Attention is often regarded as a mechanism by which attended objects become perceptually more salient, akin to increasing their contrast. We demonstrate by means of human psychophysics that attention is better described as a mechanism that reduces contextual integration, thereby ensuring that task irrelevant information is prevented from influencing the processing of task relevant information. To investigate possible neuronal bases of this phenomenon we studied the effects of attention on spatial integration in V1 of the macaque monkey. In line with our psychophysical results, attention directed to parafoveal locations reduced spatial integration by reducing the summation area of V1 neurons. Additionally we measured length tuning in V1 in the presence and absence of externally applied acetylcholine in V1 of the marmoset monkey. The effects of acetylcholine application and attention were largely similar. Acetylcholine reduced spatial integration by reducing the neuron's summation area. These data demonstrate that attention can alter perceptual and neuronal spatial integration, and that acetylcholine might contribute to task dependent receptive field dynamics.