Visual attention can be voluntarily directed towards an item based on our current behavioral goals and knowledge or it can be reflexively drawn to salient features of a visual scene. Recent functional magnetic resonance imaging (fMRI) studies have proposed two partially segregated networks for these complementary top-down and bottom-up attentional control mechanisms: The dorsal frontoparietal system, which is involved in top-down attentional allocation, and a stimulus-driven ventral frontoparietal system.
Most of these studies have investigated covert attentional mechanisms in search and detection paradigms using schematic stimuli. Here, we study the role of the two networks for overt attentional allocation during viewing of natural scenes. Therefor, we combine eye tracking with fMRI. To compare the contribution of top-down versus bottom-up attentional processing, we investigate the influence of task instructions and different strengths of luminance contrast modifications of the images on both, eye movements and neural activity.
Comparing all experimental conditions to baseline, we observe enhanced activation in intraparietal sulcus (IPS) as part of the dorsal frontoparietal network and a distinct deactivation in temporoparietal junction (TPJ) – a crucial region of the stimulus-driven ventral frontoparietal network.
We do not find an effect of luminance contrast modification on blood oxygenation level dependent (BOLD) response. However, viewing behavior is affected by strong as well as weak luminance contrast modifications, whereby the size of the effect scales linearly with the strength of modification.
The comparison of a search task with a free viewing condition reveals diverse results across subjects. Yet, significant differential responses in the regions of IPS and TPJ due to task instructions in most subjects suggest a complex interaction of top-down and bottom-up mechanisms, which might crucially rely on the present state and strategy of the individual. A fast event-related analysis dissociating the search period from target detection might reveal further insights into both attentional control mechanisms.
As one of the first fMRI studies that explores overt attentional mechanisms during free viewing, our study supports the assumption that overt and covert attentional processes involve similar brain regions even under natural viewing conditions.
Institute of Cognitive Science
Department of Neurobiopsychology
Supervisors: Prof. Dr. Peter K¨onig, Prof. Dr. Rainer Goebel