This thesis examined the influences of attention on the processing of complex and simple motion stimuli. The work consists of two physiology studies. In the first study we investigated the question of how complex motion stimuli, namely transparent motion stimuli, are processed in area MT and the influence of attention on the processing. In our second study we were interested in attentional effects on visual motion stimuli in the primary visual cortex. We find a strong feature-based component which contributes to the processing of complex motion stimuli. Furthermore we show that effects of s ...

The contributors to this book focus on such key aspects of motion processing as interaction and integration between locally measured motion units, structure from motion, heading in an optical flow, and second-order motion. They also discuss the interaction of motion processing with other high-level visual functions such as surface representation and attention.

Motion perception lies at the heart of the scientific study of vision. The motion aftereffect (MAE) is the appearance of directional movement in a stationary object or scene after the viewer has been exposed to viusal motion in the opposite direction. For example, after one has looked at a waterfall for a period of time, the scene beside the waterfall may appear to move upward when one's gaze is transfered to it. Although the phenomenon seems simple, research has revealed copmlexities in the underlying mechanisms, and offered general lessons about how the brain processes visual information. In the 1990s alone, more than 200 papers have been published on MAE, largely inspired by improved techniques for examining brain electrophysiology and by emerging new theories of motion perception.

The ability to recognize and understand the movements and actions of others is critical to everyday social interaction. Considering the ecological significance and efficiency of biological motion perception, it has often been described as an automatic or attention-free process. However, although perception of these complex stimuli may seem effortless, evidence suggests that attention does play an important role, especially when other stimuli are present. Even so, the nature of the relationship between biological motion and attention has not been well defined. The four experiments of this dissertation detail the relationship between attention and biological motion within the framework of the biased competition model of attention. The biased competition model proposes that attention acts by resolving the competition that arises when two or more objects occur simultaneously, in favor of the attended stimulus. Behavioral and event-related functional magnetic resonance imaging responses were measured while participants viewed point-light animations of human and tool motion under different attentional conditions. Results illustrated that the neural mechanisms underlying biological motion perception are strongly modulated by selective attention: when attention was focused away from biological motion, responses were reduced compared to when biological motion was selected by attention. Additionally, consistent with the biased competition model, the spatial proximity between concurrently presented items influenced the neural response to biological motion. While separation between simultaneously presented objects resulted in increased responses when attention was focused on biological motion, directing attention away from biological motion led to decreased neural responses when stimuli were separated. These results indicated that there is involvement of both object-based and spatial attention. Finally, expectations regarding specific object categories did not influence visual processing by preactivating neural responses in brain regions involved in processing biological motion. Together, the findings presented here lend further evidence of a critical role of top-down influences on the neural mechanisms underlying biological motion perception and indicate that this role is partially consistent with the predictions of the biased competition model.

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.