If we’re honest about shortcomings in human physiology, optical illusions would be labelled “Brain Failures.”
– Neil DeGrasse Tyson
When we think about the ways our perception plays tricks on us, optical illusions come to mind first. They’re not the only kind of sensory illusions, though. Tactile illusions also illustrate the fascinating ways that our perception ‘fails’ to reflect reality. In this post, we describe several different types of tactile illusions.
Perception of Taste
In 2010, psychophysics researcher Michael Barnett-Cowan documented an interesting experiment involving the influence of tactile stimuli on our perception of taste. He wrote:
In this study, blindfolded participants bit-into fresh or stale pretzels while rating their freshness/staleness and crispness/softness. Information provided to the hand was either congruent (whole pretzel fresh or stale) or incongruent (half pretzel fresh, half stale) with what was presented to the mouth. The results demonstrate that the perception of both freshness and crispness was systematically altered when incongruent information was provided: bit-into fresh pretzel tips were perceived as staler and softer when a stale pretzel tip was held in the hand and vice versa. Haptic information available when handling food thus plays a significant role in modulating food perception.
The “contingent aftereffect” refers to a distorted perception of some stimulus based on exposure to a previous stimulus. Contingent aftereffect illusions exist across the domains of perception: visual, auditory, and haptic. Walker and Shea demonstrated one such haptic contingent aftereffect in a study where they placed a narrow bar on one side of the participant and a wide bar on the other. The participants were asked to repeatedly grasp each bar with one of their hands to judge the bars’ widths. After this initial stimulus, the two bars were replaced by two new bars of identical width, and the participants were asked again to use the same hand to judge which of the two bars was wider. The researchers found that the bar placed on the side where the wider bar had previously been now felt narrower than the bar on the other side, revealing an aftereffect contingent on the location of the initial stimulus.
Other Aftereffect Illusions
Many other aftereffect illusions occur in haptic perception. Researchers Kahrimanovic, Bergmann Tiest, and Kappers discovered one such effect related to the perception of roughness. They asked participants to “scan” a rough surface with their finger by moving their finger back and forth across it. After scanning this rough surface, participants perceived the next surface they scanned as relatively less rough.
A study by James J. Gibson revealed another haptic aftereffect illusion related to the perception of curvature. In this study, Gibson instructed participants to run their fingers over a convex curved piece of cardboard for 3 minutes, after which they were asked to move their fingers now along a straight edge. All participants reported feeling that the straight edge was concave; however, they also noted a gradual decrease in the perceived concavity of the straight edge during the 3 minutes they continued to run their fingers across it.
The Tau Effect, first discussed in Science in 1930, is an illusion that occurs when the distance between two points of contact with the skin is constant, but the time elapsed between stimulations changes. When two successive taps have a short delay between them, the perceived distance will be shorter than a similar stimulation with a longer delay.
The Kappa Effect is closely related to the Tau Effect and occurs when an observer judges the elapsed time between sensory stimuli according to the distance between the stimuli. The Kappa Effect occurs when time is held constant but the distance between stimuli is increased or decreased. When perceiving a sequence of stimulations, people tend to overestimate the time between each stimulation when the distance is larger and tend to underestimate the time between each stimulation when the distance is smaller.
Cutaneous Rabbit Illusion
The cutaneous rabbit illusion, also known as cutaneous saltation or the cutaneous rabbit effect, is an illusion in haptic perception that evokes a feeling of sequential taps “hopping” across the skin. The illusion is created most effectively on areas of the skin with a low spatial resolution for tactile stimulations and is typically evoked by tapping two or more regions in rapid succession. The lack of acuity in the space between the taps causes a person’s somatosensory system to interpret the taps as the motion of an object across that surface, producing a conscious experience of tactile motion. The illusion typically occurs only with successive taps occurring within 300 milliseconds of each other, and the effectiveness of the illusion decreases when an individual directs conscious attention to focus on sensations.
Haptic funneling is the phantom vibration experienced between two points of vibration– although stimulation is provided to two regions of the skin that are within 3-6cm of each other, the vibration will be perceived at a location between the two regions. As a result, the virtual resolution of a grid of vibrotactile actuators can be greater than the actual number of motors used in a haptic display.
Tactile illusions can be created by a computer interface, too. Tactile Brush is an algorithm developed by Disney Research that uses apparent tactile motion and phantom vibration to draw shapes onto a person’s skin using a grid of vibrotactile actuators. Disney may intend to use haptic displays with the algorithm to enhance the rides and attractions at their theme park.
In the future, computer interfaces will interact with our bodies in new ways, including tricking our brains with tactile illusions through haptic feedback.