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    In the fast lane

    Dr. McDonnell

    Critical flicker-fusion frequency (CFF), as every ophthalmologist knows, is the lowest frequency at which a flickering light source is perceived to be constant (not flickering).

    Humans average a CFF of 60 Hz, but other animals measure with very different capabilities.

    Higher CFFs are thought to convey a survival advantage, as they permit faster reactions to threats and make it easier to catch prey.

    According to a recent article by a professor in Dublin, Ireland, the CFF of a species correlates with metabolic rate and size.

    All things being equal, in small species—like flies—the signals have very little distance to travel from the light-sensing neurons of the eye to the brain, so everything happens fast. Flies have a CFF of 250 Hz, making them so difficult for us humans—living in a world only one-fourth that speed—to swat those little guys.

    The professor measured CFFs of a wide array of animals:

    ·      Eels (disgusting creatures) have a CFF of only 14 Hz.

    ·      Hammerhead sharks are tied with people at 60 Hz.

    ·      Ground squirrels lead all tested vertebrates at 120 Hz.

    This means that hammerhead sharks don’t have trouble catching and biting people (as the media are always eager to report whenever that occurs somewhere in the world), because humans do not see images any faster than the shark. Similarly, these data explain why it so rare for us to read about sharks successfully attacking ground squirrels.

    Birds have CFFs of about 100 Hz, allowing them to zip through the air among tree branches without running smack into an obstacle.

    The human factor

    For humans, when walking or running, a CFF of 60 Hz is quite adequate, but in a vehicle at high speed we apparently begin to suffer in terms of our ability to identify and react to objects with sufficient speed.

    An interesting fact about us primates is that we have cells in our lateral geniculates that respond to higher frequencies than 60 Hz, even though we are not conscious of this. Some believe that this may explain why certain people experience headaches and cognitive problems when exposed to the 120 Hz flicker of older fluorescent lighting.

    A fascinating aspect of CFF is that the refresh rate on a television screen is usually set at 60 Hz, adequate for us human viewers to perceive smoothly transitioning video. One theory is that “dogs have a CFF of 80 Hz, which is probably why they do not seem to like watching television. To a dog a TV programme looks like a series of rapidly changing stills.”

    The lack of interest among dogs in watching television may relate to this scientific observation about CFF, but an alternative explanation exists. Clearly, most television shows are so moronic these days that sentient beings, like dogs, simply can’t stand to watch.

    Let’s face it; since the end of season one of “House of Cards” (filmed largely in Baltimore, by the way), there’s been nothing worth watching on TV.


    1.      How animals perceive time. The Economist. Sept. 21, 2013

    2.     Veitch J, McColl S. Modulation of fluorescent light: Flicker rate and light source effects on visual performance and visual comfort. Lighting research and technology. 1995:27:243-256.

    3.     Spekreijse H, van Norren D, van den Berg TJ: Flicker responses in monkey lateral geniculate nucleus and human perception of flicker. Proceedings of the National Academy of Sciences of the United States of America. 1971;68:2802-2805.


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    Peter J. McDonnell, MD
    He is director of The Wilmer Eye Institute, The Johns Hopkins University School of Medicine, Baltimore, and chief medical editor of ...

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    • Dr. Doug
      Love the humor injected into this interesting comparative article.


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