Divide and Conquer Binary Tree

Consider an n-node complete binary tree T, where n=2^d - 1 for some d. Each node v of T is labeled with a real number x_v. You may assume that the real numbers labeling the nodes are all distinct. A node v of T is a local minimum if the label x_v is less than the label x_w for all nodes w that are joined to v by an edge.

You are given such a complete binary tree T, but the labeling is only specified in the following implicit way: for each node v, you can determine the value x_v by probing the node v. Show how to find a local minimum of T using only O(log n) probes to the nodes of T.

(This is a problem from the book Algorithm Design by Kleinberg and Tardos, page 248, problem 6)