| Summary: | Inhibition deficits have been the subject of intensive investigation in the context of ‘impulsivity’, both in relation to normal variation and clinical disorder. Until now the focus has been on inhibition of stimulus-response (S-R) associations, using tasks such as the Go/NoGo. However, a chain of antecedent stimuli may be important in the triggering of any particular response, which can thus depend on other aspects of the context beyond the immediately preceding stimulus. The present thesis therefore examines the inhibition of stimulus-stimulus (S-S) associations in both normal participants and participants with Attention Deficit Hyperactivity Disorder (ADHD) or Tourette syndrome (TS). Two novel conditioned inhibition procedures with engaging storylines (a serially presented ‘Mission to Mars’ version and a simultaneously presented comic book based Wolverine/Weapon X task) were developed to be age appropriate in the level of understanding, and ease of use for younger participants. The tasks were first validated in a series of experiments conducted with undergraduate populations, assessing variation in performance in relation to individual differences and comparing performance with established tests of response inhibition, measured using Go/NoGo, colour-word Stroop and Simon tasks. The studies with undergraduates used the Craver and White’s (1994) BIS/BAS (behavioural inhibition system/behavioural activation system) questionnaire, as well as questionnaire measures of ADHD and TS. The results showed that in general, TS and ADHD-like individual difference behaviours had no effect on either S-S, or S-R inhibition in the normal population. There were similarly no differences in inhibition in relation to the BIS/BAS scores. However, inhibition deficits were expected to be more extreme in cases of clinically diagnosed ADHD and TS. These disorders have been attributed to inhibitory deficits resulting from dysfunction of the dopamine system and the basal ganglia. At face value, ‘prediction error’ models of learning generate the same prediction of impaired conditioned inhibition in cases of dopaminergic disorder (Schultz, Dayan, & Montague, 1997; Schultz & Dickinson, 2000). Counter to prediction, both the TS and ADHD groups showed overall normal conditioned inhibition compared to matched controls. However, unplanned comparisons revealed significant effects of medication in both ADHD and TS groups. In both tasks, there was a reduction in the expression of conditioned inhibition in TS participants medicated with clonidine, with no effect of medication on excitatory learning. In the ADHD clinical group, there was an overall improved performance of the Weapon X test discrimination by the participants on the higher dosage (or longer treatment time) of methylphenidate. Thus, it appears that the medications used for TS and ADHD had distinguishable effects on the excitatory (methylphenidate) versus inhibitory (clonidine) learning demonstrated in the conditioned inhibition tasks. The results of the current investigation demonstrate that the inhibition of S-S associations is unaffected by TS and ADHD. This provides a possible avenue for behavioural modification treatments of ADHD and TS, which modify S-S associations that are believed to bring about the unwanted behaviours observed in the clinical populations. The present results also suggest that the effectiveness of such behavioural treatments will be influenced by medication for these disorders.
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