In the present symposium, we focus on neuronal operant conditioning in which increments in neuronal activities are directly rewarded without behaviors. We discuss the potential of this approach to elucidate neuronal plasticity for volitional control over specific brain activities and its interaction with the progress in brain-machine interface (BMI) including neurorehabilitation. Here we introduce our recent study showing selective enhancement of firing synchrony of multiple neighboring neurons in rat hippocampus. Rats were trained to engage in a free-operant task in which nose-poke behaviors were rewarded in session 1, and firing rates and synchrony above preset criteria were rewarded in sessions 2 and 3, respectively. The firing rates of motor cortical and hippocampal neuron groups were found to increase rapidly in session 2 similarly to the nose-poke behavior in session 1. Placing contingency of reward on firing synchrony resulted in selective enhancement of firing synchrony of only hippocampal neurons in session 3. Control experiments revealed that the enhancement of neuronal firing was not attributable to increments of superstitious behaviors or excitation caused by reward delivery. Analysis of the firing rates and synchrony of individual neurons and neuron pairs in each group revealed that the firing rates and synchrony of some but not all neurons and neuron pairs increased in each group. No enhancement was observed in any neurons and neuron pairs recorded by neighboring electrodes not used for conditioning. These results suggest that neuronal operant conditioning enhances not only firing rates but also firing synchrony of only some neurons in small restricted areas. Finally we discuss the reasons for emphasizing firing synchrony in neuronal operant conditioning, the main reason being that they may reflect the activities of cell assemblies, which have been suggested to be basic neuronal codes representing information in the brain. We argue that research of neuronal operant conditioning, BMI, neurorehabilitation, and systems neuroscience will produce findings applicable to these interrelated fields, and neuronal synchrony including oscillation can be a common important bridge among all of them.