Problem isolating assets
As far as I know you can't restrict users to certain folders in the Assets section of Craft, you can only set their permissions for the whole Asset Source.

The only workaround I can think of for you would be to actually remove Authors permissions on the Assets section of Craft completely.

That way the only way they can make Asset updates is when they are editing an entry, and with the permissions structure you have they can only edit entries they have made themselves anyway.

As they don't have access to other people's entries, they can't edit the assets on them.

It may not be the most elegant approach but it would give you the functionality you are looking for. I'm sure others might have a better way of handling it though. Hope that helps a little.