Studies in Perception and Action III

Benoit G. Bardy, Reinoud J. Bootsma, Yves Guiard, BenoŒt G. Bardy

• Publisher: Psychology Press
• Year: 2019
• Print ISBN: 9780805818673
• eBook ISBN: 9781317729280
• Topic: Psychology
• Subtopic: Experimental Psychology
• Index: Psychology

This volume, a posterbook based on the seventh biennial Conference of the International Society for Ecological Psychology, is a collection of compact empirical and/or theoretical articles on the study of perception and action.

SECTION II. ACTION-PERCEPTION COUPLING

II.1. Affordances

Perceiving the Preferred Critical Boundary for an Affordance

Leonard S. Mark

Department of Psychology, Miami University, Oxford, OH, U.S.A.

Most studies of affordances have focused on the perception of the critical boundary, the absolute limit of the relevant environmental property beyond which a phase transition to a different type of action is required. Investigators have reported a small, but consistent, discrepancy between observers’ perceived and actual capabilities. Research (e.g., Mark, 1987; Jiang & Mark, 1994) has found that observers underestimated their own critical action boundaries by 8–15% for bipedal climbing, sitting, and stepping across a gap. This tendency may reflect a conservative bias in judging one’s action capabilities near the critical boundary (Mark, 1987); the actor leaves a margin of safety to reduce the risk associated with failing to complete the action.

This paper examines a second factor: As the critical boundary for an action is approached, the biodynamic costs associated with performing the action increase sharply. For example, considering a surface on which an actor is capable of climbing bipedally, the height of the support surface may require extreme limb movement (in order to place the lead foot on the upper surface of support) and muscular exertion (to raise the actor’s center of mass above the surface of support). As the relative cost for one action (bipedal gait) rises, the relative cost for the other action (quadrupedal gait) decreases. Greater efficiency and comfort would obtain if the phase transition takes place at a more optimal point, where the relative costs associated with the two possible actions are more nearly equivalent, a preferred critical boundary.

The reported discrepancy between actors’ perceptual judgments and their actual absolute capabilities may indicate that actors’ judgments refer to such a preferred critical boundary for (making) the transition from one type of action to another, rather than the absolute limit of their capabilities. The current study attempts to distinguish between the absolute and preferred critical boundaries by measuring: (1) actors’ judgments of their absolute critical boundaries, (2) actors’ absolute maximum action capabilities, and (3) the location of phase transitions when actors are allowed to choose the type of action used to realize a goal.

Three phase transitions will be studied: bipedal to quadrupedal climbing as the height of the support surface increases, sitting by lowering one’s weight onto the support surface to boosting oneself up as surface height increases, and stepping across a distance on the ground plane to leaping across as distance increases. If actors’ perceptual judgments of their capabilities reflect a preferred critical point for the phase transition, their judgments should closely correspond to measures of their capabilities when they are able to choose the type of action for realizing the goal (i.e., sit on, climb on, or step across).

Method

Six female students participated in each action in three separate sessions. Because of the physical exertion required to perform each action, participants came directly from an aerobics class in which they had been physically active for 30 minutes.

Sitting

A 33 × 28 cm surface that could be continuously raised and lowered in the range 50–95 cm was used (Mark, 1987). Perceptual judgments were obtained using a method of limits with six ascending and six descending judgments. The average of the six pairs of judgments will be reported. The actor stood at a distance of 2 m from the apparatus; the seat was raised (or lowered) until the actor judged it to be at the maximum height she could just sit on without having to boost herself up onto the seat with her hands (Mark, 1987). After making judgments of her maximum sitting height, the actor actually sat on the apparatus using any type of action she chose. Eleven heights were used ranging from 0.5 to 1.0 of the actor’s leg length (L) in increments of 0.05 L. Each height was presented three times in random order. Two experimenters observed each action and judged whether the sitting action involved simply lowering her weight onto the surface of support or whether the actor had to boost herself up using her hands. (There was 98% agreement between the judges.) Finally, Mark’s (1987) procedure was used to determine the actor’s absolute maximum sitting capability.

Climbing

The same procedures were followed as in the study of sitting. A larger surface, 75 × 40 cm, was attached to the apparatus so that the surface could be climbed on. There was 100% agreement between the two experimenters.

Stepping

Perceptual judgments of the widest distance an actor thought she could step across were obtained by having the observer stand on the ground plane just behind a tape mark. A 1 × 4 × 82 cm board, placed parallel to the tape mark, was moved toward (or away) from the observer. Observers indicated when the distance between the tape mark and the edge of the board was at the maximum they could just step across without leaping. The procedure was otherwise similar to that used for the other actions. Actors were also asked to step across 11 distances, ranging from 0.6 to 1.6 of their L in increments of 0.1 L. Actors had to cover the distance in a single step using an action of their own choice. Two experimenters judged whether the action was stepping (one foot was always in contact with the ground) or leaping (both feet were simultaneously off the ground); agreement exceeded 99%. Finally, maximum stepping distance was measured by having actors step as far as they could on the ground.

Results and Discussion

Perceived and preferred critical boundaries were determined by calculating the percentage of chances at each height/distance that each observer either judged she could perform, or actually performed, the action in the prescribed manner: For sitting, the actor had to place her weight on the seat without boosting herself up with her hands; for climbing, use a bipedal gait (no hands); and for stepping, at least one foot had to remain in contact with the ground (no leaping). The height or distance at which the resultant function passed through the 50% mark was taken as the measure of the phase transition.

For each action the mean absolute critical boundary was significantly greater than the mean perceived critical height or distance (Table 1), replicating the outcome of previous work. However, when actors could choose the type of action for attaining the goal, the location of the preferred phase transition between the two actions corresponded closely to their perceptual judgments.

Table 1
Mean (SEM) Critical Boundary for the Three Phase Transitions Given in Units of Mean Leg Length (L)

Critical Boundary	Sitting	Climbing	Stepping
Absolute	0.830 L (0.009)	0.865 L (0.009)	1.402 L (0.017)
Perceived	0.746 L (0.012)	0.768 L (0.008)	1.183 L (0.011)
Preferred	0.749 L (0.010)	0.778 L (0.011)	1.191 L (0.019)

This finding is consistent with our proposal that perceptual judgments of affordances may reflect the actor’s preferred value of the relevant environmental property at which to make the transition from one type of action to another. (We believe that our participants did not understand the action on which they were to base their judgments: The instructions clearly stressed absolute capabilities, and participants appeared surprised by the magnitude with which they underestimated their absolute capabilities.) This preferred critical boundary may be indicative of biodynamic efficiency and comfort with which each of the actions can be performed. Affordance research should examine the relative biodynamic efficiency and comfort of different types of actions near the critical boundary.

References

Jiang, Y., & Mark, L. S. (1994). The effect of gap depth on the perception of whether a gap is crossable. Perception and psychophysics, 56, 691–700.

Mark, L. S. (1987). Eyeheight-scaled information about affordances: A study of sitting and stair climbing. Journal of Experimental Psychology: Human Perception and Performance, 13, 361–370.

Intentional Perceptual Scaling Means π = 1

Oded M. Flascher, Robert E. Shaw, Endre E. Kadar, and Thomas A. Aromin

CESPA, University of Connecticut, Storrs, CT, U.S.A.

Affordances are opportunities for action (Gibson, 1979). Dually, effectivities are means for realizing affordance goals (Shaw, Turvey, & Mace, 1982). The role of perception in prospective control involves scaling effectivities to their affordance goals so the ratio intended affordance goal/effectivity for attaining goal = π = 1.0 (otherwise there is error in action control, information detection, or both). Thus this ecological π (dimensionless) number is obtained by scaling the intended affordance to the relevant successful effectivity; both must be written in the same language – that of action (a goal-directed behavior) – if dimensional homogeneity is to hold. Successful prospective control not only depends on perceiving the intended affordance-goal and assembling the intended effectivity-means, but also on detecting information for how to act so that their ratio yields a dimensionless (ecological) π number that equals unity (within a tolerance limit).

Previous research on affordance perception has revealed a diverse set of π numbers of unequal value (for a review, see Shaw, Flascher, & Kadar, in press). Such dimensionless numbers have been based on body-scale (geometric π numbers), biomechanics (kinematic π numbers), or bioenergetics (kinetic π numbers). Recently, we introduced an additional dimensionless number that is based on intentional dynamics (ecological π numbers) (Shaw et al., in press). Whenever the action intended is successful, these latter π numbers are hypothesized to be of equal value, namely, renormalizable to π = 1.

Method

Three able-body persons were trained to use a wheelchair in routine daily activities and specific tasks over a period of 8–10 weeks. In phase 1 the participants were placed 3 eye-heights from a gap between two boxes. Using the limits method, they were asked to judge the minimal gap-size under three conditions: absolute minimal gap-size passable, minimal gap-size for a comfortable wheeling speed, and minimal gap-size for maximum speed. In phase 2, we calculated average judged size for each task type (intention) and presented gaps that were smaller (-2, -4, -6 cm), the same, or larger (+2, +4, +6 cm) than the size each individual judged as minimal for the intended tasks. Participants were positioned 13 m away from a...

Table of contents

• Cover
• Half Title
• Title Page
• Copyright Page
• Table of Contents
• Preface
• Contributors
• Section I. Action
• Section II. Action - Perception Coupling
• Section III. Perception
• Author Index
• Keyword Index