Induced movement, illusory movement in a stationary stimulus resulting from adjoining movement, has received steady experimental investigation over the last 70 years or so. It is observed under different viewing conditions in a wide variety of displays that differ considerably in overall size and in form of inducing and induced stimuli. Explanations have been diverse, some being based on relations within the display and others invoking mediation by other aspects of the observer's perception. Probably, no one explanation can account for all forms of induced movement. Current knowledge about induced movement may have important implications for visual perception of object morion. Among possibly important factors is size of display, which can vary from two small spots (e.g., Carr & Hardy, 1920) to displays occupying most of the observer's visual field (e.g., Post, 1986).A second major purpose of this overview is to examine explanations for induced movement (Explanations section). Duncker's (1929Duncker's ( /1938 findings have had an important influence here, although many of his suggestions have subsequently been shown to require at least some modification. For example, his theory I thank J. Morss for reading an earlier version of this article, J. Ravey for translation, and two anonymous referees for helpful suggestions.

Background: Charles Bonnet Syndrome is defined as visual hallucinations in psychologically normal people and has been associated with low vision for over two hundred years. Objectives: To evaluate the prevalence and complexity of visual hallucinations in patients with low vision. Design: A crosssectional comparative analysis of clinical visual data and hallucinatory phenomena in two groups of patients with age-related macular degeneration and a third control group. Methods: A questionnaire on visual hallucinations was administered to 145 patients and 58 control individuals. Measurements: Visual hallucination type (simple, complex) was examined and group comparisons were made using analysis of variance, cross-tabulations, and percentages. Results: Those individuals reporting visual hallucinations had a significantly lower mean visual acuity than comparative groups. Materialization and frequency of visual hallucinations were dependent on hallucination type. Hallucination type was not dependent on visual acuity. While visual hallucinations were reported by 40% of patients, Charles Bonnet Syndrome as defined in this study, was prevalent in 21% of the low vision cases. Conclusion: Charles Bonnet Syndrome occurs in approximately 1/5 of patients with age-related macular degeneration. Visual hallucinations are dependent on visual acuity rather than on age or ocular condition. Greater awareness of this condition among the medical and allied professions is important so that patients can be advised and reassured accordingly. Consideration should be given to including Charles Bonnet Syndrome as a discrete Syndrome in major classification systems.

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Substantial rotatory induced movement and aftereffects associated with induced movement were observed in a large statis patterned disc bounded at its periphery by a rotating patterned annulus. The area of the annulus was less than one tenth that of the disc, so its peripheral location seemed to be important in eliciting these phenomena. This was confirmed in two experiments comparing a peripheral annulus and a relatively central anulus in their ability to elicit induced movement and aftereffects in the same large static field. Aspects of the vection (induced self-movement) phenomenon may have been involved in generation of induced movement. This suggested that the motion-inducing properties of the peripheral annulus might have derived from: (i) its eccentric location in the perceiver's visual field; or (ii) its location with regard to the display itself. Two further experiments showed that (ii) was important for the elicitation of both induced movement and the aftereffects, and (i) was important for the elicitation of induced movement. Neurons responsive to relative movement in conjunction with lateral inhibition may provide a partial explanation for these effects. However, they do not explain why the visual system can assign considerable movement to a large static field under the conditions of these experiments.

Our results are consistent with previous research employing reaction time to a neutral stimulus as dependent measure. In addition, our results suggest that phobics scan the environment as part of safety behaviour. We suggest that exposure treatments to reduce spider phobia may be facilitated by encouraging patients to stop environmental scanning.