The Representational Dynamics of Task and Object Category Processing in Humans

Object recognition is commonly described as a feedforward process, yet the tasks we carry out often affect what information in visual stimuli is diagnostic and may influence their processing. Surprisingly little is known about how task context is processed and when and how it interacts with the emerging representation of objects. Here we used magnetoencephalography (MEG) and multivariate decoding to investigate the temporal dynamics of task and object processing and their interaction. Participants viewed objects while we varied task context on a trial-by-trial basis, using both high-level conceptual and low-level perceptual tasks. Time-resolved multivariate decoding and temporal cross-classification revealed multiple distinct yet overlapping stages of task processing from the onset of a trial, likely reflecting a sequence of visual, semantic, mnemonic, and rule-related task representations. Object decoding was stronger in conceptual than perceptual tasks, with differences emerging around 530 ms after object onset. However, object decoding generalized well between task contexts, indicating differently strong but qualitatively similar brain responses. Using model-based MEG-fMRI fusion, we found that frontoparietal areas were strongly dominated by information about task context throughout the trial, while occipitotemporal regions reflected a mixture of both task and object category indicating their parallel encoding in the same brain area. Together, our results reveal the temporal evolution of task context representations and suggest that the impact of task context during object processing occurs late in time.