One of the characteristics which distinguishes cognitive psychology
from behaviourism is the acceptance of a mental life - experiences.
Part of experiences, thinking, cognising, is to form some sort of
mental representation. The nature of these representation is the
subject of fierce debate.
Imagery has long been a contender for the medium of thought. Aristotle
considered imagery of prime importance, and the introspectionists (e.g.
Galton) wallowed in it. The behaviourists were suitably unimpressed,
and imagery lay dormant until cognitive psychologists took a fresh
look. However, the vague terms and imprecise definitions of the
previous era were no longer sufficient. Cognitive psychologists wanted
to know exactly what imagery was, what mechanisms operated the process,
and what use imagery was to cognition. Two basic sides formed - one
side considering images to be picture-like representations, and the
other side considering them to be forms of propositional
representation.
Paivo suggested there were two distinct systems for representing and
processing information. One system dealt with verbal material and
stored it in a verbal form; the other dealt with image based
representation and processing. Interference studies and
neuropsychological evidence appeared to provide support for this.
However, it still failed to explain exactly what images were, and how
they were represented. Shepard performed experiments in which subjects
had to mentally rotate an object in order to answer a question. It was
found that the further the object had to be rotated, the longer it took
the subject to answer. Kosslyn asked subjects to mentally scan a map
in order to answer questions, and again, the further the distance
"scanned" on the image of the map, the longer it took to answer. These
findings were taken as evidence that images were a form of analogical
mental representation. In contrast, propositional representations
consist of an abstract symbol system which can be used to express the
contents of the mind via predicate calculus.
The imagery debate began in earnest with Pylyshyn (1973) when he
claimed that picture-like entities were an inappropriate way of
considering images. He made two basic criticisms. First, the concept
of imagery was too vague to be accepted in its present form. Second,
and more importantly, imagery was merely epiphenomenal to the process
of thinking. Rather than images being the means by which thinking is
carried out, they are an effect of the thinking process. Basically,
Pylyshyn suggested that there was only one form of representation, and
this was propositional. Furthermore, it followed that if there was a
verbal and a nonverbal form of processing, then there must be some
superordinate code into which both could translate, which would act for
both carrying out the process and representing the abstract, underlying
concept. The superordinate code was propositional. All the evidence
produced to date could be re-interpreted in line with a propositional
model. If images were like pictures, then a "mind's eye" was needed to
see them; which would need another eye; and so on infinitely. This is
the homonculus problem - who looks at the image?
Several points were answered by "imagists". They countered that images
were not pictures as such, but were generated via perceptual memories,
and contained spatial information. Also, although propositional
representations could explain the empirical findings after the fact,
imagery would actually predict the findings. The homonculus problem
cannot be easily dismissed. However, there is another side to the
issue. In Pylyshyn's account, if a propositional code is necessary to
translate between nonverbal and verbal material, what translates
between verbal and propositional? Or between the new code and
propositional? Where do the initial propositions come from? Are they
innate? If so, how can they be interpreted? Or do they have to be
learned? If so, how are they learned, if thinking is via
propositions? This is also a homonculus problem, and therefore poses a
challenge to both sides of the debate. The concepts were not well
specified, and because of the lack of empirical evidence, it made this
account little better than introspection.
One of the most difficult points to answer was that imagery was
unnecessary, because, indeed, everything that can be explained by
images can be explained by propositions, even if longwindedly. Kosslyn
approached the problem by attempting to specify the distinctive
properties of images, and also remove the homuncular quality. He
proposed a theory which acknowledged an important role for
propositions, but also had an important role for images. Images were
represented in a medium which had specific properties, including the
capacity to portray spatial information. This theory was tested in
several experiments. The subject had to image various objects, and it
was found that the difficulty or time taken for the tasks was related
to the "visual" properties of the image. However, introspective
evidence of this sort can be misleading. Apart from experimenter
expectation effects, the presence of accurate spatial data is not
necessarily limited to non-propositional images. Neuropsychological
evidence suggested that selective impairments of imagery existed.
Although individually each piece of evidence was not conclusive, they
did converge together.
Meanwhile, Pylyshyn (1984) had developed his earlier ideas. He now
claimed that cognition was computation, computation was symbol
manipulation, and this was propositional. He criticised Kosslyn's
findings on several counts. The subjects could have been imagining
that they were visually scanning an object, rather than performing
operations on an image. By giving different instructions, the results
could be changed. This is a good point, but unfortunately, Pylyshyn
made it very complicated. He claimed that the mind had a functional
architecture. This was not influenced by beliefs and goals etc. The
medium for images would have to be in this architecture, and so the
images would exist there, too. As part of the "hardware" they would
not be influenced by beliefs etc. Pylyshyn termed this as "cognitively
impenetrable". As beliefs etc are propositional ("software"), then if
images are cognitively penetrable, they cannot be in the special
medium, and will be epiphenomena based on propositions. As changing
the instructions could affect the images, then the images were
cognitively penetrable.
However, because one construct can influence another, does it make the
second an epiphenomena? It could equally be argued that beliefs are
epiphenomenal because they can be influenced by images. His view of
images versus propositions may also be an inappropriate way of
resolving the issue. Kosslyn certainly thought so (1995). He
suggested that imagery could be both propositional and visual. He
further developed this idea, and proposed that imagery was related to
visual perception. Imagery did not ride on the back of perception, as
had been suggested, but was an essential component of it. When an
object presents in a novel way, information from the input can be
matched to the representation, and the object can be recognised. If
there isn't enough information to get a proper match, then an image of
the next best is generated for comparison for comparison to the
perceptual image. This is not a template, but rather is generated in
an effort to provide the missing information. As soon as a stored
representation is generated, it begins to send an image for use as
feedback within the system. Kosslyn did provide some empirical
evidence for this proposal, but not all of it was convincing. However,
it did share similarities with the way Jeannerod addressed motor
imagery. He suggested there was a link between motor imagery and
preparation. The image is a representation of the intention to act,
and will echo the movement itself, although not exactly, because it has
no biomechanical constraints, and may be influenced by the memories of
previous movements. Motor processes are not automatic in the way
sensory processes are. However, if the nature of the image depends
upon the representational mechanisms which process the sensory input,
it may well reflect this sensory input.
The debate is far from concluded. The main problems are: first,
anything which can be described by imagery can also be described by
propositions; second, accessing imagery tends to have been
introspectional, and performance needs to be more prominent; finally,
any account of imagery has to dispense with a homonculus to be
convincing. This appears, not surprisingly, difficult, but a computer
model may be called for at this stage.
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