Matsuzaka Y, Picard N, Strick PL. Skill representation in the primary motor cortex after long-term practice. J Neurophysiol 97: 1819 -1832, 2007. First published December 20, 2006; doi:10.1152/jn.00784.2006. The acquisition of motor skills can lead to profound changes in the functional organization of the primary motor cortex (M1). For example, performance of movement sequences after prolonged practice is associated with an expansion of the effector representation in M1. Paradoxically, there is little evidence that the activity of M1 neurons reflects acquired skills, especially sequences of movements. We examined the activity of M1 neurons during skilled movement sequences in macaques trained to successively hit targets on a monitor. The targets appeared either pseudorandomly (Random mode) or in one of two repeating sequences (Repeating mode). With practice, response times for repeating sequences substantially declined and the monkeys performed the task predictively. Highly trained animals retained the acquired skill after long gaps in practice. After Ͼ2 yr of training, 40% of M1 neurons were differentially active during the two task modes. Variations in movement kinematics did not fully explain the task-dependent modulation of neuron activity. Differentially active neurons were more strongly influenced by task mode than by kinematics. Our results suggest that practice sculpts the response properties of M1 neurons. M1 may be a site of storage for the internal representation of skilled sequential movements.
I N T R O D U C T I O NThe ability to link elementary actions together to perform a meaningful sequence of movements is a key component of voluntary motor behavior. The classical view of the cortical control of sequential movements is that the premotor areas are critical for learning and storing the representations of movement sequences (Campbell 1905;Fulton 1935;Jacobsen 1934). In contrast, the primary motor cortex (M1) is thought to simply produce the patterns of muscle activity necessary to implement the plans generated by the premotor areas. However, results from a number of recent studies suggest that M1 plays a more active role in both the acquisition and retention of motor skills including sequential movements (Hund-Georgiadis and von Cramon 1999; Karni et al. 1995Karni et al. , 1998Kleim et al. 1998;Pascual-Leone et al. 1994Plautz et al. 2000;Remple et al. 2001;Sanes and Donoghue 2000;Tyčè et al. 2005). For example, Karni et al. (1995Karni et al. ( , 1998 reported that practice on a sequence of finger movements resulted in large and lasting increases in the extent of M1 activation. Similar training also enlarged the map of cortical output that could be demonstrated using transcranial magnetic stimulation (Pascual-Leone et al. 1994. The map expansion was specific to the muscles used during training. These observations suggest that learning a sequence of movements alters the excitability and functional organization of M1. Furthermore, they imply that M1 is a site of storage for the long-term memory of m...