Traits presumed to reflect dopaminergic reward and prefrontal executive systems functioning were assessed in 100 clients undergoing residential treatment for substance dependence and a community sample of 107 social drinkers. All participants completed self-report measures of impulsivity, alexithymia, frontal systems dysfunction, sensitivity to rewards and punishments, dispositional mindfulness, alcohol use, illicit drug use, mood and demographic characteristics. The percentage of in-patients meeting the criterion for alexithymia was more than twice as high as in the community sample (p < .0001). Multivariate analysis of covariance controlling for age, education, head injury and gender revealed significant differences (p < .0001) between clinical and community samples such that clients scored higher on negative moods, frontal systems dysfunction, reward sensitivity, punishment sensitivity and impulsivity, and lower on dispositional mindfulness. Time in treatment was correlated only with negative mood, supporting the stability of the trait measures; controlling for negative mood eliminated group differences on punishment sensitivity and mindfulness only. Results are consistent with the notion that addiction is linked to reward sensitivity and frontal lobe deficits, with associated implications.

Neural noise is an inherent property of all nervous systems. However, the mechanisms by which such random and fluctuating neural activity influences perception are still unclear. To elucidate the relationship between neural noise and perceptual performance we require techniques that can safely manipulate neural noise in humans. Transcranial random noise stimulation (tRNS), a form of electrical brain stimulation, has been proposed to increase neural noise according to principles of stochastic resonance (SR); where small to moderate intensities of tRNS improve performance, while higher intensities are detrimental. To date, high intensity (i.e., >2mA) tRNS effects on neural noise levels have not been directly quantified, nor have the detrimental effects proposed by SR been demonstrated in early visual processing. For this purpose, we applied a maximum current intensity of 3mA high-frequency tRNS to the visual cortex (V1) during an orientation discrimination task across increasing external visual noise levels, and fit the perceptual template model to contrast thresholds to quantify intrinsic mechanisms related to noise underlying changes in perceptual performance. We found that tRNS generally worsened perceptual performance by increasing observers internal noise and reducing the ability to filter external noise compared to sham. While most observers experienced detrimental effects, others demonstrated improved perceptual performance (i.e., reduced internal noise and better noise filtering). Preliminary evidence suggests that individual baseline internal noise levels may drive the observed beneficial or detrimental observer responses to tRNS. These findings have important implications for the application of tRNS to investigate the impact of internal noise and noise filtering processes on perception.