Functional MRI of dynamic judgments of spatial extent.

Judgments of spatial relationships are often made when the object or observer is moving. Behaviourally, there is evidence that these 'dynamic' judgments of spatial extent differ from static judgments. For example, in one of the simplest probes of spatial extent--the line bisection task--the typically observed leftward bisection bias of about 1% of line length is increased considerably after left-to-right scanning. Here we used fMRI to examine whether or not different brain regions would be involved in static and dynamic judgments of spatial extent. Dynamic (i.e., scan the line prior to bisecting) relative to static ocular bisections (i.e., line bisection by fixation) produced activations in the cuneus and precuneus bilaterally, with reduced activation relative to static judgments observed in the supramarginal gyrus bilaterally. Dynamic bisections relative to a control condition (i.e., scan a line and then saccade to a transection mark) produced activations in the precuneus/superior parietal lobe bilaterally and left cerebellum. Only marginal evidence was found for different activations due to the initial scan direction of the line. These results highlight the fact that dynamic judgments of spatial extent use distinct brain regions from those employed to make static judgments, and the same mechanism is employed independent of scan direction. It may be the case that velocity processing and time estimates are integrated primarily in the cuneus and precuneus to produce estimates of spatial extent under dynamic scanning conditions.