Cognitive scientists at CITEC discover new perceptual illusion

Cognitive scientists at CITEC discover new perceptual illusion

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) 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 2016 in the scientific journal “Current Biology.”

Prof. Dr. Marc Ernst from the Cluster of Excellence CITEC is researching how human perception can be deceived. Photo: CITEC/Bielefeld UniversityDeveloping 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. The more she presses into the object, and the softer the object is, the larger the area of contact will be. “In our new study, we wanted to understand if these contact areas, which contain characteristic information about the object and the person’s interaction with it, have an influence on perception,” says the cognitive scientist.

According to Marc Ernst, these haptic impressions also have an impact on proprioception —the way a person senses how their arms, fingers, and other limbs and parts of the body are positioned. “This happens, on the one hand, through sensors in muscles and tendons,” explains the researcher. “On the other hand, cutaneous sensory cells in skin also contribute to proprioception. When, for instance, the skin on the finger stretches or tightens when we bend it, this tells us something about how the finger is positioned. Through cutaneous sensors, the brain can assess whether the finger is straight or bent, and in which position it is.” Just how important these cutaneous receptors are in proprioception becomes especially apparent in people with extensive burn scars. These individuals frequently have problems perceiving the correct position of the damaged body part. How proprioception is influenced by tactile perception was part of this new study. 

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.

“Our study demonstrates that even the cutaneous tactile sensors in our fingertips play an important role in our perception of our body and the position of our limbs,” says Professor Dr. Marc Ernst. “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.

With a custom-designed apparatus, CITEC researcher Dr. Alessandro Moscatelli and his colleagues are testing how people perceive touch and their own bodies. Photo: CITEC/Bielefeld UniversityIn their article, Ernst and Moscatelli compare this effect—the greater the area of contact with the skin, the closer the object seems to be—with a well-known effect that also happens in vision. “When an object flies towards us, and keeps getting closer, this image also becomes larger on our retina,” explains Alessandro Moscatelli. “And vice versa: when we see an image that is becoming larger on our retina, the brain understands from this that the object is moving towards us—that the object is not just suddenly expanding.  Biologists call this visual phenomenon ‘looming.’ The Bielefeld researchers borrowed this term to describe the feeling that arises during touch when the finger comes in contact with an object and the contact area keeps increasing: the object moves closer, or rather the finger has to bend more in order to press into the soft object. Just as a person would not accept that an object suddenly expands, the brain also does not accept that the softness of an object suddenly changes. If we, however, artificially change the softness of an object, the brain falls for it and understands, mistakenly, that the finger has moved. “We call this tactile looming,” says Marc Ernst.

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.

Alessandro Moscatelli, Matteo Bianchi, Alessandro Serio, Alexander Terekhov, Vincent Hayward, Marc Ernst, Antonio Bicchi: The Change in Fingertip Contact Area as a Novel Proprioceptive Cue,, erschienen am 7. April 2016

Weitere Informationen im Internet:
„Per Tasthandschuh Objekte in der Ferne fühlen“ (Pressemitteilung vom 17.09.2013):

Prof. Dr. Marc Ernst, Bielefeld University 
Faculty of Biology
Telephone: +49 521 106-5700 

Dr. Alessandro Moscatelli, Bielefeld University 
Faculty of Biology