Shape and Size Constancy
The retinal image is different to the image we 'see'. There are many occasions when round shapes such as cups have elliptical images and doors do not present as rectangles. But our brains portray them as 'normally' shaped and sized objects, regardless of how the images appear on the retina. In effect, our brains 'correct' images to make objects appear constant.
Real objects give a larger retinal image when they are close and a smaller one when they are distant. Again, the brain corrects for this by making distant objects seem bigger.
This can be demonstrated with after-images. If you stare at a bright light for a few seconds (never look directly at the sun for this), and then look away, you will see an after-image that has a fixed size and shape on the retina. However, when you look at a nearby object and then (quickly) further away, the after-image seems to change in size - appearing to be larger when we look into the distance.
Another example of constancy comes when looking at telegraph poles or trees. The poles look vertical, no matter how you incline your head. What seems to be happening is that the brain attempts to create a stable view of the world, regardless of what the body may be doing. Consider moving around in a room, climbing stairs, travelling on a bus. Your eyes must be presenting your brain with an erratic series of images from all kinds of angles. Howebver, the brain has information from other (propriocentric) senses that tells the brain where the body is in space and what movements it is making. The brain combines this with visual information to remove the awareness of movement when appropriate.- so-called location constancy. Without it, the world would swing around in a crazy fashion as we move our heads and our eyes. Interference with this process causes some of the characteristic side-effects of alcohol.
Other forms of constancy:
Paper is always seen as white, even in dim light, and coal is black despite shining in sunlight.
Objects tend to keep their under a range of lighting conditions. The brain modifies the information it receives about light, dark and colour in order to preserve these constancies.
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