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 characterize 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.

We tested the hypothesis that long-term adaptation to the normal contingencies between walking and its multisensory consequences (including optic flow) leads to enhanced discrimination of appropriate visual speeds during self-motion. In experiments 1 (task 1) and 2 a two-interval forced-choice procedure was used to compare the perceived speed of a simulated visual flow field viewed while walking with the perceived speed of a flow field viewed while standing. Both experiments demonstrated subtractive reductions in apparent speed. In experiments 1 and 3 discrimination thresholds were measured for optic flow speed while walking and while standing. Consistent with the optimal-coding hypothesis, speed discrimination for visual speeds near walking speed was enhanced during walking. Reduced sensitivity was found for slower visual speeds. The multisensory context of walking alters the coding of optic flow in a way that enhances speed discrimination in the expected range of flow speeds.

Where do observers direct their attention in complex scenes? Previous work on the cognitive control of fixation patterns in natural environments suggests that subjects must learn where to direct attention and gaze. We examined this question in the context of a change blindness paradigm, where some objects were more likely to undergo a change in orientation than others. The experiments revealed that observers are capable of learning the frequency with which objects undergo a change, and that this learning is manifested in the distribution of gaze among objects in the scene, as well as in the reaction time for detecting visual changes, and the frequency of localizing changing objects. However, observers were much less sensitive to the conditional probability of a second feature, border color, predicting a change in orientation. We conclude that striking demonstrations of change blindness may reflect not only the constraints of attention and working memory, but also what observers have learnt about what information to attend and select for storage during the task of change detection. Such exploitation of the frequency of change suggests that gaze allocation is sensitive to the probabilistic structure of the environment.

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