Impulsivity and Rapid Decision-Making for Reward

Impulsivity is a feature of many brain disorders. Although often defined as the predisposition to act with an inadequate degree of deliberation, forethought, or control, it has proven difficult to measure. This may in part be due to the fact that it is a multifaceted construct, with impulsive decisions potentially arising as a result of a number of underlying mechanisms. Indeed, a “functional” degree of impulsivity may even promote effective behavior in healthy participants in a way that can be advantageous under certain circumstances. Although many tasks have been developed to study impulsivity, few examine decisions made rapidly, for time-sensitive rewards. In the current study we examine behavior in 59 adults on a manual “Traffic Light” task which requires participants to take risks under time pressure, if they are to maximize reward. We show that behavioral variables that index rapid anticipatory responding in this paradigm are correlated with one, specific self-report measure of impulsivity: “lack of premeditation” on the UPPS Impulsive Behavior Scale. Participants who scored more highly on this subscale performed better on the task. Moreover, anticipatory behavior reduced significantly with age (18–79 years), an effect that continued to be upheld after correction for potential age differences in the ability to judge the timing of responses. Based on these findings, we argue that the Traffic Light task provides a parametric method to study one aspect of impulsivity in health and disease: namely, rapid decision-making in pursuit of risky, time-sensitive rewards

Endurance- and Resistance-Trained Men Exhibit Lower Cardiovascular Responses to Psychosocial Stress Than Untrained Men

Evidence shows that regular physical exercise reduces physiological reactivity to psychosocial stress. However, previous research mainly focused on the effect of endurance exercise, with only a few studies looking at the effect of resistance exercise. The current study tested whether individuals who regularly participate in either endurance or resistance training differ from untrained individuals in adrenal and cardiovascular reactivity to psychosocial stress. Twelve endurance-trained men, 10 resistance-trained men, and 12 healthy but untrained men were exposed to a standardized psychosocial stressor, the Trier Social Stress Test. Measurements of heart rate, free salivary cortisol levels, and mood were obtained throughout the test and compared among the three groups. Overall, both endurance- and resistance-trained men had lower heart rate levels than untrained men, indicating higher cardiac performance of the trained groups. Trained men also exhibited lower heart rate responses to psychosocial stress compared with untrained men. There were no significant group differences in either cortisol responses or mood responses to the stressor. The heart rate results are consistent with previous studies indicating reduced cardiovascular reactivity to psychosocial stress in trained individuals. These findings suggest that long-term endurance and resistance trainings may be related to the same cardiovascular benefits, without exhibiting strong effects on the cortisol reactivity to stress

Therapeutic subthalamic nucleus deep brain stimulation reverses cortico-thalamic coupling during voluntary movements in Parkinson's disease.

Deep brain stimulation of the subthalamic nucleus (STN DBS) has become an accepted treatment for patients experiencing the motor complications of Parkinson's disease (PD). While its successes are becoming increasingly apparent, the mechanisms underlying its action remain unclear. Multiple studies using radiotracer-based imaging have investigated DBS-induced regional changes in neural activity. However, little is known about the effect of DBS on connectivity within neural networks; in other words, whether DBS impacts upon functional integration of specialized regions of cortex. In this work, we report the first findings of fMRI in 10 subjects with PD and fully implanted DBS hardware receiving efficacious stimulation. Despite the technical demands associated with the safe acquisition of fMRI data from patients with implanted hardware, robust activation changes were identified in the insula cortex and thalamus in response to therapeutic STN DBS. We then quantified the neuromodulatory effects of DBS and compared sixteen dynamic causal models of effective connectivity between the two identified nodes. Using Bayesian model comparison, we found unequivocal evidence for the modulation of extrinsic (between region), i.e. cortico-thalamic and thalamo-cortical connections. Using Bayesian model parameter averaging we found that during voluntary movements, DBS reversed the effective connectivity between regions of the cortex and thalamus. This casts the therapeutic effects of DBS in a fundamentally new light, emphasising a role in changing distributed cortico-subcortical interactions. We conclude that STN DBS does impact upon the effective connectivity between the cortex and thalamus by changing their sensitivities to extrinsic afferents. Furthermore, we confirm that fMRI is both feasible and is tolerated well by these patients provided strict safety measures are adhered to

Imaging results.

<p>A typical drop-out artefact in a single subject's GE-EPI acquisition viewed from (a) axial, (b) coronal, and (c) sagittal sections; cross-hair position = −34.8, −21.5, 53.3 mm (MNI coordinates). SPMs in (d) summarize the movement network on a rendered MNI brain (p<0.001 uncorrected). Clusters representing BOLD signal increases in the insula cortex (e, <i>green arrow</i>), and thalamus (f, <i>green arrow</i>).</p

Clinical Response to STN DBS: Details of 10 PD participants and their motor response to STN DBS, assessed immediately before the fMRI scan while off medication.

<p>‘Off/OFF’ = Off medication, OFF stimulation’; ‘Off/ON’ = Off medication, ON stimulation.</p

The interaction between movement and stimulation identified using a whole-brain analysis.

<p>BOLD activity increases comparing STN DBS ON v OFF during left hand movements (p<0.0005, uncorrected, Cluster size>5 voxels). Regions in <b>bold</b> were identified in the restricted volume analysis to the motor network described in the text (p<0.05, FWE corrected). The remaining regions are reported in view of strong support of their involvement from previous publications.</p

The dynamic causal models compared using Bayesian model selection (model space).

<p>Model 15 – the winning model – is shown enlarged. The blue node represents the right insula cortex, and red node, the right thalamus. Green arrows indicate the connection/s that DBS modulates. The ‘movement input’ is likely made up of both motor inputs arriving from M1, PM and SMA, as well as sensory inputs elicited by on-going movements. Thalamic ‘intrinsic subcortical projections’ refer to thalamic afferents from BG nuclei. Cortical ‘intrinsic insula cortical projections’ refer to cortical afferents from within the cortex. Average DCM parameters are included on the enlarged model 15, units are in Hertz (Hz). Positive A-matrix parameters represent an excitatory effect on the target, whereas negative values indicate an inhibition of the target area. Positive B-matrix parameters (value in green) represent an increased target response to input (i.e. an increased gain), whereas negative values indicate a decreased target response to input (i.e. a reduced gain). The coupling during movements with DBS is equal the sum of the A and B value on that connection, e.g. during movements with DBS, the cortico-thalamic drive switches from −0.21 Hz to (−0.21+0.30) = 0.09 Hz, i.e. it switches from an inhibitory to an excitatory drive.</p

Simplified diagram of current understanding of cortico-striato-pallido-thalamo-cortical circuitry.

<p>Green arrows represent the regions in which there were BOLD response increases. The 3 input pathways are shown; the direct (1), indirect (2), and hyperdirect (3) pathways. Thalamic ‘cortical afferents’ are likely to arrive via one of these pathways – passing through BG nuclei. The thalamic ‘BG afferents’ – discussed in the main text – arrive from other BG nuclei, independent of cortical activity. Red arrows indicate glutamatergic (excitatory) projections; blue arrows indicate GABAergic (inhibitory) projections. The grey line represents the DBS electrode. GPe/GPi = Globus Pallidus pars externa/interna. STN = Subthalamic Nucleus.</p

BMS results.

<p>FFX = Fixed Effects Assumptions, RFX = Random Effects Assumptions. (a) The relative log-evidences across all 16 specified models with model 3 showing the highest log-evidence. (b) Given the observed data and the models specified, one can be >95% certain that model 15 is the data generator. (c, d) The relative log-evidences between the 4 most likely models – again highlighting model 15 as the most likely model. This is repeated using RFX BMS, confirming the FFX findings (e, f).</p