1. The rostromesial agranular frontal cortex of macaque monkey (Macaca fuscata), traditionally defined as the supplementary motor area (SMA), was studied using various physiological techniques to delineate two different areas rostrocaudally. 2. Field and unitary responses to electrical stimulation of the primary motor cortex were distinct in the caudal part, but minimal or absent in the rostral part. Intracortical microstimulation readily evoked limb or orofacial movements in the caudal part, but only infrequently in the rostral part. Neuronal responses to visual stimuli prevailed in the rostral part, but somatosensory responses were rare. The opposite was true in the caudal part. 3. The rostral part, roughly corresponding to area 6a beta, was operationally defined as the presupplementary motor area (pre-SMA). The caudal part was redefined as the SMA proper. 4. Single-cell activity in the pre-SMA was quantitatively compared with that in the SMA proper in relation to a trained motor task. 5. Phasic responses to visual cue signals indicating the direction of forthcoming arm-reaching movement were more abundant in the pre-SMA. 6. Activity changes during the preparatory period, which lasted until the occurrence of the trigger signal for the reaching movement, were more frequent in the pre-SMA. 7. Phasic, movement-related activity was more frequent in the SMA, and its onset was often time locked to the movement onset. In the pre-SMA, the occurrences of response time locked to the movement-trigger signal were more frequent than in the SMA. 8. Among neurons in both areas, directional selectivity was found in all the cue, preparatory, and movement-related responses.

Interval timing is an essential guiding force of behavior. Previous reports have implicated the prefrontal and parietal cortex as being involved in time perception and in temporal decision making. We found that neurons in the medial motor areas, in particular the presupplementary motor area, participate in interval timing in the range of seconds. Monkeys were trained to perform an interval-generation task that required them to determine waiting periods of three different durations. Neuronal activity contributed to the process of retrieving time instructions from visual cues, signaled the initiation of action in a time-selective manner, and developed activity to represent the passage of time. These results specify how medial motor areas take part in initiating actions on the basis of self-generated time estimates.

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...

How does long-term training and the development of motor skill modify the activity of the primary motor cortex (M1)? To address this issue we trained monkeys for ~1-6 years to perform visually-guided and internally-generated sequences of reaching movements. Then, we used 14 C-2-deoxyglucose (2DG) uptake and single neuron recording to measure metabolic and neuron activity in M1. After extended practice, we observed a profound reduction of metabolic activity in M1 for the performance of internally-generated compared to visually-guided tasks. In contrast, measures of neuron firing displayed little difference during the two tasks. These findings suggest that the development of skill through extended practice results in a reduction in the synaptic activity required to produce internally-generated, but not visually-guided sequences of movements. Thus, practice leading to skilled performance results in more efficient generation of neuronal activity in M1.

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