ActionScript 3.0 :: Adding A MouseEvent Listener To A Loader?

I'm loading .JPGs into loaders, setting the alpha of the loaders to .5 and would like to add a ROLL_OVER event to set the alpha to 1 then back to .5 on ROLL_OUT.Here's what I've got so far and it works well.
 
var ldr1:Loader=new Loader();var url1:URLRequest=new URLRequest("thumbs/1t.jpg");ldr1.contentLoaderInfo.addEventListener(Event.COMPLETE, done1);ldr1.load(url1);
function done1(event:Event):void{    event.target.content.alpha=.5;    event.target.content.x=currentX;   //currentX & Y are already set    event.target.content.y=currentY;    addChild(event.target.content);    currentX+=event.target.content.width;}
 
NOW, I'm trying to add the MouseEvent and can't figure out what to add the listener to - the loader or its content?
 
ldr1.addEventListener(MouseEvent.ROLL_OVER, rollOn); <<<<--- this doesn't work.
ldr1.contentLoaderInfo.addEventListener(MouseEvent.ROLL_OVER, rollOn); <<<<---this doesn't work either.