Interactions within and between the spatial senses.

This paper reviews five types of interaction between sources of spatial information within and between sense organs; 1) nested, 2) opponent, 3) comparison, 4) covariation, and 5) multicue interactions. Efference copy is treated as a type of sensory input. Examples of each type of interaction are provided, with an emphasis on visual-vestibular interactions. In the first type of interaction, inputs from nested sensory systems are summed like vectors. For instance, the 3-D vector sum of inputs from the joints and muscle spindles of the arm allows one to judge the position of the hand. In the second type, inputs from opponent systems are combined to form a signed difference signal with respect to a norm. For instance, the push-pull linkage between the vestibular organs on the two sides of the head provides the signal for head rotation. The third type involves comparisons based on the detection of differences between stimuli presented to different regions of the same sense organ or to distinct sense organs. The fourth type involves the extraction of products or ratios between stimuli used in the detection of invariant high-level features. For instance, the linear size of an object can be derived from the constant product of the distance of the object and the size of its image. Similar systems are used to scale the response to one stimulus feature with respect to a second feature. For instance, vestibular inputs evoking eye nystagmus are scaled by viewing distance. Judgments based on all of the above mechanisms are relational, meaning that they require information from several sources. The fifth type involves multicue systems in which alternative cues are available for the same judgment. The cues are sometimes combined as a weighted mean. For instance, the direction of an object is derived from the mean position of the images in the two eyes, or a judgment of the rotation of the body may be based on combined inputs from the vestibular system and from visual motion. For distinct types of cue, averaging is less common than cue dominance, dissociation, or cue reinterpretation.