Neural correlates of stimulus-invariant decisions about motion in depth.

Perceptual decision-making is a complicated, multi-stage process. Recently human neuroimaging studies implicated a set of regions, extending from the medial frontal cortex to the inferior parietal lobule that are involved in various steps of perceptual judgments. However, relatively little is known about the dependence of perceptual decisions on the visual stimulus itself. In the current study, we used functional magnetic resonance imaging to map neural activations while subjects performed a demanding 3D heading estimation task (heading slightly to the left or right of fixation). Subjects (n=13) were presented a constantly expanding optic-flow stimulus, composed of disparate red-blue spheres, viewed stereoscopically through red-blue glasses. We varied task difficulty either by adding incoherently moving spheres to the stimuli, hence reducing the strength of the motion signal and thereby increasing the amount of noise or by reducing the relevant differential information by decreasing the deviation of the average trajectory of the spheres from straight ahead. BOLD signals were compared during "easy" and "hard" trials in both stimulation conditions to isolate the neural mechanisms underlying the decision process. We hypothesized that areas involved in perceptual decisions about motion should exhibit significantly different activation across both stimulus conditions. Our results indicate that during earlier, sensory-stimulation-related phases of decision-making the left dorsolateral prefrontal cortex, posterior cingulate and inferior parietal cortex showed more activation for the "easy" compared to the "hard" trials, while during later, response-related phases the bilateral precuneus and inferior parietal cortex, as well as the bilateral superior medial gyrus showed this pattern of activation. Our results suggest that a large, non-overlapping network of areas is involved in various steps of decisions regarding 3D motion.
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