Dissociable neural correlates of contour completion and contour representation in illusory contour perception

Abstract

Object recognition occurs even when environmental information is incomplete. Illusory contours (ICs), in which a contour is perceived though the contour edges are incomplete, have been extensively studied as an example of such a visual completion phenomenon. Despite the neural activity in response to ICs in visual cortical areas from low (V1 and V2) to high (LOC: the lateral occipital cortex) levels, the details of the neural processing underlying IC perception are largely not clarified. For example, how do the visual areas function in IC perception and how do they interact to archive the coherent contour perception? IC perception involves the process of completing the local discrete contour edges (contour completion) and the process of representing the global completed contour information (contour representation). Here, functional magnetic resonance imaging was used to dissociate contour completion and contour representation by varying each in opposite directions. The results show that the neural activity was stronger to stimuli with more contour completion than to stimuli with more contour representation in V1 and V2, which was the reverse of that in the LOC. When inspecting the neural activity change across the visual pathway, the activation remained high for the stimuli with more contour completion and increased for the stimuli with more contour representation. These results suggest distinct neural correlates of contour completion and contour representation, and the possible collaboration between the two processes during IC perception, indicating a neural connection between the discrete retinal input and the coherent visual percept.

Figure 1

Illustration of the experimental stimuli. The Kanizsa (upper) and control (lower) stimuli with large (left) and small (right) gaps are shown. The size of the inducer gaps was varied so that contour completion (interpolation between gaps) and contour representation (contour strength or contour clarity) varied in opposite directions. As the interpolation increased (i.e., more gap), the contour strength became weaker, and vice versa. For instance, the strength of the contour of a square was weaker in the large‐gap than in the small‐gap condition, but more interpolation was needed.

Figure 2

Illustration of the results of the main experiment. The difference between the activations to Kanizsa (K) and control (C) stimuli (IC response) was not significant in V1 and V2 in the small‐gap condition. The IC response was stronger in the large‐gap than in the small‐gap condition in V1, but became weaker in the lateral occipital cortex (LOC). In addition, the IC response increased from low‐ to high‐tier visual areas (P < 0.001) in the small‐ rather than in the large‐gap condition. One asterisk indicates 0.05 ≥ P > 0.01; two asterisks indicate 0.01 ≥ P > 0.001; three asterisks indicate P ≤ 0.001. Error bars indicate ± S.E.M. The data from the left and right hemispheres were averaged.

Figure 3

Illustration of the inducer localizer stimuli and the results of the control experiment. (a) Regions in V1 corresponding to the inducers or inducer gaps of one representative subject are respectively marked with circles and squares. Localizer stimuli are shown in the middle. The relationships between the inducers and the V1 regions activated by them are indicated by colors. The orange color represents the overlapping regions. (b) The differences between the activations to Kanizsa and control stimuli (IC response) in the large‐gap condition in the above V1 regions. The mean IC responses in the regions corresponding to the inducers or inducer gaps are respectively shown with the large filled circles or squares. The IC responses from the individual subjects are shown by the small triangles in different directions. The IC response occurred in the regions corresponding to the inducers gaps but not to the inducers themselves. Error bars indicate ± S.E.M. The data from the left and right hemispheres were averaged. [Color figure can be viewed in the online issue, which is available at wileyonlinelibrary.com.]