The "first order" rate distortion function

Question

Suppoer we have a random source $(X_n; n \geq 1)$ taking values in some source alphabet $A$ to be compressed int another alphabet $\hat{A}$, with respect to a distortion function $\rho: A \times \hat{A} \rightarrow [0, \infty)$. Suppose the source is stationary and $P$ be its marginal distrubtion on $A$. Then the "first order" rate-dstortion function $R_1(P, D)$ is said to be defined as, $$R_1(P,D)=\inf_{(U,V) \sim W \in W(P,D)}I(U;V)$$ where the infimum is over all $A\times\hat{A}$-valued random variables $(U,V)$ with joint distribution $W$ belonging to the set $$W(P,D) = \{W:E_W[\rho(U,V)] \leq D \}$$ Now the question: is this different from Shannon's original rate distortion function? and how?