17 January 2014

Visual Perception

Welcome back. Though hardly a visionary, I’ve blogged about stereovision, depth perception, pupil size and, stretching it, eyespots. I’ve got one more.

Have you ever seen birds chasing one another? They never slam into a tree branch or feeder. Their visual systems respond fast enough for them to see what’s coming. Luke Skywalker and Princess Leia may have chased scout troopers through the woods on speeder bikes that can go 300 mph, but could you do that without smashing into a tree? 

It goes further, of course. If I see a cat chasing a squirrel, the cat is almost a blur. But the escaping squirrel’s visual system perceives the cat to be moving pretty slowly. 

Video that gives an idea of how squirrels see cats
moving. www.youtube.com/watch?v=J1vpB6h3ek4

If I may continue with cinema, Neo’s visual perception allowed him to dodge and watch bullets fly by. Don’t try it. He’s The One; you’re not.

New research from Ireland’s Trinity College Dublin and Scotland’s Edinburgh and St. Andrews universities suggests that there’s an order to all this. In the animal kingdom, at least among vertebrates, it appears that temporal resolution of visual systems--how well a species perceives time-changing information--is generally related to body size and metabolic rate.

Critical Flicker Fusion Frequency

Using data from the literature, the collaborators conducted a comparative analysis of the visual system temporal resolution of 34 vertebrate species together with mass-specific metabolic rate and body mass. They also added data on brain mass for 28 of the species.

For their measure of temporal resolution, the investigators used maximum values of critical flicker fusion frequency (aka flicker fusion threshold, flicker fusion frequency, flicker fusion rate and the like).

Think of switching a light on and off, faster and faster, until it’s fast enough that the light appears to stay on. The frequency of light ons and offs per second when the light first appears to be on continuously to the observer--whether the observer is a human, squirrel, bird or cat--is the critical flicker fusion frequency. That frequency is dependent on a number of factors, including light brightness.

Vertebrate Scale

The analysis discovered a pattern of visual system temporal resolution that increased with metabolic rate and decreased with body mass.

Small animals with high mass specific metabolic rates in high light environments had the highest maximum flicker fusion frequencies and thus the greatest ability to perceive visually dynamic information. In contrast, large animals with low metabolic rates in low light environments had the lowest flicker fusion frequencies.

The frequencies, measured in cycles per second or Hertz, ranged from European eels at 14 to gold-mantled ground squirrels at 120, with humans checking in at 60. Cat lovers will be aghast to learn that the temporal resolution of their whiz-by cat is only 55, much less than that of dogs, 80, chickens, 87, or pigeons, 100.

Wrap Up

The researchers point out the study’s implications for the evolution of predator-prey interactions as well as less obvious possible influences on ecological niche. But if just learning that littler creatures perceive dynamic information faster than bigger creatures doesn’t impress you, at least recognize that when it comes to how fast you or I can go safely with or without a Star Wars speeder bike, it’s not as fast as pigeons--or, if pressed, chickens. Thanks for stopping by.


- Study paper in Animal Behaviour:  www.sciencedirect.com/science/article/pii/S0003347213003060
- Articles on the study on Popular Science and the BBC News websites:

- Background on flicker fusion frequency:
- Examples of earlier research on flicker fusion frequency measurements:
insects (1958): www.sciencedirect.com/science/article/pii/002219105890012X
cats (1975): www.sciencedirect.com/science/article/pii/0042698975903028
dogs (1989): www.sciencedirect.com/science/article/pii/0031938489900929

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