Briefly gaze off to the side of the screen, to something way off in your side vision (try not to rotate your head).  Maintain your gaze on that spot for a few seconds, and then quickly fixate your gaze back on the screen and onto the second hand of the clock below.  Did you notice it? If you timed it right, you may have noticed the first tick after fixating on the second hand seemed to take longer than all the other ticks. (If you didn't notice it, keep trying, it has to be timed just right).


This phenomenon is called chronostasis, and the exact mechanism behind it is up for debate.  One of the most cited explanations is that when we change gaze from one object to the next we perform a rapid eye movement called a saccade.  During a saccade, our brain stops processing visual input (effectively blinding us), and only starts again once the saccade stops and we begin fixating on a new target.  You can experience this for yourself by looking at yourself in a mirror, and then quickly glancing from looking at your right eye to your left eye.  Notice how you didn't see your eyes move in the mirror?  They definitely did move, however during the saccade your brain effectively "shutdown" your vision, only turning it on again once you halted your saccade and fixated on the left eye.  The brain does this to keep us from getting overwhelmed by the tremendous amount of motion blur we would perceive throughout the day during the tens of thousands of saccades we make each day.  It would be analogous to watching a movie without any cuts, and instead having the cameraman constantly swinging the camera around as fast as possible trying to pick up the next scene.  This would obviously be very annoying, and would be similar to the motion blur we would perceive during a saccade.   Therefore the brain does us a favor and suppresses this motion blur until a new target is fixated on. 

So how does this explain chronostasis? Well, when we flick our eyes from target to target, the constant suppression of our sight during saccade would create significant "black out" periods if we did indeed not process anything during the saccade. Since this probably is only marginally less annoying than seeing constant motion blur, our brain rectifies this by taking the target object that we resume our fixation on, and "backfilling" the blackout period with this image. So if you gaze at an object off to your left for 2 seconds, and then quickly shift gaze to an object on your right for 2 seconds, your eyes technically sent data to the brain saying "2 seconds of object A, 0.5 seconds of blur during transit, and 2 seconds of object B" for a total of 4.5seconds. Since your brain suppresses the 0.5 seconds of motion blur, what your brain does is backfill that time with the image of object B, so your brain tells you that you saw 2 seconds of object A, and then 2.5 seconds of object B.  Using this example, you can now see why the first tick on the clock experiment above appeared longer, since once we pick up fixation on the second hand, the brain backfills our perception, and tricks us into thinking that we have been looking at it for 0.5s, even though it had just ticked*.  So by the time it ticks again, we feel like it's been 1.5s since the last tick.  This explanation does seem to have some merit, neatly tying in the saccade blindness phenomena, while also explaining why the effect is more prominent the longer the saccade is.    However, the same phenomena has also been noted with our hearing too, and we all probably have experienced this too (quickly putting an already ringing phone to our ear sometimes gives the impression that the first ring is much longer than the rest).  The fact that this phenomena happens with the auditory system too makes the saccade explanation seem possibly incomplete, although still intriguing. 


*This is why we need to glance at the second hand immediately after it has ticked to experience the effect.  If you glance at it during mid-tick, you brain will add the 0.5s saccade backfill to it, making it feel like a full 1 seconds between ticks, which wouldn't raise any suspicion since it feels like the normal rhythm of the clock.  Also, the further (and longer) the saccade, the more time that gets backfilled, and the more the effect is noticeable. 


Dr. Burke is an optometrist practicing at Calgary Vision Centre.  He first noticed this phenomenon for himself on a coffee break when he was a teenager working at his first job.  He kept glancing back and forth at the clock since he thought it made his coffee breaks longer. Opinions above do not constitute medical advice, and readers should consult with their optometrist if they have questions or concerns about their eye health.