The effects of feature-based attention on perception:

Feature-based attention is one of the mechanisms that can facilitate the processing of many aspects of our visual perception. A variety of paradigms were employed in the current dissertation to further investigate how feature-based attention modulates motion perception, visual search and temporal processing of stimuli. The first set of experiments aimed to explore the effects of feature-based attention on the processing of motion speed and motion direction separately. Speed and direction discrimination tasks were used in separate experiments. Results showed that feature-based attention has more dramatic influence on direction perception than on speed perception. This may be taken as evidence that humans are more sensitive to motion speed change than to motion direction change. The second set of experiments was designed to study how performance in color-orientation conjunctive searches changes when observers attend to a pre-cued location, or a pre-cued feature (color or orientation), as well as the temporal characteristics of these precue effects. Color (sensory and symbolic) and location precues improved search performance. The magnitude of improvement did not vary as the inter-stimulus interval (ISI) changed for color and location cues. The sensory color and location cues exhibited their effect in directing visual search as early as 0 ms of ISI. However, orientation precue did not facilitate nor inhibit the search processing. These results may imply that color is a better feature to base the segmentation processing on and thereby facilitate the visual search processing. The third set of experiments explored the existence of feature-based attentional prior-entry effect, which refers to the hypothesis that attended objects are perceived prior to unattended ones. Temporal order judgment (TOJ) and simultaneity judgment (SJ) tasks were employed to test this hypothesis. Prior-entry effect for objects with attended feature was found in TOJ task, the most frequently used paradigm in the literature to claim the spatial prior-entry effect, but the effect was absent in the SJ task. This could be due to a second-order response bias in the TOJ task, or to the fact that the SJ task is not as sensitive as the TOJ task.Ph.D.Includes bibliographical references (p. 71-81)by Xiaohua Zhuan