The theory that talks about "the medial temporal lobe structures are needed initially for encoding and retrieval, but their role diminishes over time", is Standard Collision theory.
There are three main pillars of the collision theory They are; participating molecules, phenomena of collision, states of energy, and proper orientation of time. Hence, the reactants must collide with one another in perfect ambient stages of energy and time to produce desired products. The force on the participating reactants also play a role.
Hence, as per the statements given, the Standard Collision Theory, states, "the medial temporal lobe structures are needed initially for encoding and retrieval, but their role diminishes over time".
To know more about the Standard Collision theory: