Recent human neuroimaging studies suggest the involvement of parietal cortex, the V5/MT+ complex and prefrontal cortex in the regulation of visual attention for one or more moving targets (Culham et al. 1998). But how does the hemodynamic activity in these areas change with changed attentional load, that is, task difficulty? We addressed this question by asking subjects to covertly track a variable number of balls (2, 3, 4 or 5 out of 10) at the some criterion level, while measuring their brain activity using BOLD fMRI in a parametric design study. We find that the BOLD response in a variety of areas increases linearly with the number of balls tracked. The strongest effects were seen in the superior parietal lobe and anterior intraparietal sulcus. Less strong attentional load effects can be observed in the inferior precentral sulcus and visual motion-responsive areas as well as in the anterior cingulate and in the cerebellum. Our results indicate that the modulation of attentional load in a visual tracking task is not merely achieved by modulating the input to the motion system at the initial stages of processing. Rather, our data shows that very strong and robust attentional load effects take place in later posterior parietal areas. These findings thus provide evidence for the existence and precise anatomical location of a neural substrate underlying attentional load processes in a covert visual tracking attention task. Moreover, these results also support the hypothesis (Wojciulik & Kanwisher 1999) that the intra-parietal sulcus plays a general role in visual attention, compatible with its putative role in the control of attentional load determined by task difficulty.
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