Searching for an oddball: neural correlates of singleton detection mode in parietal cortex.

Editor's Note: These short reviews of a recent paper in the Journal, written exclusively by graduate students or postdoctoral fellows, are intended to mimic the journal clubs that exist in your own departments or institutions. For more information on the format and purpose of the Journal Club, please see We spend much of the day engaged in visual search tasks. Both the nature of the target and the context of the search can have a dramatic effect on search strategy and efficiency. Imagine looking for a single red apple among a pile of lemons, as depicted in Figure 1 A; because there is only one red apple, searching for the unique item is an efficient strategy [re-ferred to as a " singleton detection mode " (SDM)]. However, if the red apple is in a basket of lemons, oranges, and pears, then search must be based on a combination of specific features that distinguish the apple from its neighbors [color, texture, size, etc., which is referred to as " feature search mode " (FSM) (Bacon and Egeth, 1994)] (Fig. 1 B). Thus, SDM relies primarily on " bottom-up " information to mark locations that contain unique objects, whereas FSM relies primarily on " top-down " information to specify target-defining features. Although previous reports have demonstrated that top-down attention to a feature value (e.g., red) modulates the activity of neurons tuned to the attended feature (Martinez-Trujillo and Treue, 2004), it is less clear how search for something " unique " might be implemented in the visual system. Because searching for a unique object requires adopting an atten-tional set that generalizes across basic feature categories, we might expect the neu-ral correlate of SDM to be found in higher-order visual areas that are not strongly feature selective. New evidence reported by Balan and Gottlieb (2006) in The Journal of Neuroscience provides an intriguing neural mechanism for SDM in the lateral intraparietal area (LIP), a high-order visual area containing neurons that have clearly defined spatial receptive fields but weak feature preferences. The distribution of activity across LIP is thought to form an attentional priority map in which the current focus of spatial attention may be inferred based on the spatial receptive fields of the neurons with the highest instantaneous firing rate (Bisley and Goldberg, 2003). For example, an attended target stimulus evokes a stronger response in LIP neurons than an irrelevant distractor [Balan and Gottlieb, …