On the continuity of perceptual experience: implications for perception and attention

Canadian Psychology, Feb 1996 by Yantis, Steven

When an observer views a real - world scene, some objects in the scene are partly occluded by intervening surfaces. Similarly, as the observer moves through the environment, some objects are temporarily occluded and revealed. Yet, although the retinal image is fragmented in space and time, perceptual experience is coherent and continuous. Continuity of experience is achieved by perceptual organization mechanisms, which have their effect before attention. Several experiments are described to illustrate the close interaction between perceptual organization and attention. This interaction determines the representational basis for selection, and is therefore a crucial precursor to understanding object - based mechanisms of visual selection.

The analysis takes place in the context of attentional control mechanisms. Attention can be controlled in either of two ways: through top - down deployment, or through bottom - up capture. It is widely believed that visually salient stimuli capture attention. However, in most experiments investigating this issue, the salient feature is task - relevant (e.g., it is the target of search), and it therefore cannot be said to capture attention in a bottom - up fashion. In several experiments we have found that even highly salient feature singletons such as a uniquely coloured element or a moving element in a background of stationary elements do not capture attention, so long as the singletons are task - irrelevant.

In contrast, we have found that the appearance of a new perceptual object in the visual field does capture attention, even when novelty is task - irrelevant. We suggest that attentional capture by new perceptual objects is a manifestation of early perceptual organization processes that parse a spatiotemporally fragmented image into coherent perceptual object representations. The temporally leading edge of a stimulus event is perceptually important and demands attention. It is the creation of a new perceptual representation that is critical in this process.

In order to further assess this idea, we have carried out experiments examining the role of spatiotemporal continuity in visual attention and perception. Several seemingly different perceptual phenomena illustrate the visual system's tendency to override temporal and spatial discontinuity in the image so as to achieve and maintain perceptual coherence.

The first line of evidence comes from a study in which observers view a bistable apparent - motion display producing what is known as the Ternus effect. This display provides an index of the perceived spatiotemporal continuity of a briefly interrupted perceptual object. We then carried out a companion experiment in which one element disappeared and reappeared for various temporal intervals; brief interruptions had no effect, but the reappearance of an element after longer interruptions captured visual attention. The timecourse of these two effects were virtually identical. This result leads us to conclude that the two phenomena are manifestations of a common spatiotemporal perceptual organization mechanism.

A second line of evidence comes from some recent experiments that reveal perceived continuity of a temporarily occluded visual object. This experiment, which also relies on reported perceptual experiences in the Ternus display, suggests that the disappearance of an object due to an occluding surface is discounted by the visual system. The perceived continuity of an occluded visual object is termed amodal integration. Perceptual representations are not rigidly tied to the fragmented retinal image; instead, they correspond to the most likely interpretation of the scene.

Finally, we have obtained evidence that attention can be directed to image regions that are separated by an occluding surface, so long as the two image regions are perceived as belonging to the same surface. This experiment shows that attention spreads from a cued image region to other parts of the same surface via amodal completion.

Together, the reported experiments provide further evidence concerning the interaction between perceptual organization and object - based theories of attention. The representational basis of selection is an important starting point for understanding how object - based selection is accomplished, and also provides insights about how the coherence of perceptual experience is achieved and maintained.