OADES, R.D., BUNK, D., & EGGERS, C. (1992). Paranoid schizophrenics may not use irrelevant signals: The use of measures of blocking and of urinary dopamine. Acta Paedopsychiatrica, 55, 183-184.(pdf file)

Conditioned blocking (CB) tests whether in associative learning, superfluous / irrelevant information is suppressed - if one learns about the additional stimulus then normal blocking is impaired.

Methods: CB was examined in an associative learning test in a visuospatial, mouse-in-house, computer task (find a safe corner associated with a coloured cue). Further 24 urine collections were taken between the 2 parts of the CB task in 15 patients with schizophrenia (8 paranoid, 7 nonparanoid type), 7 with obsessive compuslive disorder (OCD) and 15 healthy contrtols, to monitor general levels of monoamine activity.

Results: CB was attenuated only in the nonparanoid patiuents with schizophrenia (not in those with OCD or paranoid diagnoses).
Normal CB was associated with lower and attenuated blocking with higher dopamine activity (i.e. HVA/DA).[An associationwith IQ was not observed.]

Conclusions: Higher levels of dopamine activity are not necessarily associated with psychosis.
Less selective attention-related processing of external events is associated with non-paranoid schizophrenia, and this is related to an increase of DA activity which, consistent with the present result, has been argued to facilitate switching between channels of information.