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Life Sciences => Physiology & Medicine => Topic started by: Lionel Marrion on 23/05/2011 18:30:03

Title: Do my eyes have anti-shake vision?
Post by: Lionel Marrion on 23/05/2011 18:30:03

Lionel Marrion  asked the Naked Scientists:
   
Hi Chris,

I recently bought a mini video camera for my bike trips to and from work. I noticed when i watch the video back that there can be considerable camera shake when i go over bumps, for example,  (I do believe some of the more expensive cameras have anti shake devices) and blurriness when changing direction quickly. In fact i can get a bit queezy looking at the video replay.

So how is it this doesn't happen with the images that are going into my head. I know that the eyes fixing on a given spot might help, but i suspect that my brain brain is doing some pretty sophistcated editing. What's going on; does my brain have anti shake, too? Why doesn't it use anti-shake when i wtch the video back? And might this be to do with the brain predicting images before they actually happen?

All the best, 

Lionel, Spain.

What do you think?

Title: Do my eyes have anti-shake vision?
Post by: RD on 23/05/2011 19:01:33

Q. Do my eyes have anti-shake vision?

A. Yes ... http://en.wikipedia.org/wiki/Vestibulo-ocular_reflex

Quote from: Lionel Marrion  on 23/05/2011 18:30:03

Why doesn't it use anti-shake when i wtch the video back?

Your head isn't shaking when watching the playback

Title: Do my eyes have anti-shake vision?
Post by: Lionel on 06/06/2011 17:17:58

that answers the main question very well(thank you very much), but at the risk of moving the goalposts, I'm not sure that covers the blurring aspect. If i look at the car in front of me, then turn my head to look at a bird in a tree, i see the car, then the bird in the tree, but i don't get a whoosh of blur in between (like I see on the camera playback).

Surely this is not explained by the same mechanism. I am tempted to think it has more to do with like (I'm told) when a brain edits out the blackout when we blink. (I wonder what pirouetting ballet dancers see as their heads quickly spin round to face the front again?)

Title: Do my eyes have anti-shake vision?
Post by: chris on 06/06/2011 23:10:16

Hi Lionel

There are two aspects to this latter part of your question.

When you move your head suddenly to fixate your gaze on a new visual target the first thing that happens is that the eyes perform a very rapid movemnt called a saccade. The eyes literally flick from one object of vision to the next, moving between the two at the rate of over 1000 degrees of arc per second i.e. very rapidly. While this is happening there is a suppression of visual processing so the two visual images - the one you were looking at and the new target - cross fade like you see on the telly. This is because there is a long latency in the visual system - it takes over 150ms for a visually-presented stimulus to be consciously attended to.

I cannot recall where the processing suppression occurs - it could be thalamus, but I'm not certain - maybe someone else can help me?

Chris

Title: Do my eyes have anti-shake vision?
Post by: RD on 07/06/2011 02:01:54

Wikipedia has a page on "saccadic masking" ...

Quote

Because saccadic suppression starts before the actual onset of the saccade,
 it cannot be triggered by retinal motion and must be centrally activated by the brain.

http://en.wikipedia.org/wiki/Saccadic_masking#Mechanism_for_saccadic_masking