WHO WON THE IMAGERY DEBATE AND WHY?
The issue in the imagery is not whether brain images exist; (we each
have our own personal experience of imagery as testimony to this), but
whether mental imagery can best be explained by manipulation of
arbitrary symbols (as propositionalists such as Pylyshyn claim) or by
analogue proccessing using depictive representations (as argued by
Kosslyn).
According to computationalists all internal processes can be explained
by mindless mechanical procedures in which it is not the shape of the
symbols which plays any importance, but the rules (algorithms) which
are followed to achieve a result. They claim that the actual images
we experience are epiphenomenal to these procedures. The strength of
this argument is that computers using propositions seem to be able to
do everything that depictions can do and also provide working models
to corroborate their case. Kosslyn himself readily admits in his
'Image and Brain' paper that "one can always form a propositional
theory to mimic a depictive one" (pg.12.) It has been suggested,
however, that this standpoint infers that what each of us is actually
aware of does not play any role at all and if this is the case, then
one should ask why there is a mind present in each of us if mindless
processes can do it all.
The initial argument against images is that they cannot be directly
measured or displayed and that there can be no little person in the
brain (homunculus) to look at them, but Kosslyn was able to show
through a computer program that analogue processing does not need a
homunculus. In addition, by carrying out mental scanning experiments
he was able to confirm his hypothesis that if mental imges are
depictions (which unlike propositions retain the spatial properties
of what they represent,) then more time is needed to move attention
greater distances across imaged objects. Furthermore, the use of
depictions seems to make sense in the light of the neurophysiological
discovery that area V1 in the brain is retinotopically mapped
(preserving spatial relations) and that by using PET scans there have
been comparisons for activation sites for both seen objects and
mentally visualised objects (e.g. for size.)
If, as Pylyshyn argued, images are epiphenomenal, then damage in area
V1 should not affect imagery. It has been found, however, that
patients with hemispatial neglect also fail to report objects on that
side in an imagined scene and that if part of the representation of
the visual field is missing (e.g. in occipital lesion,) then the size
of the mental visual field is reduced.
Despite the above findings, the evidence for analogue processing in
the brain is not conclusive. Supporters of symbol manipulation as a
unitary amodal representational system have explained the mental
scanning experiments in terms of lists of propositions and networks
and there has been at least one clinical case of complete loss of
visual imagery with preservation of normal visual perception.
It can be seen then that there has not yet been a clear-cut winner of
the imagery debate which looks set to continue until there is a
compelling case which convinces everyone that one theory is correct
and the other is incorrect. In the meantime I am happy to accept
taht mindless symbol manipulation is the best way to describe what is
going on in my head (without ruling out the possibility that analogue
processing might be used for visual and motor imagery and bearing in
mind that although computers may be able to do what we do, it has not
been definitively proved that the way they do it IS actually the way
we do it.)
This archive was generated by hypermail 2b30 : Tue Feb 13 2001 - 16:23:42 GMT