Systematic research on delusional disorder (DD) is limited. The goal of this study was to assess DD patients in the following areas: sensory capacities, decision-making style, and complex reasoning. Ten DD patients and 10 matched normal controls completed the following (1) smell, taste, and vision testing; (2) a probabilistic inference test in which subjects made probability decisions; and (3) a gambling task assessing complex reasoning. No significant difference was found between DD subjects and normals for taste acuity, olfactory acuity, or olfactory discrimination. No difference in visual acuity was noted, but sample size was limited. In addition, DD subjects required significantly less data to make probability decisions than normal controls. Despite using less data, DD subjects were as certain as controls regarding the accuracy of their decisions. As for complex reasoning, DD subjects performed as well as normal controls, but tended to surmise the purpose of the task sooner than normals, a difference that approached significance. In conclusion, these results suggest no differences between DD and normal subjects regarding olfaction, taste, and vision. The reasoning studies suggest that DD subjects may have a "cognitive set" that predisposes them to make conclusions with significantly less data than normals. Further, the study suggests that this reasoning difference generalizes to events outside the DD subjects' delusional realm and can be evoked in an experimental environment.
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