I administer NP batteries to HIV study subjects. For those who score in the impaired categories, it can be difficult to determine whether the deficit is due to HIV, or to some confound. I've noticed that some impaired Ss, that have a history of head injury, display perseveration on the WAIS-III Letter-Number-Sequencing task, and others. Such perseveration doesn't seem to appear in other (e.g. "just HIV") cases. Can anyone relate to this; perhaps suggest some publications that address it?