despite experimental findings that suggest a role for dendritic spine dynamics in mediating learning and memory, the degree of structural plasticity's importance remains debatable. for instance, studies estimate that only a small portion of spines formed during training actually contribute to lifelong learning. in addition, the formation of new spines may not significantly contribute to the connectivity of the brain, and spine formation may not bear as much of an influence on memory retention as other properties of structural plasticity, such as the increase in size of spine heads.