Uncovering the neural basis of movement transitions

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You're feeling thirsty. Your arm moves outwards, reaching for the cup of coffee on your desk, grasps the handle, and moves the cup smoothly back to your waiting lips. You're walking to the bus stop, on your way to an important interview. Legs swinging, left then right, left then right, when the bus unexpectedly turns the corner ahead of you. Panicked, you break into a run, legs pumping, feet leaving the floor with each stride. Both are transitions between movements, between the discrete movements of the arm - out, then stop, then back - and between the rhythmic movements of walking to running. But while we know much about how the brain represents and controls single movements, we know little about how it controls the transitions between them. Understanding this would help us build better intelligent prosthetics for the paralysed and disabled; and build better, more natural controllers for moving robots. The challenge is that different movements are controlled by the same set of neurons in the brain. There are a set of neurons in your motor cortex that control arm movement. Elsewhere there are a set of neurons that create the rhythms of leg movement. Somehow, the activity of those same neurons changes from representing one movement to another, and does so smoothly, so that you do not freeze in place.Our proposed work thus aims to tackle the intriguing problem of how a single group of neurons changes between patterns of activity so different that they each generate different movements, yet does so smoothly. To tackle this problem for discrete movements, we will study neural activity in the motor cortex of monkeys moving their arms to control a joystick. The monkey's goal is to move the joystick to hit each of four targets in a row, each movement between targets thus creating a discrete arm movement. To tackle this problem for rhythmic movements, we will study neural activity in the crawling circuit of sea-slugs escaping, changing from being still, to galloping, to crawling normally. Studying rhythmic transitions in sea-slugs has the unique advantages that we can reliably cause this escape response in the lab, and at the same time can record every output from about ten percent of all the essential neurons.These data will let us answer some deep questions about how brains control transitions between movements. The first is to work out which pattern of neural activity creates which movement. We will develop methods to find when and how the patterns change, and compare these changes to the movements in monkeys and sea-slugs. This will reveal the basic neural "code" for transitions in movements.The second is to understand if single neurons are important for transitions. The pattern of activity that is responsible for, say, galloping is shared among a set of neurons; and approximately the same pattern can be created by different combinations of those neurons. So it may be that only the pattern is consistently created, and not the activity of individual neurons. Knowing this will help us better understand how to decode movements from brain activity.The third is to discover what physical changes to the circuit create the changes in activity pattern. We will use models of circuits to pick apart whether the timing and type of changes between movements are caused by changes to the inputs to the circuit, changes to the wiring between neurons, or something else. These insights this will help us design better ways to control changes between movements by controlling changes in brain activity.By revealing how brains successfully and smoothly move bodies between movements, our results could provide a wealth of new options for the control of artificial or robotic limbs by patients, and for designing controllers for movement in robots.