Identifying rate-limiting nodes in large-scale cortical networks for visuospatial processing: An illustration using fMRI

With the advent of functional neuroimaging techniques, in particular functi onal magnetic resonance imaging (fMRI), we have gained greater insight into the neural correlates of visuospatial function. However, it may not always be easy to identify the cerebral regions most specifically associated with performance on a given task. One approach is to examine the quantitative r elationships between regional activation and behavioral performance measure s. In the present study, we investigated the functional neuroanatomy of two different visuospatial processing tasks, judgement of line orientation and mental rotation. Twenty-four normal participants were scanned with fMRI us ing blocked periodic designs for experimental task presentation. Accuracy a nd reaction time (RT) to each trial of both activation and baseline conditi ons in each experiment was recorded. Both experiments activated dorsal and ventral visual cortical areas as well as dorsolateral prefrontal cortex. Mo re regionally specific associations with task performance were identified b y estimating the association between (sinusoidal) power of functional respo nse and mean RT to the activation condition; a permutation test based on sp atial statistics was used for inference. There was significant behavioral-p hysiological association in right ventral extrastriate cortex for the line orientation task and in bilateral (predominantly right) superior parietal l obule for the mental rotation task. Comparable associations were not found between power of response and RT to the baseline conditions of the tasks. T hese data suggest that one region in a neurocognitive network may be most s trongly associated with behavioral performance and this may be regarded as the computationally least efficient or rate-limiting node of the network.