The Orthodox have a more synergistic doctrine that has the process of purification continuing throughout the mystical ascent to God. Louth cites Mme Lot-Borodine, who says that this difference is well illustrated in the doctrine of the Immaculate Conception, a Catholic doctrine that states that Mary was preserved from the corruption of original sin (that she was "immaculately conceived") and thus enabled to give her assent to the Incarnation. The Orthodox reject this doctrine, in part because it is itself a rejection of the sort of synergism that is essential to Orthodoxy.
But Louth questions whether there really is a fundamental difference between Western and Eastern Christianity here. It's possible that the Eastern coolness to the doctrine of the Dark Night is a result of the monergism versus synergism debate - but perhaps the difference is a matter of emphasis:
For there is no fundamental contrast between the idea of our responding to God and the idea of our working with God. There would indeed be such a contrast if God were external to me, if God were not the One who has created me and holds me in being, if God were not interior intimo meo. But, in responding to God, "in whose service is perfect freedom", I find true freedom and so become a fellow-worker (synergos) with God. It is a paradox that St Paul lays hold of when he says, "Work out your own salvation with fear and trembling. For it is God which worketh in you both to will and to do of his good pleasure": here the ideas of your own effort, God's grace, and the fact that the fruits of our efforts in obedience are the work of God, both at the level of deed and at the deeper level of the inspiring will, are united. Here is true synergism that cannot be opposed to the idea of response.I find this convincing, but then I've always had a strong syncretic impulse. To me it seems to be a matter of missing the forest for the trees. All movement toward God is the result of God's prior action. Augustine's famous line, "Thou has made us for Thyself, and our hearts are restless till they rest in Thee", locates the source of the desire for God in God's creation of us. In the course of our lives ("on the ground") our movement toward God seems to be an action of our own will alone. From another perspective, however, we can see that our movement toward God is in fact a response to the prior action of God - in Creation, in the Incarnation, in the institution of the Church and its sacramental ministry, etc. Maybe someone could show me where I am wrong, but Louth's assessment, that it is more a difference in style than fundamental disagreement, seems right to me.
There may, however, be a difference of style according to whether one is influenced by teaching on synergism or response as keys to interpret mystical experience, and these different styles draw out different areas of mystical experience. If East and West display different styles in the way they explain the same experience of the souls' engagement with God, this is but evidence of a tension within a deeper unity, and suggests that East and West have much to learn from one another here.