Frontal–Occipital Connectivity During Visual Search

Although expectation- and attention-related interactions between ventral and medial prefrontal cortex and stim-ulus category-selective visual regions have been identified during visual detection and discrimination, it is not known if similar neural mechanisms apply to other tasks such as visual search. The current work tested the hy-pothesis that high-level frontal regions, previously implicated in expectation and visual imagery of object catego-ries, interact with visual regions associated with object recognition during visual search. Using functional magnetic resonance imaging, subjects searched for a specific object that varied in size and location within a com-plex natural scene. A model-free, spatial-independent component analysis isolated multiple task-related compo-nents, one of which included visual cortex, as well as a cluster within ventromedial prefrontal cortex (vmPFC), consistent with the engagement of both top-down and bottom-up processes. Analyses of psychophysiological in-teractions showed increased functional connectivity between vmPFC and object-sensitive lateral occipital cortex (LOC), and results from dynamic causal modeling and Bayesian Model Selection suggested bidirectional connec-tions between vmPFC and LOC that were positively modulated by the task. Using image-guided diffusion-tensor imaging, functionally seeded, probabilistic white-matter tracts between vmPFC and LOC, which presumably un-derlie this effective interconnectivity, were also observed. These connectivity findings extend previous models of visual search processes to include specific frontal–occipital neuronal interactions during a natural and complex search task. Key words: diffusion tensor imaging; dynamic causal modeling; fMRI; independent component analysis; lateral occipital cortex; object detection; ventromedial prefrontal cortex; visual searc