In the Backster framework, it is a 20- to 35- second block of polygraph chart time initiated by a symptomatic question having a unique psychological focusing appeal to the examinee who is fearful that the examiner may ask an unreviewed question dealing with an outside issue. Because subsequent research has not been supportive of the symptomatic question, the value of the black zone has been called into question. See: Backster (2001); Krapohl & Ryan (2001).