370: Optic Flow

OPTIC FLOW

As you move through your environment you see everything moving by you 

but your perception is that they are stationary.  

APPROACHES TO OPTIC FLOW

Behavioral Approach

There is a local disturbance in the optic array (e.g., you fixate on the environment and something moves across your vision)  There is a local disturbance in the optic array when you move your eyes to follow the stimuli that moves across the scene.  A global disturbance is when everything seems to be moving because in fact you are the one that is moving.

Physiological Approach

Vestibular information is your sense of motion so when spin for a while even when you stop spinning you will feel as though you are still spinning.  This is an example of when vestibular information is contrasting with your visual information.  Why would you feel nauseous in these types of situation?  The reason for throwing up is to get rid of toxins in your body so if you activate this area of postrema (the area that controls vomiting) you will throw up.  If you feel dizzy then that is a good indication that you have toxins so your body will interpret that something is wrong and probably a toxin so it is adaptive to throw up.

COROLLARY DISCHARGE THEORY

The idea is that if you move your eyes there is a signal sent to you eyes to move eye muscles that we call a motor signal (MS).  There is a copy of the motor signal called the corollary discharge signal (CDS) that goes to the visual system and converges with the image movement signal (IMS), that comes from the eye, and if there is a difference between the IMS and the CDS then we perceive movement.

Some of the prediction of this theory include...

If you were to immobilize your eye muscles you could still get a CDS but not an IMS which means that the world will appear to be spinning around you.  This is because the eye will remain stationary when you try to move it.  

If I shine a light at your eye and then turn off all the lights and then you move your eye what would happen with the after image?  You will get a CDS but no change in the IMS so the after image will appear to be moving.

If an object moves and your eyes move then the image of the object will not move.  You will get an CDS but not an IMS change since you are following the image so you will perceive movement.

REAL-MOTION NEURONS

How do we go from V1 neurons only being able to see one portion of the scene to seeing everything?  It needs lots of inputs.  We have a global motion detector that takes these inputs from a layer right above V1 and combines all the information together to determine the global bit of motion that is going on.  Basically you are just taking a lot of bits of information and combining it to get the whole picture.  

MIDDLE TEMPORAL AREA (MT)

How sensitive are MT neurons to motion.  The idea is that you show these monkeys randomly moving dots and the question is  how coherent does the motion have to be to perceive movement.  You can vary experimentally the motion and monkeys are taught to judge direction of dot motion.  As the coherence of dot motion increased so does the firing rate of the MT neurons.  The actual percentage of coherence that you need to perceive motion is as low as 1-2%.  If there is an MT lesion then monkeys don't respond until about 20%.  

What if you stimulate MT?  Can you simulate motion?  The short answer is yes, you can. 

BIOLOGICAL MOTION

Biological motion is processed in the superior temporal sulcus (STS).  We are very good at identify biological motion even if we aren't quite sure what the actual object is.  An application of this type of biological motion is the recording of joints on the actor who played Gollum.  The reason that movies use people instead of just relying on animators is because even when there is a slight mistake in motion we are able to detect that something is off.