Posted on 11 July 2017
The team discovered that visual acuityâthe ability to see fine detailâcan be enhanced by an illusion known as the âexpanding motion aftereffectâ.
While under its spell, viewers can read letters that are too small for them to read normally.
Visual acuity is normally thought to be dictated by the shape and condition of the eye but these new findings suggest that it may also be influenced by perceptual processes in the brain.
Interest in the intersection between perception and reality led the research team from the Universities of York and Glasgow to wonder about visual illusions and how they might affect visual acuity.
The expanding motion aftereffect can make objects appear larger than they really are and the scientists wanted to know whether this apparent increase in size could bring about the visual benefits associated with actual increases in size. In particular, could it make small letters easier to read?
Spirals
To find out, the researchers employed a tool that can be found in any optometristâs office: the classic logMAR eye chart. On this chart, letters are arranged in rows and the letters become increasingly smaller and more difficult to read as you move down the chart. Optometrists calculate visual acuity based on the size at which a person can no longer reliably identify the letters.
In two related experiments, the researchers presented a total of 74 observers with a spiral pattern that rotated either clockwise or counterclockwise for 30 seconds followed by a set of letters, which participants were asked to identify. The font size of the letters became increasingly smaller over subsequent trials.
The experiment revealed that participantsâ visual acuity differed depending on which spiral they saw.
Participants who started with normal visual acuity and saw clockwise spiralsâwhich induce adaptation to contracting motion and cause subsequent static images to appear as if they are expandingâshowed improved visual acuity. That is, they were able to identify letters at smaller font sizes after exposure to the clockwise spiral.
Those who saw counterclockwise spiralsâwhich induce adaptation to expanding motion and cause later images to appear as if they are contractingâactually performed worse after exposure to the spirals.
Font
A third experiment, in which each participant saw both types of spirals over two sessions, showed similar results: Seeing clockwise spirals that induced an expanding motion aftereffect enabled participants to read letters at smaller font sizes.
Co-author, Dr Rob Jenkins from the University of Yorkâs Department of Psychology, said: âWe were pretty impressed by the consistency of the effect. No matter how you break it downâby letter size, by letter positionâthe performance boost is there.â
âAnd there was a correlation with initial ability: the harder people found the task, the more the illusion helped them.â
But donât throw out your eyeglasses just yet: the researchers note that the overall boost to visual acuity is small and fleeting.
Nonetheless, this common visual illusion reveals a fundamental aspect of how we see, showing us that our ability to discriminate fine detail isnât solely governed by the optics of our eyes but can also be shaped by perceptual processes in the brain.
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