Trigger event won't imitate mouse action

Hello,

I got a problem regarding firing a trigger event. First, look at the following code:

<span class="test_span" onmouseover="Test()">
	test
</span>

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You can see that the <span> element has a mouseover event which will call the Test() function.
Within my Javascript file I have the following code:

$( document ).ready(function() {	
	$(".test_span").trigger('mouseover');
})

function Test() {			
	var target = window.event.target || window.event.srcElement;	
	console.log(target);
}

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The problem that I encounter is that when the page is loaded and the trigger has been fired the value for my target variable is "document":

//trigger
var target = window.event.target || window.event.srcElement;	// will return as object"document"

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But If I use my mouse to hover over the element I get the object that I want, namely:

//mouse hover over object
var target = window.event.target || window.event.srcElement;	 // will return as object: " <span>...</span>"

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Can anyone tell me what the difference is between trigger() and the mouse event action? So my question is, is there a way to make sure that using the trigger will result in the same action as you do with the mouse?

Due to implementation limitations is it for me not allowed to use any kind of implementations like: "onmouseover="Test(event)"

Perhaps another solution might help me? Or is the only way to achieve this by passing the "event" parameter?