Abstract
Attention can be selectively directed to both external sensory inputs and internally generated representations. The relationship between these external and internal attentional states is not fixed but varies between competition, concurrence, and cooperation. However, the conditions that allow for each of these relationships to occur remain poorly understood. We distinguish between the level of brain states and behavior and synthesize evidence for each of these relationships at both levels. Our review shows that shared, capacity-limited mechanisms and opposing computational modes (e.g., encoding vs. retrieval) often produce competitive dynamics, particularly in the presence of simultaneous and incompatible demands. At the same time, several studies have revealed partially independent substrates that permit concurrent selection of internal and external information, especially when controlled, intentional processing is not required. More recently, researcher have started exploring conditions under which external and internal can support or facilitate each other, enabling cooperative integration in the service of a single goal. We argue that task demands, degree of control, representational overlap, and the considered timescale jointly determine whether external and internal attention compete, co-occur, or cooperate, and we outline key directions for future work aimed at a more nuanced understanding of how these states jointly shape cognition.