Magnitude of Middle Temporal N300 Reflects Contour Fidelity in Contour Integration

Contour integration (CI) reflects the ability of the visual system to bind individual elements into a global coherent shape. Previous neuroimaging studies of CI compared behavioral and neural data using stimuli with and without contours. However, these studies had multiple potential confounding variables, including the degree of visual awareness, temporal prediction, and task-relevance. To address these, we presented an array of 27x15 line-segments (each 2x0.5°) that changed their orientations independently and randomly at a rate of 15 Hz. Each trial comprised 25 frames (1.67 s). During one of these frames, the orientations of 12 line-segments were aligned to form a contour (outline of a box with 3 segments on each side), at 6° to the right or left of fixation. Subjects (n=10) fixated on a central dot throughout the trial and indicated whether the contour was on the right or left. EEG signals were collected and analyzed along with task performance. In Experiment 1, we varied the onset timing of the contour to test the effects of temporal prediction. Results showed that ERPs including Frontal P200, Occipital N200 and P400, Parietal P400 and contralateral N200, as well as Middle Temporal (MT) N300 were synchronized with the onset timing of the contour. In Experiment 2, we fixed the onset timing of the contour but varied its fidelity by adding various levels of random orientation jitter to the line segments that formed the contour. Results showed that among all the ERPs observed in Experiment 1, only the magnitudes of MT N300 and Parietal P400 were dependent on contour fidelity. As reported in the literature, Frontal P200 and Posterior P400 are likely to reflect awareness/attention and task-related-efforts, respectively, and Posterior N200 is likely to be a correlate of visual phenomenal consciousness. We therefore propose MT N300 as a neural correlate of contour integration.