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A series of experiments was performed to test the predictions of adaptation level, efferent readiness, and framing explanations of illusions of extent. The framing notion, that the ratio of total figure length to shaft length (i.e., the framing ratio) determines the magnitude of illusions, was supported for the Müller-Lyer, Baldwin, and divided line illusions. Peak overestimation of shaft length obtained when the framing ratio was 3:2. Variation of proximal figure size, achieved by altering either viewing distance or distal figure size, was found to be directly related to the magnitude of the illusions. A model was proposed to account for the effects of both the framing ratio and proximal shaft length on judgments of focal length.

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When two lines were presented simultaneously, in an illusion paradigm, the judged length of one of the lines (the focal line) assimilated toward the length of the contextual line. When the lines were presented sequentially, in an aftereffect paradigm, the apparent length of the focal line was displaced away from the length of a previously presented contextual line (i.e. contrast). The largest effects obtained at inverse contextual-to-focal length ratios: 1.67:1 for contextual lines longer than the focal line and 0.60:1 for contextual lines shorter than the focal line. Overestimation of focal length was larger than underestimation for both illusions and aftereffects despite the fact that identical stimuli produced effects in opposite directions in the two paradigms. Results were discussed in terms of a neural length-coding model and were related to visual size constancy.

The parallel lines configuration produces assimilation oflinear extent when the contextual and focal lines are presented simultaneously and contrast when the contextual line is presented prior to the focal judgment. The interpretation of the shift from assimilation to contrast is confounded by the covariation of mode of presentation (simultaneous or sequential) of contextual and focal stimuli and the duration of contextual exposure in previous research. This confound was removed by varying contextual exposure duration within the sequential mode ofpresentation (Experiment 1) and by combining the simultaneous and sequential modes of presentation (Experiment 2). The results indicated that the shift from assimilation to contrast is determined by the mode of presentation of contextual and focal stimuli. The data are discussed as they relate to Coren and Girgus's (1978) "pool and store" model of assimilation and contrast.Visual contours often interact to produce systematic distortions in the apparent value of some parameter of form. These interactions can be observed in two manners. First, the contours can be presented simultaneously so that one stimulus (the contextual stimulus) produces a distortion of the stimulus being judged (the focal stimulus). This phenomenon is usually referred to as visual illusion. A second kind of interaction, called an aftereffect, occurs when the focal stimulus is perceptually distorted subsequent to inspection of a contextual stimulus. Both illusions and aftereffects can occur in two directions. When the perception of the focal stimulus is shifted away from the value of the contextual stimulus, the effect is termed "contrast," and when the perception of the focal stimulus is shifted toward the contextual stimulus, the effect is termed "assimilation."lllusions and aftereffects occur in the same direction for many parameters of form. For example, both orientation illusion (Blakemore, Carpenter, & Georgeson, 1970; Jastrow, 1982) and orientation aftereffect (the tilt aftereffect; Gibson & Radner, 1937) are contrast effects. Also, spatial-frequency illusion (Klein, Stromeyer, & Ganz, 1974) and spatial-frequency aftereffect (Blakemore, Nachmias, & Sutton, 1970) are contrast effects. Distortions of linear extent represent an interesting exception to this general rule, because the directions of illusion and aftereffect are opposite; length illusion (e.g., the MUllerLyer illusion and the illusion of parallel lines) is an asThe authors would like to thank Diane Schiano and two anonymous reviewers for their comments on earlier versions of the manuscript. Requests for reprints should be sent to Kevin Jordan, Department of Psychology, San Jose State University, San Jose, CA 95192. 447similation effect, whereas length aftereffect (the aftereffect of parallel lines; Brigell & Uhlarik, 1979) is a contrast effect.There are two differences between the procedures generally used to establish length illusions and aftereffects which could account for the effects' being opposite in direction. The most obviou...