Try this: observe your eyes in a mirror. Keep your head stationary. Try to observe your eyes in motion, e.g., when looking from one eye to another or from a distant object to a closer one. It can’t be done. We can’t observe our own eyes in motion.
This is due to a phenomenon called saccadic supression. The eyes usually alternate between saccades (short & quick movements) and fixations. The eyes remain fixated on a given spot only for a very short period of time. We move our eyes many times every second, even when trying to keep them still. To avoid motion blur during eye movement, visual processing is selectively blocked during saccades.
But we can trick the brain, as demonstrated by Benjamin Tatler and Tom Troscianko from the University of Sussex. We can hack the visual system to allow us to catch a glimpse of our eyes at the end of their motion. This hinges on a phenomenon first introduced in the so-called the Pulfrich effect. Earlier this year, I wondered what the “exposure time” of the human eye might be. The Pulfrich effect is a phenomenon where a pendulum moving side-to-side in front of a subject with one eye darkened by a filter will appear to be moving on an elliptical path. This is due to a delay in visual processing in the darkened eye (proportional to the amount of darkening), clearly demonstrating that the eye takes longer to collect information when it’s dark (“longer exposure time”).
Even though the pendulum is moving from side to side, it appears to be moving closer and further away, due to the asynchronicity between the visual processing of the two eyes. This mismatch can be exploited to see our eyes in motion. You can see the setup in the picture above: a neutral density filter is held in front of one eye, while looking into a mirror with both eyes. The researchers were able to glimpse the last part of the movement. That’s kind of neat, but this simple experiment also provides some evidence in favor of one hypothesis over another:
If the suppression signal is generated centrally, then the introduction of a delay in the perceptual mechanisms in one eye, by the use of a dark filter, might result in a temporal mismatch between suppression and perception, allowing a glimpse of the eye in motion. Conversely, if suppression is retinal in origin, then the sudden dark- adaptation due to photoreceptor shear should itself be delayed by the introduction of a dark filter in front of one eye. Hence perceptual inflow and saccadic suppression would remain coupled in the filtered eye and so no movement of the eye would be visible. It is with these possibilities that we returned to our mirror to repeat Erdmann and Dodge’s self-observation, this time equipped with a neutral density filter and a desk lamp. With careful observation and a little patience and perseverance, we found that we were able to see the final phase of saccadic movements as we looked from one eye to the next. (…)
That we can use our simple technique to observe the terminal phase of our own saccades is consistent only with accounts in which it is proposed that saccadic suppression is generated by central mechanisms. Our straightforward demonstration argues against the recent suggestions by Castet and colleagues that suppression might be retinal in origin. Given central suppressive mechanisms, the explanation of this effect is simple and fits with what we know about the Pulfrich effect and the probable genera- tion of saccadic suppression. Saccadic suppression is likely to be generated from an efference copy or corollary discharge of the eye-movement signal. The movements of the two eyes are synchronous and occur at the time consistent with the efference copy. In the light-adapted eye, the result is as usual: the suppression signal switches off perceptual inflow for the duration of the saccade. In the dark-adapted eye, however, the perceptual inflow is delayed and hence asynchronous with the suppression signal.
dailymeh posted this on December 9, 2011
