This paper introduces the concept of psychophysiological interactions, which are explained responses in one cortical area due to the interaction between the influence of another area and an experimental parameter. These interactions are distinct from psychological and physiological interactions and are framed in terms of functional integration. The authors use functional magnetic resonance imaging (fMRI) and positron emission tomography (PET) to study these interactions, focusing on extrastriate, inferotemporal, and posterior parietal regions during visual processing under different attentional and perceptual conditions. The paper discusses the interpretation of psychophysiological interactions, including effective connectivity, contribution, and regression analyses. It also explores factorial designs and the potential neurobiological mechanisms behind these interactions, providing examples from studies on attention to visual motion and perceptual priming. The authors highlight the importance of these interactions in understanding functional specialization and integration in the brain.This paper introduces the concept of psychophysiological interactions, which are explained responses in one cortical area due to the interaction between the influence of another area and an experimental parameter. These interactions are distinct from psychological and physiological interactions and are framed in terms of functional integration. The authors use functional magnetic resonance imaging (fMRI) and positron emission tomography (PET) to study these interactions, focusing on extrastriate, inferotemporal, and posterior parietal regions during visual processing under different attentional and perceptual conditions. The paper discusses the interpretation of psychophysiological interactions, including effective connectivity, contribution, and regression analyses. It also explores factorial designs and the potential neurobiological mechanisms behind these interactions, providing examples from studies on attention to visual motion and perceptual priming. The authors highlight the importance of these interactions in understanding functional specialization and integration in the brain.