Discovery and description of the position-tracking motion-detection mechanism

Perception of the dynamic visual world is only possible with mechanisms that provide consistency between objects over space and time. Motion perception serves as the glue that binds spatiotemporal information from the visual input to produce a unified percept of the world. Two theoretical mechanisms for supplying motion have been proposed throughout the history of perception. The first is an elementary process that directly computes velocity from the visual image, as a fundamental attribute of perception. Previous research has shown that direct perception of motion is mediated by an array of detectors at early levels of visual analysis. In the second process, motion is derived or inferred from identification of object locations as they change across time. Until now, this higher-level tracking process has been presumed, but not demonstrated as a motion detection system. The current thesis demonstrated that this type of position-tracking mechanism is, in fact, an important part of the perception of motion.; The thesis is divided into three sections, each with its own goal with respect to position tracking. In each experiment, observers were asked to detect the motion of patterns made of various attributes, such as luminance, texture, color or depth. Results showed that while it was the velocity of the luminance patterns that uniquely determined thresholds, it was the position-change of the other patterns that determined thresholds. Additional experiments explored the domain in which each mechanism is used and revealed characteristic differences between the two systems. Further experiments indicated that stimulus conditions requiring position tracking also require attentional scrutiny to see motion. Taken together, the three sections of this thesis find that (1) a position tracking motion mechanism does exist for use by the human observer, (2) this mechanism is used for equiluminant stimuli at low contrasts and speeds and (3) the most likely basis for such a position tracking mechanism is the movement of attention. Specification of the position-tracking mechanism and its attributes in this thesis provides new organization for understanding previous research and broadens our understanding of motion perception as a whole.