Frank H. Durgin

Who Is Being Deceived? The Experimental Demands Of Wearing A Backpack

A growing literature argues that wearing a heavy backpack makes slopes look steeper and distances seem longer (e.g., Proffitt, 2006). To test for effects of experimental demand characteristics in a backpack experiment, we manipulated the experimental demand of the backpack and then used a postexperiment questionnaire to assess participants’ beliefs about the purpose of the backpack. For participants in the low-demand condition, an elaborate deception was used to provide an alternative explanation of the requirement to wear a heavy backpack (i.e., that it held EMG equipment). The highest slope judgments were found for those undeceived participants who guessed that the backpack was intended to affect their slope perception and also reported that they thought they were affected by it. When persuaded that the backpack served another purpose, participants’ slope estimates were no different from those of participants not wearing a backpack. These findings suggest that backpack effects, and other reported effects of effort on perception, are judgmental biases that result from the social, not physical, demands of the experimental context.

Distance Perception and the Visual Horizon in Head-Mounted Displays

Can distance perception be studied using virtual reality (VR) if distances are systematically underestimated in VR head-mounted displays (HMDs)? In an experiment in which a real environment was observed through an HMD, via live video, distances, as measured by visually directed walking, were underestimated even when the perceived environment was known to be real and present. However, the underestimation was linear, which means that higher-order space perception effects might be preserved in VR. This is illustrated in a second experiment, in which the visual horizon was artificially manipulated in a simulated outdoor field presented in immersive VR. As predicted by the claim that angle of declination from the horizon may serve as a strong cue to distance, lowering the horizon line produced “expansiv ” judgments of distance (power function exponents greater than one) both in verbal and in motor estimates.

Texture density adaptation and visual number revisited

Summary Burr and Ross [1] have recently proposed that the visual dimension of number is itself directly adaptable. The aftereffect they describe is one that my colleagues and I previously used to investigate the perception of texture density [2–4]. Burr and Ross [1] argue that the effect is new because it concerns visual number, rather than texture density, but they did not report critical tests to support this claim. Here, I shall briefly describe the striking similarity between our prior work and that of Burr and Ross [1], and discuss how some of our results rule out Burr and Rosss [1] interpretation that numerosity, and not density, is at play. I shall also provide a new demonstration that confirms that these effects are based on density, using a display that explicitly dissociates density from numerosity. Taken together, this line of arguments suggests that Burr and Rosss [1] recent study may best be thought of as replicating support within a well-established line of work on texture density.

The perception of walking speed in a virtual environment

Studies of locomotion in virtual environments assume that correct geometric principles define the relationship between walking speed and environmental flow. However, we have observed that geometrically correct optic flow appears to be too slow during simulated locomotion on a treadmill. Experiment 1 documents the effect in a head-mounted display. Experiment 2 shows that the effect is eliminated when the gaze is directed downward or to the side, or when the walking speed is slow. Experiment 3 shows that the effect is unchanged by stride length. Experiment 4 verifies that the effect is not attributable to image jitter. The change in perceived speed from straight ahead to side or down gaze coincides with a shift from expanding optic flow to lamellar flow. Therefore, we hypothesize that lamellar flow is necessary for accurate speed perception, and that a limited field of view eliminates this cue during straight-ahead gaze.

Rubber Hands Feel the Touch of Light

Two experiments involving a total of 220 subjects are reported. The experiments document that “stroking” a false hand with the bright beam of light from a laser pointer can produce tactile and thermal sensations when the hand can be seen as ones own. Overall, 66% of subjects reported somatic sensations from the light. Felt hand location was recalibrated toward the location of the false hand for those subjects who felt the light. Moreover, the proprioceptive recalibration from the laser experience was comparable to that produced by actual coordinated brushing of the false hand and of the unseen real hand after 2 min of stimulation. The illusion may be experienced on ones real hand as well. The results are discussed in terms of multisensory integration.

Perception of Visual Speed While Moving.

During self-motion, the world normally appears stationary. In part, this may be due to reductions in visual motion signals during self-motion. In 8 experiments, the authors used magnitude estimation to characterize changes in visual speed perception as a result of biomechanical self-motion alone (treadmill walking), physical translation alone (passive transport), and both biomechanical self-motion and physical translation together (walking). Their results show that each factor alone produces subtractive reductions in visual speed but that subtraction is greatest with both factors together, approximating the sum of the 2 separately. The similarity of results for biomechanical and passive self-motion support H. B. Barlows (1990) inhibition theory of sensory correlation as a mechanism for implementing H. Wallachs (1987) compensation for self-motion.

Self-motion perception during locomotor recalibration: more than meets the eye.

Do locomotor aftereffects depend specifically on visual feedback? In 7 experiments, 116 college students were tested, with closed eyes, at stationary running or at walking to a previewed target after adaptation, with closed eyes, to treadmill locomotion. Subjects showed faster inadvertent drift during stationary running and increased distance (overshoot) when walking to a target. Overshoot seemed to saturate (i.e., reach a ceiling) at 17% after as little as 1 min of adaptation. Sidestepping at test reduced overshoot, suggesting motor specificity. But inadvertent drift effects were decreased if the eyes were open and the treadmill was drawn through the environment during adaptation, indicating that these effects involve self-motion perception. Differences in expression of inadvertent drift and of overshoot after adaptation to treadmill locomotion may have been due to different sets of ancillary cues available for the 2 tasks. Self-motion perception is multimodal.

The social psychology of perception experiments: hills, backpacks, glucose, and the problem of generalizability.

Experiments take place in a physical environment but also a social environment. Generalizability from experimental manipulations to more typical contexts may be limited by violations of ecological validity with respect to either the physical or the social environment. A replication and extension of a recent study (a blood glucose manipulation) was conducted to investigate the effects of experimental demand (a social artifact) on participant behaviors judging the geographical slant of a large-scale outdoor hill. Three different assessments of experimental demand indicate that even when the physical environment is naturalistic, and the goal of the main experimental manipulation was primarily concealed, artificial aspects of the social environment (such as an explicit requirement to wear a heavy backpack while estimating the slant of a hill) may still be primarily responsible for altered judgments of hill orientation.

Palm boards are not action measures: An alternative to the two-systems theory of geographical slant perception

Whereas most reports of the perception of outdoor hills demonstrate dramatic overestimation, estimates made by adjusting a palm board are much closer to the true hill orientation. We test the dominant hypothesis that palm board accuracy is related to the need for motor action to be accurately guided and conclude instead that the perceptual experience of palm-board orientation is biased and variable due to poorly calibrated proprioception of wrist flexion. Experiments 1 and 3 show that wrist-flexion palm boards grossly underestimate the orientations of near, reachable surfaces whereas gesturing with a free hand is fairly accurate. Experiment 2 shows that palm board estimates are much lower than free hand estimates for an outdoor hill as well. Experiments 4 shows that wrist flexion is biased and noisy compared to elbow flexion, while Experiment 5 shows that small changes in palm board height produce large changes in palm board estimates. Together, these studies suggest that palm boards are biased and insensitive measures. The existing literature arguing that there are two systems in the perception of geographical slant is re-evaluated, and a new theoretical framework is proposed in which a single exaggerated representation of ground-surface orientation guides both action and perception.

Comparing Depth From Motion With Depth From Binocular Disparity

The accuracy of depth judgments that are based on binocular disparity or structure from motion (motion parallax and object rotation) was studied in 3 experiments. In Experiment 1, depth judgments were recorded for computer simulations of cones specified by binocular disparity, motion parallax, or stereokinesis. In Experiment 2, judgments were recorded for real cones in a structured environment, with depth information from binocular disparity, motion parallax, or object rotation about the y-axis. In both of these experiments, judgments from binocular disparity information were quite accurate, but judgments on the basis of geometrically equivalent or more robust motion information reflected poor recovery of quantitative depth information. A 3rd experiment demonstrated stereoscopic depth constancy for distances of 1 to 3 m using real objects in a well-illuminated, structured viewing environment in which monocular depth cues (e.g., shading) were minimized.