Perceptual and computational mechanisms of feature -based attentio

Visual attention modulates visual processing along at least three dimensions: A spatial dimension which enhances the representation of stimuli within the focus of attention, a feature dimension thought to enhance attended visual features throughout the visual field and an object dimension by which attention enhance the whole object as a unit. In this thesis, we focus on the feature dimension by studying the feature-based attention. We investigated the overall perceptual consequences of feature-based attention, by using dual-task human psychophysics and two distant drifting Gabor stimuli to systematically explore 64 combinations of visual features (orientations and drift speeds) and tasks (discriminating orientation or drift speed). The resulting single, consistent dataset suggests a functional model, which predicts a maximum-rule by which only the dominant product of feature enhancement and feature benefit by feature relevance may benefit perception. We also used fMRI to investigate whether feature-based attention is a specific form of object-based attention or a new type of lower feature based attentional selection. We studied the feature-based attentional enhancement in two conditions: either the two stimuli appeared to belong to same object or as two different objects. Results showed both in same-object condition and in different-object condition the four subjects consistently had significant enhancement of the ignored stimulus in early visual areas. Hence it indicated feature-based attentional enhancement exists even when two stimuli belong to two different objects, suggesting it is a new type of attentional selection which takes place in the low feature levels, not dependent on the objectness of the two features. We also present two unpublished studies showing that Gabor stimuli are not the suitable stimuli to trigger the feature-based attentional enhancement using fMRI. In summary, this thesis provides a functional model to explain the overall psychophysical perceptual mechanism of the feature-based attention, and evidence that the feature-based attention is new kind of attentional selection biasing on the lower feature level.