Causal evidence for parietal lobule dynamics supporting intention readout

The ability to understand other people’s intentions by observing their actions is crucial to interpret and anticipate their behavior. However, the specific neural computations involved in this ability remain unclear and causally untested. One major reason is the difficulty of identifying intention readout patterns associated with ever-changing kinematic features. Movement is “repetition without repetition”. Statistical analyses averaging across trials can blur away how intention information encoded in single trial variations is readout in real-time. Here we developed a novel approach combining motion tracking with continuous theta burst stimulation (cTBS) and new computational analyses to determine how the transient disruption of a target region – the left inferior parietal lobule (IPL) – influences intention readout computations with single-trial resolution. In separate sessions, participants received neuronavigation-guided cTBS to the left IPL or the left Inferior frontal gyrus (IFG) before completing a two-alternative, forced-choice visual discrimination of intention. Single-trial analyses combined with a set of task manipulations revealed that cTBS to the left IPL, but not to left IFG, selectively impaired the ability to infer the intention of an observed action from variations in visual kinematics. Importantly, IPL cTBS did not interfere with the ability to ‘see’ changes in movement kinematics, nor did it alter the weight given to informative versus non-informative kinematic features. Rather, it selectively impaired the ability to link variations in informative features to the correct intention. These results support a model in which the selection of the most informative kinematic feature occurs outside of the left IPL and in which the left IPL is selectively responsible for the correct readout of such features.