Observe_field on hidden field

I have an observe_form running well …

<% form_tag … %>

<%= hidden_field_tag :contact -%>
<% end -%>

<%= observe_form(‘display_networkForm’, :url => {:action =>
‘display_rnetwork’}, :submit => ‘display_networkForm’ ) -%>

Inside this form, I need to have an Ajax request (link_to_remote,…) to
update the hidden field
but when the hidden_field is changed this doesn’t fire up the observer
to submit the form

any trick ?

thanks

kad