(This was very much a skim read, and I confess I'm relying on Stevan
to rescue the especially poor bits .....)
There are two main issues to the imagery debate:
1. What is an image? Is it purely propositional, or partially pictorial?
2. Who "looks" at the image, if there is one?
Kosslyn cites several pieces of evidence suggesting there may be a
depictive (pictorial) element.
1. Maintaining a visual image impairs a contrasting visual
perception, but facilitates an identical visual perception.
2. Imagery can induce the McCullough effect, although only weakly
(afterimages of the complementary colour)
3. Imagery can induce perceptual illusions.
4. Times to process evaluations of imaged and perceived patterns
are similar; scan times of images also correlate with distances.
5. Brain scans suggest that the same areas are used for both
imaging and visual perception.
6. Patients with unilateral visual neglect often had a
corresponding lack of half a mental image.
7. Other perceptual deficits have been mirrored in imagery (eg
someone who can't recognise faces also can't recognise
faces).
(N.B. problems with introspective evidence; experimenter expectancy
- although Kosslyn did try and control for this; using brain damaged
individuals as evidence for normal functioning; and relating the
evidence from brain scans to the actual processes in the brain; also
much of the evidence can support either a pictorial or purely
propositional explanation).
However, this evidence led Kosslyn to ask whether visual imagery had
common mechanisms with visual perception? Kosslyn himself believes
that not only are images produced by the same processes that are
responsible for visual perception, but that effective visual
processing is dependent on the imaging process.
Visual perception: the high level visual system (ie at the level of
object recognition) has seven subsystems. These are used in both
perception and imagery.
Visual buffer (occipital lobe): input to the rest of the system
Attention window: selects activity from the visual buffer for
further processing.
Ventral system (inferior temporal lobes): receives information from
the attentional window and encodes shape, colour and texture.
Dorsal system (posterior parietal lobes): receives information from
the attention window and encodes spatial properties.
Associative memory (?posterior, superior temporal lobes): receives
information from the ventral and dorsal systems. If a single
representation is not produced, a search is started for the best
match.
Information look-up system (dorsolateral prefrontal cortex): based
on activation from the associative memory, a distinctive
characteristic of the object is chosen.
Attention shifting subsystem (frontal, parietal, sub-cortical areas):
directs the attention window to this characteristic. The ventral
system is then activated to encode the characteristic. The cycle
repeats until the object is recognised. This latter activation of
the ventral system (and activation of the dorsal system) can also
activate a stored representation, and a mental image results. The
image can then go throught he same cycles.
Evidence that these systems are involved in imagery:
Visual buffer: properties affect both perception and imagery in
similar ways, eg imaging an object to the point of "overflow" - the
larger the object the greater the distance at overflow.
Attention window: the smaller the area attended, the easier to
detect probe dots, in both perceived and imaged patterns.
Attention shifting subsystem: does not appear involved in imager. ??
a way of distinguishing imnage from reality.
(N.B. back to the problems of introspective evidence etc; also
control of the image - don't we have a certain amount of control over
when our images overflow, or whether we can detect dots, which may
affect the results?)
So, does imagery ride on the back of perception?
Objects can be recognised from many orientations, and imagery may be
involved in this.
When an object presents in a novel way, information from the input
can be matched to the stored representation, and the object can be
recognised. But, if there isn't enough information to get a proper
match, then an image of the next best match is generated for
comparison to the perceptual image. The generated image isn't so
much a template, as to provide the missing information. Furthermore,
as soon as a stored representation is activated (ie as the best match
inhibits the others) it begins to send an image for use as feedback
within this system.
To return to the original questions:
1. What is an image? Is it purely propositional, or partially pictorial?
2. Who "looks" at the image, if there is one?
Kosslyn suggests:
1. It has pictorial elements, although it may be processed propositionally.
2. It is part of the perceptual process, and the same mechanisms
involved in perception "look at" (process) the image.
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