A visual-search paradigm was used to explore the relative ease with which the direction of gaze can be detected. Straight-gaze stimuli were presented as targets within a variable number of distractors with left-averted or right-averted gaze. Reaction time in this case was compared with that when either the left-averted or right-averted gaze stimuli were the targets among distractors of the two remaining gaze directions. The data were examined for the existence of a search asymmetry favoring the straight-gaze targets. Such an asymmetry was found with stimuli that were realistically drawn renditions of pairs of human eyes, as well as with similar schematic stimuli representing pairs of human eyes. The asymmetry, however, was not found with geometric control stimuli, which also presented the critical feature in the central, the left-lateral, or the right-lateral position within the stimulus, but were not eyelike. It was also not found for schematic stimuli consisting of only one eye. It was concluded that the straight gaze direction is a special stimulus with eyelike stimuli, which the visual system is set up to process faster and with fewer errors than averted gaze directions. The results are discussed in terms of the evolutionary significance of the straight gaze direction.

No abstract

The cerebral cortex of the cat contains between 1 and 2 dozen representations of the visual field with different functional specializations. Six visual field maps lie along both banks of the suprasylvian sulcus, lateral and anterior to the visual areas in the occipital cortex. We have studied single-unit receptive field properties and their global organization across the visual field in 2 of these lateral suprasylvian areas, PMLS (essentially the Clare-Bishop area) and PLLS. Most neurons in PMLS and PLLS display selectivity for the direction of a light stimulus moving across their receptive fields with various degrees of directional tuning. We have used light spots of different size and velocity projected on a tangent screen in order to determine the direction preference of cells in these 2 areas. A strong tendency was found for neurons to respond best to centrifugal directions, i.e., to movement away from the area centralis. Thus, for these cells direction preference depends on the location of their receptive fields within the visual field. Velocity preference and binocular interaction in these neurons is also globally organized: Velocity preference increases with eccentricity, binocular synergism is strongest in the center of the visual field. Cluster analysis of recording tracks with respect to "radial" and "circular" cell categories reveals a grouping of cells with like properties in the lateral suprasylvian cortex. These new categories are formed by combining "centrifugal" and "centripetal" cells on the one hand and cells with direction preferences orthogonal to these on the other. The radial or centrifugal organization of direction preferences in conjunction with the global arrangement of velocity preference and binocular interaction suggests that PMLS and PLLS are involved in the processing of expanding visual flow fields of motion. Such flow fields are commonly encountered when a visual object moves towards an observer or during forward locomotion.

In a series of experiments, we demonstrate the effects of two spatially distinct primers on motion induction (MI) and the influence of attribute characteristics on the resulting collision site. MI means that a primer such as a spot produces a motion sensation in a subsequently presented geometrical pattern such as a line or a rectangle. This pattern will appear to grow out of the spot. In the present paper we report that when two different locations of the visual field are activated simultaneously by presenting two spots prior to a bar between these spots, there is a motion sensation of two bars growing away from the spots and colliding in the centre (split priming effect). Attribute characteristics can have profound effects on this illusion. When two differently coloured isoluminant spots are presented and the subsequent bar is composed of either one of these colours, the induced motion is away from the spot of identical colour. We call this effect attribute priming. Manipulating the delay between the spot presentations (SOA) showed that timing had a strong effect on split priming, but very little on attribute priming. For split priming experiments with dichoptic presentations, we show that at shorter SOAs there is a dominant effect of the primer which is presented to the same eye as the bar, as opposed to the usual dominance of the later primer. For longer SOAs, however, the temporal sequence of the primers also plays a role in motion induction. Further, we report that geometrical arrangements can strongly influence the direction of perceived motion when more than a single primer is used. Generally, in motion induction with two primers, unlike what is found with a single primer, there appears to be a dominance of low-level effects such as geometry, attributes, and eye of presentation. For dichoptic presentations, however, this can be overcome for longer SOAs. The differences between the single and split priming paradigms are discussed in terms of the differential contribution of bottom-up and top-down processes.

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).