Author ' s personal copy Decomposing the neural mechanisms of visual search through model-based analysis of fMRI : Top-down excitation , active ignoring and the use of saliency by the right TPJ

Despite being studied intensively over the past 30 years, the neural processes underlying visual search are not yet fully understood. In the current study we extend prior work using model-based analysis to decompose fMRI data. fMRI data on human search were assessed using activation functions predicted from the spiking Search over Time and Space model (sSoTS; Mavritsaki et al., 2006). Going beyond previous work, we show for the first time that activity in a central location map in the model, which computes the saliency of a target relative to distractors, correlated with the BOLD response in the right temporo-parietal junction (TPJ)--a key region implicated in clinical studies of unilateral neglect. This is consistent with the right TPJ responding to the relative saliency of visual stimuli. In addition, a re-analysis of search performance, with a larger participant set and a psychologically plausible response rule, showed distinct neural regions in parietal and occipital cortices linked to top-down excitation and the to active ignoring of distractors. The results indicate that excitatory and inhibitory circuits for visual selection can be separated, and that the right TPJ may be critical for responding to salient targets. The value of using a model-based approach is discussed.