Visual orientation judgments made during body tilt (E-and A-effects), during centrifugation (oculogravic effect), and following prolonged tilt (aftereffect) are interpreted in terms of visual orientation constancy. Data from two classes of experiments, those using labyrinthine-defective subjects and those involving different combinations of posture, indicate that three systems are involved in orientation constancy. These are the vestibular otolith system and the proprioceptive systems of the neck and trunk. Although the three systems are involved in the E-, A-, and oculogravic effects, adaptation of only the neck and trunk systems generates the aftereffect.

3 explanations of apparent reversals (oscillation) of rotary motion in depth attribute this effect to misjudgment of orientation. These explanations are based mainly on observations of a trapezoidal "window" in rotation. The experiments reported here show that perspective effects in a trapezoidal window do not increase reversal frequencies and that other shapes in addition to a trapezoid exhibit the effect with similar frequencies. The experiments also failed to confirm that misjudgments of orientation are a causal condition of apparent reversals. A general theory in terms of an identity of projected (retinal) motion characteristics for clockwise and anticlockwise motion is proposed with supporting evidence. Apparent orientation is held to be a consequence of rather than a necessary condition for apparent reversal. This theory is sufficiently general to explain apparent reversals ("fluctuations") in the orientation in depth of static figures and objects and to explain also the kinetic depth effect. All these phenomena are held to derive from an identity of retinal projections for 2 or more motions or orientations of an object in space.

An analysis of spatial transformations of perceived space is made in terms of angular and parallel modifications of the median, horizontal, and frontal planes of O, and the perceptual and behavioral outcomes of such transformations examined. It is argued that there are 2 independent outcomes: behavioral compensation and sensory spatial adaptation with aftereffect. The 1st can be regarded as a special case of motor learning similar to that studied in early investigations with frontal plane transformation (mirror tracing), and the 2nd is essentially similar to spatial adaptation which may occur with appropriate nontransformed stimulation. Both effects can occur simultaneously in the same direction, but the experimental data presented show that they can be studied independently. The effects observed by Ivo Kohler are treated as special instances of sensory adaptation which occur with transformations dependent upon sense-organ position and movement. The felt-position hypothesis and the reafference theory proposed by Held are shown to be reinterpretable in terms of motor learning and transfer of learning. Various methodological issues in the investigation of motor learning and sensory adaptation are examined.

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