Task interference underlies the task switch cost in perceptual decision-making

To survive in the uncertain world, humans and animals must pick the right task to perform in each situation and flexibly switch between tasks when they infer that the environment has changed. Yet such flexibility comes at a cost. Task performance decreases when the relevance of the task is uncertain. Here we present multiple lines of evidence from monkey electrophysiology, human psychophysics, and artificial neural networks demonstrating a neuronal mechanism that inflicts a cost of flexibility. In a behavioral paradigm designed to measure and manipulate subjects’ belief about the relevance of either of two perceptual tasks, we found that human and monkey subjects make less accurate perceptual decisions under task uncertainty. To generate hypotheses about a neuronal basis for the task switching cost, we compared two recurrent neural networks (RNNs), trained to produce the correct choice or to reproduce the choices of monkey subjects. The ‘correct-choice’ RNN learned to flexibly switch tasks without incurring a task switch cost, while the ‘monkey-choice’ RNN displayed the expected cost of task switching. The different activity of the recurrent layers of the two models suggested a mechanism that produces the task switch cost. The relevant subspaces for the two tasks remained separate for the ‘correct-choice’ model, but collapsed together for the ‘monkey-choice’ model, leading to interference between tasks especially when task is uncertain. We confirmed predictions of the model in further behavioral and physiological experiments, demonstrating that behavior and neural activity shows signs of task interference under uncertain task conditions. These results provide a neuronal mechanism for flexible decision-making in neurotypical subjects, as well as its dysfunction in common neurological disorders. They support the general, tantalizing hypothesis that limits in cognitive capacity arise from interference between the neural representations of different stimuli, tasks, or memories.