Condition-dependent and condition-independent target selection in the macaque posterior parietal cortex.

During a visual search, information about the visual attributes of an object and associated behavioral requirements is essential for discriminating a target object from others in the visual field. On the other hand, information about the object's position appears to be more important when orienting the eyes toward the target. To understand the neural mechanisms underlying such a transition (i.e., from nonspatial- to spatial-based target selection), we examined the dependence of neuronal activity in the macaque posterior parietal cortex (PPC) on visual sensory properties and ongoing task demands. Monkeys were trained to perform a visual search task in which either a shape or color singleton within an array was the target, depending on the ongoing search dimension. The visual properties and the task demands were manipulated by independently changing the stimulus features (shape and color), singleton type, and search dimension. We found that a subset of PPC neurons significantly discriminated the target from other stimuli only when the target was defined by a particular stimulus dimension and had specific stimulus features, such as a shape-singleton, bar stimulus (condition-dependent target selection), whereas another subset did so irrespective of the stimulus features and the target-defining dimension (condition-independent target selection). There was thus a great deal of variety in the neural representations specifying the locus of the target. The coexistence of these distinctly different types of target-discrimination processes suggests that the PPC may be situated at the level where the transition from nonspatial- to spatial-based target selection takes place.