AJAX :: AjaxToolkit Accordion Clicked Postback And No Slide Effect

When I clicked Header area, Content opened and have "Slide effect". but when I clicked Header's "ImageButton", it do something at codebehidn, Content opened but no "Slide effect". I want slide effect, how to do it,

[aspx]
<asp:UpdatePanel ID="UpdatePanel1" runat="server">
<ContentTemplate>
<AjaxToolkit:Accordion ID="AccordionMenu" runat="server"
SelectedIndex="0" HeaderCssClass="accordionHeader"
ContentCssClass="accordionContent" AutoSize="None"
FadeTransitions="true" Width="200px" SuppressHeaderPostbacks="false">
<Panes>
<AjaxToolkit:AccordionPane ID="AccordionPane1" runat="server">
<Header>Pane 1
<asp:ImageButton ID="ImageButton1" runat="server" ImageUrl="~/images/expand.jpg" />
</Header>
<Content>
The Details of Item1.
</Content>
</AjaxToolkit:AccordionPane>
<AjaxToolkit:AccordionPane ID="AccordionPane2" runat="server">
<Header>Pane 2
<asp:ImageButton ID="ImageButton2" runat="server" ImageUrl="~/images/expand.jpg" />
</Header>
<Content>
The Details of Item2.
</Content>
</AjaxToolkit:AccordionPane>
</Panes>
</AjaxToolkit:Accordion>
<p></p>
<asp:Label ID="Label1" runat="server" />
</ContentTemplate>
</asp:UpdatePanel>
[.vb]
Protected Sub ImageButton1_Click(ByVal sender As Object, ByVal e As System.Web.UI.ImageClickEventArgs) Handles ImageButton1.Click
Me.Label1.Text = "[Call Header Function 1]"
End Sub
Protected Sub ImageButton2_Click(ByVal sender As Object, ByVal e As System.Web.UI.ImageClickEventArgs) Handles ImageButton2.Click
Me.Label1.Text = "[Call Header Function 2]"
End Sub