Quality of Life after DBS in PD

Recent comparative studies between PD patients who underwent either GPi or STN DBS report a small but significant difference in motor outcomes (10--15%) STN>GPi. However, the same set of patients report similar improvement in Quality of life. Various hypotheses may be raised, such as:

A. Difference exists but QoL measures are not accurate enough to show it.

B. Difference exists but patients' reports are biases by other factors (e.g. dopaminergic medication doses).

C. Different really does not exist because QoL is somehow a measure of happiness, so patients are equally happy with DBS despite the target.

D. QoL scales/questionnaire are not adequate for the particular population because they are based in questions not related to motor performance (somewhat resembles A.).

There are many specialists in the field who participate in this network. I would like to hear your thoughts about this matter, including different hypothesis if you agree/disagree with the primary assumption on different motor outcome after GPi/STN DBS.