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.     

Multifractal roots of suprapostural dexterity

Visually guided postural control emerges in response to task constraints. Task constraints generate physiological fluctuations that foster the exploration of available sensory information at many scales. Temporally correlated fluctuations quantified using fractal and multifractal metrics have been shown to carry perceptual information across the body. The risk of temporally correlated fluctuations is that stable sway appears to depend on a healthy balance of standard deviation (SD): too much or too little SD entails destabilization of posture. This study presses on the visual guidance of posture by prompting participants to quietly stand and fixate at distances within, less than, and beyond comfortable viewing distance. Manipulations of the visual precision demands associated with fixating nearer and farther than comfortable viewing distance reveals an adaptive relationship between SD and temporal correlations in postural fluctuations. Changing the viewing distance of the fixation target shows that increases in temporal correlations and SD predict subsequent reductions in each other. These findings indicate that the balance of SD within stable bounds may depend on a tendency for temporal correlations to self-correct across time. Notably, these relationships became stronger with greater distance from the most comfortable viewing and reaching distance, suggesting that this self-correcting relationship allows the visual layout to press the postural system into a poise for engaging with objects and events. Incorporating multifractal analysis showed that all effects attributable to monofractal evidence were better attributed to multifractal evidence of nonlinear interactions across scales. These results offer a glimpse of how current nonlinear dynamical models of self-correction may play out in biological goal-oriented behavior. We interpret these findings as part of the growing evidence that multifractal nonlinearity is a modeling strategy that resonates strongly with ecological-psychological approaches to perception and action.     

The functional role of central and peripheral vision in the control of posture

Three experiments were conducted to investigate the role of central and peripheral vision (CV and PV) in postural control. In Experiment 1, either the central or peripheral visual field were selectively stimulated using a circular random dot pattern that was either static or alternated at 5 Hz. Center of foot pressure (CoP) was used to examine postural sway during quiet standing under both CV and PV conditions. The results showed that, when the visual stimulus was presented in the periphery, the CoP area decreased and more so in the anterior-posterior (AP) than in the medio-lateral (ML) direction, indicating a characteristic directional specificity. There was no significant difference between the static and dynamic (alternating) conditions. Experiment 2 investigated the directional specificity of body sway found in Experiment 1 by having the trunk either be faced toward the stimulus display or perpendicularly to it, with the head always facing the display. The results showed that the stabilizing effect of peripheral vision was present in the direction of stimulus observation (i.e., the head/gaze direction), irrespective of trunk orientation. This suggested that head/gaze direction toward the stimulus presentation, rather than a biomechanical factor like greater mobility of the ankle joint in AP direction than in ML direction, was essential to postural stability. Experiment 3 further examined whether the stabilizing effect of peripheral vision found in Experiments 1 and 2 was caused because more dots (500) were presented as visual cues to the peripheral visual field than to the central visual field (20 dots) by presenting the same number of dots (20) in both conditions. It was found that, in spite of the equal number of dots, the postural sway amplitudes were larger for the central vision conditions than for the peripheral vision conditions. In conclusion, the present study showed that peripheral rather than central vision contributes to maintaining a stable standing posture, with postural sway being influenced more in the direction of stimulus observation, or head/gaze direction, than in the direction of trunk orientation, which suggests that peripheral vision operates primarily in a viewer-centered frame of reference characterized by the head/gaze direction rather than in a body-centered frame of reference characterized by the anatomical planes of the body.     

Postural costs of suprapostural task load

In an immersive visualization experiment, participants performed a conjunction search task while standing either in open (heels 10 cm apart, feet at a comfortable angle) or closed stance (feet pressed together). In the world-frame condition, the search display maintained its position in space as the participant swayed, generating optic flow informative about sway. In the head-frame condition, the display maintained constant distance and orientation with respect to the participant's head, providing no visual information about sway. In both conditions, participants (surprisingly) searched faster while in the more difficult closed stance. Interpretation of this result is unclear. Participants also swayed more as search-load increased, and made more errors in the high search-load condition. It is suggested that this performance tradeoff is a result of the sharing of a limited-capacity, modality-non-specific spatial-attentional resource between postural and suprapostural tasks.     

Effects of peripheral eccentricity and head orientation on gaze discrimination

Visual search tasks support a special role for direct gaze in human cognition, while classic gaze judgement tasks suggest the congruency between head orientation and gaze direction plays a central role in gaze perception. Moreover, whether gaze direction can be accurately discriminated in the periphery using covert attention is unknown. In the present study, individual faces in frontal and in deviated head orientations with a direct or an averted gaze were flashed for 150 ms across the visual field; participants focused on a centred fixation while judging the gaze direction. Gaze discrimination speed and accuracy varied with head orientation and eccentricity. The limit of accurate gaze discrimination was less than ±6° eccentricity. Response times suggested a processing facilitation for direct gaze in fovea, irrespective of head orientation, however, by ±3° eccentricity, head orientation started biasing gaze judgements, and this bias increased with eccentricity. Results also suggested a special processing of frontal heads with direct gaze in central vision, rather than a general congruency effect between eye and head cues. Thus, while both head and eye cues contribute to gaze discrimination, their role differs with eccentricity.     

Analysis of the Movement-Inducing Effects of Music through the Fractality of Head Sway during Standstill

Abstract

The links between music and human movement have been shown to provide insight into crucial aspects of human’s perception, cognition, and sensorimotor systems. In this study, we examined the influence of music on movement during standstill, aiming at further characterizing the correspondences between movement, music, and perception, by analyzing head sway fractality. Eighty seven participants were asked to stand as still as possible for 500?seconds while being presented with alternating silence and audio stimuli. The audio stimuli were all rhythmic in nature, ranging from a metronome track to complex electronic dance music. The head position of each participant was captured with an optical motion capture system. Long-range correlations of head movement were estimated by detrended fluctuation analysis (DFA). Results agree with previous work on the movement-inducing effect of music, showing significantly greater head sway and lower head sway fractality during the music stimuli. In addition, patterns across stimuli suggest a two-way adaptation process to the effects of music, with musical stimuli influencing head sway while at the same time fractality modulated movement responses. Results indicate that fluctuations in head movement in both conditions exhibit long-range correlations, suggesting that the effects of music on head movement depended not only on the value of the most recent measured intervals, but also on the values of those intervals at distant times.     

Interstimulus Intervals and Sensory Modality Modulate the Impact of a Cognitive Task on Postural Control

The present literature not only reveals the use of a wide variety of cognitive tasks but variability in their interaction with postural control. The question then arises, as to, whether postural control is sensitive to specific features of a cognitive task. The present experiment assessed the impact of cognitive tasks with interstimulus intervals (ISI) of varied duration and sensory modality on postural control in young adults. Seventeen participants (23.71 ± 1.99 years old) were instructed to stand on a force platform while concurrently performing cognitive tasks with ISIs of two and 5 s. The tasks were presented both, auditorily and visually. The visual tasks consisted of counting the total occurrence of a single digit. The auditory tasks consisted of counting the total occurrence of a single letter. Performing the cognitive tasks with an ISI of 2 s resulted only in an increase in the anteroposterior mean power frequency. Presenting the tasks visually also significantly reduced area of 95% confidence ellipse and AP and mediolateral sway variability. These results may suggest that ISIs can modify postural performance by altering the allocation of attentional focus. Also, presenting tasks using a visual sensory modality appears to yield lower postural sway.     

Measurement of auditory cues in drivers' distraction

A driving simulator was used to examine the effects on driving performance of auditory cues in an in-vehicle information search task. Drivers' distraction by the search tasks was measured on a peripheral detection task. The difficulty of the search task was systematically varied to test the distraction caused by a quantified visual load. 58 participants completed the task. Performance on both search tasks and peripheral detection tasks was measured by mean response time and percent error. Analyses indicated that in-vehicle information search performance can be severely degraded when a target is located within a group of diverse distractors. Inclusion of an auditory cue in the visual search increased the mean response time as a result of a change in modality from auditory to visual. Inclusion of such an auditory cue seemed to influence distraction as measured by performance on the peripheral detection task; accuracy was lower when auditory cues were provided, and responses were slower when no auditory cues were provided. Distraction by the auditory cue varied according to the difficulty of the search task.     

Reducing postural sway by concurrently performing challenging cognitive tasks

The present experiment varied cognitive complexity and sensory modality on postural control in young adults. Seventeen participants (23.71 ± 1.99 years) were instructed to stand feet together on a force platform while concurrently performing cognitive tasks of varying degrees of difficulty (easy, moderate and difficult). The cognitive tasks were presented both, auditorily and visually. Auditory tasks consisted of counting the occurrence of one or two letters and repeating a string of words. Visual tasks consisted of counting the occurrence of one or two numbers. With increasing cognitive demand, area of 95% confidence ellipse and ML sway variability was significantly reduced. The visual tasks reduced ML sway variability, whereas the auditory tasks increased COP irregularity. We suggest that these findings are primarily due to an increase in sensorimotor integration as a result of a shift in attentional focus.     

Responses to Optical Looming in the Retinal Center and Periphery

Stoffregen (1985, 1986) found that the periphery of the retina is insensitive to radially structured optic flow specifying postural sway. This raised the issue of whether the retinal periphery is sensitive to radially structured flow per se, that is, to radial flow specifying events other than postural sway. In the experiment discussed herein we addressed this question in the context of optical looming. Computer-generated optical displays were presented to the retinal center or periphery of standing participants, who were required to dodge out of the path of the depicted object at the last moment before impact. Depicted trajectories were slightly eccentric, requiring participants to employ a directional response. Results showed that the trajectory and time-to-contact of the simulated object strongly influenced responses for peripheral as well as central looms. In general, responses to peripheral looms closely matched those to central looms. These findings indicate that the periphery of the retina is sensitive to optically specified time-to-contact, despite the fact that such information is carried in radially structured flow patterns. We argue that impending collision and postural sway can be distin- guished on the basis of dynamics that are characteristic of each event.     

Postural Sway and the Amplitude of Horizontal Eye Movements

In 2 experiments, we evaluated relations between postural activity and the amplitude of visually guided eye movements. Participants shifted their gaze to follow horizontal oscillation of a visible target. The target moved with amplitude 9° or 24°. In different experiments, the frequency of target oscillation was 0.5 Hz or 1.1 Hz. In both experiments, the variability of head and torso motion was reduced (in the anterior-posterior axis) when participants viewed moving targets, relative to sway during viewing of stationary targets. Sway variability was not influenced by the amplitude of target motion. The results are compatible with the hypothesis that postural activity was modulated relative to the demands of suprapostural visual tasks.     

Is voluntary control of natural postural sway possible?

The authors explored whether standing human participants could voluntarily decrease the amplitude of their natural postural sway when presented with explicit visual feedback and a target. Participants (N = 9) stood quietly, without any feedback and with feedback on the center of pressure coordinate or the head orientation. They were unable to decrease sway amplitude when presented with visual feedback and a target. Decreasing target size led to contrasting effects on the 2 fractions of sway: rambling and trembling. The smaller target was associated with a decrease in rambling and an increase in trembling. Those observations suggest that sway represents a superposition of at least 2 independent processes. They also suggest that providing visual feedback on a variable tied to body sway may not be an effective way to decrease postural sway in young healthy people.     

Effects of visual and auditory short-term memory tasks on the spatiotemporal dynamics and variability of postural sway

The authors measured postural sway while participants (N = 20 in each experiment) stood on a rigid or a compliant surface, with their eyes open or closed, and while they did or did not perform a short-term memory (STM) task. In Experiment 1, the STM stimuli were presented visually; in Experiment 2, the stimuli were presented auditorily. In both experiments, fine-scaled, mediolateral postural-sway variability decreased as the cognitive load imposed by the STM task increased. That effect was independent of support surface and vision manipulations. The spatiotemporal profile of postural sway was affected by both visual and auditory STM tasks, but to a greater degree by the auditory task. The authors discuss implications of the results for theories and models of postural control.     

The impact of visual flow stimulation on anxiety, dizziness, and body sway in individuals with and without fear of heights

Individuals with acrophobia experience anxiety and dizziness when exposed to heights. It may be that their balance system is disturbed and that they therefore have to rely more strongly on visual information.We tested this hypothesis by exposing 20 individuals with high fear of heights and 20 healthy control participants to nine different visual flow stimuli through a head mounted display, thereby inducing a conflict between visual input and somatosensory information. Anxiety and dizziness were assessed repeatedly by means of self-reports, while resultant body sway was measured continuously with a force plate individuals stood on.Individuals with fear of heights felt more anxious and dizzier, and also showed stronger body sway than healthy control participants.Merely receiving visual balance information contradictory to somatosensory balance information is sufficient to induce anxiety, dizziness, and body sway in individuals with fear of heights. An underlying balance dysfunction may contribute to the development of height phobia.     

How Information Availability Interacts with Visual Attention during Judgment and Decision Tasks

Decisions in front of a supermarket shelf probably involve a mix of visually available information and associated memories—and interactions between those two. Several cognitive processes, such as decision making, search, and various judgments, are therefore likely to co‐occur, and each process will influence visual attention. We conducted two eye‐tracking experiments capturing parts of these features by having participants make either judgments or decisions concerning products that had been previously encoded. Half the time, participants made their choices with full information about the available products and half the time with crucial task‐relevant information removed. By comparing participants' use of visual attention during decisions and search‐based and memory‐based judgments, we can better understand how visual attention is differently employed between tasks and how it depends on the visual environment. We found that participants' visual attention during decisions is sensitive to evaluations already made during encoding and strongly characterized by preferential looking to the options later to be chosen. When the task environment is rich enough, participants engage in advanced integrative visual behavior and improve their decision quality. In contrast, visual attention during judgments made on the same products reflects a search‐like behavior when all information is available and a more focused type of visual behavior when information is removed. Our findings contribute not only to the literature on how visual attention is used during decision making but also to methodological questions concerning how to measure and identify task‐specific features of visual attention in ecologically valid ways. Copyright © 2015 John Wiley & Sons, Ltd.     

0 + 1 > 1: How adding noninformative sound improves performance on a visual task

It is well known that the nervous system combines information from different cues within and across sensory modalities to improve performance on perceptual tasks. In this article, we present results showing that in a visual motion-detection task, concurrent auditory motion stimuli improve accuracy even when they do not provide any useful information for the task. When participants judged which of two stimulus intervals contained visual coherent motion, the addition of identical moving sounds to both intervals improved accuracy. However, this enhancement occurred only with sounds that moved in the same direction as the visual motion. Therefore, it appears that the observed benefit of auditory stimulation is due to auditory-visual interactions at a sensory level. Thus, auditory and visual motion-processing pathways interact at a sensory-representation level in addition to the level at which perceptual estimates are combined.     

Exploring interhemispheric collaboration in older compared to younger adults

Physical and Name Identity letter-matching tasks were used to explore differences in interhemispheric collaboration in younger and older adults. To determine whether other factors might also be related to across/within-hemisphere processing or visual field asymmetries, neuropsychological tests measuring frontal/executive functioning were administered, and comparisons were made for participants split into low and high efficiency groups based on performance on the letter-matching tasks. A Task by Across/Within interaction was found for both groups, but with a stronger within-hemisphere advantage for the Physical Task and a weaker across-hemisphere advantage for the Name Task for older participants. More efficient groups and better performers on several neuropsychological tasks showed a reduced across-hemisphere advantage for the Name Identity task. Findings suggest that computational complexity, specific task demands, and perhaps trade-offs between age-related changes in gray and white matter all contribute to whether processing loads are distributed across or within hemispheres as we age.     

Retinal location and its effect on the processing of target and distractor information

Two experiments were conducted to study the perceptual facilitation and inhibition that occurs between foveal and parafoveal or peripheral regions of the eye. Experiment 1 used a Stroop task to compare color detection latencies of foveal and parafoveal targets. The target and distractor components of the Stroop stimuli were separated and presented with varied stimulus onset asynchronies. Experiment 2 used a Stroop-like task to replicate and extend the findings into the visual periphery. Subjects were found to process foveally presented distractor information while attending to targets presented in parafoveal or peripheral regions of the eye. Distractor information that was incompatible was suppressed while compatible information was used to facilitate target processing. When the targets were presented foveally along with the distractor information, subjects appeared to automatically process the distractor information. The findings are discussed within the framework of past studies that presented subjects with competing tasks across retinal location. The implications of these findings to a two-process theory of attention are also considered.     

Not just standing there: the use of postural coordination to aid visual tasks

Postural control is an integral part of all physical behavior. Recent research has indicated that postural control functions in a manner that facilitates other higher order (suprapostural) tasks. These studies, while showing that postural sway is modulated in a task specific manner, have not examined the form of postural coordination that allows for the achievement of these higher behavioral goals. The current study examined the relation between visual task constraints (viewing distance), environmental constraints (changes in the surface of support), and the postural coordination employed to complete the task. Thirty-one participants were asked to perform a reading task while standing on various surfaces. Postural motion was recorded from the head, cervico-thoracic spine, sacrum (hip), and ankle. It was found that body segment coordination changed as a function of surface characteristics and task constraints. Additionally, the overall pattern of postural sway (head motion) replicated that which was found by Stoffregen et al. [J. Exp. Psychol. Human Percep. Perform. 25 (6) (1999) 1641]. These findings suggest that postural adaptation involves more than basic reduction or increase of motion; it involves the functional coordination of body segments to achieve a particular goal. The data further suggest that there is a need to examine postural control in the absence of external perturbations.     

Psychophysics of lateral tachistoscopic presentation

Human visual performance depends upon the retinal position to which a target is delivered. A general finding is that performance measured in a variety of psychophysical tasks deteriorates as a target is presented to more eccentric retinal regions. One purpose of this paper is to describe differences between foveal and peripheral vision in a number of psychophysical tasks. A second purpose is to review studies which have attempted to account for the fall off in visual performance between central and peripheral target presentations. A third purpose is to consider the contribution of the periphery to perception since targets which are sufficiently large project not only on receptors in the fovea but also on those in the periphery. In addition, stimuli presented to the peripheral retina can influence the processing of a target presented to the central retinal region. A fourth purpose is to review studies which have attempted to compensate for foveal and peripheral differences by scaling the target in size or some other attribute in proportion to the cortical magnification factor. A final purpose of this paper is to consider whether the fovea and the periphery are specialized for different functions.