Kosslyn’s (1980) initial study of mental scanning used four groups of subjects to investigate the effects of distance in scanning mental images. After visually encoding 10 line drawings, participants were shown properties that either were or were not present in the line drawings. Two groups were asked to visualise the image during the test, one focusing on a corner of the image, and the other keeping the whole image in mind. The other two groups were asked not to use imagery, but to approach the drawings in terms of descriptions of the images; again, these were divided into those concentrating on one area, and those concentrating on the whole description. Kosslyn hypothesised that if we do see images in our mind in a spatial medium, it should take longer to scan to objects that are further away. This effect of longer scanning times was found, as were longer scanning times for those focusing on one part of the image description. Horizontal scanning was also much easier in those using mental imagery, and there were no effects of property location in both conditions focusing on the entirety of the image/description. These findings could be interpreted to show that images are depicted in a spatial medium, rather than simply being encoded as a set of physical descriptions. The fact that horizontal scanning was easier when visualising the image suggests not only that we do use mental imagery, but that mental imagery can aid recall, and it is more productive than a set of descriptions.
Conflicting evidence for the existence of mental images through scanning comes from Lea (1975). He conducted a similar study to that of Kosslyn, yet here, the results showed that the time taken to scan between points depended on the number of intervening items, not the distance, appearing to provide evidence for the existence of ordered lists or links instead of mental images. Kosslyn criticised Lea’s findings based on the fact that participants were asked to encode the image based on the number of steps from the focus to other locations, and therefore not encoding the image in the form of an image at all. He suggests that it is plausible that participants encoded the image as both an image and a list, where both forms would have their separate advantages in terms of recall. Kosslyn argues that encoding information about geographical locations in terms of a visual map as well as a list of distances would be useful; if we wanted to know how far one city was from another, we could consult the list, but if we were interested in quickly assessing what cities were near a particular location, a visual map would be more useful (Kosslyn, 1980).
Kosslyn did, however, recognise that his previous study did not account for the variable of intervening items, and so removed this in future experiments which will be discussed now (Kosslyn, Ball and Reiser, 1978). Kosslyn et al’s (1978) first experiment used letters staggered on a line to test both distance and intervening objects, and see if effects were present. They found, interestingly, that there were indeed effects in both distance and intervening objects. They defended their theory, however, suggesting that the effect of intervening letters was merely participants slowing down to process these items when passing over them, and that if they were accessing a list instead of an image, there should have been no distance effects at all. Further support came from the finding that scanning left to right and right to left took the same time, opposed to the verbal conditions in Kosslyn’s initial experiment where left-right was easier, suggesting that mental scanning involves different processes to reading.
The second experiment from Kosslyn et al., (1978) involved scanning 21 possible pairs of seven locations on a fictional map. Participants were asked to imagine a black speck following the most direct route between locations so as to ensure the shortest distance was being measured. Again, the results showed that father distances required more time, supporting the claim that images are quasi-pictorial representations. Kosslyn et al’s (1978) final experiment used three drawings of faces, and involved participants encoding them in terms of their ‘full size’ (the whole image being fully visible at a glance), ‘half-size’ (the image being half the size of the original) and an ‘overflow’ (enlarging the image so that only one portion of the face could be visible at a time). They found that time increased with further separation of features, and also that more time was taken to scan larger images. This suggests that the images visualised in the mind do have spatial dimensions like those of a visual image, supporting their aforementioned theory.
Although Kosslyn’s studies provide evidence that appears to account for the existence of mental images, many criticisms of the concept have arisen. Pylyshyn (1981) amongst others, suggest reasons why mental images could not be quasi-pictorial. Criticisms such as people being unable to count the stripes on a tiger in a mental picture seem to suggest that we are not actually viewing images, but merely referring to a list that states that tigers have stripes (Dennett, 1969). A counter-argument, however, is that not only do we have little need for fine detail in mental images, but also that details such as these were never actually initially encoded in memory (Fodor, 1975). Another criticism lies in the obvious point that we don’t have eyeballs in the mind that could scan these images. Sperling’s (1960) research on scanning afterimages, however, show that eye movements were irrelevant, invalidating this particular criticism. Pylyshyn (1981) argues that processing and storing images in our brains would require a huge amount of the brain to be devoted to this, and retrieval would be incredibly slow. Kosslyn’s reply to this, however, is that we have no idea about the capacity of the brain, nor about how much space would be required to store an image. Images could be easily categorised in some other way, making retrieval easy. Evidence from neuroscience shows an area of the visual cortex (V1) to be active when viewing mental images, providing evidence that the visual system is used, and that we really are visualising these mental images. Pylyshyn claims that this is not conclusive evidence, and it does not explain anything about the form or nature of these images. He states that both vision and mental imagery could use the same form, but neither is pictorial, and so it provides little evidence for Kossyln’s theory. Neuroscience also provides evidence which supports Pylyshyn’s view, however, in that people can have normal mental imagery with severe visual problems, and conversely, people with no visual problems who have little or no ability to produce mental images.
The above criticisms highlight the complex debate on mental scanning as an investigative approach, as well as a phenomenon in its own right. Scanning studies do provide strong support for this mental imagery, particularly with the consistent effects of distance. It seems conceivable that images exist as spatial representations, and the evidence implying the visual system’s involvement indicates an area that perhaps needs more research. Studies such as Bower’s (1970) use of mnemonics highlight how useful mental images appear to be for learning, and this combined with the highly vivid images so many of us use everyday, it seems difficult to resign oneself to the thought that we are merely accessing a list every time we visualise an elephant. However, without any conclusive evidence, it seems likely that mental imagery as well as the concept of mental scanning will continue to be a point of debate for some time.
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