Switching attention within and between objects : The role of subjective organization

Abstract An object effect refers to faster and/or more accurate responses to targets when they are part of a single object than when they are parts of two different objects Duncan, 1984). Egly, Driver, and Rafal (1994) recently showed that switching attention within an object is faster than switching attention between objects. In the experiments reported here, an object effect was found if a stimulus pattern was subjectively organized into two objects, but disappeared if the same pattern was seen as a single object. The study provides converging evidence to the existence of multiple levels of visual representations. It suggests that attention selects the internal representation of both space and object, and that subjective organization is an important factor in the manifestation of the object effect. Encoding selectivity is an important function of the visual system. Such selectivity can be achieved by attending only to a particular region of space and whatever it contains, or by attending to a particular object, regardless of where it is. Although no consensus exists among researchers as to whether attention is best described as primarily spacebased (e.g. Posner, Snyder, & Davidson, 1980; Tsal & Lavie, 1988, 1993), object-based (e.g. Duncan, 1984), or both (e.g. Egly, Driver, & Rafal, 1994; Kramer & Jacobson, 1991; Vecera, 1994; Vecera & Farah, 1994), many people would perhaps agree that the subjective parsing of a visual scene is an important factor affecting the distribution of attention. For instance, when viewing Rubin's (1921) faces-vase ambiguous figure, an observer's allocation of attention should depend heavily on whether she sees the figure as two faces or as a vase. Prinzmetal and Keysar (1989, Exp. 2) also showed that when participants were induced to group a 4 x 4 evenly spaced stimulus array as either rows or columns, they made twice as many within group illusory conjunction errors (Treisman & Schmidt, 1982) than between group ones. In other words, participants were more likely to erroneously combine features from adjacent items within the same row once they subjectively organized the stimulus array into rows rather than columns, and vice versa. However, although much research has been dedicated to the effects of subjective organization on visual perception (e.g. Carrasco & Chang, 1995; Coren & Girgus, 1980; Jenkins & Ross, 1977; Jonides & Gleitman, 1972; Yantis, 1992), few people have directly investigated the relationship between the subjective organization of a visual scene and an object-based allocation of attention. The experiments reported in this paper focus on this issue. There is no doubt that space plays an extremely important role in visual selection (e.g. Downing & Pinker, 1985; Eriksen & Eriksen, 1974; Eriksen & Hoffman, 1972; 1973; Eriksen & St. James, 1986; Hoffman & Nelson, 1981; Posner, 1980; Posner et al., 1980; Treisman & Gelade, 1980; Tsal & Lavie, 1988, 1993). Posner and his colleagues (Posner, 1980; Posner et al., 1980) demonstrated this clearly in their studies. In a typical Posner experiment, participants' task was to detect a luminance change as quickly as possible after a briefly flashed central cue. The cue could be either informative or uninformative. When it was informative and valid (this type of trial typically comprises 70% to 80% of all informative cue trials), the subsequent target appeared at the cued location. When the cue was informative but invalid, the target occurred at one of the uncued locations. When the cue was uninformative, i.e., neutral, the target was equally likely to appear at any of the possible target locations. Relative to neutral cue trials, participants were faster and/or more accurate at detecting the target given a valid cue, and they were slower and/or less accurate given an invalid cue. Although the importance of space in the distribution of attention continues to gain evidence from behavioural studies on healthy people (e. …