In visual search experiments, we examined the existence of a search asymmetry for the direction with which three-dimensional objects are viewed. It was found that an upward-tilted target object among downward-tilted distracting objects was detected faster than when the orientation of target and distractors was reversed. This indicates that the early visual process regards objects tilted downward with respect to the observer as the situation that is more likely to be encountered. That is, the system is set up to expect to see the tops of these objects. We also found a visual field anisotropy, in that the asymmetry was more pronounced in the lower visual field. These findings are consistent with the idea that the tops of objects are usually situated in the lower visual field and less often in the upper field. Examination ofthe conditions under which the asymmetry and the anisotropy occur demonstrated the importance of the three-dimensional nature of the stimulus objects. Early visual processing thus makes use of heuristics that take into account specific relationships between the relative locations in space of the observer and 3-D objects. 211One of the primary tasks that the visual system has to accomplish when it tries to make sense ofa particular view ofthe world is to discriminate and identify objects. To accomplish this, the visual process first has to segregate visual surfaces in the three-dimensional (3-D) world. Since it is often not initially known where in the field of view the interesting or important objects are going to be, this initial process needs to take place across the whole visual field. It is also of importance that this first analysis occurs as rapidly as possible. These requirements put a lot of demands on the system. In order to facilitate the task, the primary process could be guided by later processes in a topdown fashion; these processes, including memory, might be better able to locate the kinds ofobjects and events that might be of importance (Wolfe, Cave, & Franzel, 1989). In addition to this relatively short-term information, there are basic facts about the world in which we live that can restrict the possibilities or set clear probabilities, so that the task could be simplified. It seems that the visual system has evolved in exactly such a way as to have incorporated certain "basic truths" about the world. That is to say, certain assumptions can be made about the world that are usually true, and that do not have to be rediscovered over and over again (utilitarian theory of perception; Ramachandran, 1989).

By combining the paradigms of motion induction (presentation of an inducing stimulus, followed after a short delay by the presentation of an elongated bar next to it) and visual search (many-item displays with or without a pop-out target), it was possible to demonstrate the existence of two separate contributions to the motion induction effect. Illusory motion in the test bar could be produced either preattentively or by facilitation due to attentional capture. The former effect is fast, independent of the delay between the inducers and the test bar and operating simultaneously at all locations across the visual display, the latter is slower (full strength in 200-300 msec) and confined to the vicinity of the pop-out inducer. The two possibly also differ in their spatial extent, the attentional capture effect extending over a larger area around the inducer. We conclude that the motion induction effect can be used to show the existence of several effects due to the sudden presentation of a visual stimulus.

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Effects of the similarity between target and distractors in a visual search task were investigated in several experiments. Both familiar (numerals and letters) and unfamiliar (connected figures in a 5 x 5 matrix) stimuli were used. The observer had to report on the presence or absence of a target among a variable number of homogeneous distractors as fast and as accurately as possible. It was found that physical difference had the same clear effect on processing time for familiar and for unfamiliar stimuli: processing time decreased monotonically with increasing physical difference. Distractors unrelated to the target and those related to the target by a simple transformation (180 0 rotation, horizontal or vertical reflection) were also compared, while the physical difference was kept constant. For familiar stimuli, transformational relatedness increased processing time in comparison with that for unrelated stimulus pairs. It was further shown in a scaling experiment that this effect could be accounted for by the amount of perceived similarity of the target-distractor pairs. For unfamiliar stimuli, transformational relatedness did have a smaller and less pronounced effect. Various comparable unrelated distractors resulted in a full range of processing times. Results from a similarity scaling experiment correlated well with the outcome of the experiments with unfamiliar stimuli. These results are interpreted in terms of an underlying continuum of perceived similarity as the basis ofthe speed of visual search, rather than a dichotomy of parallel versus serial processing.Visual search tasks have been used extensively to study the processes of early vision by which the visual system comes to make a quick analysis of the visual scene. The purpose of such an analysis is to extract some more or less rudimentary structure from the impinging visual array in order that later processes can apply more stringent and more accurate analyses. A quick but distributed analysis (perhaps over the whole visual field) can guide slower and more focused analyses (Hoffman, 1979;Wolfe, Cave,

Abstract-A new method, using phase-reversing sinusoidal gratings to cancel perceived motion, was developed to measure the motion aftereffect (MAE). This technique was used to show the existence of a remote MAE, i.e. an MAE in areas that were not directly stimulated during adaptation. In several experiments, this remote MAE was compared to the local MAE. The remote effect was generally weaker and of shorter duration. It showed no directional tuning within the investigated range, as compared to a tuning of ± 60 deg of the local MAE. There was no adaptation effect to the component gratings of a plaid, indicating that the plaid was treated as a coherent pattern. The local MAE showed clear spatial frequency tuning, whereas the remote MAE varied little with spatial frequency difference, although there was a tendency towards frequencies lower than the adaptation frequency. The possibility is considered that both local and remote MAEs are generated in extrastriate areas.