Multi-select does not retain value after post in codeigniter
(I'm assuming $pre_selected is an array of values?)
You can reset selected values after failed form submit using the $_POST array.
Since you're already using $pre_selected, you should be able to use the following:
return form_multiselect('authors[response][]', $faculty->get_all_for_multiselect(), array_unique(array_merge($pre_selected, $_POST['response'])), $additional_attributes);