Finger Fooled: New Perceptual Illusion Discovered

Fingers are a human’s most important tactile sensors, but they do not always sense accurately and can even be deceived. Researchers at the Cluster of Excellence Cognitive Interaction Technology (CITEC) of Bielefeld University demonstrated this in a new study in which they ‘outwit’ human perception. Test subjects placed their index finger in an apparatus and touched an object whose softness changed at random without the person noticing. While touching the object, the test subjects were under the illusion that it was the position of their finger that changed, not the softness of the object. The curious thing here was that the test subjects felt an “illusory” finger displacement, much larger in extent than the actual, “real” displacement. The researchers published their findings this Thursday, 7 April in the scientific journal “Current Biology.”

Developing a virtual sense of touch is the long-term aim of Professor Dr. Marc Ernst, who headed the Cognitive Neurosciences research group in Bielefeld until the end of March 2016″. On the EU research project “WEARHAP,” Ernst is working with colleagues from all around Europe to achieve this goal. “We now have a better understanding of how we can virtually convey the impression of whether an object feels soft or hard,” explains the neuroscientist. “In the future, this should help in developing a virtual sense of touch with which one can ‘touch’ things across distances, such as how a sweater or another product feels while shopping online.”

“A fundamental question in this project is what role haptic stimuli play in perception,” says Ernst. With the term ‘haptic stimuli,’ the cognitive scientist is referring to the sensations that arise from touch. “A special feature of our finger pads is that they are fleshy – they can ‘deform’ by giving way when touching something,” says Marc Ernst. For instance, when a person touches a sponge, she feels its composition and consistency through the tactile sensors in her skin.

For their experiment, the researchers constructed an apparatus at the end of which an elastic fabric strap was stretched horizontally. The tightness of the strap was adjustable. The test subject laid her hand and lower arm in the guiding casing of the apparatus. The person was instructed to say when they thought their finger was bent more. In reality, the position of the finger did only change a little – only the tightness of the elastic band did. “Astonishingly, all study participants estimated their finger to be most bent when the elastic band was loose. This is apparently because the loose band has comparatively more contact area with the skin,” explains Dr. Alessandro Moscatelli, who performed the experiment.

The researcher's finger in the device.
With a custom-designed apparatus, CITEC researcher Dr. Alessandro Moscatelli and his colleagues are testing how people perceive touch and their own bodies. Credit: CITEC/Bielefeld University.

“The key here is how much surface area of the object comes into contact with our skin. The greater the area of contact, the closer the object seems to be, and therefore, the more the finger appears to be bent. Why is this finding significant? “If we were not to know exactly how our body is positioned, we would not be able to grasp or catch, and therefore could not interact with objects or other people,” explains Marc Ernst.

About this psychology research

Bielefeld University is cooperating with research institutions from across Europe on the research project WEARHAP (WEARable HAPtics for Humans and Robots), which will run until February 2017. The European Union has invested 7.7 million Euro in this project, of which almost 1 million Euro is going to Bielefeld University.

Funding: Funding provided by the European Union.

Source: Dr. Marc Ernst – Bielefeld University
Image Credit: The image is credited to CITEC/Bielefeld University.
Original Research: Full open access research for “The Change in Fingertip Contact Area as a Novel Proprioceptive Cue” by Alessandro Moscatelli, Matteo Bianchi, Alessandro Serio, Alexander Terekhov, Vincent Hayward, Marc O. Ernst, and Antonio Bicchi in Current Biology. Published online April 7 2016 doi:10.1016/j.cub.2016.02.052


Abstract

The Change in Fingertip Contact Area as a Novel Proprioceptive Cue

Highlights
•Pushing a finger against a soft surface provokes an increase of the contact area
•This increase in contact area potentially provides a cue to finger displacement
•We ran psychophysical experiments to test this hypothesis
•Results revealed a novel proprioceptive cue, i.e., the change in contact area

Summary
Humans, many animals, and certain robotic hands have deformable fingertip pads [ 1, 2 ]. Deformable pads have the advantage of conforming to the objects that are being touched, ensuring a stable grasp for a large range of forces and shapes. Pad deformations change with finger displacements during touch. Pushing a finger against an external surface typically provokes an increase of the gross contact area [ 3 ], potentially providing a relative motion cue, a situation comparable to looming in vision [ 4 ]. The rate of increase of the area of contact also depends on the compliance of the object [ 5 ]. Because objects normally do not suddenly change compliance, participants may interpret an artificially induced variation in compliance, which coincides with a change in the gross contact area, as a change in finger displacement, and consequently they may misestimate their finger’s position relative to the touched object. To test this, we asked participants to compare the perceived displacements of their finger while contacting an object varying pseudo-randomly in compliance from trial to trial. Results indicate a bias in the perception of finger displacement induced by the change in compliance, hence in contact area, indicating that participants interpreted the altered cutaneous input as a cue to proprioception. This situation highlights the capacity of the brain to take advantage of knowledge of the mechanical properties of the body and of the external environment.

“The Change in Fingertip Contact Area as a Novel Proprioceptive Cue” by Alessandro Moscatelli, Matteo Bianchi, Alessandro Serio, Alexander Terekhov, Vincent Hayward, Marc O. Ernst, and Antonio Bicchi in Current Biology. Published online April 7 2016 doi:10.1016/j.cub.2016.02.052

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