Written by Dr. Doris Chow, postdoctoral fellow in the GPN. Twitter: @_DorisChow
What do you experience when you watch a moving cloud of dots like in this video? Do you feel like you are moving through a tunnel of lights or voyaging in space? Where do you find yourself looking as you watch?
Our visual world is rarely still. We walk, bike, and drive, from one place to another (perhaps less often now during the pandemic). Even when we sit still, our eyes move a lot–on average, they make large sudden movements called saccades three times per second[1]. This changes what we see on our retina and creates “motion signals” that the brain processes and interprets. How these motion signals impact our actions in everyday situations remains one of the most challenging problems in sensory neuroscience.
Using a computer-generated stimulus like the short video clip mentioned above (and found here) psychologists and neuroscientists study how humans and non-human animals perceive motion signals in laboratory settings. This stimulus is a simplified version of what you might experience in everyday life, for example when you sit inside a moving train. In this case, the dots radiate from a point on the screen called the “focus-of-expansion.” You can see that the focus-of-expansion point moves throughout the video. This focus-of-expansion point generally aligns with the “heading direction” which is the direction we are heading in, either toward or away from.
By studying the focus-of-expansion and heading direction, we have learned there are parts of the brain that respond to different speeds and different directions of motion[2]. We have also discovered we are quite good at judging even minor differences in heading directions[3], as small as one degree of visual angle. This difference in angle is about the width of your index finger when you hold your arm straight out in front of your eyes.
In addition to studies on how we perceive a moving stimulus, studies have looked at how a moving stimulus affects how we interact with the world. For example, if presented with the cloud of dots while on a treadmill, we walk more quickly if we see the dots moving faster—this simulates faster self-motion[3]. Even newborns lean their head backward as they watch a moving stimulus that simulates forward motion[4].
Our group’s latest research reports that humans tend to track heading direction changes intuitively with their eyes[5]. In our study, participants viewed the moving cloud of dots on a computer monitor in front of a video-based eye-tracker that recorded the observers’ eye positions (Fig. B, left). They received no instructions on what to expect of the moving cloud or what to do with their eyes. Results showed that the participants’ eye positions changed as the heading direction changed—and this was consistent in more than 80% of the observers. The lack of instructions means we can understand this to be an “intuitive” reaction to the heading direction change.
It seems that as humans, we do look where we’re going. So, where do we go from here? For starters, this study confirms our task as a promising candidate for vision test assessment which would be suitable for many different populations. With intuitive tracking, explicit instruction is not required, unlike most laboratory tasks. Developing the right tool for testing dynamic vision is important, especially when the ability to perceive motion might deteriorate with healthy ageing and age-related eye diseases[6]. We don’t know yet whether intuitive tracking is related to other measures of motion processing, like the ability to judge heading directions or the strength of neural signals in response to motion. This is the next research goal for us. If you want to learn more, check out how this research, among other research in our lab and other labs[7,8], which explores how eye movements can be a sensitive indicator of our ability to see and think.
References:
1 Otero-Millan, J., Troncoso, X. G., Macknik, S. L., Serrano-Pedraza, I., & Martinez-Conde, S. (2008). Saccades and microsaccades during visual fixation, exploration, and search: Foundations for a common saccadic generator. Journal of Vision, 8(14):21, 1–18.
2 Duffy, C., & Wurtz, R. (1995). Response of monkey MST neurons to optic flow stimuli with shifted centers of motion. The Journal of Neuroscience, 15(7), 5192.
3 Warren, W. H., & Hannon, D. J. (1988). Direction of self-motion is perceived from optical flow. Nature, 336(6195), 162–163.
4 Jouen, F., Lepecq, J-C., Gapenne, O., & Bertenthal, B. (2000). Optic flow sensitivity in neonates. Infant Behavior & Development, 23, 271-284.
5 Chow, H.M., Knöll, J., Madsen, M., & Spering, M. (2021). Look where you go: Characterizing eye movements toward optic flow. Journal of Vision, 21(3):19, 1-15.
6 Bennett, P. J., Sekuler, R., & Sekuler, A. B. (2007). The effects of aging on motion detection and direction identification. Vision Research, 47(6), 799–809.
7 Dakin, S. C., & Turnbull, P. R. K. (2016). Similar contrast sensitivity functions measured using psychophysics and optokinetic nystagmus. Scientific Reports, 6(1), 34514.
8 Mooney, S.W. J., Hill, N. J., Tuzun, M. S., Alam, N. M., Carmel, J. B., & Prusky, G. T. (2018). Curveball: A tool for rapid measurement of contrast sensitivity based on smooth eye movements. Journal of Vision, 18(12):7, 1–19.
