Motor learning-induced new dendritic spines are preferentially involved in the learned task than existing spines

Learning induces the formation of new synapses in addition to changes of existing synapse strength. However, it remains unclear whether new synapses serve different functions from existing synapses. By performing two-photon structural and Ca²⁺ imaging of postsynaptic dendritic spines in layer 2/3 pyramidal neurons, we show that new spine formation increases in the mouse motor cortex 8-24 h after motor training. New spines, not existing spine populations, are preferentially active when mice perform the learned task rather than a new task. New spine activity is also more synchronized with dendritic/somatic activity when the learned task, not a new task, is carried out. Furthermore, new spines are formed to increase the task specificity in a subset of neurons, and their survival is not affected when a new task is learned. These findings suggest that newly formed synapses preferentially increase the task specificity of neurons over existing synapses at the retention stage of motor learning.