Positron emission tomography (PET) was used to identify regions of the human visual system which were selectively modulated by attention during feature discrimination and recognition tasks. In a first experiment, subjects were cued to the shape, colour or speed of visual stimulus arrays during a same-different match-to-sample paradigm. The psychophysical sensitivity for discriminating subtle attribute variations was enhanced by selective attention. Correspondingly, the neural activity (as measured by blood flow changes) in different visual associative regions was enhanced when subjects attended to different attributes of the same stimulus (intraparietal sulcus for speed; collateral sulcus and dorsolateral occipital cortex for colour; collateral sulcus, fusiform and parahippocampal gyri, superior temporal sulcus for shape). These regions appeared to be specialized for processing the selected attribute. Attention to a visual feature, therefore, enhances the psychophysical sensitivity as well as the neural activity of specialized processing regions of the human visual system. In a second experiment the effect of target probability (which biases attentional selection) was studied during visual search tasks involving the recognition of a single-feature (i.e. colour) or a feature-conjunction (i.e. colour x orientation) target. Target probability positively modulated neural activity of extrastriate visual regions, which were related to the single-feature or feature-conjunction processing level. These results suggest that selective attention can influence different processing levels in the visual system, possibly reflecting a facilitatory effect on different visual computations or task components.