Everyday activities that we take for granted, such walking, riding a bike or even sitting still, depend on our sense of equilibrium. Gravity provides an ever-present downward pull that we need to sense in order to balance effectively. Visual cues, such as knowing a tree trunk is rooted in the ground, assist in this process.
Gravity provides an ever-present downward pull that we need to sense in order to balance effectively
Pearl Guterman
Pearl Guterman (BSc '05, BA '06, MA '09, PhD '16) a grad student at the time of the research, and Lassonde School of Engineering Professor Robert Allison made an important discovery in the field of vision and perception: they confirmed the A-effect, the phenomenon of perceiving a vertical line as tilted towards the body when tilting your head sideways in the dark, and showed that a similar misperception applied to perceived direction of visual motion. They also found that the A-effect is stronger when people feel like they’re moving compared to when they see this motion as coming from the external world.
Robert Allison
The findings were published in Vision (2019). The research was funded by National Science and Engineering Research Council of Canada and the Canadian Space Agency.
Allison is a core member of the Vision: Science to Application (VISTA) program, associate director of the Centre for Vision Research (CVR) and a York Research Chair in Stereoscopic Vision and Depth Perception. Guterman, now a PhD, is a principal in Applied Intelligence at Accenture.
The two researchers sat down with Brainstorm to talk about the significance of this work.
Q: What were the objectives of this study?
PG: We wanted to see whether the A-effect occurs when scene motion shown to stationary observers generated a compelling illusion of self-motion (such as walking or driving) called vection. Vection is similar to the feeling of motion that you experience when sitting in a stationary train and viewing another train moving on an adjacent track: you feel like you’re moving as well.
What’s interesting about vection is that it occurs despite a conflict between what you are seeing and what your inner ear is sensing.
Vection is the compelling illusion of self-motion
RA: With the A-effect, if you tilt to one side, something that’s vertical appears to tilt with you. So you might think that you misestimated how much your body is tilted. The interesting thing about vection is that it’s very body centric. You feel like you’re moving, but it’s an external vision signal.
“The CRV is a world-renowned research leader in biological and machine vision research.” – Robert Allison
Q: Please describe the experiment.
Apparatus for the upright and tilted posture used in the experiment
PG: We conducted two experiments where participants (from the York community) in various postures viewed a line or dot motion scene that was vertical or tilted (from vertical relative to gravity). The motion was either in 2D, and it looked like a dotted wallpaper, or in 3D, which was more consistent with real self-motion.
In this experiment, participants only had to do one thing: indicate whether the line or scene appeared to be tilted clockwise or counterclockwise from vertical.
In the first experiment, with 20 participants, we just wanted to see whether there was an A-effect for motion in general. In the second experiment, with eight participants, we were interested in whether this effect also occurred when you felt like you were moving (experiencing vection), so we compared 3D motion of short and long duration.
Q: What was the key finding?
PG: We found that the A-effect is stronger when people felt like they were moving compared to when they saw this motion as coming from the external world. This makes sense because vection already involves a conflict in terms of what you’re seeing versus what you’re experiencing.
“The highly interdisciplinary and collaborative nature of research at the CVR, along with leading-edge facilities, has made it a font of scientific discoveries and technological innovations.” – Pearl Guterman
Q: Did anything surprise you?
RA: This vection result did surprise us as we had made the opposite hypothesis. We have an explanation for the results, but it wasn’t what we expected.
PG: What also surprised us was that the vection tilt judgments were more precise and consistently so. This suggests that a different strategy (involving other transformations in the brain) is being used to determine the tilt, when you feel like you’re moving, in estimating self-motion direction than for motion in general.
In this experiment, the motion was either in 2D, and it looked like a dotted wallpaper, or in 3D which was more consistent with real self-motion
Q: Is this original work?
RA: Yes. Only one other group has ever looked at motion, and no one has ever considered self-motion.
Q: How could this research be applied?
PG: This has many applications, particularly operating in space, which is why this work was supported by the Canadian Space Agency. For instance, it could be applied to remotely operating robotics since the operator’s moving view could potentially cause them to misinterpret the direction of their device or other objects.
The findings of this study could also help us to better understand why the perception of vertical tends to be misjudged in a wide range of neurological conditions.
Q: What can you say about York’s leadership in vision research?
RA: York has a long history, stretching back to the 1970s, of expertise in this particular area. We’ve got unique facilities like the tumbling room. We’ve got people like myself, Laurence Harris, Michael Jenkin and the late Ian Howard, the founder of the CVR, who have looked at this specific issue of orientation with respect to gravity.
This centre is a world-renowned research leader in biological and machine vision research, consisting of researchers from all disciplines including the sciences and media arts.
PG: The highly interdisciplinary and collaborative nature of research at the CVR, along with leading-edge facilities, has made it a fount of scientific discoveries and technological innovations.
RA: York's research on vection is continuing in space with ongoing experiments on self-motion perception in astronauts aboard the International Space Station.
To read the article in Vision (2019), visit the website. To learn more about Allison, visit the lab website.
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By Megan Mueller, senior manager, Research Communications, Office of the Vice-President Research & Innovation, York University, muellerm@yorku.ca