Pylyshyn was a supporter of the propositional theory, which he believed had no place in cognitive psychology. He believes that mental images are just occurrences that happen in a conscious state. Whereas Kosslyn believes the analog images occur only in an unconscious state, and were very much a part of cognitive psychology.
The analog representations, supported by Kosslyn, are more like pictures formed in the brain. “similar mechanisms in the visual system are activated when objects or events are imagined as when they are the same objects or events are actively perceived” (Finke, 1989).
Research into analog coding includes that of Kosslyn at all (1978), where they asked participants to study a map and focus on a names object, then to pick a second object and mentally focus on this, and when done so pressing a button. Results showed that the reaction time for the participant to mentally focus of the second object was longer when the second object was further away from the first one. This research shows participants going through a process involving the distance between imaged analog objects to the actual physical operation of the physical objects.
Other research carried out by Shepard & Metzler, (1971) into mental rotation, participants were asked to identify if two drawings of 3D objects were the same or different, when rotated. The results show that the participants had to make some sort of mental transformation with the second drawing, to decide whether it was the same as the first or not. Also that the participants were using 3D mental images of the drawings, therefore rotating the images in a 3D space in their brains.
During visual tasks the occipital visual cortex is used, which is on the right cerebral hemisphere. Having a smaller image to represent the closer to the centre of the visual field, as the image is smaller therefore is more concentrated to the centre. This would suggest that when using mental imagery the right cerebral hemisphere is used more. However research carried out into participants with brain lesions can show exactly which parts of the brain are being used.
Research has been carried out into whether people with brain damage can still have mental imagery. For example Kosslyn (1985) studied two patients who had a severed corpus callosum, and found that the left hemisphere could produce single and multipart images, and could review imaged patterns. Whereas the right hemisphere could produce an image and its overall shape, but had problems in creating multipart images.
Further research by Grossi et al (1989) studied to men, one with a lesion on the right hemisphere, and a second with a left occipital lesion. Results of the study showed that the participant with a lesion on the left hemisphere found it difficult to perform both perceptual and imagery tasks. Whereas the participants with a lesion on the right hemisphere, could perform perception tasks but not imagery. These results would suggest that the right hemisphere has spatial exploration at both perceptive and imaginal levels, whereas the left hemisphere is used to create mental images.
This research highlights which hemisphere helps in mental imagery, showing that the left cerebral hemisphere can be used in forming parts of mental images, but the right can produce an overall shape of the image, but it can’t be broken down. Therefore combining the two hemispheres will allow for a complete mental image, which can be broken into smaller images, and therefore altered.
Research into the propositional theory suggests evidence which may contradict that found from the analog theory. Some findings show that when there are lesions on the visual cortex the visual field may not function properly, hindering mental imagery. This would therefore suggest that there is some aspect of propositional representations involved in mental imagery
The propositional theory is based on the use of a more verbal area in mental imaging, and whether the use of verbal labelling can help mental imaging. This is favoured by the likes of Pylyshyn. It suggests that an image is formed but is backed by verbal cues. Research has shown that seeing and imaging use similar areas of the brain which support the fact the imagery and processing share similar processing mechanisms, also that which region of the brain is being used during imagery depends of the content of the image.
This propositional theory is backed by behavioural evidence, for example a study by Farah presented participants with the letter ‘H’ followed by a brief interval. They were then shown another screen for a very short amount of time with either a ‘H’, ‘T’ or nothing on it. The results showed that participants are more likely identify ‘H’ than ‘T’ when asked what they saw.
Other evidence supporting the propositional theory is that when someone has a lesion on the visual cortexes that have led to perceptual problems they also have problems with mental imagery. Also PET scans show that only visual imagery leads to the activation of the visual cortex not the auditory imagery.
The propositional theory is contradicted by the research carried out in the analog representations into rotation and scanning. As the theory suggests that the image is back using a description, whereas if this was the case it wouldn’t be possible to rotate an image in your head.
Some conclusions drawn from research show that complex or abstract mental images may need to be backed with verbal labelling, whereas more simple mental images may not need to. Therefore it would appear that both propositional and analog codes work in mental imagery, and may work together.
Overall conclusions seem to suggest that mental imagery relies upon a mix of both propositional representations and analog representations. The research above suggests that both the propositional and analog theories can create mental imagery, however some of the evidence such as the ability to rotate or the fact lesions on the visual cortex hinders mental imagery suggests that it is not solely propositional or analog. They appear to work together to then be able to create a full and exact mental image. Also that using both hemispheres allows creating a full and exact image, which can also be altered and taken apart as it can be seen in multiparts.
References:
Finke, R, Pinker, S, Farah, M (1989), Reinterpreting Visual Patterns in Mental Imagery, Cognitive Science 13(1) 51-78.
Grossi, D, Modafferi, APelosi, L, Trojano, L (1989), On the different roles of the cerebral hemispheres in mental imagery: the ‘o’clock Test in two clinical cases, Brain and cognition (10) 1 18-27
Kosslyn, S, Ball, T, Reiser, B (1978) Visual Images Preserve Metric Spatial Information: Evidence from Studies of Image Scanning, Journal of Experimental Psychology: Human Perception and Performance, 4(1), 47-60.
Kosslyn, S, Holtzman, J, Farah, M, Gazziniga, M (1985), A computational analysis of mental image generation: Evidence from functional dissociations in split-brain patients. Journal of Experimental psychology 114 (3), 311-341
