ABSTRACTS OF PAPERS ACCEPTED FOR PUBLICATION 395

Recomtmcting the Third Dimensk Intemctioas b&am Cdor, Texture, Motion, Binocahr Diqwi& and Shape. PATRICK CAVANAGH. Departement de Psychologie, Universiti de Montreal, Mont&l, Quebec, Canada H3C 3J7. Received March 25,1986.

The effectiveness of depth cues such as occlusion and shading was examined in images defined by color, texture, binocular disparity, or motion. Line drawings represented in any of these modalities were able to signal shape and occlusion showing that contour occlusions are analyzed at a high level, following the reintegration of the separate representations of visual attributes such as color and motion. Subjective contours, on the other hand, could be seen only if the figures were defined by luminance differences. Figures whose depth depended on the interpretation of shadows also required luminance differences: shadow regions had to be darker than the surrounding, non-shadow regions. Shadows areas filled with colors or textures that could not occur in natural scenes were perceived as shadows as readily as real shadows. Even when shadow and non-shadow regions had different depths or had textures that moved in different directions, the depth from shading was still seen as long as there was an appropriate brightness difference. These findings indicate a variety of analysis mechanisms to cue three-dimensional structure. Occlusion cues in line drawings appeared to be analyzed by a general purpose mechanism having access to all pathways of the visual system. Subjective contours and shadows appeared to depend on special purpose processes accessing only the luminance pathway. Finally, although natural constraints have proved useful in solving many visual problems, they did not play a significant role in the interpretation of the depth cues examined here.

Conhgen~ Ajbwffects and Isdnmti Psychqhysicd Eoidurrc for Sepamtion of Cdor, OrienMoq and Marion. ROBERT L. ~AVOY. Rowland Institute for Science, 100 Cambridge Parkway, Cambridge, Massachusetts 02142. Received April 20,1986.

Anatomical, physiological, and psychophysical evidence is converging to indicate that early image analysns in human vision can be conceptualized as being performed by a variety of distinct systems. These systems are surprisingly independent, except in so far as the output from one is the input to another. The present work reviews some general information about one of these systems (the luminance contrast system) and argues that it interacts with its complement (the color system) in a way that gives rise to some contingent aftereffects (the McCollough effect in particular). A key point in the argument is that, when input to the huninance contrast system is minimized (by using isohtminant stimuli), the contingent color aftereffect is not generated. Isoluminant stimuli elicit (or fail to elicit) a number of other visual responses. These will be reviewed, as well as some general considerations in the generation of isoluminant stimuli.

Nested S- of Confr& An Zntdtitx View. MICHABL LEYTON. Harvard University, Cambridge, Massachusetts 02138 and Department of Computer Science, State University of New York, Buffalo, New York 14260. Received May 20,1986.

An intuitive exposition is provided of a theory (rigorously elaborated elsewhere) that perceptual organization is the description of phenomena as nested structures of control. The theory is used to explain a number of perceptual-organizational phenomena such as complex shape prototypitication, generalized-cone analysis, the importance of curvature singularities, grouping phenomena, motion phenomena, etc. The nested-control theory maintains that any perceptual organization is given a stratification where each level of the stratification is operated on (1) asymmetrically, and (2) as a whole, by the level in which it is embedded. The levels-within-levels hierarchy is divided into two subhierarchies, one describing the stimulus set as gene-rated via a sequence of deformations, and the other describing the initial prototype as generated from a subset of itself. The structure of the latter sub-hierarchy yields the grouping structure of the stimulus set. Principles are proposed that determine the way these two sub-hierarchies are interrelated: Symmetry axe in the grouping structure are converted into lines of flexibility in the prototype modifications. Using these concepts, solutions are offered to some classical problems in perceptual organization, and a theory of complex shape is developed.