NeuroImage 11, Number 5, 2000, Part 2 of 2 Parts I II [ PERCEPTION Motion of illusory contours enhances activation in V1 Mohamed Seghier*, Michel Dojat*, Chantal Delon-Martin*, Christoph Segebarth*, Jean Bulliert *INSERM U438/LRC-CEA/UJF, CHU, Grenoble, France t CERCO, CNRS- UPS, Toulouse, France Introduction. Functional imaging studies in humans have suggested that illusory contours (ICs) activate mainly V2 [1,2] or higher-order areas [3]. These studies, in which paradigms with static stimuli were applied, did not clearly reveal involvement of V1 during perception of the ICs. Recently, using moving ICs, we reported a strong activation within V1 [4]. In order to verify whether the enhanced activation within V1 was due to the motion, we have in the present study compared the activations obtained during perception of moving and of stationary ICs. Material & Methods. Three subjects were examined. The volume of interest (8 coronal planes, 40mm thick overall), cut the posterior part of the calearine sulcus (Figure 2d). An EPI GRE MR sequence was used (1.5T, TRfl'E/Flip=2s/50ms/80 °, FOV= 192mm, matrix=64x64), with a surface coil for detection. Four block paradigm experiments were realized. During the stimulation epochs of Experiment 1, subjects viewed successive sets of "pacman" inducers which were configured such as to induce the perception of an illusory rectangle moving vertically (Figure la). During the control epochs, the pacmen were tilted with respect to the corresponding configurations of the stimulation epochs, so as to prevent the perception of ICs (Figure lb). In Experiment 2, the paradigm proposed in [2] was applied. During the stimulation epochs, a single static Kanisza illusory rectangle was displayed. During the control epochs, the four inducers were again Iilted to destroy the perception of the rectangle. Experiment 3 was aimed at detecting the borders between V1 and V2. Therefore, an hourglass-shaped stimulus filled with a contrast-reversing checkerboard and centered on the vertical meridian was shown during the stimulation epochs. Experiment 4 was aimed at localizing V5. Continuously expanding concentric rings were therefore presented during the stimulation epochs [5]. During the control epochs of both Experiments 3&4, a fixation point was displayed. Data processing relied upon cross-correlation analysis. Results. Figure 2 shows typical results obtained for one subject. The activations obtained in response to the moving ICs (Experiment 1) are shown in Figure 2c, those obtained to the static contours (Experiment 2) are shown in Figure 2a. The area activated at the border of V1 and V2 is shown in Figure 2b (Experiment 3) and has been delineated also on the former two figures. Thus, with this subject, the moving ICs strongly activate cortical regions inside area V1 (Figure 2c). Al- though less strong, activations are also found within V1, near the border with V2, during the perception of static ICs (Figure 2a). The region activated by the moving ICs nearby V5 (Figure 2f) may be compared with the activations due to expanding tings (Figure 2e). The activations within and around V5 were found for moving ICs only. For two among the three subjects, the moving ICs also activated a region corresponding to LOS/KO and reportedly responding to moving stimuli and to kinetic contours [6,7]. Figure 1 Discussion. The involvement of V 1 during the perception of moving and of static ICs has been demonstrated. This result suggests that perceptual grouping mechanisms involve neural processing within the striate cortex. The fact that the motion was of translational type might have contributed to the enhancement of the responses found within V1 [8].The significantly stronger activation found within V1 for moving than for static ICs may be due to feedback from V5 [9]. References. 1. Ffytche DH & Zeki S. NeuroImage, 1996, 3:104-108. 2. Hirsch Jet al., PNAS, 1995, 92:6469-6473. 3. Mendola JD et al. I Neurosci, 1999, 19:8560-8572. 4. Seghier Met al. Cerebral Cortex, (to appear). 5. Tootell RBH et ah, Nature, 1995, 375:139-141. 6. Orban GA et al. PNAS(USA), 1995, 92:993-997. 7. Sunaert Set al. Exp Brain Res, 1999, 127:355-370. 8. Watanabe T et al., PNAS, 1998, 95:11489-11492. 9. Hup6 JM et al. Nature, 1998, 394:784-787. Figure 2 $697