Dennis R. Proffitt

Embodied Perception and the Economy of Action

Perception informs people about the opportunities for action and their associated costs. To this end, explicit awareness of spatial layout varies not only with relevant optical and ocular-motor variables, but also as a function of the costs associated with performing intended actions. Although explicit awareness is mutable in this respect, visually guided actions directed at the immediate environment are not. When the metabolic costs associated with walking an extent increase—perhaps because one is wearing a heavy backpack—hills appear steeper and distances to targets appear greater. When one is standing on a high balcony, the apparent distance to the ground is correlated with ones fear of falling. Perceiving spatial layout combines the geometry of the world with behavioral goals and the costs associated with achieving these goals.

Visual-motor recalibration in geographical slant perception

In 4 experiments, it was shown that hills appear steeper to people who are encumbered by wearing a heavy backpack (Experiment 1), are fatigued (Experiment 2), are of low physical fitness (Experiment 3), or are elderly and/or in declining health (Experiment 4). Visually guided actions are unaffected by these manipulations of physiological potential. Although dissociable, the awareness and action systems were also shown to be interconnected. Recalibration of the transformation relating awareness and actions was found to occur over long-term changes in physiological potential (fitness level, age, and health) but not with transitory changes (fatigue and load). Findings are discussed in terms of a time-dependent coordination between the separate systems that control explicit visual awareness and visually guided action.

The Role of Effort in Perceiving Distance

Berkeley proposed that space is perceived in terms of effort. Consistent with his proposal, the present studies show that perceived egocentric distance increases when people are encumbered by wearing a heavy backpack or have completed a visual-motor adaptation that reduces the anticipated optic flow coinciding with walking effort. In accord with Berkeleys proposal and Gibsons theory of affordances, these studies show that the perception of spatial layout is influenced by locomotor effort.

Perceiving geographical slant.

People judged the inclination of hills viewed either out-of-doors or in a computer-simulated virtual environment. Angle judgments were obtained by having people (1) provide verbal estimates, (2) adjust a representation of the hill’s cross-section, and (3) adjust a tilt board with their unseen hand. Geographical slant was greatly overestimated according to the first two measures, but not the third. Apparent slant judgments conformed to ratio scales, thereby enhancing sensitivity to the small inclines that must actually be traversed in everyday experience. It is proposed that the perceived exaggeration of geographical slant preserves the relationship between distal inclination and people’s behavioral potential. Hills are harder to traverse as people become tired; hence, apparent slant increased with fatigue. Visually guided actions must be accommodated to the actual distal properties of the environment; consequently, the tilt board adjustments did not reflect apparent slant overestimations, nor were they influenced by fatigue. Consistent with the fact that steep hills are more difficult to descend than to ascend, these hills appeared steeper when viewed from the top.

Tool Use Affects Perceived Distance, But Only When You Intend to Use It

Recent research demonstrates neurologic and behavioral differences in peoples responses to the space that is within and beyond reach. The present studies demonstrated a perceptual difference as well. Reachability was manipulated by having participants reach with and without a tool. Across 2 conditions, in which participants either held a tool or not, targets were presented at the same distances. Perceived distances to targets within reach holding the tool were compressed compared with targets that were beyond reach without it. These results suggest that reachability serves as a metric for perception. The 3rd experiment found that reachability only influenced perceived distance when the perceiver intended to reach. These experiments suggest that the authors perceive the environment in terms of our intentions and abilities to act within it.

Quantifying immersion in virtual reality

Virtual Reality (VR) has generated much excitement but little formal proof that it is useful. Because VR interfaces are difficult and expensive to build, the computer graphics community needs to be able to predict which applications will benefit from VR. In this paper, we show that users with a VR interface complete a search task faster than users with a stationary monitor and a hand-based input device. We placed users in the center of the virtual room shown in Figure 1 and told them to look for camouflaged targets. VR users did not do significantly better than desktop users. However, when asked to search the room and conclude if a target existed, VR users were substantially better at determining when they had searched the entire room. Desktop users took 41% more time, re-examining areas they had already searched. We also found a positive transfer of training from VR to stationary displays and a negative transfer of training from stationary displays to VR.

Perceiving distance: A role of effort and intent

Perceiving egocentric distance is not only a function of the optical variables to which it relates, but also a function of peoples current physiological potential to perform intended actions. In a set of experiments, we showed that, as the effort associated with walking increases, perceived distance increases if the perceiver intends to walk the extent, but not if the perceiver intends to throw. Conversely, as the effort associated with throwing increases, perceived distance increases if people intend to throw to the target, but not if they intend to walk. Perceiving distance combines the geometry of the world with our behavior goals and the potential of our body to achieve these goals.

Infant sensitivity to figural coherence in biomechanical motions

Two experiments assessed infant sensitivity to figural coherence in point-light displays moving as if attached to the major joints of a walking person. Experiment 1 tested whether 3- and 5-month-old infants could discriminate between upright and inverted versions of the walker in both moving and static displays. Using an infant-control habituation paradigm, it was found that both ages discriminated the moving but not the static displays. Experiment 2 was designed to clarify whether or not structural invariants were extracted from these displays. The results revealed that (1) moving point-light displays with equivalent motions but different topographic relations were discriminated while (2) static versions were not, and (3) arrays that varied in the amount of motion present in different portions of the display were also not discriminated. These results are interpreted as indicating that young infants are sensitive to figural coherence in displays of biomechanical motion.

Two-handed virtual manipulation

We discuss a two-handed user interface designed to support three-dimesional neurosurgical visualization. By itself, this system is a “point design,” an example of an advanced user interface technique. In this work, we argue that in order to understand why interaction techniques do or do not work, and to suggest possibilities for new techniques, it is important to move beyond point design and to introduce careful scientific measurement of human behavioral principles. In particular, we argue that the common-sense viewpoint that “two hands save time by working in parallel” may not always be an effective way to think about two-handed interface design because the hands do not necessarily work in parallel (there is a structure to two-handed manipulation) and because two hands do more than just save time over one hand (two hands provide the user with more information and can structure how the user thinks about a task). To support these claims, we present an interface design developed in collaboration with neurosurgeons which has undergone extensive informal usability testing, as well as a pair of formal experimental studies which investigate behavioral aspects of two-handed virtual object manipulation. Our hope is that this discussion will help others to apply the lessons in our neurosurgery application to future two-handed user interface designs.

Usability analysis of 3D rotation techniques

We report results from a formal user study of interactive 3D rotation using the mouse-driven Virtual Sphere and Arcball techniques, as well as multidimensional input techniques based on magnetic orientation sensors. MultidimensionaI input is often assumed to allow users to work quickly, but at the cost of precision, due to the instability of the hand moving in the open air. We show that, at least for the orientation matching task used in this experiment, users can take advantage of the integrated degrees of freedom provided by multidimensional input without necessarily sacrificing precision: using multidimensional input, users completed the experimental task up to 36% faster without any statistically detectable loss of accuracy. We also report detailed observations of common usability problems when first encountering the techniques. Our observations suggest some design issues for 3D input devices. For example, the physical form-factors of the 3D input device significantly influenced user acceptance of otherwise identical input sensors. The device should afford some tactile cues, so the user can feel its orientation without looking at it. In the absence of such cues, some test users were unsure of how to use the device.