Learning can be associated with the establishment of new synapses that provide long-lasting memory traces of learned knowledge. In support of this notion, repeated imaging studies of spines in situ have provided evidence that sensory adjustments, skill learning, and Pavlovian conditioning induce new synapses and loss of pre-existing synapses [1–4]. Moreover, subsequent behavioral retrieval of the same skill does not induce more new synapses but does induce re-strengthening of previously assembled synapses [1–3,5].
