Our ability to perceive rapid changes in visual scenes over time 鈥� our 鈥渟peed of sight鈥� 鈥� varies a surprising amount between people, according to the first study to systematically investigate the question.
This suggests that some people can track fast-moving objects better than others because of their innate superior vision, which may contribute to people鈥檚 different abilities in sports like baseball and cricket, says at Trinity College Dublin.
Our speed of sight is also known as the flicker fusion threshold, because if a light is flashing on and off at a frequency above someone鈥檚 threshold, it appears to them to be shining steadily.
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鈥淚t鈥檚 comparable to how many 鈥榝rames per second鈥� our visual system processes,鈥� says Haarlem.
This threshold varies widely in the animal kingdom, being higher in creatures that can move faster, especially if they hunt other speedy animals. For instance, the peregrine falcon, the fastest creature on Earth thanks to its plummeting hunting dives, has a flicker fusion frequency of about 130 images per second, or 130 hertz.
The upper threshold for humans is often taken as a flicker rate of about 60 Hz.
To find out how much this ability varies between individuals, Haarlem and his colleagues asked 88 people from the university to view a flickering light, the frequency of which could be adjusted. They used three methods: asking the person to turn the frequency up until the light stopped flickering, asking them to turn the frequency down until flickering began or asking them to say whether the light was flickering over a series of randomly chosen frequencies.
The participants repeated all the tests on three occasions to see if their threshold changed from day to day.
Using the last method, thought to be the most accurate, the average flicker threshold for the whole group was about 50 Hz, but it varied from 34 to 61 Hz.
鈥淭he range is a lot larger than I expected,鈥� says team member , also at Trinity College Dublin. 鈥淔or most things in the world, that probably wouldn鈥檛 make any difference to the way they鈥檙e perceived because they鈥檙e not moving fast enough. But there are some things moving very fast, where [it] might well make a difference.鈥�
鈥淚f something were to fly past your eyes very rapidly, but you have a threshold high enough to capture that image, your brain would probably turn it into a blur going in a certain direction,鈥� says Haarlem. 鈥淲hereas in someone with a very low threshold, that flying object might not be captured at all.鈥�
It isn鈥檛 known what causes people鈥檚 flicker threshold to vary, but it could involve innate features of the cells at the back of the eye that react to light or the parts of the brain that process vision. 鈥淭here are multiple levels of processing that get an image presented to our conscious perception,鈥� says Haarlem. 鈥淚 think it鈥檚 a combination.鈥�
鈥淭his is a very nice study because it鈥檚 completely new,鈥� says biologist . He speculates that people may be able to increase their flicker threshold with training, for instance, if they play fast-moving sports such as baseball or squash.
bioRxiv
