Washington, July 18 : A collaboration between experts from the Massachusetts Institute of Technology, Harvard University, and McGill University has led to the creation of a touch-based illusion, which may help gain fresh insights into perception.
The researchers say that this illusion may help understand how different senses, such as touch and sight, work together.
"The most familiar illusions involve vision. But we're interested in discovering general principles of perception, and we wanted to see whether similar illusions can occur in the tactile domain," says Christopher Moore, a principal investigator at the McGovern Institute for Brain Research at MIT and an assistant professor in MIT's Department of Brain and Cognitive Sciences.
A research article on the Current Biology web site describes the visual illusion as a motion quartet, wherein two dots are presented at diagonally opposite corners of an imaginary square.
When the pattern alternates between the two diagonals-top left/bottom right followed by top right/bottom left-people perceive the dots as moving back and forth either horizontally or vertically.
After a period of time, typically a minute or two, most observers report that the axis of motion appears to flip from vertical to horizontal or vice versa.
Olivia Carter, a postdoctoral researcher at Harvard University, and Talia Konkle, a graduate student in Moore's MIT lab, helped create a tactile version of the illusion by using a new piezoelectric stimulator device.
Created by Qi Wang and Vincent Hayward at McGill University, this device was originally designed as a computer Braille display. It uses a centimetre-square array composed of 60 "tactors" to deliver precisely controlled touch stimuli to the fingertips of volunteer subjects.
When volunteer subjects were given the diagonally alternating stimuli, they perceived them as moving smoothly back and forth-and just as with the visual illusion, the direction of apparent motion flipped back and forth from vertical to horizontal, on average about twice per minute, even though there was no change in the stimulus itself.
After a period of adaptation to an unambiguous horizontal or vertical stimulation produced by activating a row of tactors in succession, the subjects were more likely to perceive a subsequent ambiguous stimulus as being in the orthogonal direction.
The researchers say point out that similar after-effects are common in vision, and were once thought to reflect fatigue in the brain circuits responsible for a particular perceptual interpretation, but are now thought to reflect a continual recalibration of the brain to its sensory environment.
In another experiment, an ambiguous touch stimulus was interrupted by a three-second break, after which subjects tended to experience the same direction as before the break, suggesting that the prior interpretation was somehow retained in memory and used to reinterpret the ambiguous stimulus.
The authors used their tactile illusion to explore the interaction between touch and vision, and instructed their subjects to make vertical or horizontal eye movements during the ambiguous touch stimuli.
The subjects perceived that the direction of tactile motion shifted into alignment with the direction of the eye movements, but only if the head and finger were also aligned.
Tilting the head sideways 90 degrees produced a shift to the other direction, suggesting that the tactile and visuomotor systems were somehow aligned with respect to the external world.
"We don't yet understand what's happening in the brain during these illusions. But we think this illusion will be a useful new tool to understand more about the similarities between different sensory modalities and how they all work together," says Konkle.