Visual selection during covert visual search in the human cortex

Introduction: During visual search for a target object within a cluttered scene, a 'frontoparietal network' is engaged. The core components of the frontoparietal network are the frontal eye fields (FEF) and several sub-regions of the intraparietal sulcus (IPS) (1,2). It is still unclear if the activation in the frontoparietal network during visual search reflects a spatial selection process (3,4). The aim of this fMRI-study was to analyze to which degree the population activity of these areas in the human brain during visual search reflects spatial attention and other operations involved in visuo-motor integration during search.

Subjects and Methods: Each trial began with the presentation of an object defined by a conjunction of shape and color. Subjects memorized this target for a delay period until a display containing one central object and an array of four peripheral objects was presented. In the control condition, subjects had to decide whether the central object matched the target. In the experimental condition, subjects had to search covertly for the target among the peripheral objects and to memorize its location. After a second delay subjects indicated the location of the target. Five healthy human volunteers were scanned at 1.5 T (Siemens Magnetom Vision) with echo planar imaging (TR/TE=3000/60ms, FA=90°, voxel size=4×4x5mm). Data analysis (Brain Voyager 4.4©) included Talairach -transformation and cortical surface reconstruction. Multiple regression analysis was performed to compute statistical activation maps. Activity reflecting spatial attention was assessed by contrasting activation during the search period in the experimental and the control condition.

Results: The group analysis showed overlapping cortical distributions of fMRI-activity during experimental condition (peripheral task with spatial uncertainty in the visual field periphery) and the control condition (identification task without spatial uncertainty in the visual field center). Particularly pronounced overlaps were present within the core components of the 'frontoparietal network'. The magnitudes of the mean fMRI responses during the experimental and the control condition were similar in all components of the frontoparietal network. However, FEF and, less robustly one sub-region of the IPS, displayed a subtle response increase during the experimental, as compared to the control, task.

Conclusions: A large overall response during both tasks, constituted the largest part of the activation in the frontoparietal network. Specific attentional top-down effects definitely contribute to the activity in this network, but this contribution may only add a subtle amount to the overall population activity in this network. Processes related to perceptual choice and arbitrary stimulus-response mapping may dominate this activity.

Acknowledgment: Supported by the Deutsche Forschungsgemeinschaft (GRK Clinical and Cognitive Neuroscience).

References:

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[4] Corbetta & Shulman, Nat Rev Neurosci. (2002) 3: 201–15.