Much has been learned over the past 25 years about how attention influences neuronal responses to stimuli in the visual cortex of monkeys and humans. The most recent studies have used parametric manipulations of stimulus attributes such as orientation, direction of motion, and contrast to elucidate the form of the attentional mechanism. The results of these studies do not always agree. However, some of this inconsistency may be caused which attentional effects are considered, such as contrast gain, response gain, or a baseline shift in firing rate with attention. Here, seven studies of spatial and feature-based attention, ranging from monkey electrophysiological studies in V4 and MT to fMRI studies in human visual cortex, are reevaluated in the context of a single parametric model that incorporates a variety of ways in which attention can influence neuronal responses. This reanalysis shows that most, though not all, of these results can be explained by a similar combination of attentional mechanisms.