We review different functions in visual perception associated with attention and memory that have been integrated by a model based on the biased competition hypothesis. The model integrates, in a unifying form, the explanation of several existing types of experimental data obtained at different levels of investigation. At the microscopic level, single cell recordings are simulated. At the mesoscopic level of cortical areas, results of functional magnetic resonance imaging (fMRI) studies are reproduced. Finally, at the macroscopic level, the outcome of psychophysical experiments like visual search tasks are also described by the model. In particular, the model directly addresses how bottom-up and top-down processes interact in visual cognition, and shows how some apparently serial processes reflect the operation of interacting parallel distributed systems. Attentional top-down bias guides the dynamics to focus attention at a given spatial location or on given features.