RewriteCond for password protected folder

Hi, i see from the docs that for the Password Protected Folders feature you use the Authtype of "basic".   I did some reading on that via the apache docs also but i did not find my answer.

I want to do a custom 401 page for the password protected folder but am not having any luck. I have custom pages for 403, 404 but the custom page for 401 is not working.  

In my htaccess file i have the following: 

AddType    text/html     shtml

#custom 401 error document
ErrorDocument 401  /401.shtml

That should be enough for it to work.   So what tried next was to use mod rewrite on it like i have done with a custom 404 page before, here is an example of that 404 code

# Set 404 missing file Custom Error File
# using Current Folder
RewriteEngine On
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d  
RewriteRule .* 404.shtml [L]

However, in order to do this for the 401 page i need to know the proper RewriteCond for the password protection.   So what i am asking is for this:

RewriteCond %{WHAT GOES HERE} !-f
RewriteCond %{WHAT GOES HERE} !-d 

I have not been able to find an example of what goes inside the brackets for this particular need.

Thanks :)