Humans are remarkably limited in: (i) how many control-dependent tasks they can execute simultaneously, and (ii) how intensely they can focus on a single task. These limitations are universal assumptions of most theories of cognition. Yet, a rationale for why humans are subject to these constraints remains elusive. This feature review draws on recent insights from psychology, neuroscience, and machine learning, to suggest that constraints on cognitive control may result from a rational adaptation to fundamental, computational dilemmas in neural architectures. The reviewed literature implies that limitations in multitasking may result from a trade-off between learning efficacy and processing efficiency and that limitations in the intensity of commitment to a single task may reflect a trade-off between cognitive stability and flexibility.
The role of capacity constraints in human cognitionOne of the most remarkable features of human cognition is the ability to rapidly adapt behavior in a changing world. This is often attributed to the capacity for cognitive control: the ability to flexibly direct behavior in pursuit of a goal (Box 1). Cognitive control is engaged by all of the higher mental faculties that distinguish humans from other species, including reasoning, problem solving, planning, and the use of symbolic language [1]. Yet, humans are strikingly limited in how many control-demanding tasks (see Glossary) they can perform simultaneously (e.g., reading a document while listening to a friend) or how intensely they can focus on a single task (e.g., parsing a mathematical equation in a noisy environment). The significance of these limitations is not only apparent in daily life. They are also a fundamental premise of general theories of human cognition (e.g. [2-7],). These theories posit that the exertion of cognitive control is associated with a cost, and that humans consider this cost when making decisions about how to allocate control [7][8][9]. The notion of a cost and concomitant constraints on control, can help integrate a wide range of empirical findings concerning the allocation of mental effort [10][11][12][13], the selection between cognitive heuristics [5], planning [14,15], or cognitive impairments in depression [16]. Yet, none of these theories provides an explanation for why control-dependent processing would be subject to these limitations in the first place.Here, we review two fundamental, computational dilemmas that arise in neural systems and suggest that these provide a rational account of constraints on cognitive control. First, we review empirical and computational evidence suggesting a trade-off between the rapid acquisition of novel tasks (learning efficacy), that is promoted by sharing representations across tasks, on the one hand; and multitasking capability (processing efficiency) that is achieved by separating representations and dedicating them to individual tasks, on the other hand. The work reviewed suggests that limitations in the ability to simultaneously execute multiple tasks may...