Onmouseover & z-index of -1

I have an onmouseover function and the z-index of the div the image is wrapped in is -1, here is the code ...
Having the div with z-index of -1, the onmouseover no longer works.  Is there a fix to this issue ?
 
<asp:Timer ID="Timer1" Interval="5000" OnTick="Timer_Tick" runat="server"></asp:Timer> <asp:UpdatePanel ID="UpdatePanel1" runat="server"> <ContentTemplate> <table width="100%"> <tr> <td style="width:100%; text-align:center;"> <div id="divSlideshow" style="position:relative; z-index:-1" onmouseover="javascript:ShowButtons();" onmouseout="javascript:HideButtons();"> <asp:Image ID="iSlide" runat="server" ImageUrl="~/Images/SlideShow/slide-0.jpg ...