shift invariance

[DeLiang Wang]

Jerry Feldman writes

>2) Understanding how the visual system achieves shift invariance.
>
>  This thread has been non-argumentative. The problem of invariances and
>constancies in the visual system remains central in visual science. I can't
>think of any useful message-sized summary, but this is an area where
>connectionist models should play a crucial role in expressing and testing
>theories. But, as several people have pointed out, we can't expect much from
>tabula rasa learning.

I'd like to know the evidence that the visual system achieves shift
(translation) invariance (I'd appreciate references if any). It seems
that the eye "focuses" on the object of interest first. In other
words, the eye seems to shift with the object, not that the visual system is 
recognizing the object wherever it occurs on the retina.

There seem to be problems with a system that DOES recognize an object no 
matter where it occurs, when the system faces more than an object as we
confront all the time.

> The unlearnability of shift invarince is not a problem in practice because
>people use preprocessing, weight sharing or other techniques to get shift
>invariance where it is known to be needed. However, it does pose a problem for
>the brain and for theories that are overly dependent on learning.

Why does it pose a problem to the brain? Perhaps the brain is doing what's
regarded as "preprocessing" (a black hole containing many "troubling" things).

I do agree that there are limits to tabula rasa learning. The
reason that we can learn things we do is, perhaps, critically linked to the 
prewiring of our brain. We know that we have a lot of difficulty in 
training a chimpanzee's brain to learn our language, let alone 3-layer 
perceptrons with backprop.

DeLiang Wang