Tuesday, February 15, 2011

Gibson on Information and Misinformation 2

Let me note another feature of the following:
It is because the affordances of things for an observer are specified in stimulus information. They seem to be perceived directly because they are perceived directly.

The central question for the theory of affordances is not whether they exist and are real but whether information is available in ambient light for perceiving them.  The skeptic may now be convinced that there is information in light for some properties of a surface but not for such a property as being good to eat. The taste of a thing, he will say, is not specified in light; you can see its form and color and texture but not its palatability; you have to taste it for that.  (Gibson, 1979, p. 140)

If there is information in the ambient light for the affordances of things, can there also be misinformation? According to the theory being developed, if information is picked up perception results; if misinformation is picked up misperception results.
The brink of a cliff affords falling off; it is in fact dangerous and it looks dangerous to us. It seems to look dangerous to many other terrestrial animals besides ourselves including infant animals. Experimental studies have been made of this fact. If a sturdy sheet of plate glass is extended out over the edge it no longer affords falling and in fact is not dangerous, but it may still look dangerous. The optical information to specify depth-downward-at-an-edge is still present in the ambient light; for this reason the device was called a visual cliff by E. J. Gibson and R. D. Walk (1960). Haptic information was available to specify an adequate surface of support, but this was contradictory to the optical information. When human infants at the crawling stage of locomotion were tested with this apparatus, many of them would pat the glass with their hands but would not venture out on the surface. The babies misperceived the affordance of a transparent surface for support, and this result is not surprising.
Similarly, an adult can misperceive the affordance of a sheet of glass by mistaking a closed glass door for an open doorway and attempting to walk through it. He then crashes into the barrier and is injured. The affordance of collision was not specified by the outflow of optical texture in the array, or it was insufficiently specified. He mistook glass for air. The occluding edges of the doorway were specified and the empty visual solid angle opened up symmetrically in the normal manner as he approached, so his behavior was properly controlled, but the imminence of collision was not noticed. A little dirt on the surface, or highlights, would have saved him.
These two cases are instructive. In the first a surface of support was mistaken for air because the optic array specified air. In the second case a barrier was mistaken for air for the same reason. Air downward affords falling and is dangerous. Air forward affords passage and is safe. The mistaken perceptions led to inappropriate actions.  (Gibson, 1979, p. 142)
In the first passages on p. 140, one implicitly knows how affordances are supposed to structure light, then one can use this to predict what a subject will perceive.  On p. 142, however, one has cases in which the prediction is that the affordance for "walk-on-ability" and "walk-through-ability" should structure light in such a way as to have babies go over the "invisible cliff" or have adults not walk into the glass occluder.  Yet, rather than acknowledging the failed predictions, Gibson instead postulates information in the light the will save the phenomena. This is the kind of move that Fodor and Pylyshyn were complaining about when saying that the notion of what is directly perceived is unconstrained.  It's the kind of problem that TSRM hint that they are trying to solve, but which I have been arguing they do not.

1 comment:

  1. I don't understand the objection. The first paragraphs aren't about predicting what information affordances produce, and the examples are situations where the surface failed to structure light in a manner to specify the actual affordance in question, with (predictable) results. I don't see the problem you say is there.

    This is the kind of move that Fodor and Pylyshyn were complaining about when saying that the notion of what is directly perceived is unconstrained.
    No it's not. They thought Gibson's theory allowed for perception to simply detect the one unique property every object has (namely, being that object).