shift invariance

[Eric Postma]

DeLiang Wang wrote
>I'd like to know the evidence that the visual system achieves shift
>(translation) invariance (I'd appreciate references if any). 

Biederman and Cooper (1991) found that an object presented at one location
of the retina facilitated recognition of that object at other locations. The
visual system does not achieve perfect ranslation invariance as shown by
Nazir and O'Regan (1991).

Biederman, I. & Cooper, E.E.  (1991).                                    
Evidence for complete translational and reflectional invariance in visual
object priming.
Perception, 20, 585-593.

Nazir, T.A. & O'Regan, J.K. (1990).
Some results on translation invariance in the human visual system.
Spatial Vision, 5, 81-100.

DeLiang Wang wrote
>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.
 ...and...
>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.

In addition to the selection of objects through direction of gaze (overt
attention), 
there exists an attentional process which operates independent of eye
movements. This process is known as covert attention and may be likened (to
a certain extent) 
to a searchlight. When fixing your gaze on a single letter of this text, you
may still be able to select and identify the adjacent letters and words. 

Inspired by Anderson and Van Essen's (1987) shifter circuits, we developed a
scalable model of covert attention capable of translation-invariant pattern 
processing (Postma, van den Herik, and Hudson, 1994, 1996 submitted).
Our model is similar to the model proposed by Olshausen, Anderson, and Van 
Essen (1993, 1995) and is based on the idea that attentional selection provides 
a solution to the problem of translation invariance and the problem of
selecting 
(parts of) objects. The attentional searchlight selects parts of a scene and
maps 
their contents into a pattern-recognition stage without affecting the spatial 
ordering of the selected pattern.

Anderson, C.H. & Van Essen, D.C. (1987).                                  
Shifter circuits: A computational strategy for dynamic aspects of          
visual processing.                                                         
{\em Proceedings of the National Academy of Sciences U.S.A.}, {\bf 84},    
6297-6301.                                                                 
                                               
Olshausen, B.A., Anderson, C.H., & Van Essen, D.C. (1993).           
A neurobiological model of visual attention and invariant pattern     
recognition based on dynamic routing of information.                  
{\em The Journal of Neuroscience}, {\bf 13}, 4700-4719.               
                                                                      
Olshausen, B.A., Anderson, C.H., & Van Essen, D.C. (1995).
A multiscale routing circuit for forming size- and position-invariant 
object representations.      
{\em The Journal of Computational Neuroscience}, {\bf 2}, 45-62.

Postma, E.O., Van den Herik, H.J., & Hudson, P.T.W. (1994).           
Attentional scanning.
In A. Cohn (Ed.), {\em ECAI 94, 11th European Conference on Artificial 
Intelligence} (pp. 173-177).
New York: John Wiley and Sons.

Postma, E.O., Van den Herik, H.J., & Hudson, P.T.W. (1996).
SCAN: s scalable model of attentional selection.
submitted to Neural Networks.


        Eric Postma
Eric Postma
Computer Science Department
Faculty of General Sciences
University of Limburg
P.O. Box  616
6200 MD  Maastricht
The Netherlands