and suprapostural tasks and therefore how the postural control system would give priority to one task over another. Results showed decrease in body sway during saccades compared to fixation, which is the same result encountered in young adults. Interestingly, no difference was found between narrow and wide stance conditions according to the frequency of saccades, which was present for young adults. Authors concluded that older adults present a more rigid postural control strategy, which does not allow larger body sway during a more complex postural task [59]. However, Laufer et al. [60] investigated postural stability of young and older adults during upright stance on restricted basis of supported 85 cm above ground level with both opened and closed eyes. Postural threat did not affect young adults; however, older adults showed a greater mean power frequency of displacement of center of pressure [60]. The authors concluded that older adults adopted an inadequate postural response during postural threat that increased body sway. The increasing of complexity of the postural task may be a challenge to older adults since the postural control system is affected by aging. In Aguiar et al.’s experiment [59], even with a more complex postural task (restricted basis of support) older adults maintained their body sway unaltered with different frequencies of horizontal saccadic eye movements. According to the authors, older adults performed a more rigid postural strategy to prioritize the postural performance during a threatening situation, which did not allow the influence of saccadic eye movements on body sway. Interestingly, Paquette and Fung [62] found that older adults performed more corrective saccadic movements and with greater latency between stimulus onset and saccadic eye-movement onset with a moving surface compared to young adults. The authors suggested that older adults have visual and postural performance impaired during threatening stability situations. In general, saccadic eye movements reduce body sway of young and older adults; however, during more challenging postural tasks the relationship between eye movements and postural control seems to be altered with aging .

6 Final Considerations

Understanding the effects of vision on postural control in healthy individuals requires an extended notion of visual system which includes postural elements. For instance, the notion that “eyes-in-the-head-on-the-body-resting-on-the-ground” [81] are continuously searching for relevant information in the optic flow favors the simultaneous consideration of gaze and postural data to discuss balance control. Postural adjustments seem to support optimal gaze behavior during simple and complex actions. Although optic flow results from translational components of head movement in space and eye movements add rotational components to the flow on the retina [82, 83], a process of minimization of rotational consequences to the flow, called gaze stabilization [84], seems advantageous to optimize translational information acquisition with respect to the perceiver. As human visual input depends on the dynamics of all body parts, studies on posture should explore this collective postural effort (resulting from movements of eyes, head, trunk, and whole body) to acquire the relevant information available in the optic flow, needed for successful action.

Following this suggestion, the series of studies presented in this chapter showed a variety of circumstances in which the postural control system faced both stronger, mechanical perturbations and subtle, sensory challenges. During quiet stance, posture is more stabilized when the fixed object is nearby as compared to a distant one, the principle which holds for complex stances, such as karate kiba-dachi; besides distinct control mechanisms, saccadic and smooth pursuit eye movements directed to predictable and continuous stimulus also tend to reduce body sway.

In contexts with more complex postural requirements, adaptations in the perception–action linkage are multivariate. Grasping action leads to increased effect of optic flow (moving room) on induced body sway; posture was more affected when the room was moved back compared to the approaching movement, suggesting that postural control takes advantage of the structured optical flow available, even when posture might be destabilized due to one’s own movement, to maintain postural balance. In cascade juggling, expertise reduces body sway of participants; the more challenging (narrow) basis of support caused significant attenuation in body sway only for the intermediate group, suggesting that experts have a refined postural control. During a pirouette in ballet or a forehand drive in table tennis, eye and head stabilization was strategically adopted in order to facilitate the extraction of useful information to control the motor behavior; increasing of task complexity (removing visual information in ballet or delaying the visual cue in table tennis) caused impairment on eye–head stabilization which disrupted performance. In simulated driving, an increase in cognitive load generated by the use of cell phone resulted in higher relative number of fixations and spatial spread of fixation locations, as well as increased variance of head position and orientation, suggesting a worsening of postural stability and visual information acquisition.

Particularly, characterizing the effects of vision on postural control in healthy older adults adds even more complexity to the debate. The natural perceptual, motor, and cognitive deficits due to the aging process push the limits of adaptation. In general, the older adults show a more rigid postural control strategy, with increased probabilities of instability and risk of falls. In summary, considering the multiple levels of the visual system functioning (with eyes, head, trunk, and whole body movements) along with lifespan postural changes is essential to advance our knowledge of the process of acquisition and use of visual information to stabilize posture.