Perceptual constancies

•Whether we are perceiving as a top-down or a bottom-up process, our brain has a way of organising the incoming stimuli so that we can make sense of it.

•How do we perceive things in 3 D when the image on our eyes is 2D? •Gibson (bottom-up) –We use visual cues directly from the visual array e.g texture gradient •Gregory (top down) –We learn about visual cues from experience   •What do you think? •If we use both eyes to get our cues, we are using binocular vision, if we use 1 eye, this is monocular vision

We have 2 eyes! Therefore we have 2 images.  This is known as retinal disparity – the slight difference in images received by your retina. Your brain ‘fuses’ them together.

Retinal (binocular) disparity: the closer the object is to the viewer, the more disparity between the images from each eye – helps us realise something is closer.   •Binocular convergence: –To see something clearly muscles move your eyes to make the image from the object fall directly onto the fovea (look at your picture of the eye to see where the fovea is). –As the object comes closer, your eyes have to turn in slightly to focus on it. The more they turn, the nearer the object is. –So your eye muscles are helping you perceive the distance of the object.

 These are used to imply depth.

• None are particularly strong alone but if you put them together they are powerful cues to depth and distance.

Interposition (overlapping) - when the first object is placed over a second object, the first object appears closer than the second, which is partially blocked

Relative Height - the object closer to the horizon is perceived as farther away, and the object further from the horizon is perceived as closer

Familiar Size - when an object is familiar to us, our brain compares the perceived size of the object to this expected size and thus acquires information about the distance of the object

Texture Gradient - all surfaces have a texture, and as the surface goes into the distance, it becomes smoother and finer. This can be shown by the animation below - as the rectangles change in dimension they appear to project into the screen.

Linear Perspective - when looking down two parallel lines, they are perceived to come closer and meet at one point

Shadow - objects don’t usually allow light to pass through and therefore cast a shadow. Some general rules for this phenomenon are:

with one source of light, all shadows lie in same direction

with an object on the ground, the shadow appears on the other side of the source

with a hole in the ground, the shadow appears on the same side as the source

the object covered by the shadow is perceived to be further away then the object in the light