Moving illusory contours activate primary visual cortex: an fMRI study

Abstract

Identifying the cortical areas activated by illusory contours provides valuable information on the mechanisms of object perception. We applied functional magnetic resonance imaging to identify the visual areas of the human brain involved in the perception of a moving Kanizsa-type illusory contour. Our results indicate that, in addition to other cortical regions, areas V5 and V1 are activated. Activity in area V1 was particularly prominent.

Figure 1

Figure 1-a. Stimuli presented during the “stimulation” epochs. Four frames are shown. The arrow indicates the direction of the succession of the frames. The saccadic rotation of the pacmen provokes the illusion of a downward motion of the illusory rectangle. A small white circle at low contrast served as a fixation mark. Figure 1-b. Stimuli presented during the “reference” epochs. The pacmen are titled with respect to the “stimulation” epochs in order to avoid the perception of illusory edges. All the pacmen are animated with a rotational movement. A small white circle at low contrast served as a fixation mark.

Figure 1

Figure 1-a. Stimuli presented during the “stimulation” epochs. Four frames are shown. The arrow indicates the direction of the succession of the frames. The saccadic rotation of the pacmen provokes the illusion of a downward motion of the illusory rectangle. A small white circle at low contrast served as a fixation mark. Figure 1-b. Stimuli presented during the “reference” epochs. The pacmen are titled with respect to the “stimulation” epochs in order to avoid the perception of illusory edges. All the pacmen are animated with a rotational movement. A small white circle at low contrast served as a fixation mark.

Figure 2

Typical functional map obtained in response to the moving illusory contours (subject S9). Left. Scout image in the sagittal plane. The position of the volume (6 slices) acquired during the functional scans has been delineated. For the sake of clarity, the calcarine sulcus has been manually darkened. Center. Four functional maps corresponding to slices 2 to 5. The color code represents the cross-correlation value of the voxels (red for 0.8 and 0.5 for green). The dashed line on the functional maps represents the intersection of the image planes with the coronal plane represented by the dashed line on the scout image. Right. Projection map corresponding to the four functional maps shown in the center.

Figure 3

Functional projection maps obtained in response to the moving illusory contours for six subjects (S4, S5, S7, S8, S10 and S11). Top and bottom. Scout images in the sagittal plane for each of the six subjects. The positioning of the volume acquired during the functional scans has been delinated. For the sake of clarity, the calcarine sulcus has been manually darkened. Center. The functional projection maps obtained for each subject. Those projection maps were obtained by projecting the functional maps corresponding to slices 2 to 5. The dashed line on the functional projection maps represents the intersection of these maps with the coronal plane represented by the dashed line on the corresponding scout image.

Figure 4

Functional projection maps for the control subject C1. The upper part of the figure (Fig. 4a–c) shows the activations recorded in response to the static illusory contour (Fig. 4a), to the hourglass-shaped stimulus aimed at delineating the border between areas V1 and V2 (Fig. 4b) and to the moving illusory contours (Fig. 4c). These maps represent a projection of two coronal slices (4–5). The position of the volume (8 slices) acquired during the functional scans has been delineated on the scout image in the sagittal plane (Fig. 4d). For the sake of clarity, the calcarine sulcus has been manually darkened. The activations induced by the moving illusory contour near the V5 region are shown in Fig. 4f. The activations induced by expanding rings is shown in Fig. 4e. Figs. 4e and 4f represent the projection maps obtained from three coronal slices (1–3)