Figure 914

A schematic diagram of how the image of (a) an elephant and a rabbit, and (b) a fly and a rabbit might result in the rabbit being imaged at different levels of detail. Adapted from Ghosts in the mind's machine: Creating and using images in the brain by Stephen Kosslyn. Reproduced by permission of the author. Copyright © 1983 by Stephen M.Kosslyn.

smaller (i.e., an elephant and a fly, respectively). The rationale here was that in the case where the elephant and the rabbit were imaged together, the elephant would take up most of the space and as a result the rabbit would be represented as being much smaller relative to the elephant. In contrast, in the case where the fly and the rabbit were imaged together, the rabbit would take up most of the space relative to the fly (see Figure 9.14a and b). Given the hypothesis that the spatial medium has granularity, the two different images of the animal-pairs should result in differences in the "visible" properties of the rabbit. In the rabbit-elephant pair many of the rabbit's properties should be hard to "see" whereas in the rabbit-fly pair most of its properties should be easy to "see". This difficulty in "seeing" properties should translate itself into differential response times in deciding on the presence of a property (e.g., whether the rabbit has a pointed nose).

This is exactly what Kosslyn found in his studies. Subjects take longer to see parts of the rabbit in the rabbit-elephant pair relative to seeing the same parts in the rabbit-fly pair. Furthermore, Kosslyn noted that subjects' introspective reports suggested that they were "zooming in" to see the parts of the subjectively smaller images.

More recently, Kosslyn, Sukel, and Bly (1999) have performed further tests on the resolution of the spatial medium using a task in which subjects either viewed or visualised arrays divided into four quadrants with each quadrant containing stripes. By varying the width of the stripes in the array it was possible to create high- and low-resolution stimuli. Kosslyn et al. found that subjects made more errors in both perception and imagery when evaluating oblique patterns, with more time being taken when imaging. The results suggest that although there are common mechanisms used by both imagery and perception it is more difficult to represent high-resolution information in imagery than in perception (see also Rouw, Kosslyn, & Hamel, 1997).

Experiments in the spatial medium

A further set of experiments by Kosslyn (1978) examined the idea of the limited spatial extent of the medium. Assume our visual field consists of a 100 degree visual arc in front of us. If we are looking at something in this visual field then at a given distance, the object will take up a portion of this arc. If we move closer to the object and it is a large object—like a double-decker bus—then eventually it will fill

Diagram of the relative amounts of the visual arc that are taken up by different-sized animals. Adapted from Ghosts in the mind's machine: Creating and using images in the brain by Stephen Kosslyn. Reproduced by permission of the author. Copyright © 1983 by Stephen M.Kosslyn.

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