add_filter comment_author not working

Hey,

Im trying to make a simple plugin that changes someones name when they post a message, from: Joe Bloggs to Joe B. I want todo this so peoples comments are not found with a google search for there name.

This is my Test Code:

function my_function($text){
  return 'test';
}

add_filter('comment_author','my_function');

It doesnt do anything! If I change the hook to comment_text that works.

Can someone please tell me what im doing wrong? This is my first plugin incase you cant tell!

Many Thanks,
David