Steven A. Cholewiak

A Frequency-Domain Analysis of Haptic Gratings

The detectability and discriminability of virtual haptic gratings were analyzed in the frequency domain. Detection (Exp. 1) and discrimination (Exp. 2) thresholds for virtual haptic gratings were estimated using a force-feedback device that simulated sinusoidal and square-wave gratings with spatial periods from 0.2 to 38.4 mm. The detection threshold results indicated that for spatial periods up to 6.4 mm (i.e., spatial frequencies >0.156 cycle/mm), the detectability of square-wave gratings could be predicted quantitatively from the detection thresholds of their corresponding fundamental components. The discrimination experiment confirmed that at higher spatial frequencies, the square-wave gratings were initially indistinguishable from the corresponding fundamental components until the third harmonics were detectable. At lower spatial frequencies, the third harmonic components of square-wave gratings had lower detection thresholds than the corresponding fundamental components. Therefore, the square-wave gratings were detectable as soon as the third harmonic components were detectable. Results from a third experiment where gratings consisting of two superimposed sinusoidal components were compared (Exp. 3) showed that people were insensitive to the relative phase between the two components. Our results have important implications for engineering applications, where complex haptic signals are transmitted at high update rates over networks with limited bandwidths.

Discrimination of Real and Virtual Surfaces with Sinusoidal and Triangular Gratings Using the Fingertip and Stylus

Two-interval two-alternative forced-choice discrimination experiments were conducted separately for sinusoidal and triangular textured surface gratings from which amplitude (i.e., height) discrimination thresholds were estimated. Participants (group sizes: n = 4 to 7) explored one of these texture types either by fingertip on real gratings (Finger real), by stylus on real gratings (Stylus real), or by stylus on virtual gratings (Stylus virtual). The real gratings were fabricated from stainless steel by an electrical discharge machining process while the virtual gratings were rendered via a programmable force-feedback device. All gratings had a 2.5-mm spatial period. On each trial, participants compared test gratings with 55, 60, 65, or 70 μm amplitudes against a 50-μm reference. The results indicate that discrimination thresholds did not differ significantly between sinusoidal and triangular gratings. With sinusoidal and triangular data combined, the average (mean + standard error) for the Stylus-real threshold (2.5 ± 0.2 μm) was significantly smaller (p <; 0.01) than that for the Stylus-virtual condition (4.9 ± 0.2 μm). Differences between the Finger-real threshold (3.8 ± 0.2 μm) and those from the other two conditions were not statistically significant. Further studies are needed to better understand the differences in perceptual cues resulting from interactions with real and virtual gratings.

Intra- and Intermanual Curvature Aftereffect Can Be Obtained via Tool-Touch

We examined the perception of virtual curved surfaces explored with a tool. We found a reliable curvature aftereffect, suggesting neural representation of the curvature in the absence of direct touch. Intermanual transfer of the aftereffect suggests that this representation is somewhat independent of the hand used to explore the surface.

Frequency Analysis of the Detectability of Virtual Haptic Gratings

The tactile detectability of sinusoidal and square-wave virtual texture gratings were measured and analyzed. Using a three-interval one-up three-down adaptive tracking procedure, detection thresholds for virtual gratings were estimated using a custom-designed high position-resolution 3-degrees-of-freedom force-feedback haptic device. Two types of gratings were used, defined by sinusoidal and square waveforms, with spatial wavelengths of 0.2 to 25.6 mm. The results indicated that the participants demonstrated a higher sensitivity (i.e., lower detection threshold) to square-wave gratings than to sinusoidal ones at all the wavelengths tested. When the square-wave gratings were represented by the explicative Fourier series, it became apparent that the detectability of the square-wave gratings could be determined by that of the sinusoidal gratings at the corresponding fundamental frequencies. This was true for any square-wave grating as long as the detection threshold for the fundamental component was below those of the harmonic components

Perception of intentions and mental states in autonomous virtual agents

Comprehension of goal-directed, intentional motion is an important but understudied visual function. To study it, we created a two-dimensional virtual environment populated by independently-programmed autonomous virtual agents, which navigate the environment, collecting food and competing with one another. Their behavior is modulated by a small number of distinct “mental states”: exploring, gathering food, attacking, and fleeing. In two experiments, we studied subjects’ ability to detect and classify the agents’ continually changing mental states on the basis of their motions and interactions. Our analyses compared subjects’ classifications to the ground truth state occupied by the observed agent’s autonomous program. Although the true mental state is inherently hidden and must be inferred, subjects showed both high validity (correlation with ground truth) and high reliability (correlation with one another). The data provide intriguing evidence about the factors that influence estimates of mental state—a key step towards a true “psychophysics of intention.”

The tipping point: visual estimation of the physical stability of three-dimensional objects

OF THE DISSERTATION The tipping point: Visual estimation of the physical stability of three-dimensional objects by STEVEN A. CHOLEWIAK Dissertation Director: Manish Singh Vision research generally focuses on the currently visible surface properties of objects, such as color, texture, luminance, orientation, and shape. In addition, however, observers can also visually predict the physical behavior of objects, which often requires inferring the action of hidden forces, such as gravity and support relations. One of the main conclusions from the näıve physics literature is that people often have inaccurate physical intuitions; however, more recent research has shown that with dynamic simulated displays, observers can correctly infer physical forces (e.g., timing hand movements to catch a falling ball correctly takes into account Newtons laws of motion). One ecologically important judgment about physical objects is whether they are physically stable or not. This research project examines how people perceive physical stability and addresses (1) How do visual estimates of stability compare to physical predictions? Can observers track the influence of specific shape manipulations on object stability? (2) Can observers match stability across objects with different shapes? How is the overall stability of an object estimated? (3) Are visual estimates of object stability subject to adaptation effects? Is stability a perceptual variable? The experimental findings indicate that: (1) Observers are able to judge the stability of objects quite well and are close to the physical predictions on average. They can track how changing a shape will

Perception of Physical Stability of Asymmetrical Three-Dimensional Objects

Visual estimation of object stability is an ecologically important judgment that allows observers to predict the physical behavior of objects. A natural method that has been used in previous work to measure perceived object stability is the estimation of perceived “critical angle” – the angle at which an object appears equally likely to fall over versus return to its upright stable position. For an asymmetric object, however, the critical angle is not a single value, but varies with the direction in which the object is tilted. The current study addressed two questions: (1) Can observers reliably track the change in critical angle as a function of tilt direction? (2) How do they visually estimate the overall stability of an object, given the different critical angles in various directions? To address these questions, we employed two experimental tasks using simple asymmetric 3D objects (skewed conical frustums): settings of critical angle in different directions relative to the intrinsic skew of the 3D object (Experiment 1), and stability matching across 3D objects with different shapes (Experiments 2 & 3). Our results showed that (1) Observers can perceptually track the varying critical angle in different directions quite well; and (2) Their estimates of overall object stability are strongly biased toward the minimum critical angle (i.e., the critical angle in the least stable direction). Moreover, the fact that observers can reliably match perceived object stability across different 3D shapes suggests that perceived stability is likely to be represented along a single dimension.