todo eso se simplifica asi:
Código HTML:
<HTML>
<HEAD>
<script>
function OcultarCapa (Capa) {
document.getElementById(Capa).style.visibility = "hidden"
}
function OcultarListadoCapas(v) {
var tmp = v.split(",");
for (i=0;i<tmp.length;i++)
OcultarCapa(tmp[i]);
}
</script>
</HEAD>
<BODY>
<form method="get" name="formulario">
<select onChange="OcultarListadoCapas(this.value)" name="categorias">
<option value=capaA1,capaA3,capaA5>Opcion 1</option>
<option value=capaA2,capaA4>Opcion 2</option>
</select>
</form>
<div id="capaA1" Style="visibility:visible; background:red;">mi capa 1</div>
<div id="capaA2" Style="visibility:visible; background:blue;">mi capa 2</div>
<div id="capaA3" Style="visibility:visible; background:green;">mi capa 3</div>
<div id="capaA4" Style="visibility:visible; background:yellow;">mi capa 4</div>
<div id="capaA5" Style="visibility:visible; background:orange;">mi capa 5</div>
</BODY>
</HTML>