Jeffrey B. Wagman

Haptically Creating Affordances: The User-Tool Interface

Successful use of a hand-held tool requires overcoming the rotational inertia of the hand-plus-tool system. Where an object is grasped affects this rotational inertia. Appropriate choice of grip position may be crucial in the safe, effective, and efficient control of a hand-held tool. In 3 experiments, the authors investigated how choice of grip position on a tool was constrained by task demands. The results suggest that choice of grasp position serves to establish relationships among 3 variables derived from the inertial ellipsoid of the hand-object system (volume, symmetry, and eigenvector angle) in a way that specifically reflected the power or precision constraints of the given task. These variables have previously been shown to play a role in haptic perception of tool function. Changing grasp position on a tool is a way to exert control over the nuances of the user-tool interface.

Attunement, calibration, and exploration in fast haptic perceptual learning.

Abstract Often, a relatively small number of trials suffices to enhance ones task-specific perceptual capability. In the present experiment, fast perceptual learning was investigated with respect to the perception of the heights or widths of wielded nonvisible rectangular objects. In that haptic perceptual task, inertial differences (mass and moments of inertia) are the basis for perceived size differences. The authors hypothesized that rapid improvement might occur in attunement (attending to the task-relevant inertial variable), calibration (scaling spatial extent to the task-relevant inertial variable), and exploratory behavior (wielding so as to differentiate the task-relevant inertial variable). Twenty-four students performed 25 trials with a set of practice objects; those trials were followed and preceded by 18 trials with a set of test objects. Practice, with knowledge of results (KR), improved both attunement, as measured by regression of perceived spatial extent on the inertial variables, and calibration, as measured by constant and variable error. Of the preceding measures, only variable error improved with practice in the absence of KR. In both KR conditions, however, exploratory behavior decreased in duration and complexity, as measured by recurrence quantification analysis. The present results suggest that the mechanisms involved in fast perceptual learning are more varied and complex than are those encompassed by current accounts.

Perceiving Affordances for Aperture Crossing for the Person-Plus-Object System

In 3 experiments we investigated perception of affordances for aperture crossing for the person-plus-object system. Perceivers wielded occluded hand-held objects and determined whether they would be able to carry those objects through an aperture of a particular size. The results suggest (a) perceivers are sensitive to this affordance both when the object is wielded but not visible and when the object is visible but not wielded, (b) perception of affordances for aperture crossing while carrying an object is constrained by the same higher order inertial variable that constrains perception of extent of a hand-held wielded object (Iij ), and (c) perceivers are sensitive to this affordance both when the shoulder width of the participant contributes to the width of the person-plus-object system and when it does not. In general, perceivers treat wielded objects as an extension of their bodies and are sensitive to affordances for the person-plus-object system.

Athletic experience influences shoulder rotations when running through apertures

In order to pass through apertures safely and efficiently, individuals must perceive the width of the aperture relative to (1) the width of the person-plus-object system and to (2) their (anticipated) movement speed. The present study investigated whether athletes who have extensive experience playing sports that require running through narrow spaces while wearing shoulder pads control their shoulder rotations differently while performing this behavior than athletes who lack such experience. Groups of athletes with experience competing in different sports (American football, rugby, and control athletes) performed a behavioral task in which they ran or walked between two tucking dummies with or without wearing shoulder pads. They also performed a psychophysical task in which they reported perceived width of the body and shoulder pads. When running through the apertures, the athletes who played American football exhibited smaller magnitudes and later onset of shoulder rotations than control athletes. No such difference was found when walking through the apertures. There was no difference in perception of the width of the shoulder pads among three groups. These findings suggest that performance of this behavior is action-scaled and task-specific.

Perceptual Behavior: Recurrence Analysis of a Haptic Exploratory Procedure

Various object properties are perceptible by wielding. We asked whether the dynamics of wielding differed as a function of the to-be-perceived property. Wielding motions were analyzed to determine if they differed under the intention to perceive or not perceive rod length (experiment 1), to perceive object height versus object width (experiment 2), and to perceive the length forward of where the rod was grasped versus the position of the grasp (experiment 3). Perceiving these different properties is known to depend on different components of the objects inertia tensor. Analyses of the subtle recurrent patterns in the phase space of the hand motions revealed differences in wielding across the different perceptual intentions. Haptic exploratory procedures may exhibit distinct exploratory dynamics.

Perception of affordances for standing on an inclined surface depends on height of center of mass

We investigated whether perception of affordances for standing on an inclined surface depended on the height of center of mass of the perceiver-actor. Participants adjusted the angle of inclination of a surface until they felt that it was just barely possible for them to stand on that surface. They performed this task while wearing a backpack apparatus to which masses were attached in one of three configurations—high-mass, low-mass, and no-mass. Moreover, participants performed this task by viewing the inclined surface or by probing it with a hand-held rod (while blindfolded). Perception of affordances for standing on the inclined surface reflected the changes in center of mass brought on by the weighted backpack apparatus (the perceptual boundary occurred at a smaller angle of inclination in the high-mass condition than in the low-mass condition and in the no-mass condition). Moreover, perception of this affordance reflected such changes both when the surface was viewed and when the surface was probed with a hand-held rod (while blindfolded). The results highlight that perception of affordances is dynamic and task-dependent and suggest that the stimulation patterns that support perception of affordances are invariant and modality-independent.

Kinetic potential influences visual and remote haptic perception of affordances for standing on an inclined surface

The ability of a perceiver–actor to perform a particular behaviour depends on their ability to generate and control the muscular forces required to perform that behaviour. If an intended behaviour is to be successful, perception must be relative to this ability. We investigated whether perceiver–actors were sensitive to how changes in their mass distribution influenced their ability to stand on an inclined surface. Participants reported whether they would be able to stand on an inclined surface while wearing a weighted backpack on their back, while wearing a weighted backpack on their front, and while not wearing a weighted backpack. In addition, participants performed this task by either viewing the surface or exploring it with a hand-held rod (while blindfolded). The results showed that perception of affordances for standing on the inclined surface depended on how the backpack influenced the ability of the participant to stand on the surface. Specifically, perceptual boundaries occurred at steeper inclinations when participants wore the backpack on their front than when they wore it on their back. Moreover, this pattern occurred regardless of the perceptual modality by which the participants explored the inclined surface.

Perception of Whether an Object Can Be Carried Through an Aperture Depends on Anticipated Speed

We investigated whether anticipated speed of locomotion through an aperture influences perception of whether an object can be carried through that aperture. Participants reported whether they would be able to carry objects through an aperture (a) if they were to attempt to walk through the aperture and (b) if they were to attempt to run through the aperture. Furthermore, they did so when the object was held but not seen and when the object was seen but not held. In general, perception was influenced by object width and by anticipated speed but not by perceptual modality. Perceptual boundaries occurred at smaller object widths when participants anticipated running through the aperture than when they anticipated walking through the aperture. The results build on work showing that perception of affordances is influenced by kinetic potential as well as geometric properties and that perception may be supported by the detection of modality-neutral stimulation patterns.

Geometric, Kinetic-Kinematic, and Intentional Constraints Influence Willingness to Pass Under a Barrier

Completing a goal directed behavior in a safe and efficient manner requires that a perceiver-actor is sensitive to the various constraints on performing that behavior and adjust his or her movements accordingly. When attempting to pass under a barrier, people adjust their ducking behavior based on the likelihood and potential costs of a collision (van der Meer, 1997). In three experiments, we investigated whether participants are sensitive to geometric (standing height), kinetic-kinematic (anticipated movement speed), and intentional (material properties of the barrier) constraints on passing under a barrier even before attempting to perform this behavior. Although Experiment 1 failed to show that anticipated movement speed influenced perception of whether a barrier could be passed under, Experiment 2 found that this factor influences willingness to attempt the behavior. Experiments 3a and 3b found that the material properties of the barrier itself also influence willingness to attempt the behavior. Together, the results highlight the contribution of geometric, kinetic-kinematic, and intentional constraints to perception.

Perception of Affordances for Walking Under a Barrier From Proximal and Distal Points of Observation

We investigated perception of affordances for a given behavior from points of observation other than that from which the behavior is to be performed. Participants viewed a horizontal barrier from different points of observation and reported whether they would be able to walk under that barrier. In Experiment 1, they viewed the barrier both while standing on the floor and while sitting on the floor. In Experiment 2, they viewed the barrier both while standing on the floor and while standing on a step stool. The results show that although perception of this affordance is body-scaled regardless of point of observation, there are subtle differences in the perceptual boundaries in each case. Such differences are likely due to differential attunement to the information specifying the affordance at each point of observation.