Visual effects with observe_field

<%= observe_field 'search_form',
        :frequency => 0.5,
        :update => 'results',
        :url => { :controller => 'users', :action => 'get_results' },
        :loading => "Element.show('indicator_gif_id')",
        :complete => "Element.hide('indicator_gif_id')",
        :with => "'search_text=' + value" %>

How do I add effects in this?]

Thanks