Afficher nouveau champs si "autre"

<select name="option" id="option" onChange="if(this.value=='Autre'){document.option.autre.style.display=''}">
                  <option selected="selected"></option>
                  <?php
include("../connexion.php");
$sql = "SELECT designation FROM  specialite order by designation asc" ;
$res = mysql_query($sql) or die('Erreur SQL !<br />'.$sql.'<br />'.mysql_error());
while ($data=mysql_fetch_array($res))
{
  $design = $data['designation'];
	
   if($_POST["designation"]=="$design")
   echo "SELECTED";
    echo "<option value=\"$design\">$design</option>\n" ;
	}
	
?>
  </select>
  <input type="text" name="autre" style="display:none" value="voir" />