CONSTRAINTS ON IMAGERY I David Pearson Room T10, William Guild Building

Over last 30 years the majority of research on mental imagery has tended to focus on similarities between imagery and perception rather than potential differences. Finke argues for a functional role for imagery during cognition: i.e., that imagery can support the same kind of discovery and interpretation processes that occur during normal perception.

For this to occur: (a)mental images must depict the geometry of a shape sufficiently well enough to allow for mental reinterpretation. (b)individuals must be able to apply shape classification procedures to the information depicted by mental images.

Reed and Johnsen (1975) contrasted the detection of embedded figures in abstract patterns in either a perceptual or imagery condition. Perceptual condition: participants shown possible part, followed by an abstract pattern. Given 10 seconds to decide whether previously presented part could be detected in pattern or not. Detection of Embedded Patterns

IMAGERY CONDITION

Results of Reed & Johnsen (1975) In perceptual condition participants failed to detect parts embedded in the patterns on 14% of trials. In imagery condition this rose to 48%. This increased to 72% if trials were eliminated in which the parts had been detected without inspecting a mental image. Results suggest that people may find it difficult to apply shape classification procedures to mental images in order to recognise embedded patterns. This mainly applies to the detection of implicit information within the pattern; i.e., shapes which do not form part of the perceptually dominant structure.

Restructuring & Combining Within Imagery Verstijnen et al. (1998) argue that mental discoveries result from two main processes: restructuring the initial conception of a pattern, and combining separate parts into a new pattern. Verstijnen et al. carried out a series of experiments using a Component Detection task.

Results (Verstijnen et al., 1998)

Design students experience in drawing techniques allowed them to overcome the limitations of imagery by externally representing their mental image. This is known as providing stimulus support. In a second experiment, Verstijnen et al. examined how often novice and expert participants used sketching during the task with both explicit and implicit target component parts.

%age of time sketching for explicit and implicit targets

Both groups used significantly more sketching for implicit rather than explicit targets. Provides support for idea that the restructuring of a pattern is difficult to accomplish in imagery without external support.

Interpretation of ambiguous figures in imagery Other research which suggests that images do not function exactly as percepts has been carried out using perceptually ambiguous figures.

When ambiguous figures are visually presented to a naive viewer, a perceptual reversal occurs - viewer spontaneously shifts their perception of the figure between alternative interpretations. Ambiguous figures demonstrate conscious percept of an external stimulus contains more information than is specified in the retinal image alone. Percepts result from an interpretation of the information conveyed through our senses. If images are functionally equivalent to percepts, then ambiguous figures should spontaneously reverse in both imagery and perception.

This was directly tested by Chambers & Reisberg (1985) using the Jastrow ambiguous figure (1900).

Participants shown figure for 5 seconds. Participants asked to form an image of the figure and then discover the alternative interpretation of the figure just by inspecting the mental image. Participants had previously been given examples of other ambiguous figures, and were given specific hints on how to reconstrue the figure. If participants were unable to reconstrue the image, they then drew it out onto a blank sheet of paper.

Results show: Successful figure reversal using imagery alone 0% Successful figure reversal using drawing 100% This study was hugely influential as it seemed to demonstrate a massive difference between images and percepts..... to say the least, this seems to be a regard in which visual images are conspicuously not like pictures. Reisberg (1996)

Result appear to conflict with those of Finke (Finke & Slayton, 1988; Finke, 1990). Verstijnen argues that creative synthesis and invention require a combination process, while the reconstrual of ambiguous patterns and the discovery of embedded figures relies on the restructuring of an existing pattern. Evidence suggests restructuring is much harder to accomplish using imagery, unless some form of external support (like drawing) is available.

Pearson, Logie and Green (1996) examined performance of the guided image manipulation task (Finke et al., 1989) in either imagery or drawing conditions.

Number of correct identifications with and without drawing

Evidence suggests that under certain conditions imagery fails to support novel discoveries which can easily be made under comparable perceptual conditions. Two main factors may potentially limit imagery performance: (a) constraints imposed by the resource limitations of the cognitive processes which underlie imagery. (b) constraints imposed by the differences between internally-generated images and externally- created percepts.

The Resource Limitations Hypothesis Fundamental concept in modern cognitive psychology 1.All forms of cognitive activity require mental resources to be carried out. 2.These resources are finite in nature, and subject to capacity limitations. 3.If the resources necessary to carry out a cognitive activity exceed the amount of resources available, then serious disruption of cognitive activity will result.

Resource Limitations in Imagery The generation and maintenance of mental images requires mental resources. The more complex the imagery task, the more resources will be consumed. If the resource requirements of an imagery task exceed the capacity of the imagery system, the accurate performance of the imagery task will begin to fail.

Effects of complexity on mental rotation (Rock, 1973)

The generation and maintenance of percepts does not place great demands on mental resources because they are externally- created.

More mental resources will therefore be available when trying to make a discovery from a percept than will be available if using an equivalent mental image.