Introduction: Conditioned blocking (CB) is the undermining of conditioning to a stimulus by conjoint exposure to a stimulus already associated with the unconditioned stimulus. CB is one of several tests of "learned inattention" in which performance has been found to depend on certain features of personality and on monoamine activity recorded in animals performing CB. Methods: In part 1, the performance of 25 healthy young adults (mean age 21.6 years) on a new test form for demonstrating CB is described ("mouse-in-house"). The personality inventories used included the Hamburger neuroticism/extroversion scale (HANES) and the short version of the Minnesota multiphasic personality inventory (MMPI). From 24h urine samples collected at the time of testing the levels of four monoamine and three of their metabolites were measured as an indicator of the overall level of neurotransmitter activity. In part 2, the development of CB was studied across children and adolescents clustered into 4 age groups (means 10, 14, 17 and 22 years of age). Results: Part 1 demonstrated CB in normal healthy young adults and the expression of CB correlated with extroversion and increased catecholamine utilisation (turnover). Part 2 showed CB was least marked prepubertally, but developed across adolescence independent of IQ. Although performance did not correlate with developing personality features, like the adults, CB was positively correlated with dopamine activity. Unlike adults CB was inversely correlated with noradrenaline activity. We show the development of increasing CB across the 4 age-groups and correlations with monoamine activity: also we illustrate how the measures of the 3 monoamines, their metabolites and their activity (turnover) develops across age-groups, with a separate pattern for each transmitter. Conclusions: The maturation of attention related information processing was discussed in terms of the development and maturation of structures in the limbic part of the central nervous system and, in particular, the separate function of dopamine in switching between and noradrenaline in tuning in/out stimulus features during selection processes
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