Electrophysiological Correlates of Object-Based Selective Attention
A visual stimulus may be selectively attended on the basis of its location in space, a specific feature, or as a whole object. While a relatively large amount of research has examined the neural basis of location-based and feature-based attentional selection, few studies have examined the neural correlates of the spread of attention from one feature of an object to another, reflecting the selection of an object as an "integrated feature ensemble" (Schoenfeld et al., 2003; O'Craven, Downing, & Kanwisher, 1999). The research in this dissertation examined the electrophysiological correlates of the spread of selective attention to a task-irrelevant color feature of an object using the event-related potential (ERP) brain imaging technique. The temporal dynamics of this spread of attention were examined for two classes of objects, grouped-arrays of lines (Study 1) and geometric shapes (Study 2). In each study, overlapping objects were presented and the participants' task was to sustain attention to one object versus another to judge a change in the thickness of one of the lines of the grid (Study 1) or a change in object shape (Study 2). On some trials, either the attended object or the unattended object was colored, and color was irrelevant to the task. Difference ERPs indicated that color was first encoded in the visual cortex by 100 ms. However, the effect of attention on the task-irrelevant color feature was evident in later ERP modulations. A bilateral parietal/occipital positivity beginning at 200 ms (Study 2) and a midline occipital negativity beginning at 280 ms (Studies 1 & 2) were associated with the selection of the irrelevant color of the attended object. Source localization analyses suggested that the neural generators of the attention-related facilitation of the irrelevant color feature were situated in ventral occipital cortical regions near color-selective cortical areas. These data provide further evidence for the "integrated competition" model (Duncan, 1996; 1997), which posits that the neural basis for the perceptual integration of an attended object consists of enhanced activity in the network of specialized modules that encode its individual features, including those that are not relevant to the immediate task.
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