EWS 2.0: get User from OnNotificationEvent

Hi,

i develop with the Exchange Web Services Managed API 2.0. 
Now i try to handle the StreamingSubscriptionConnection.OnNotificationEvent.

Is there a way to find out from which user this event is raised?

Because i am working with impersonation like the link below and i want to find out from which user this event is raised.

http://msdn.microsoft.com/en-us/library/office/dd633680(v=exchg.80).aspx

Thanks :)