Comparing the perception of 2D motion patterns versus optic flow

Abstract

When an observer is moving in the environment, objects would project on the observer’s retina and generate a dynamic light motion pattern, named optic flow (Gibson, 1950). In the field of human perception and action, there are two long-standing questions: (1) whether people solely rely on 2D features of optic flow to perceive heading or also adopt 3D information to achieve this task, and (2) whether the brain areas that respond to global motion patterns show unique responses specific to optic flow. Three psychophysical studies and one fMRI study were conducted to address these questions. In Study 1, the detection sensitivities of six types of 2D motion patterns and 3D-cloud optic flow stimuli (i.e., translation, rotation, contraction and expansion radial, contraction and expansion spiral patterns) were examined. In each trial, participants judged whether there was a motion pattern. Results showed that the detection sensitivities to 2D motion patterns were different from those to 3D optic flow. Specifically, for 2D motion patterns, the detection sensitivity was significantly higher for translation patterns than for other patterns and was significantly higher for contraction patterns than for expansion patterns. No significant difference was found between spiral, rotation, and radial patterns. In contrast, for 3D flow stimuli, the detection sensitivity was significantly higher for rotation and expansion radial flow stimuli than for other flow stimuli and was significantly higher for spiral flow than for rotation and radial flow stimuli. In Study 2, the center of motion discrimination sensitivities to contraction and expansion for 2D motion patterns and 3D cloud optic flow stimuli were compared. In each trial, participants judged whether the center in the 1st interval was to the left or right of the center in the 2nd interval. Results showed that observers had higher discrimination sensitivity to contraction than expansion for 2D motion patterns, but the trend was not found for 3D flow stimuli. Built upon Studies 1 and 2, Study 3 sought to examine the detection and center discrimination sensitivities to contraction and expansion for 2D motion patterns and optic flow stimuli with strictly controlled stimuli: (1) the mode of image speeds of optic flow was controlled to be equal to the image speed of 2D motion patterns; (2) the dots were uniformly distributed in each frame of the display; (3) a new type of optic flow, i.e., fronto-parallel plane optic flow was tested; (4) the presentation time of each display varied (i.e., 150 ms, 300 ms, 600 ms, and 800 ms). Results showed that observers had higher detection sensitivity to contraction than expansion for 2D motion patterns, while an opposite trend was found for optic flow; observers had higher center discrimination sensitivity to contraction than expansion for 2D motion patterns, whereas the trend was not found for optic flow stimuli. When the presentation time was extended from 150 ms to 300 ms, the center discrimination sensitivity improved for optic flow stimuli but remained unchanged for 2D motion patterns. In Study 4, the cortical areas responsible for the detection performance were examined using the fMRI technique. Results showed that the neural decoding accuracy of area MST showed the same pattern of behavioral performance as in Studies 1 and 3. Specifically, the decoding accuracy of area MST increased with the increase of motion coherence. The decoding accuracy was higher for contraction than expansion 2D motion patterns, while an opposite trend was observed for optic flow stimuli. Importantly, area MST’s decoding accuracy positively correlated with behavioral accuracy. These results suggest that area MST might be one magic area responsible for the perceptual processing of optic flow. In conclusion, the present thesis provides evidence to support the claim that the perceptual processing of optic flow (i.e., pattern detection and center discrimination) relies on not only 2D features (e.g., the center of motion) but also 3D information in optic flow (e.g., motion parallax and speed gradient). Importantly, area MST in the human brain shows unique responses specific to optic flow.