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Abstract

Self- and object-motion processing greatly relies on visual cues. There are at least two entirely independent kinds of self-induced visual motion that combine to optic flow in the visual field: expansion flow, such as that induced by forward motion in depth, or planar motion, such as induced by translational self-motion or by pursuit eye movements across a visual scene. In real life, both signals may occur in combination, yet, only one of the cues may be of behavioral relevance, thus requiring to be selectively attended to. In this fMRI study we attempt to address the question whether differential neural substrates get modulated by selective attention to either one of these motion cues. We created a stimulus combining an expansion flow pattern with translational motion on the same set of dots. In a feature-based detection task, subjects selectively attended either to the expansion or to the translation component of the stimulus and reported changes in the speed of the attended motion component. In control conditions that used the same stimuli subjects attended to color hue changes of the fixation cross, or passively fixated the stimulus without any attentional demand. In each of the three attention conditions, the attentional load was kept constant across conditions by a continuously updating staircase procedure.
We found that attention to expansion modulated the separately localized areas MT/V5, MST, and V3A significantly more than attention to translation. This is in line with stimulus-driven studies that showed a preference to expansion/contraction stimuli in these areas (Smith et al., 2006). In contrast, V7 and the cingulate sulcus visual area (CSv) differed from all other regions, in that they did not show any selective modulation by attention to expansion flow. Most interestingly, we found motion selective modulation in the foveal confluence of V1, despite a physical match between stimulus conditions. This might be due to differential attentional enhancement within V1, or by differential feedback from higher regions such as MT/V5, MST or V3A. Our results therefore show a differential attentional modulation within the motion-processing pathway, depending on the type of motion-component that is attended to within the same flow stimulus.
Smith AT, Wall MB, Williams AL, Singh KD (2006) Sensitivity to optic flow in human cortical areas MT and MST.