Spatiotemporal Priority and Object Identity [ Commentary on F . Bedford ’ s “ A Law of Numerical Object Identity ” ] BRIAN

What is an object? This question can only be addressed by relativizing it to a particular type of cognitive process; for example, the ‘objects’ of lowerlevel visual processes may not always correspond to higher-level intuitions about the nature of objecthood. Here I will be concerned with the nature of visual objects (keeping in mind that there may also be different types of these, relative to different visual processes — e.g. those which do and do not involve attention). So what is a visual object? Perhaps the most natural way to ask this question is to explore the types of feature clusters in a scene which are segmented and processed together — i.e. to ask what types of stimuli ‘count’ as objects for the visual system in the first place (e.g. Scholl, Pylyshyn, & Feldman, 2001). Another way of asking about visual objecthood, however, introduces a temporal component: what properties mediate the representation of some portion of the visual field as the same object, persisting through time and perhaps motion (e.g. Scholl & Pylyshyn, 1999). Answering such questions is a critical project for vision science, since these properties essentially determine the ‘currency’ over which many later processes operate. This ‘temporal’ aspect of objecthood is one of the many related problems that is dealt with in Bedford’s ‘nested geometries’ model of object identity (Bedford, this issue). This model attempts to account for a wide variety of phenomena in which a decision must be made about whether two states of the world constitute or derive from the same object. Bedford suggests that a host of such ‘correspondence’ problems — including those in apparent motion,