Strong attentional modulation of V1/V2 activity implements a robust, contrast-invariant control mechanism for selective information processing

When selective attention is devoted to one of multiple stimuli within receptive fields of neurons in visual area V4, cells respond as if only the attended stimulus was present. The underlying neural mechanisms are still debated, but computational studies suggest that a small rate advantage for neural populations passing the attended signal to V4 suffices to establish such selective processing. We challenged this theory by pairing stimuli with different luminance contrasts, such that attention on a weak target stimulus would have to overcome a large activation difference to a strong distracter. In this situation we found unexpectedly large attentional target facilitation in macaque V1/V2 which far surpasses known magnitudes of attentional modulation. Target facilitation scales with contrast difference and combines with distracter suppression to achieve the required rate advantage. These effects can be explained by a contrast-independent attentional control mechanism with excitatory centre and suppressive surround targeting divisive normalization units.