Neuronal activity can be triggered by either external stimuli or by stimuli that are a result of your own actions.  In his second talk, Engert explains how his lab measured and compared neuronal activity in response to these two types of stimuli. Using a set up where larval zebra fish are immobilized with drugs, yet the neurons which innervate the muscles fire normally, they measured neuronal activity in response to visual stimuli. By moving a virtual landscape quickly or slowly they could mimic fast or slow swimming by the fish.  Interestingly, they found that the fish would adjust its neuronal activity up or down so its perceived swim speed matched the movement of the virtual landscape or, in other words, so the external stimuli matched the expected result of its actions. Engert’s group mapped the neurons responsible for visual processing and locomotion, as well as those responsible for gain control up or down to match the fish’s swimming strength with the external stimuli.