On the edge: haptic discrimination of edge sharpness

Distribution Characteristics and Correlation of Edge Sharpness Threshold and Contact Area

It is currently unclear how sharpness discrimination ability is distributed across a wide range of edge sharpness and the effect of contact area on haptic perception. We 3D printed triangular prisms with various edge sharpness and half-edge widths in the full-scale range and conducted 2AFC tasks to gain the haptic threshold distribution. Results show that the distribution curves of the sharpness threshold and its contact area have a similar inflection point at 115 °, implying a boundary between medium-low and high stimuli. It is also found that Weber fractions in the medium stimulus range follow Weber's Law and are consistent with previous studies but lower than the mean of Weber fractions in the high stimulus range. Besides, there is no significant difference in upper and lower thresholds in the medium-low stimulus range but a significant difference in the high stimulus range with the higher upper threshold. Variations in contact area do not affect sharpness discrimination ability when the half-edge width exceeds 2 mm. However, decreasing the half-edge width from 2 mm to 1 mm significantly reduces haptic sensitivity. Our findings offer preliminary evidence contributing to understanding haptic perception in edge sharpness discrimination, encompassing the properties of objects and object-individual interfaces.

Perceived Duration Increases with Contrast, but Only a Little

Recent adaptation studies provide evidence for early visual areas playing a role in duration perception. One explanation for the pronounced duration compression commonly found with adaptation is that it reflects adaptation-driven stimulus-specific reduction in neural activity in early visual areas. If this level of stimulus-associated neural activity does drive duration, then we would expect a strong effect of contrast on perceived duration as electrophysiological studies shows neural activity in early visual areas to be strongly related to contrast. We employed a spatially isotropic noise stimulus where the luminance of each noise element was independently sinusoidally modulated at 4 Hz. Participants matched the perceived duration of a high (0.9) or low (0.1) contrast stimulus to a previously presented standard stimulus (600 ms, contrast = 0.3). To achieve perceptually equivalent durations, the low contrast stimulus had to be presented for longer than the high contrast stimulus. This occurred when we controlled for stimulus size and when we adjusted for individual differences in perceived temporal frequency. Further, we show that the effect cannot be explained by shifts in perceived onset and offset and is not explained by a simple contrast-driven response bias. The direction of our results is clearly consistent with the idea that level of neural activity drives duration. However, the magnitude of the effect (~10% duration difference over a 0.9-0.1 contrast reduction) is in marked contrast to the larger duration distortions that can be found with repetition suppression and the oddball effect; particularly when these may be associated with smaller differences in neural activity than that expected from our contrast difference. Taken together, these results indicate that level of stimulus-related neural activity in early visual areas is unlikely to provide a general mechanism for explaining differences in perceived duration.

Proximal-distal, not medial-lateral, movement across an edge increases discrimination of edge sharpness

Edges are fundamental properties of our environment and the objects we interact with. There is a lack of research on the haptic perception of edges, especially the sharpness of an edge. Skinner et al. [2013 PLoS ONE, 8(9): e73283] found that haptic discriminability of sharpness was clearly superior when using a relatively unrestrained, free exploration strategy compared with a static single touch strategy. In the free exploration condition two distinct movement patterns were frequently used by participants: a proximal-distal movement of the fingerpad across the test edge and a medial-lateral movement of the fingerpad along the test edge. Here, using the same stimuli and two-alternative forced-choice method of constant stimuli as Skinner et al. (2013), we demonstrate that a proximal-distal movement results in substantially lower sharpness discrimination thresholds than a medial-lateral movement. The underlying neurophysiology and implications for the design of haptic displays are considered.

Psychophysics and the evolution of behavior

Sensory information allows animals to interpret their environment and make decisions. The ways in which animals perceive and measure stimuli from the social and physical environment guide nearly every decision they make. Thus, sensory perception and associated cognitive processing have a strong impact on behavioral evolution. Research in this area often focuses on the unique properties of the sensory system of an individual species, yet certain relevant features of perception and cognition generally hold across taxa. One such general feature is the proportionally based translation of physical stimulus magnitude into perceived stimulus magnitude. This process has been recognized for over a century, but recent studies have begun to consider how a law of proportional psychophysics, Weber's law, exerts selective force in behavioral evolution.