In our first experiment (Limanowski & Friston 2020 Cerebral Cortex), we used fMRI and DCM to investigate the neuronal mechanisms underlying the task-dependent weighting of visual vs proprioceptive information from the moving body, as described above. We found increased activity of visual and multisensory brain areas during the 'virtual hand' task and increased activity of proprioceptive brain areas during the 'real hand' task. DCM then showed that these activity changes were the result of selective, diametrical modulations of excitability of sensory (visual vs proprioceptive) brain areas (Figure 2). These results showed that endogenous attention can balance the excitability (i.e. cortical 'gain') of visual vs proprioceptive brain areas during action.
In a combined simulation and behavioural study (Limanowski & Friston 2020 Scientific Reports), we simulated a simple agent based on predictive coding formulations of active inference as situated within a free energy principle of brain function. The behaviour of our 'real' participants and the results of the computational simulations jointly confirmed that precision estimates of vision vs proprioception within the agent’s model of its body directly determined the degree to which each modality was used for driving goal-directed action (Figure 3). Thus, we established the hypothesized link between sensory precision weighting and behaviour.
Building upon the fMRI and simulation results, we next examined cortical oscillations with MEG while participants performed an analogous task (Limanowski, Litvak, & Friston 2020 bioRxiv). Crucially, the rich temporal structure of MEG data allowed us to use a neural mass model for DCM comprising three interconnected cell populations, which thus distinguished between ‘extrinsic’ (‘forward’ and ‘backward’) between-area connections, and ‘intrinsic’ connections. The latter connections model effects of self-inhibition, determining the input-output balance or ‘excitability’ of a given source, and are therefore usually associated with cortical gain control. We could thus, in our model comparison, test whether the condition-specific effects were best explained by changes in extrinsic (forward and/or backward between-region) and/or intrinsic (within-region) connectivity. Our MEG spectral results revealed that relative to the congruent movement conditions, occipital oscillatory power in the ‘beta’ range (12-30 Hz) was suppressed in the incongruent ‘virtual hand’ task but enhanced in incongruent ‘real hand’ task. Our DCM analysis identified diametrical changes in the cortical gain of visual areas as the most likely causes of these spectral differences; i.e. increased gain during the incongruent ‘virtual hand’ task and decreased gain during the incongruent ‘real hand’ task relative to movements without visuo-proprioceptive conflict (Figure 4). These results strongly support the hypothesis that visual (vs proprioceptive) bodily action information can be differently weighted depending on the prevalent cognitive-attentional set; i.e. for integration with the current action plan.
The implications of our experimental work for the understanding of minimal selfhood within the larger framework of active inference were further discussed in two theoretical papers (Limanowski & Friston 2018 Frontiers in Psychology, 2020 Philosophy and the Mind Sciences).
