Previous studies have proposed a variety of mechanisms by which attention influences neuronal activity. Here we investigated the mechanisms of attention in the striate cortex of monkeys performing a spatial or an object-based attention task at various stimulus contrasts and compared neuronal contrast response functions with and without attention. Our data are best described by an "additive" interaction: The influence of attention on the neuronal response is relatively independent of the stimulus contrast, at least when the stimulus has enough contrast to become visible. This shows that attention adds to the neuronal responses in a largely contrast invariant manner. These data support recent functional magnetic resonance imaging studies and suggest that feedback from higher areas exerts a constant attentional drive that is mostly task not stimulus driven.