Orientation relative to the retina determines perceptual organization

Two experiments are reported which show that perceptual unit formation, as measured by Gillam’s method of using common reversals of ambiguous lines as an index of their coherence, depends strongly on the orientation of the configuration. Measurements under conditions of 90° head and body tilt show that the critical orientation is retinal, not physical. The results are discussed in relation to other findings and to possible explanations.