On the Edge: Haptic Discrimination of Edge Sharpness

Abstract

The increasing ubiquity of haptic displays (e.g., smart phones and tablets) necessitates a better understanding of the perceptual capabilities of the human haptic system. Haptic displays will soon be capable of locally deforming to create simple 3D shapes. This study investigated the sensitivity of our haptic system to a fundamental component of shapes: edges. A novel set of eight high quality shape stimuli with test edges that varied in sharpness were fabricated in a 3D printer. In a two alternative, forced choice task, blindfolded participants were presented with two of these shapes side by side (one the reference, the other selected randomly from the remaining set of seven) and after actively exploring the test edge of each shape with the tip of their index finger, reported which shape had the sharper edge. We used a model selection approach to fit optimal psychometric functions to performance data, and from these obtained just noticeable differences and Weber fractions. In Experiment 1, participants performed the task with four different references. With sharpness defined as the angle at which one surface meets the horizontal plane, the four JNDs closely followed Weber’s Law, giving a Weber fraction of 0.11. Comparisons to previously reported Weber fractions from other haptic manipulations (e.g. amplitude of vibration) suggests we are sufficiently sensitive to changes in edge sharpness for this to be of potential utility in the design of future haptic displays. In Experiment 2, two groups of participants performed the task with a single reference but different exploration strategies; one was limited to a single touch, the other unconstrained and free to explore as they wished. As predicted, the JND in the free exploration condition was lower than that in the single touch condition, indicating exploration strategy affects sensitivity to edge sharpness.

Perceived Duration Increases with Contrast, but Only a Little

“…We formalized this observation by applying a model selection technique to our psychophysical data (Skinner et al, 2013; Kent et al, 2014). In addition to our probit function, with its two parameters (location and spread), we employed an additional random button press model.…”

Proximal-Distal, Not Medial-Lateral, Movement across an Edge Increases Discrimination of Edge Sharpness

“…These cells respond to sustained skin deformation from points, edges, and curvature, and respond linearly to the depth of indentation (eg Phillips & Johnson, 1981); SA2 receptors are much more sensitive to stretch than indentation, so likely play a lesser role. However, our previous study (Skinner et al, 2013) demonstrated that discrimination relying on a static touch-a strategy for exploration that, because of its relatively slow and sustained nature, is likely to activate predominately SA1 receptors-resulted in poorer discrimination of edge sharpness than more active strategies, which are likely to additionally recruit more rapidly adapting receptors. It therefore seems likely that perception of edge sharpness additionally benefits from input from rapidly adapting mechanoreceptors that respond to dynamic touch (and are insensitive to static touch).…”

“…In particular, the sharpness of an edge, defined as the angle at which two faces meet, as distinct from the curvature of an object (see Kappers, 2011, for a review of curvature), has been underexplored. In one study directly investigating edge sharpness, Skinner et al (2013) showed that perception of edge sharpness was affected by exploration strategy. Specifically, Skinner et al found that the discrimination threshold for the sharpness of an edge (using only the index finger) decreased when participants were allowed to explore the edge using a relatively free exploration strategy with the index finger compared with when participants were allowed only a single static touch of the edge.…”

“…The simple aim of the study reported here was to test which of the two movement patterns, promixal-distal or medial-lateral, is most informative in aiding participants to discriminate edge sharpness. Participants completed a two-alternative forced-choice (2AFC) task combined with the method of constant stimuli, identical to the task used by Skinner et al (2013) except that exploration movement direction was manipulated such that participants explored using either a proximal-distal or medial-lateral movement. In each condition the just-noticeable difference (JND) was determined, and these JNDs were used to compare discrimination performance for the two movements.…”

“…2013, we fitted a series of logistic and cumulative Weibull functions to our data; the purpose of this exercise was to determine the best model to fit those data and to determine the JND. This process is described in more detail by Skinner et al (2013); but briefly, the logistic function models the relation between proportion correct (P correct ) and sharpness difference (D i ) as…”