Investigators from Harvard and MIT have been looking into the difference in the brain’s response when presented with a similar illusion, but acting on different senses. The ‘apparent motion quartet’, seen on the right, at first tends to make people see the dots jumping vertically or horizontally, but after some time the motion switches from horizontal to vertical, or vertical to horizontal. What’s interesting is that when the tactile version was used, similar results were achieved.
To create a tactile version of this illusion, Olivia Carter, a postdoctoral researcher at Harvard University, and Talia Konkle, a graduate student in Moore’s MIT lab, used a new piezoelectric stimulator device developed by Qi Wang and Vincent Hayward at McGill University. This device, originally designed as a computer Braille display, uses a centimeter-square array composed of 60 “tactors” to deliver precisely controlled touch stimuli to the finger tips 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.
The authors went on to show that after a period of adaptation to an unambiguous horizontal or vertical stimulation (produced by activating a row of tactors in succession), subjects were more likely to perceive a subsequent ambiguous stimulus as being in the orthogonal direction. 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.
Real-world objects often stimulate multiple senses simultaneously, and our brains must combine these disparate stimuli into a unified interpretation of the world. The authors used their tactile illusion to explore the interaction between touch and vision. They instructed their subjects to make vertical or horizontal eye movements during the ambiguous touch stimuli. 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 are somehow aligned with respect to the external world.
Press release: MIT-led team creates touch-based illusion