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Signal detection theory is appropriate for analyzing the problem of whether auditory hallucination arises from a sensory or judgmental disorder. In the present study the sensory discrimination ability and decision bias of psychiatric patients with auditory hallucination was investigated by using the visual continuous performance test. Based on signal detection theory the dЈ (d prime) and the lnb value (the natural logarithm of the b value) were calculated. The dЈ is indicator of discriminating ability, whereas the lnb value is an indicator of decision bias. The psychiatric inpatients with the diagnosis of schizophrenia mainly were divided into 11 patients with auditory hallucinations (AH(+)) and 11 patients without hallucinations (AH(-)). As compared with the AH(-) group, the AH(+) group showed the same dЈ value and a significantly lower ln b value. As compared with the normal control group, the AH(+) group showed a significantly lower dЈ value and a nearly normal lnb value. These results suggest that auditory hallucination is characterized by a cognitive deficit of unreasonably maintaining a relatively risktaking decision bias in spite of decreased discriminating ability, which opposes the interpretation of Bentall and Slade (1985). The AH(+) group was divided into subgroups with bizarre delusions (BD(+)) and without bizarre delusions (BD(-)). The BD(+) group showed a stronger tendency toward a lower dЈ value and normal ln b value, which the AH(+) group had.

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