Tracking an object through feature space

Visual attention allows an observer to select certain visual information fo r specialized processing. Selection is readily apparent in 'tracking' tasks where even with the eyes fixed, observers can track a target as it moves a mong identical distractor items(1). In such a case, a target is distinguish ed by its spatial trajectory. Here we show that one can keep track of a sta tionary item solely on the basis of its changing appearance-specified by it s trajectory along colour, orientation, and spatial frequency dimensions-ev en when a distractor shares the same spatial location. This ability to trac k through feature space bears directly on competing theories of attention, that is, on whether attention can select locations in space(2-4), features such as colour or shape(5-7), or particular visual objects composed of cons tellations of visual features. Our results affirm, consistent with a growin g body of psychophysical(8-13) and neurophysiological(14-16) evidence, that attention can indeed select specific visual objects. Furthermore, feature- space tracking extends the definition of visual object(17) to include not o nly items with well defined spatio-temporal trajectories(18), but also thos e with well defined featuro-temporal trajectories.