The Space Between Our Ears: How the Brain Represents Visual Space
by Michael Morgan
The winner of The Wellcome Trust Prize
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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
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.