Controlled retrieval relies on directed interactions between semantic control regions and visual cortex: MEG evidence from oscillatory dynamics

To navigate the world, we store knowledge about the relationships between concepts and retrieve this information flexibly to suit our goals. The semantic control network, comprising left inferior frontal gyrus (IFG) and posterior middle temporal gyrus (pMTG), is thought to orchestrate this flexible retrieval by modulating sensory inputs. However, interactions between semantic control and input regions are not sufficiently understood. Moreover, pMTG's well-formed structural connections to both IFG and visual cortex suggests it as a candidate region to integrate control and input processes. We used magnetoencephalography to investigate oscillatory dynamics during semantic decisions to pairs of words, presented one at a time, when participants did or did not know the type of semantic relation between them. IFG showed early increases and decreases in oscillatory responses to prior task knowledge, while pMTG only showed positive effects of task knowledge at a later time window. Furthermore, both regions provided feedback to visual cortex when goals were absent, while IFG also provided feedback when goals were known. This goal-dependent feedback coincided with an earlier onset of feedforward signalling from visual cortex to pMTG, indicating rapid retrieval of task-relevant features. Knowledge of task goals also enhanced simultaneous inputs to pMTG from both IFG and visual cortex, consistent with the view that pMTG integrates top-down control with bottom-up input. Our findings elucidate the separate roles of anterior and posterior components of the semantic control network and reveal the spectro-temporal cascade of interactions between semantic control and visual regions that underlie our ability to flexibly adapt cognition to the current goals.