REVERSED ILLUSION WITH 3-DIMENSIONAL MULLER-LYER SHAPES

The purpose of this study was to determine whether the Muller-Lyer ill usion is produced by a mechanism which uses information defined in the retinal coordinates, or by a mechanism taking into account the three- dimensional (3-D) shape of the illusion figure. The classical Muller-L yer figure could not be used to address this question since it is two- dimensional. Three-dimensional Muller-Lyer figures were created to see if the illusion they produce is correlated with the shape of the proj ected retinal image, or with the shape of these figures defined in a 3 -D coordinate frame. In the experiments retinal image shape was juxtap osed against the 3-D shape of the illusion displays. For some displays the direction in which the fins pointed, relative to the shafts, in t he 3-D frame was the 'opposite' of the direction in which they pointed in the retinal images. For such displays, the illusion predicted on t he basis of the 3-D structure was the opposite of that predicted on th e basis of retinal image shapes. For another 3-D display the fins were oriented such that each projected a single straight line in the retin al image, thus the typical retinal image (< >, > <) was replaced by st raight lines (\ \, \ \). For all the displays the observed illusion wa s consistent with how the fins were oriented relative to the shaft in the 3-D coordinate frame, ie with the 3-D shape of the illusion displa ys. The retinal image shape appeared to play little, if any, role. One conclusion that emerges is that the specific retinal image shape proj ected by the classical line-drawn pattern is neither necessary nor suf ficient for producing the illusion. The present findings are inconsist ent with two well known theories of the Muller-Lyer illusion: inapprop riate constancy scaling and selective filtering.