This article compares the properties of apparent motion between a light and a touch with apparent motion between either two lights or two touches. Visual and tactile stimulators were attached to the tips of the two index fingers that were held apart at different distances. Subjects rated the quality of apparent motion between each stimulus combination for a range of stimulus onset asynchronies (SOAs). Subjects reported perceiving apparent motion between all three stimulus combinations. For light-light visual apparent motion, the preferred SOA and the direction threshold SOAs increased as the distance between the stimuli increased (consistent with Korte's third law of apparent motion). Touch-touch apparent motion also obeyed Korte's third law, but over a smaller range of distances, showing that proprioceptive information concerning the position of the fingers is integrated into the tactile motion system. The threshold and preferred SOAs for visuotactile apparent motion did not vary with distance, suggesting a different mechanism for multimodal apparent motion.
Apparent motion is the illusory perception of motion created by the discrete stimulation of points appropriately separated in space and time. Apparent motion can be experienced between pairs of lights, touches, or sounds (see Kolers, 1972, for a review). In 1915, Korte published a set of seminal studies showing that the optimal timing for experiencing visual apparent motion varies with the temporal and spatial separations of the stimuli and then- duration and intensity. He expressed this as a series of equations. One such equation states that the stimulus onset asynchrony (SOA) between two lights that produces optimal apparent motion is proportional to the distance between them. Although Korte did not explicitly state this as a law, but rather as a relationship, the variation of SOA with distance has become known as his third law after Koffka (1931).
Korte's third law was based on observations of visual apparent motion, and it also holds for tactile apparent motion (Burtt, 1917b; Kirman, 1974a; Sherrick, 1968). Auditory apparent motion, on the other hand, does not follow Korte's third law (Burtt, 1917a, dark soundproof room with two speakers at 200 cm from the observer, 5° -20° apart, and SOAs from 6 to 54 msec; Strybel, Manligas, Chan, & Perrott, 1990, dark audiometric, semianechoic, chamber with two speakers at 86 cm from the observer, 6°-160° apart, and SOAs from O to 500 msec). Since both visual and tactile apparent motion follow the same rule, they may share a single motion mechanism. If there were a common visual and tactile motion system, then stimulating vision at one location and touch at another might activate this common motion mechanism and produce a sensation of multimodal visuotactile apparent motion that should also follow Korte's third law.
Experimental evidence for multimodal apparent motion was first reported by Zapparoli and Reatto in 1969, who investigated apparent motion between lights and sounds. They used lights that were 12 mm in diameter, separated by 7.7 cm, and that corresponded to an angular separation of 3°, with sounds either played through loudspeakers beside each light or delivered through headphones. When all four stimuli were played (a light and sound on the left played together, followed by a light and sound on the right played together), subjects reported a single object or an event with two attributes (audio and visual) moving. The moving object/event was described in various ways-for example, a "sonorous light" or a "luminous sound." When only two stimuli were used (e.g., a single light followed by a single sound), some subjects described perceiving intermodal apparent motion. One subject described "a light tunnel which grows longer and shorter while the sound passes through it" (Zapparoli & Reatto, 1969, p. 262). Other subjects described their perception as a "causal nexus between the two stimuli" (the light appeared to cause the sound or vice versa). …