Figure 15

Left: Brain regions and connections comprising an effective connectivity model formulated in terms of a Volterra series (see ). Right: Characterization of the effects of V2 inputs on V5 and their modulation by posterior parietal cortex (PPC). The broken lines represent estimates of V5 responses when PPC activity is zero, according to a second order Volterra model of effective connectivity with inputs to V5 from V2, PPC and the pulvinar (PUL). The solid curves represent the same response when PPC activity is one standard deviation of its variation over conditions. It is evident that V2 has an activating effect on V5 and that PPC increases the responsiveness of V5 to these inputs. In this study subjects were studied with fMRI under identical stimulus conditions (visual motion subtended by radially moving dots) whilst manipulating the attentional component of the task (detection of velocity changes). The insert shows all the voxels in V5 that evidenced a modulatory effect (p < 0.05 uncorrected). These voxels were identified by thresholding a SPM{F} testing for the contribution of second order kernels involving V2 and PPC while treating all other components as nuisance variables.