Perception of Visual Speed While Moving.

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Abstract
During self-motion, the world normally appears stationary. In part, this may be due to reductions in visual motion signals during self-motion. In 8 experiments, the authors used magnitude estimation to charac- terize changes in visual speed perception as a result of biomechanical self-motion alone (treadmill walking), physical translation alone (passive transport), and both biomechanical self-motion and physical translation together (walking). Their results show that each factor alone produces subtractive reductions in visual speed but that subtraction is greatest with both factors together, approximating the sum of the 2 separately. The similarity of results for biomechanical and passive self-motion support H. B. Barlow's (1990) inhibition theory of sensory correlation as a mechanism for implementing H. Wallach's (1987) compensation for self-motion. It has been reported that the perceived speed of an expanding flow field is reduced if that flow field is viewed by a person walking on a treadmill (Distler, Pelah, Bell, & Thurrell, 1998; Pelah & Thurrell, 2001; Pelah, Thurrell, & Berry, 2002; Thurrell & Pelah, 2002; Thurrell, Pelah, & Distler, 1998). Such reductions are predicted by Barlow's (1990) model of contingent adaptation. According to this theory, highly correlated events, such as walking and expanding flow fields, mutually specify each other—as can be learned by perceptual experience—and therefore produce shifts in coding strategies that take advantage of the redundancy. According to Barlow, these coding shifts are produced by the strengthening of inhibitory connections between neural units that are simulta- neously active. Such inhibitory strengthening can lead both to sparse coding and to contingent adaptation, such as that in the McCollough effect (McCollough, 1965). The perceived reduction of speed while walking, on this account, serves the function of de-emphasizing predictable events in favor of detecting deviations from the norm. However, a reduction in perceived speed while walking is also consistent with motor prediction theory (Wolpert & Flanagan, 2001). According to this theory, the perceptual consequences of motor actions can be anticipated and subtracted from sensory signals. This theory articulates the role of motor prediction in terms of the need for fast, precise action when direct perceptual feedback is too slow. In conjunction with perceptual feedback, motor prediction provides error correction in motor control. An error signal is produced when control fails, and a revision of motor prediction results. Although the value of motor prediction in motor calibration seems clear, the value of the perceived speed reduction is less clear. In theory, motor calibration could take place with or without the perceptual reduction in predicted sensory values. In normal walking, however, if the subtraction was essentially complete, then any apparent motion of the world produced by walking could be interpreted as a control error, with prediction errors being a pos- sible source of the error. Deviation signals are emphasized in motor prediction theory and in Barlow's (1990) correlation theory, and both theories provide a framework for understanding why the perceived speed of optic flow might be reduced while walking. One advantage of Barlow's (1990) theory is that it is more general. For example, Barlow's interpretation, rather than motor prediction theory, would be favored if similar reductions in per- ceived speed were found under conditions in which other sensory signals specified self-motion without the involvement of locomo- tor activity. Thus, evidence that the perceived speed of optic flow is reduced under conditions of passive forward movement seems to implicate a more general theory in which inertial and other sensory signals might contribute to the sense of self-motion. Indeed, Bar- low's theory might serve as a mechanism for motor prediction itself.

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