Processes underlying formal thought disorder in psychiatric inpatients.

Three theories about the cognitive processes underlying symptoms of formal thought disorder in psychiatric inpatients were tested. Chapman and Chapman's "excessive yielding to normal bias" theory and a response competition theory were tested by using two ambiguity tasks. Chapman and Chapman's bias theory predicts a smaller ambiguity effect for thought-disordered patients; a response competition hypothesis predicts a larger ambiguity effect. Results showed no difference between thought-disordered and non-thought-disordered patients. To test a distractibility theory of thought disorder, subjects performed a Stroop-type task in which they counted the number of digits (e.g., 3333) or symbols (####) in a set. Thought-disordered patients did show an increased effect of the presence of the digits, and this finding was replicated in a second inpatient sample. Thus, results were consistent with the distractibility theory and with a reformulation of the bias theory, in which the bias shown by thought-disordered patients is not a tendency toward a particular response, but rather toward a particular rule, or set, for responding.