Wednesday, June 20, 2012

Saccadic Masking, Chronostasis, and Concsiousness

Time. What is it? To some, Time is a kid-friendly news magazine characterized by info-graphics and colorful pictures. To others, Time represents a continuum of continued progress of events from the past, present, and future. While Time is indefinitely continuous, its rate can appear quite variable, at least to humans.

Below is a clock face with an active seconds hand. Focus your gaze on something nearby (e.g., something just above your CPU monitor) then shift your gaze to the clock and watch time elapse for 5 seconds. What you should notice is that the first second to elapse appears to take longer than the subsequent 4 seconds (It's subtle, you may have to try this a few times to notice the illusion).
If you did notice time lag, then you just experienced chronostasis. Chronostasis is the experience or illusion that occurs immediately after a saccade that appears to stop time momentarily. This occurs so much that every day we experience 40 minutes of chronostasis, though it goes entirely unnoticed.

So what's going here? When our eyes move, the image reflected on to the retina is also in motion. This creates 
motion blur (see picture at beginning of this post). A blurred image being utterly incomprehensible (and of no use) to us sighted humans, our brains have a mechanism to circumvent the blur an create a comprehensible image. This phenomenon is known as saccadic masking. During saccadic masking, the blur is suppressed, along with visual processing, and the gap in visual processing that should be experienced as your eyes move from on side to another. The brain then replaces the blur with an image of the very next thing that your eyes fixate on.  

This explains chronostasis experienced in the clock illusion. As you shift your gaze to fixate on the clock, instead of seeing a incomprehensible blur, the blur is supplanted with an image of the clock that your currently fixated. That's why that first second to elapse appears longer than all other subsequent seconds.

The process is depicted below:

The first figure depicts what actually occurs, but only our subconscious brain perceives. Figure two shows what what we actually see. If you imagine that point two is the image of the clock, our brain then fills in the time that had motion blur with the same image. 

If the brain is replacing a past image with a current image, does that mean what I'm seeing is not really in the present but the past? Yes, in fact, human awareness or what we experience as the "present" is actually the very recent past; more specifically our consciousness lags 80 milliseconds behind actual events. This is how saccadic masking and chronostasis are possible; before we become aware, our brain has to make sense of stimuli first, which takes just about 80 milliseconds.

As it turns out, saccadic masking and the 80 millisecond lag in awareness explain many of the visual illusions that many may already be aware of such as the moving snake illusion. As you move your eyes looking at the image below, the illusion of motion is apparent. If you fix your gaze on a black dot, the illusion of motion will cease. 
There are other phenomenon as it relates to time perception: Time passing more quickly as we age and time slowing down as we become scared. Of course, the latter phenomenon should not be confused with time dilation, which is an actual difference of elapsed time (as opposed to perceived difference) between two events as measured by observers moving relative to each other. 

2 comments:

BeemerRider said...

Now I have to. Try and figure out how to save this on the damn iPad! Great post

Glenn

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