Theories of Perception.

!i~al perception may be defined as a biological process whose goal is to rapidly compute a three-dimensional representation of the world that the organism can use for navigation and for object manipul~How is this achieved? In the nineteenth century the German physicist Hermann Von Helmholtz pointed out that the visual image is inherently ambiguous and that perception is essentially a matter of resolving ambiguities by using knowledge of the external world. I can think of no better illustration of this principle than the so-called "Ames room" -an example of which may be seen at San Francisco's Exploratorium. The Ames Room is a life-size distorted room, one corner of which is made much bigger than all the others. But if you stand outside the room and look inside through a small peephole using one eye the room looks completely normal! The standard textbook explanation of this illusion is that from this vantage point the retinal image produced by the distorted room is identical with (and therefore indistinguishable from) that of a normal room. But surely this explanation begs the question of how the visual system "knows" which of the two interpretations is normal. Or to put it differently, how does it know the room is not distorted? If you shut one eye and look at the room that you are in right now, the image that exists on your retina is in fact compatible with an infinity of distorted Ames rooms. How does the visual system discard this infinity of Ames rooms and home in on a single interpretation? This is the central problem of perception. If you examine the history of ideas on perception during the last century or so you will notice that there have been three major trends in thinking:*