There are 3 types of notification queries available from the Activity Stream. These are queries that in some way specifically relate to the querying user as opposed to events relating to people or objects that a user has chosen to follow.
Filtering of a user's stream based on notification events is done with pseudo groups -
Notifications for me: /activitystreams/@me/@notesforme
Notifications from me: /activitystreams/@me/@notesfromme
Responses to my content: /activitystreams/@me/@responses