PROJECT SUMMARY Research in several species shows that rapidly stopping action activates the subthalamic nucleus (STN) of the basal ganglia (BG) ? and that the STN may be recruited via a hyperdirect pathway (HDP) from prefrontal cortical stopping nodes. Human research also shows that unexpected events (such as a surprising tones) also recruit the stopping system (including the STN), and interrupt working memory (WM). At the core of this proposal is the idea that WM is maintained via a recurrent cortico-BG loop, and that this can be interrupted via HDP recruitment of the STN. The interruption is induced via stop signals and unexpected events, and could function to `clear cognition'. This is a radical new theory: that a fronto-BG circuit for stopping underlies cognitive interruptions. Validating this theory has broad implications for understanding the BG, for mechanisms of cognitive interruptions, and for potential cognitive under-and-over flexibility in Parkinson's disease (PD) and ADHD. Here we propose to systematically test this theory using parallel human-mouse studies, based on extensive preliminary data and using several novel approaches, including event-related Deep Brain Stimulation (eDBS). Aim 1 (motor stop) will use STN eDBS in humans and optogenetic stimulation in mice to test whether the STN is causally important for stopping action. Aim 2 (cognitive stop) will use eDBS and functional MRI in humans and optogenetics in mice to test whether the STN is causally important for decrementing WM. Aim 3 (HDP) will use combinatorial optogenetics in mice and novel combinations of transcranial magnetic stimulation with concurrent local field recordings in human STN to test whether motor and cognitive Stops are implemented via HDP inputs to STN from frontal cortex. Throughout the proposal we focus on designing complementary experiments across species, so that what is learned in mice can directly inform our understanding of human neural circuit function and behavior.