Science Questions

Do my eyes have anti-shake vision?

Sun, 5th Jun 2011

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Question

Lionel Marrion asked:

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.

Answer

Chris -   You can actually find the answer lurking in your inner ear on each side.  Itís in a system called your vestibular system and this is your organ of balance.  What you have are three tiny semi-circular canals.  These are actually smaller than a one penny piece each. They're a tiny canal, they contain fluid and they're orientated at 90 degrees to each other.  So you have one which is a hoop going over your head towards the front, one which is at 90 degrees to that so itís going from one ear to the other, and then one like a dinner plate lying flat.  

And these three together can detect the movement of the head in any direction and the rate of movement.  They send signals via a nerve supply to the brain and they're connected to the nerves that control your eye movements.  And so what they do, whenever you move your head in any direction, this movement is picked up by this vestibular system and it then makes your eyes move in completely the opposite direction at the same rate and amount to directly compensate for the movement of your head and this is called the vestibule-ocular reflex and itís the reason that you're going to hold a finger out in front of you, fix it up with your eyes and then shake your head backwards and forwards, and maintain a continuous gaze on your eye without everything looking shaky.

And if something goes wrong with that vestibular system, you do feel very giddy because you've lost your own built-in fuzzy logic.

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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? Lionel Marrion , Mon, 23rd May 2011

Q. Do my eyes have anti-shake vision?

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



Your head isn't shaking when watching the playback RD, Mon, 23rd May 2011

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?) Lionel , Mon, 6th Jun 2011

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

chris, Mon, 6th Jun 2011

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

http://en.wikipedia.org/wiki/Saccadic_masking#Mechanism_for_saccadic_masking
RD, Tue, 7th Jun 2011

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