Thomas A. Stoffregen

Affordances as Properties of the Animal-Environment System

In this article, I argue that affordances are properties of the animal-environment system, that is, that they are emergent properties that do not inhere in either the environment or the animal. I critique and review the formal definition of affordance offered by Turvey (1992). Turvey defined affordances as properties of the environment; I discuss some consequences of this and argue that Turveys strategy of grounding the definition of affordance in terms of dispositional properties is problematic. I also suggest that Turveys definition of affordance may lead to problems for the specification and direct perception of affordances. Motivated by these problems, I propose a new definition of affordance, in which affordances are properties of the animal-environment system. This definition does not rely on the concept of dispositional properties and is consistent with direct perception.

Modulating postural control to facilitate visual performance

Abstract We explored relations between visual performance and postural control. Variability in postural sway was analyzed in the context of variations in supra-postural visual tasks. We varied target distance (near vs. far) and visual task (inspecting a blank target vs. counting the frequency of letters in a block of text). Variability in postural sway was reduced when participants fixated near targets as opposed to far targets. Also, postural sway during the visual search task was reduced relative to sway during inspection of blank targets. We argue that the search task placed more restrictive constraints on the visual system, and that postural sway was reduced to facilitate visual search. The results support the hypothesis that postural control is not an autonomous system, but is organized as part of an integrated perception–action system. Postural control can be used to improve visual performance.

Postural coordination modes considered as emergent phenomena.

The coordination of multiple body segments (torso and legs) in the control of standing posture during a suprapostural task was studied. The analysis was motivated by dynamical theories of motor coordination. In 2 experiments it was found that multisegment postural coordination could be described by the relative phase of rotations around the hip and ankle joints. The effective length of the feet, the height of the center of mass, and the amplitude of head motions in a visual tracking task were varied. Across these variations, 2 modes of hip-ankle coordination were observed: in-phase and anti-phase. The emergence of these modes was influenced by constraints imposed by the suprapostural tracking task, supporting the idea that such tasks influence postural control in an adaptive manner. Results are interpreted in terms of a dynamical approach to coordination in which postural coordination modes can be viewed as emergent phenomena.

Postural stabilization for the control of touching

Abstract We attempted to distinguish between task-related (supra-postural) and perceptual influences on postural motions. Two groups of participants had to make very light tactile contact with an adjacent pliable surface while standing with their eyes closed. In the absence of vision, such light touching with a finger is known to reduce sway. For one group, tactile contact with the surface was merely the result of extending the right forearm. For the other group, variability in the point of tactile contact had to be kept to a minimum. Touching reduced postural sway relative to non-touching only for participants in the latter group. The present results, in combination with others addressing similar task differences, question the assumption that information detected haptically and/or visually is used solely to reduce postural fluctuations. It seems that postural fluctuations are modulated to facilitate performance of tasks over and above the task of standing upright and still.

Dynamics of human postural transitions.

In the present study, the authors examined transitions between postural coordination modes involved in human stance. The analysis was motivated by dynamical theories of pattern formation, in which coordination modes and transitions between modes are emergent, self-organized properties of the dynamics of animal-environment systems. In 2 experiments, standing participants tracked a moving target with the head. Results are consistent with the hypothesis that changes in body coordination follow typical nonequilibrium phase transitions, exhibiting multistability, bifurcation, critical fluctuations, hysteresis, and critical slowing down. The findings suggest that posture may be organized in terms of dynamical principles and favor the existence of general and common principles governing pattern formation and flexibility in complex systems.

Postural Sway during Dual Tasks in Young and Elderly Adults

Background: Previous studies have shown that healthy young adults are able to decrease their standing postural sway when an additional postural visual task, such as reading, is performed. Objective: In this study, we investigated postural sway during dual tasks in young and elderly adults. Methods: Twelve healthy active elderly adults (aged 65–75 years) and 12 healthy young adults (aged 22–39 years) participated in the study. The subjects performed different visual tasks while standing on a force plate. We varied the nature of the visual tasks (looking at a blank target versus a visual search task) and the distance of visual targets (near versus far). Center of pressure displacement obtained from the force plate data and kinematics of body segments obtained from a video analysis system were investigated. Results: Both groups presented significantly larger postural sway in the mediolateral direction during the eyes-closed condition as compared with the eyes-open condition. In the anteroposterior direction, this effect was observed only for the elderly group. Both groups had the same percentage correct in counting letters, but the elderly adults were significantly slower as compared with the young adults. The amplitude of postural sway was greater for the elderly adults than for the young adults in all conditions. However, both the young and the elderly adults exhibited significant reductions in sway during performance of the search task relative to sway during viewing of a blank target. The sway was also reduced for both groups during viewing a near target when compared to a distant target. Conclusions: The results suggest that, despite the overall increase in postural sway with aging, subtle integration of visual information by the postural control system is not affected by aging. The present results support the idea that dual tasks do not necessarily lead to an increase in postural sway. This effect, found here in elderly adults, raises questions about widely held views in which age-related changes in postural sway are related to competition between postural control and other activities for central processing resources.

Affordances and Events

Affordance is a fundamental concept in the ecological approach to perception and action, but it is not yet fully developed. In this article, I attempt to further the development of the concept by contrasting it with the concept of events. My specific purpose is to raise for further discussion the following question: Can a theory that predicts the perception of affordances also predict the perception of events? I argue that affordances and events are not identical and that they differ qualitatively. I also discuss what I call mutuality relations between the animal and its environment, which have inspired much research on perception-action couplings (e.g., the perception and control of interceptive action, or the guidance of locomotion). There has been little direct discussion of relations between perception-action coupling and affordances. I suggest that mutuality relations are not affordances and that they may be a category of events. As an example of this argument, I reinterpret common analyses of time-to-contact in terms of affordances for interceptive action. I argue that the utility of affordance perception is clear but that it is uncertain how an ecological theory could predict the perception of events. I conclude with a brief discussion of some additional issues that remain to be resolved in the concept of affordances.

Postural Instability and Motion Sickness in a Fixed-Base Flight Simulator

We evaluated the prediction that postural instability would precede the subjective symptoms of motion sickness in a fixed-base flight simulator. Participants sat in a cockpit in a video projection dome and were exposed to optical flow that oscillated in the roll axis with exposure durations typical of flight simulation. The frequencies of oscillation were those that characterize spontaneous postural sway during stance. Head motion was measured prior to and during exposure to imposed optical flow. Of 14 participants, 6 were classified as motion sick, either during or after exposure to the optical oscillation. Prior to the onset of subjective symptoms, head motion among participants who later became sick was significantly greater than among participants who did not become motion sick. We argue that the results support the postural instability theory of motion sickness. Actual or potential applications include the prevention or mitigation of motion sickness in virtual environments.

Visually Induced Motion Sickness Predicted by Postural Instability

We investigated whether postural instability can predict motion sickness and studied relations among instability, motion sickness, and vection. Nine men and 4 women (mean age = 19.85 years) were exposed, while standing, to an optical simulation of body sway. Head motion was recorded using a magnetic tracking system. Postural instabilities were observed prior to the onset of motion sickness. Vection was reported by most participants, including all who became ill. A discriminant analysis revealed that parameters of postural motion accurately predicted motion sickness. The results confirm that postural instability precedes motion sickness and suggest that measures of postural motion may serve as reliable predictors of motion sickness. Potential applications of this research include the development of on-line diagnostic tools that will allow for the prevention of motion sickness in operational and training settings.

The role of balance dynamics in the active perception of orientation

The Stoffregen and Riccio (1988) hypothesis that perceived orientation is determined primarily by balance dynamics was tested. Perception of orientation was evaluated in the context of a task that required Ss to control the roll orientation of a device in which they were seated. The devices direction of balance was manipulated across trials and thus was independent of gravity. Eighteen Ss participated in the investigation. After each trial, Ss estimated their mean tilt with respect to upright. Correlations of perceived tilt with tilt from balance were consistently higher than the correlations with gravity tilt. The dominance of balance over gravity depended on the magnitude of tilt from balance.