PHP Ajax发送数组

I'm sending an array to a PHP file named update.php and in all previous versions of this script it's worked, but for some bizarre reason it isn't working in this instance.

Here are the files:

var phpLocation = '/admin/_backend' + $( this ).attr("action"),
    usergroup = $('input:hidden[name="usergroup"]').val(),
    serializedData = $('input:checkbox[name="permission"]:checked').map(function () {
                         return this.value;
                      }).get();

var ajaxData = {'usergroup_permissions': serializedData, 'usergroup' : usergroup};

$.ajax({
    type: "POST",
    url: phpLocation,
    data: ajaxData,
    success: function(data){
        $("#post_reply").html(data);

        console.log( ajaxData );
    }
});

When attempting to grab and turn usergroup_permissions from an array into a list separated by commas, it doesn't seem to work.

Even when using $_POST['usergroup_permissons'] it doesn't seem to work..

$permission_list = implode(",", $_POST['usergroup_permissions']);

Edit
This is the console.log output

print_r( $_POST ); returns:

I worked out that somewhere else in the script I am calling a serialize of the whole form and so the permission in the second image is coming from that (because it's the name of the checkbox).

I've now rectified this and it is working fine.

Thank you very much for all of your help.