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Type of Document	Dissertation
Author	Ho, Ching Elizabeth
URN	etd-04072005-145953
Persistent URL	http://resolver.caltech.edu/CaltechETD:etd-04072005-145953
Title	Multiple mechanisms of apparent motion perception
Degree	PhD
Option	Computation and Neural Systems
Advisory Committee	Advisor Name Title George Sperling Committee Co-Chair Shinsuke Shimojo Committee Co-Chair
Keywords	none
Date of Defense	2000-01-01
Availability	restricted
Abstract Motion perception is a complex phenomenon. Recently, multiple categories of motion perception have been defined through properties of the stimulus: e.g., short-range and long-range motion (Braddick, 1980), or first-order (luminance-defined), second-order (texture-defined) (Chubb & Sperling, 1988; Chubb & Sperling, 1989; Cavanagh & Mather, 1989), and third-order (pattern-tracking) (Lu & Sperling, 1995a; Lu & Sperling, 1995b) motion. This thesis elucidates the mechanisms of motion perception for a class of ambiguous motion stimuli. In particular, two competing motion systems were found to be involved in the perception of nominal second-order motion stimuli. These systems were hypothesized to explain the inter-observer differences in the perceived direction of motion, and to explain the differences in performance under a dual task paradigm. The interference effects seen in the dual task performances implied the involvement of multiple forms of/distributed attention. Choice of systems could be influenced by attentional instructions and training in addition to stimulus conditions. In viewing an ambiguous texture-defined motion stimulus first devised by Werkhoven et al (1993), the observers fell into two distinct groups based on the direction of perceived motion. The differences were interpreted in terms of the algorithms used to extract motion: one group using a second-order motion process, the other, a third-order motion process. This was investigated fuither using a dual-task paradigm in which the interference between two tasks indicated the nature of processing involved. Observers who used third-order motion processing experienced interference with a letter-recognition task, and a more severe interference in dual third-order motion tasks. Observers who used second-order motion processing experienced interference with another second-order motion detection task, but not with the letter-recognition task. These observations suggest that two systems, a second-order system and a third-order system, are involved in the perception of the nominal second-order motion stimuli. The performance of observers can be interpreted in terms of a simple architecture of motion processing and attentional resource. Insofar as task interference implies competition for attentional resource, the complex and apparently paradoxical interference effects of second-order motion perception suggest that there are multiple attentional resources, or in another word, attention is distributed. Whether two tasks interfere or not depends on whether they require the same attentional resource. A quantitative method, Structure-Attention-Mapping (SAM) was formulated, and a model architecture for the motion pathways was proposed using this method. It was found to be able to explain the data in the experiment. While different observers have different innate tendencies to use one pathway over the other, the selection of pathway is not fixed but can be induced by attentional instructions, training, temporal frequency, and viewing conditions. High temporal frequencies and monocular displays favored the second-order system, low temporal frequencies and interocular displays favor the third-order system. In addition, stimuli composed of patches with orthogonal slant patterns (one slant could be selectively attended) favored the third-order direction; same-slant stimuli favored the second-order direction. A study of luminance-defined motion showed that it can also be perceived by a third-order motion system as well as a first-order motion system. The competition between first- and third-order systems qualitatively resembles that between second- and third-order motion systems.
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