Poppy phenomena 
I wrote this back in February, but saved it with the intention of honing it further. The examples, which had been on the whiteboard in my office, have now been replaced by some logical formula. So it must be time to post it.

In Patterns of Discovery, Norwood Russell Hanson provides a figure like this one:

He writes: "Normal retinas and cameras are impressed similarly by figure 1. Our visual sense-data will be the same too. [Yet...] Do we all see the same thing? Some will see a perspex cube viewed from below. Others will see it from above. Still others will see it as a kind of polygonally-cut gem. Some people see only criss-crossed lines in a plane. It may be seen as a block of ice, an aquarium, a wire frame for a kite-- or any number of other things."

Take a moment to look at the figure and see it in each of the ways Hanson suggests. Don't merely acknowledge that such a figure could be seen in such a way, but try to actually see it in each of the ways he lists. I want to make a point about the phenomenology, and it is no good for me to tell you what your phenomenology is like. (If your phenomenology fails to support the distinction I draw below, please mention this in the comments.)

When I switch between seeing it as a cube from above, a cube from below, a gem with only the center square jutting forward, or mere lines on a page, there is a shift. It is a kind of 'pop' as the figure changes aspect.* When I suppose that it is a cube seen from above and change what I imagine its composition to be, I experience no such change. I imagine it to be a block of ice, a glass box, or a wire frame with no change in my perception of it.

The contrast can be drawn out clearly by considering another case:

You will probably see this as just a square. Yet, it may be seen as a napkin, a handkerchief, a ceramic tile-- or any number of other things. Take a moment and run through these possibilities.

Although figure 2 might be seen in these different ways, the switch between them does not involve the pop that occurs when the faces of figure 1 shift perspective. Moreover, I cannot imagine any two ways of seeing figure 2 such that the shift between them causes such a pop.

Here is a trickier case, adapted from another of Hanson's examples:**

Take a look at this for a moment before reading on.

If you can make no sense of it, then it is just a jumble of lines. It is, however, a drawing of a tree trunk with a koala bear climbing on the other side. Perhaps there is a pop when one first sees the figure in this way. With just this interpretation in mind, however, I feel no pop when shifting back to seeing it as mere lines or back again to seeing it as a koala on a tree.

In order to get any pop out of figure 3, I need to think of it as something else. For example, I can see it as the cross section of a vertical tunnel which is lined with sharp gears. When I switch between this and the koala on the tree, I feel the pop.

Hanson overlooks this difference between the changes that pop and those that do not. Yet it seems like an important feature of observation that some differences are visceral and others more intellectual. Arguments from the theory-ladenness of perception often supposed that theory makes a popping, visceral difference in how the things show up to us. I do not know the current state of psychology, but I wonder whether the difference between the visceral and intellectual shifts indicates some different underlying mechanism or is merely epiphenomenal.

* It might be more precise to call this pop a 'gestalt switch.' A similar shift occurs when considering an inkblot. Someone describes a particular splodge as a dog, pointing to the head, the legs, and whatall else. If I merely entertain the possibility that it might be a dog, then it still just looks like a splodge of ink to me. If I see it as a dog, then it gets collected together in my perception of it.

** Thanks for John Styles for suggesting the revised version of the example.

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