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17 September 2014
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Is Seeing Believing

1. Spinning disks - After Effect

When you look at this spiral for several seconds and then look away to a stationary object, it may appear to expand or contract depending on the direction the spiral was turning when you were staring at it. This is called a Motion After-effect.

This happens because we have different visual neurons in our brain that are sensitive to either expansion or contraction (as well as other directions of motion). If we focus on a stimulus that is consistently moving in only one direction, then neurons sensitive to that direction of motion will become less responsiveThis 'adaptation' makes sure the brain remains sensitive to any changes from what it's currently seeing.

The consequence, however, is that when we look at a stationary object after a period of adaptation, the net balance of the activity of neurons sensitive to different directions of motion shifts towards those sensitive to directions opposite to the adapting direction of motion. With the result that we see objects moving in exactly the opposite direction.

2. Impossible Slopes - depth perception

We see the balls appear to roll up the slopes of the model towards the middle.

This is because all the information in the image is consistent with the supporting legs of the model standing at right angles to the floor. Because of this, the brain interprets the middle leg as being the tallest leg in the model. Which means the ramps must actually slope upwards. As a result, the rolling balls appear to roll up - now down, which they really are.

Because the image hitting the retina is only 2 dimensional, in order to see the world in 3D, our brain must create its 3D perception based on its past experience of the most likely 3D configuration given the 2D image. Sometimes this means even perceiving a world that goes against the laws of physics.

When you change your perspective you see the slopes actually point downwards into the middle.

3. Flicker Test - Change blindness

This is called the Flicker Paradigm. It's difficult to spot the difference between the photos because a single blank frame has been inserted on the screen between the photos.

The reason why this happens is related to why the cinema works. When the brain sees subtle differences between two consecutive photos, if the time between presenting the two photos is quick enough, the brain sees that difference as motion: as if the object has actually moved. This perceived motion, then, draws out attention to that location in the photo, helping us to become aware of the change between the two. However, when we insert a blank frame between the two photos, the brain is no longer able to see the subtle difference as motion. As a result, our attention is not drawn to that part of the photo containing the difference, which causes us to not become conscious of the change. If we remove the blank frame, your eyes will now instantly be drawn to the change in the picture.

We often aren't aware of what we don't pay attention to because our brain is unable to take in all the information presented in the image in one quick observation, and must instead focus on a small part of the image at any given time. How this actually works is a complete mystery. In fact we don't really have a good definition of what attention really is.

4. Magic Singh - Inattention blindness

What you have just experienced is Inattention blindness. You don't see things that are in plain sight because your attention is not focused on it.

Our visual system has to take information in quickly but we have limited ability to encode everything out there. We literally can't see everything at once.

Instead we both 'see the whole before the part', and then we systematically select the information within the whole that is of most behavioural importance to us. That region we tend to focus on with high acuity is called the Fovea, which only covers about the same area as your thumb, when your arm is held out straight in front of you.

5. Brightness Illusion - colours out of place

If you put the mask on the picture you will see that they are in fact the same brightness.

This happens because our brain does not create its perceptions of brightness from only the intensity of a single surface or light in a scene, but according to the relationships between all the surfaces and lights in the scene. In other words the brain always uses context to figure out what's in front of it.

6. Lilac Chaser

If you can fix your eyes on the central cross long enough you can see a demonstration of several characteristics of the visual system.

The green disk is an illusory colour afterimage produced by seeing the complementary colour lilac. The illusion of movement is produced by successively seen stationary images like on TV. The lilac disks disappear because unmoving blurry objects in peripheral vision fade away and the effect is called Troxler's Fading.

7. Spinning diamond

It can appear to spin in both directions because the sequence of 2-dimensional images on your eye is identical for a 3-D diamond spinning either to the left or to the right. When we look at the spinning diamond, our brain doesn't actually know which way it is spinning. And because both directions of motion are possible, it bounces between the two options depending on where in the diamond we are looking.

8. Coffee Bean illusion

The tiny detail of the face can only be seen if you look directly at the face. Looking just to one side or the other by a tiny amount means that you can't see the detail. This shows that only our very central vision can see the finest detail. It can take several minutes to find it because the image is crammed with information and our face-detection neural pathways initially become overloaded. But once you know where it's located, you can't help but see it straight away.

9. Shepards Table

The dimensions of the green table top, are the same as the table on the right, the red table. In fact the red table is the green table rotated. All that's different between them are the angles at the corners of the tabletops.

The reason we cannot see these two tabletops as having the same length and width the information in the image, for instance the shadows, texture on the floor, angles of the legs provide us with a perspective cues that cause us to see them differently.

Their different perspective influences our interpretation of the tables three dimensional shape because we are using our experience with these cues to guide our perceptions.

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