setInterval: Interval only occurs once

My script currently generates a floored random number between 0-4 on mousemove, which is used in turn to select one of four images to display.

At the moment the images are changing too quickly, but I can't work out how to slow down the selection.

I tried to set an interval between selections in the following way, but it only introduced an initial delay after which the function behaved as normal:

var test = setInterval(Test, 250);

function Test() {
    var $imgs = $('#imgs'),
    $img = $('img', $imgs);

$(document).on("mousemove", function(e) {
    e.preventDefault();
    $img.hide().eq((Math.random()*4)|0).show();
});
};

Previously I was using another method of selecting images that I found online here, which worked very well, albeit not in a random fashion as I'd like.

$function() {
    var $imgs = $('#imgs'),
    $img = $('img', $imgs),
    n = $img.length;

 $(document).on("mousemove", function(e) {
      e.preventDefault();
      $img.hide().eq(((e.pageX*0.025)|0)%n).show();

I would greatly appreciate some help!