Brain circuits that control habitual learning identified

EU-funded scientists have identified two brain circuits that are involved in habitual learning such as finding our way to and from work. The new findings, published in the journal Neuron, have implications for the study of Parkinson's disease, substance abuse and many psychiatric disorders.

The discovery is an outcome of the SELECT-AND-ACT ('The role of striatum in selection of behaviour and motor learning: neuronal code, microcircuits and modelling') project, which is funded with EUR 2.5 million through the Health Theme of the EU's Seventh Framework Programme (FP7).

Daily habits are formed by the simple process of repetition and exploring through trial and error. We often carry out routines without thinking about them or noticing them very much. For example, one might walk home from the usual train stop while daydreaming about something else entirely.

Scientists have long explored how we are able to learn a routine well enough to perform it without thinking. In this latest study, researchers focused on the basal ganglia, a remarkable group of neurons in the mammalian brain that are involved in various functions ranging from movement to emotion and thinking.

Previous studies have indicated that the largest structure in the basal ganglia, the striatum, may be important in reward-based learning. One part of the striatum controls movement and is connected to the sensorimotor cortex, which is involved in planning and executing voluntary functions. Another circuit in the striatum controls flexible behaviour and is connected to the 'association cortex', which processes and integrates sensory information.

Until now, little has been known about how these two separate circuits contribute to learning new behaviours. In this study, researchers based at the Massachusetts Institute of Technology (MIT) in the US recorded the activity of the two striatal circuits in rats learning to find their way to a cache of chocolate-flavoured sprinkles in a maze. To make their way to the sprinkles, the rats had to figure out the meaning of sound and touch cues delivered at a T-junction in the maze. They kept trying until the journey became routine.

As the rats' performance improved, the circuits showed distinctive activity patterns that evolved during the learning process. One became most active when the rats had to take specific actions (e.g. start, stop or turn) and got stronger as the routine was learned. The other became very active when the rat had to decide which way to turn, but as the rat learned the route the signal became weaker.

'We think the two basal ganglionic circuits must work in parallel,' said Catherine Thorn of MIT, first author of the study. 'We see what looks like competition between the two circuits until the learned behaviour becomes ingrained as a habit.'

'These brain circuits are affected in Parkinson's disease, substance abuse and many psychiatric disorders,' explained MIT's Ann Graybiel. 'If we can learn how to tilt the competition in one direction or the other, we might help bring new focus to existing therapies, and possibly aid in the development of new therapies.'