Richfaces combobox on selection changed event

Question

I have this code:

<richfaces:comboBox
  value="#{ChoixContratBean.selectedFormule"}
  suggestionValues="#{ChoixContratBean.formules}">

  <a4j:support event="onmouseover"
    action="#{ChoixContratBean.getDescriptionFormule}"
    reRender="reponse" />
</richfaces:comboBox>

I would like that when the user hovers with the mouse the different values in the combo, a new description of the product appears in :

<jsf:outputText
   id="reponse"
   value="#{ChoixContratBean.descriptionFormuleSelected}" />

No event is triggered when the mouse hovers the richfaces combo values; instead, the event that is triggered is the simple mouseover on the combo itself.

What must I to do have the values receive the event trigger?