Attention can selectively enhance neuronal responses and exclude external noise, but the neuronal computations underlying these effects remain unknown. At the neuronal level noise exclusion might result in altered spatial integration properties. We tested this proposal by recording neuronal activity and length tuning in macaque V1 when attention was directed towards or away from stimuli presented in the neuron’s classical receptive field. For cells with central-parafoveal receptive fields, attention indeed reduced spatial integration demonstrated by a reduction in preferred length and in the size of the spatial summation area. Conversely, in cells representing more peripheral locations, attention increased spatial integration by increasing the cell’s summation area. This previously unknown dichotomy between central and peripheral vision could support accurate analysis of attended foveal objects and target selection for impending eye-movements to peripheral objects.