Dopamine Activation Preserves Visual Motion Perception Despite Noise Interference of Human V5/MT

Cortical processes of multisensory plausibility modulation of vibrotactile perception in virtual environments in middled-aged and older adults

Digital technologies, such as virtual or augmented reality, can potentially support neurocognitive functions of the aging populations worldwide and complement existing intervention methods. However, aging-related declines in the frontal-parietal network and dopaminergic modulation which progress gradually across the later periods of the adult lifespan may affect the processing of multisensory congruence and expectancy based contextual plausibility. We assessed hemodynamic brain responses while middle-aged and old adults experienced car-riding virtual-reality scenarios where the plausibility of vibrotactile stimulations was manipulated by delivering stimulus intensities that were either congruent or incongruent with the digitalized audio-visual contexts of the respective scenarios. Relative to previous findings observed in young adults, although highly plausible vibrotactile stimulations confirming with contextual expectations also elicited higher brain hemodynamic responses in middle-aged and old adults, this effect was limited to virtual scenarios with extreme expectancy violations. Moreover, individual differences in plausibility-related frontal activity did not correlate with plausibility violation costs in the sensorimotor cortex, indicating less systematic frontal context-based sensory filtering in older ages. These findings have practical implications for advancing digital technologies to support aging societies.

Effects of Catecholaminergic and Transcranial Direct Current Stimulation on Response Inhibition

BACKGROUND

The principle of gain control determines the efficiency of neuronal processing and can be enhanced with pharmacological or brain stimulation methods. It is a key factor for cognitive control, but the degree of how much gain control may be enhanced underlies a physical limit.

METHODS

To investigate whether methylphenidate (MPH) and transcranial direct current stimulation (tDCS) share common underlying mechanisms and cognitive effects, we administered MPH and anodal tDCS (atDCS) over the right inferior frontal gyrus both separately and combined, while healthy adult participants (n = 104) performed a response selection and inhibition task. The recorded EEG data were analyzed with a focus on theta band activity, and source estimation analyses were conducted.

RESULTS

The behavioral data show that MPH and atDCS revealed interactive effects on the ability to inhibit responses. Both MPH and atDCS modulated task-related theta oscillations in the supplementary motor area when applied separately, making a common underlying mechanism likely. When both stimulation methods were combined, there was no doubling of effects in the supplementary motor area but a shift to inferior frontal areas in the cortical network responsible for theta-driven processing.

CONCLUSIONS

The results indicate that both MPH and atDCS likely share a common underlying neuronal mechanism, and interestingly, they demonstrate interactive effects when combined, which are most likely due to the physical limitations of gain control increases. The current study provides critical groundwork for future combined applications of MPH and non-invasive brain stimulation.

Optical coherence tomography as a potential surrogate marker of dopaminergic modulation across the life span

The retina has been considered a "window to the brain" and shares similar innervation by the dopaminergic system with the cortex in terms of an unequal distribution of D1 and D2 receptors. Here, we provide a comprehensive overview that Optical Coherence Tomography (OCT), a non-invasive imaging technique, which provides an "in vivo" representation of the retina, shows promise to be used as a surrogate marker of dopaminergic neuromodulation in cognition. Overall, most evidence supports reduced retinal thickness in individuals with dopaminergic dysregulation (e.g., patients with Parkinson's Disease, non-demented older adults) and with poor cognitive functioning. By using the theoretical framework of metacontrol, we derive hypotheses that retinal thinning associated to decreased dopamine (DA) levels affecting D1 families, might lead to a decrease in the signal-to-noise ratio (SNR) affecting cognitive persistence (depending on D1-modulated DA activity) but not cognitive flexibility (depending on D2-modulated DA activity). We argue that the use of OCT parameters might not only be an insightful for cognitive neuroscience research, but also a potentially effective tool for individualized medicine with a focus on cognition. As our society progressively ages in the forthcoming years and decades, the preservation of cognitive abilities and promoting healthy aging will hold of crucial significance. OCT has the potential to function as a swift, non-invasive, and economical method for promptly recognizing individuals with a heightened vulnerability to cognitive deterioration throughout all stages of life.

The Ability to Voluntarily Regulate Theta Band Activity Affects How Pharmacological Manipulation of the Catecholaminergic System Impacts Cognitive Control

BACKGROUND

The catecholaminergic system influences response inhibition, but the magnitude of the impact of catecholaminergic manipulation is heterogeneous. Theoretical considerations suggest that the voluntary modulability of theta band activity can explain this variance. The study aimed to investigate to what extent interindividual differences in catecholaminergic effects on response inhibition depend on voluntary theta band activity modulation.

METHODS

A total of 67 healthy adults were tested in a randomized, double-blind, cross-over study design. At each appointment, they received a single dose of methylphenidate or placebo and performed a Go/Nogo task with stimuli of varying complexity. Before the first appointment, the individual's ability to modulate theta band activity was measured. Recorded EEG data were analyzed using temporal decomposition and multivariate pattern analysis.

RESULTS

Methylphenidate effects and voluntary modulability of theta band activity showed an interactive effect on the false alarm rates of the different Nogo conditions. The multivariate pattern analysis revealed that methylphenidate effects interacted with voluntary modulability of theta band activity at a stimulus processing level, whereas during response selection methylphenidate effects interacted with the complexity of the Nogo condition.

CONCLUSIONS

The findings reveal that the individual's theta band modulability affects the responsiveness of an individual's catecholaminergic system to pharmacological modulation. Thus, the impact of pharmacological manipulation of the catecholaminergic system on cognitive control most likely depends on the existing ability to self-modulate relevant brain oscillatory patterns underlying the cognitive processes being targeted by pharmacological modulations.

Digitally embodied lifespan neurocognitive development and Tactile Internet: Transdisciplinary challenges and opportunities

Mechanisms underlying perceptual processing and inference undergo substantial changes across the lifespan. If utilized properly, technologies could support and buffer the relatively more limited neurocognitive functions in the still developing or aging brains. Over the past decade, a new type of digital communication infrastructure, known as the "Tactile Internet (TI)," is emerging in the fields of telecommunication, sensor and actuator technologies and machine learning. A key aim of the TI is to enable humans to experience and interact with remote and virtual environments through digitalized multimodal sensory signals that also include the haptic (tactile and kinesthetic) sense. Besides their applied focus, such technologies may offer new opportunities for the research tapping into mechanisms of digitally embodied perception and cognition as well as how they may differ across age cohorts. However, there are challenges in translating empirical findings and theories about neurocognitive mechanisms of perception and lifespan development into the day-to-day practices of engineering research and technological development. On the one hand, the capacity and efficiency of digital communication are affected by signal transmission noise according to Shannon's (1949) Information Theory. On the other hand, neurotransmitters, which have been postulated as means that regulate the signal-to-noise ratio of neural information processing (e.g., Servan-Schreiber et al., 1990), decline substantially during aging. Thus, here we highlight neuronal gain control of perceptual processing and perceptual inference to illustrate potential interfaces for developing age-adjusted technologies to enable plausible multisensory digital embodiments for perceptual and cognitive interactions in remote or virtual environments.

Cognitive science theory-driven pharmacology elucidates the neurobiological basis of perception-motor integration

An efficient integration of sensory and motor processes is crucial to goal-directed behavior. Despite this high relevance, and although cognitive theories provide clear conceptual frameworks, the neurobiological basis of these processes remains insufficiently understood. In a double-blind, randomized placebo-controlled pharmacological study, we examine the relevance of catecholamines for perception-motor integration processes. Using EEG data, we perform an in-depth analysis of the underlying neurophysiological mechanisms, focusing on sensorimotor integration processes during response inhibition. We show that the catecholaminergic system affects sensorimotor integration during response inhibition by modulating the stability of the representational content. Importantly, catecholamine levels do not affect the stability of all aspects of information processing during sensorimotor integration, but rather-as suggested by cognitive theory-of specific codes in the neurophysiological signal. Particularly fronto-parietal cortical regions are associated with the identified mechanisms. The study shows how cognitive science theory-driven pharmacology can shed light on the neurobiological basis of perception-motor integration and how catecholamines affect specific information codes relevant to cognitive control.

The metacontrol hypothesis as diagnostic framework of OCD and ADHD: A dimensional approach based on shared neurobiological vulnerability

Obsessive-compulsive disorder (OCD) and attention-deficit hyperactivity disorder (ADHD) are multi-faceted neuropsychiatric conditions that in many aspects appear to be each other's antipodes. We suggest a dimensional approach, according to which these partially opposing disorders fall onto a continuum that reflects variability regarding alterations of cortico-striato-thalamo-cortical (CSTC) circuits and of the processing of neural noise during cognition. By using theoretical accounts of human cognitive metacontrol, we develop a framework according to which OCD can be characterized by a chronic bias towards exaggerated cognitive persistence, equivalent to a high signal-to-noise ratio (SNR)-which facilitates perseverative behaviour but impairs mental flexibility. In contrast, ADHD is characterized by a chronic bias towards inflated cognitive flexibility, equivalent to a low SNR-which increases behavioural variability but impairs the focusing on one goal and on relevant information. We argue that, when pharmacology is not feasible, novel treatments of these disorders may involve methods to manipulate the signal-to-noise ratio via non-invasive brain stimulation techniques, in order to normalize the situational imbalance between cognitive persistence and cognitive flexibility.

Thunderstruck: The ACDC model of flexible sequences and rhythms in recurrent neural circuits

Adaptive sequential behavior is a hallmark of human cognition. In particular, humans can learn to produce precise spatiotemporal sequences given a certain context. For instance, musicians can not only reproduce learned action sequences in a context-dependent manner, they can also quickly and flexibly reapply them in any desired tempo or rhythm without overwriting previous learning. Existing neural network models fail to account for these properties. We argue that this limitation emerges from the fact that sequence information (i.e., the position of the action) and timing (i.e., the moment of response execution) are typically stored in the same neural network weights. Here, we augment a biologically plausible recurrent neural network of cortical dynamics to include a basal ganglia-thalamic module which uses reinforcement learning to dynamically modulate action. This “associative cluster-dependent chain” (ACDC) model modularly stores sequence and timing information in distinct loci of the network. This feature increases computational power and allows ACDC to display a wide range of temporal properties (e.g., multiple sequences, temporal shifting, rescaling, and compositionality), while still accounting for several behavioral and neurophysiological empirical observations. Finally, we apply this ACDC network to show how it can learn the famous “Thunderstruck” song intro and then flexibly play it in a “bossa nova” rhythm without further training.

How do humans flexibly adapt action sequences? For instance, musicians can learn a song and quickly speed up or slow down the tempo, or even play the song following a completely different rhythm (e.g., a rock song using a bossa nova rhythm). In this work, we build a biologically plausible network of cortico-basal ganglia interactions that explains how this temporal flexibility may emerge in the brain. Crucially, our model factorizes sequence order and action timing, respectively represented in cortical and basal ganglia dynamics. This factorization allows full temporal flexibility, i.e. the timing of a learned action sequence can be recomposed without interfering with the order of the sequence. As such, our model is capable of learning asynchronous action sequences, and flexibly shift, rescale, and recompose them, while accounting for biological data.

Neuromodulatory Control of Early Visual Processing in Macaque

Visual processing is dynamically controlled by multiple neuromodulatory molecules that modify the responsiveness of neurons and the strength of the connections between them. In particular, modulatory control of processing in the lateral geniculate nucleus of the thalamus, V1, and V2 will alter the outcome of all subsequent processing of visual information, including the extent to and manner in which individual inputs contribute to perception and decision making and are stored in memory. This review addresses five small-molecule neuromodulators-acetylcholine, dopamine, serotonin, noradrenaline, and histamine-considering the structural basis for their action, and the effects of their release, in the early visual pathway of the macaque monkey. Traditionally, neuromodulators are studied in isolation and in discrete circuits; this review makes a case for considering the joint action of modulatory molecules and differences in modulatory effects across brain areas as a better means of understanding the diverse roles that these molecules serve.

Dopamine Modulation of Motor and Sensory Cortical Plasticity among Vertebrates

Goal-directed learning is a key contributor to evolutionary fitness in animals. The neural mechanisms that mediate learning often involve the neuromodulator dopamine. In higher order cortical regions, most of what is known about dopamine's role is derived from brain regions involved in motivation and decision-making, while significantly less is known about dopamine's potential role in motor and/or sensory brain regions to guide performance. Research on rodents and primates represents over 95% of publications in the field, while little beyond basic anatomy is known in other vertebrate groups. This significantly limits our general understanding of how dopamine signaling systems have evolved as organisms adapt to their environments. This review takes a pan-vertebrate view of the literature on the role of dopamine in motor/sensory cortical regions, highlighting, when available, research on non-mammalian vertebrates. We provide a broad perspective on dopamine function and emphasize that dopamine-induced plasticity mechanisms are widespread across all cortical systems and associated with motor and sensory adaptations. The available evidence illustrates that there is a strong anatomical basis-dopamine fibers and receptor distributions-to hypothesize that pallial dopamine effects are widespread among vertebrates. Continued research progress in non-mammalian species will be crucial to further our understanding of how the dopamine system evolved to shape the diverse array of brain structures and behaviors among the vertebrate lineage.

Neurochemistry of Visual Attention

Visual attention is the cognitive process that mediates the selection of important information from the environment. This selection is usually controlled by bottom-up and top-down attentional biasing. Since for most humans vision is the dominant sense, visual attention is critically important for higher-order cognitive functions and related deficits are a core symptom of many neuropsychiatric and neurological disorders. Here, we summarize the importance and relative contributions of different neuromodulators and neurotransmitters to the neural mechanisms of top-down and bottom-up attentional control. We will not only review the roles of widely accepted neuromodulators, such as acetylcholine, dopamine and noradrenaline, but also the contributions of other modulatory substances. In doing so, we hope to shed some light on the current understanding of the role of neurochemistry in shaping neuron properties contributing to the allocation of attention in the visual field.

A neural noise account of Gilles de la Tourette syndrome

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A neural noise account on Tourette syndrome is conceptualized.

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We outline how neurophysiological methods can be used to test this account.

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The neural noise account may lead to novel treatment options.

Tics, often preceded by premonitory urges, are the clinical hallmark of Tourette syndrome. They resemble spontaneous movements, but are exaggerated, repetitive and appear misplaced in a given communication context. Given that tics often go unnoticed, it has been suggested that they represent a surplus of action, or motor noise. In this conceptual position paper, we propose that tics and urges, but also patterns of the cognitive profile in Tourette syndrome might be explained by the principle of processing of neural noise and adaptation to it during information processing. We review evidence for this notion in the light of Tourette pathophysiology and outline why neurophysiological and imaging approaches are central to examine a possibly novel view on Tourette syndrome. We discuss how neurophysiological data at multiple levels of inspections, i.e., from local field potentials using intra-cranial recording to scalp-measured EEG data, in combination with imaging approaches, can be used to examine the neural noise account in Tourette syndrome. We outline what signal processing methods may be suitable for that. We argue that, as a starting point, the analysis of 1/f neural noise or scale-free activity may be suitable to investigate the role of neural noise and its adaptation during information processing in Tourette syndrome. We outline, how the neural noise perspective, if substantiated by further neurophysiological studies and re-analyses of existing data, may pave the way to novel interventions directly targeting neural noise levels and patterns in Tourette syndrome.

Dissociating direct and indirect effects: a theoretical framework of how latent toxoplasmosis affects cognitive profile across the lifespan

About one-third of the world's population has latent toxoplasmosis, which is typically most prevalent in old age due to its lifelong persistence. Most infected people do not reveal clinically relevant symptoms, but T. gondii might trigger cognitive changes in otherwise asymptomatic individuals. As intact cognitive processes are essential for various achievements and successful aging, this review focuses on the cognitive profile associated with latent toxoplasmosis across the lifespan. It could be explained by a shift in balance between direct effects (increased dopamine synthesis) and indirect effects (neurodegeneration and chronic inflammation, which can decrease dopamine levels). Based thereon, we provide a possibly comprehensive framework of how T. gondii can differently affect cognitive performance across the lifespan (i.e., from increased catecholaminergic signaling in young age to decreased signaling in old age). We outline how future studies may inform our knowledge on the role of individual differences in response to T. gondii and how longitudinal studies can help trace the temporal dynamics in the shift of the balance between direct and indirect effects.

The Neurobiology of Pathological Fatigue: New Models, New Questions

The last decade has seen the emergence of new theoretical frameworks to explain pathological fatigue, a much neglected, yet highly significant symptom across a wide range of diseases. While the new models of fatigue provide new hypotheses to test, they also raise a number of questions. The primary purpose of this essay is to examine the predictions of three recently proposed models of fatigue, the overlap and differences between them, and the evidence from diseases that may lend support to the models of fatigue. I also present expansions for the sensory attenuation model of fatigue. Further questions examined here are the following: What are the neural substrates of fatigue? How can sensory attenuation, which underpins agency also explain fatigue? Are fatigue and agency related?

A novel approach to intra-individual performance variability in ADHD

Patients with attention deficit/(hyperactivity) disorder (AD(H)D) show increased intra-individual variability (IIV) in behavioral performance. This likely reflects dopaminergic deficiencies. However, the precise performance profile across time and the pattern of fluctuations within it have not yet been considered, partly due to insufficient methods. Yet, such an analysis may yield important theory-based implications for clinical practice. Thus, in a case-control cross-sectional study, we introduce a new method to investigate performance fluctuations in patients with ADD (n = 76) and ADHD (n = 67) compared to healthy controls (n = 45) in a time estimation task. In addition, we also evaluate the effects of methylphenidate (MPH) treatment on this performance pattern in 29 patients with AD(H)D. Trial-by-trial differences in performance between healthy controls and patients with AD(H)D do not persist continuously over longer time periods. Periods during which no differences in performance between healthy controls and patients occur alternate with periods in which such differences are present. AD(H)D subtype and surprisingly also medication status does not affect this pattern. The presented findings likely reflect (phasic) deficiencies of the dopaminergic system in patients with AD(H)D which are not sufficiently ameliorated by first-line pharmacological treatment. The presented findings carry important clinical and scientific implications.

Cognitive profile in Restless Legs Syndrome: A signal-to-noise ratio account

Restless legs syndrome (RLS) is a common neurological disorder characterized by a sensorimotor condition, where patients feel an uncontrollable urge to move the lower limbs in the evening and/or during the night. RLS does not only have a profound impact on quality of life due to the disturbed night-time sleep, but there is growing evidence that untreated or insufficiently managed RLS might also cause cognitive changes in patients affected by this syndrome. It has been proposed that RLS is caused by alterations in the signal-to-noise ratio (SNR) and in dopamine (DA) neurotransmission in the nervous system. Based on this evidence, we propose the “SNR-DA hypothesis” as an explanation of how RLS could affect cognitive performance. According to this hypothesis, variations/reductions in the SNR underlie RLS-associated cognitive deficits, which follow an inverted U-shaped function: In unmedicated patients, low dopamine levels worsen the SNR, which eventually impairs cognition. Pharmacological treatment enhances DA levels in medicated patients, which likely improves/normalizes the SNR in case of optimal doses, thus restoring cognition to a normal level. However, overmedication might push patients past the optimal point on the inverted U-shaped curve, where an exaggerated SNR potentially impairs cognitive performance relying on cortical noise such as cognitive flexibility. Based on these assumptions of SNR alterations, we propose to directly measure neural noise via “1/f noise” and related metrics to use transcranial random noise stimulation (tRNS), a noninvasive brain stimulation method which manipulates the SNR, as a research tool and potential treatment option for RLS.

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Restless legs syndrome (RLS) is a common neurological disorder.

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RLS is caused by alterations in the SNR ratio and in DA neurotransmission.

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The SNR- DA hypothesis how RLS affects cognitive performance is presented.

The influence of syringic acid treatment on total dopamine levels of the hippocampus and on cognitive behavioral skills

BACKGROUND

Natural polyphenols have been investigated and are claimed to be mediators of the relationship between dopamine (DA) and memory. Therefore, we aimed to measure and evaluate the effect of syringic acid (SA) on DA expression by behavioral tests related to short-term and recognition memory in Wistar rats.

METHODS

Rats were randomly assigned to control (0.5 cc corn oil, n = 10), SA (25 mg/kg/day, o.g, n = 10), Deltamethrin (DTM) (1.28 mg/kg/day o.g, n = 10) and DTM (1.28 mg/kg/day o.g, n = 10) + SA (25 mg/kg/day) groups. The Y-maze and Novel Object Recognition (NOR) tests were performed to assess cognitive and behavioral functions in the rats. Dopamine levels in the hippocampus were measured by mass spectrometry.

RESULTS

Syringic acid significantly increased DA (5.45 ± 1.06 ng/ml, p = 0.0026, p < 0.05) compared with the other groups. SA increased the percent alternation (34.85 ± 0.72%, p < 0.05), time spent in the novel arm (2.88 ± 0.18 min, p < 0.05), and frequency of novel arm entries (44.91 ± 2.28%, p < 0.05), of the rats after the Y-maze test. The SA elevated the discrimination index (70.42 ± 3.59%, p < 0.001), and exploration time (30.44 ± 1.8 sec, p < 0.05) in the NOR test, and increased the short term and recognition memory in behavioral tests.

CONCLUSION

Our findings support the hypothesis that SA-induced DA levels of the hippocampus may facilitate recognition and short-term memory in Wistar rats through the activation of dopaminergic receptors or pathways during the learning process, and that this can be seen in the cognitive behavior of SA-treated rats.

Conscious perception and the modulatory role of dopamine: no effect of the dopamine D2 agonist cabergoline on visual masking, the attentional blink, and probabilistic discrimination

RATIONALE

Conscious perception is thought to depend on global amplification of sensory input. In recent years, striatal dopamine has been proposed to be involved in gating information and conscious access, due to its modulatory influence on thalamocortical connectivity.

OBJECTIVES

Since much of the evidence that implicates striatal dopamine is correlational, we conducted a double-blind crossover pharmacological study in which we administered cabergoline-a dopamine D2 agonist-and placebo to 30 healthy participants. Under both conditions, we subjected participants to several well-established experimental conscious-perception paradigms, such as backward masking and the attentional blink task.

RESULTS

We found no evidence in support of an effect of cabergoline on conscious perception: key behavioral and event-related potential (ERP) findings associated with each of these tasks were unaffected by cabergoline.

CONCLUSIONS

Our results cast doubt on a causal role for dopamine in visual perception. It remains an open possibility that dopamine has causal effects in other tasks, perhaps where perceptual uncertainty is more prominent.

Differential Effects of Pergolide and Bromocriptine on Working Memory Performance and Brain Activation after Mild Traumatic Brain Injury

Dopamine D1 and D2 receptors differ with respect to patterns of regional brain distribution and behavioral effects. Pre-clinical work suggests that D1 agonists enhance working memory, but the absence of selective D1 agonists has constrained using this approach in humans. This study examines working memory performance in mild traumatic brain injury (mTBI) patients when given pergolide, a mixed D1/D2 agonist, compared with bromocriptine, a selective D2 agonist. Fifteen individuals were studied 1 month after mTBI and compared with 17 healthy controls. At separate visits, participants were administered 1.25 mg bromocriptine or 0.05 mg pergolide prior to functional magnetic resonance imaging (MRI) using a working memory task (visual-verbal n-back). Results indicated a significant group-by-drug interaction for mean performance across n-back task conditions, where the mTBI group showed better performance on pergolide relative to bromocriptine, whereas controls showed the opposite pattern. There was also a significant effect of diagnosis, where mTBI patients performed worse than controls, particularly while on bromocriptine, as shown in our prior work. Functional MRI activation during the most challenging task condition (3-back > 0-back contrast) showed a significant group-by-drug interaction, with the mTBI group showing increased activation relative to controls in working memory circuitry while on pergolide, including in the left inferior frontal gyrus. Across participants there was a positive correlation between change in activation in this region and change in performance between drug conditions. Results suggest that activation of the D1 receptor may improve working memory performance after mTBI. This has implications for the development of pharmacological strategies to treat cognitive deficits after mTBI.

Task experience eliminates catecholaminergic effects on inhibitory control - A randomized, double-blind cross-over neurophysiological study

Catecholaminergic neural transmission plays an important role during the inhibition of prepotent responses. Methylphenidate (MPH) is an important drug that modulates the catecholaminergic system. However, theoretical considerations suggest that the effects of drugs (e.g. MPH) on cognitive control may depend on prior learning effects. Here we investigate this in a conflict-modulated Go/Nogo task and evaluate neurophysiological processes associated with this dynamic using EEG signal decomposition methods and source localization analysis. The behavioral data show that prior learning experiences eliminate effects of MPH on response inhibition processes. On a neurophysiological level, we show that MPH modulates specific processes in medial frontal brain regions. Although MPH seems to consistently modulate neurophysiological processes associated with response inhibition, this is no longer sufficient to modulate behavioral performance once learning or task familiarization processes have taken place. An important consequence of this study finding is that it may be important to adjust MPH dosage depending on learning effects in a specific setting to constantly increase cognitive control functions in that setting. This has important implications for clinical practice, since MPH is the first-line pharmacological therapy in attention-deficit hyperactivity disorder (ADHD). Cross-over study designs with constant doses of MPH can mask effects on cognitive functions. The impact of learning needs careful consideration in cross-over study designs examining catecholaminergic drug effects.

A Neurofunctional Domains Approach to Evaluate D1/D5 Dopamine Receptor Partial Agonism on Cognition and Motivation in Healthy Volunteers With Low Working Memory Capacity

BACKGROUND

Dopamine D1 receptor signaling plays key roles in core domains of neural function, including cognition and reward processing; however, many questions remain about the functions of circuits modulated by dopamine D1 receptor, largely because clinically viable, selective agonists have yet to be tested in humans.

METHODS

Using a novel, exploratory neurofunctional domains study design, we assessed the safety, tolerability, pharmacodynamics, and pharmacokinetics of PF-06412562, a selective D1/D5R partial agonist, in healthy male volunteers who met prespecified criteria for low working memory capacity. Functional magnetic resonance imaging, electrophysiologic endpoints, and behavioral paradigms were used to assess working memory, executive function, and motivation/reward processing following multiple-dose administration of PF-06412562. A total of 77 patients were assigned PF-06412562 (3 mg twice daily and 15 mg twice daily) or placebo administered for 5 to 7 days. Due to the exploratory nature of the study, it was neither powered for any specific treatment effect nor corrected for multiple comparisons.

RESULTS

Nominally significant improvements from baseline in cognitive endpoints were observed in all 3 groups; however, improvements in PF-06412562-treated patients were less than in placebo-treated participants. Motivation/reward processing endpoints were variable. PF-06412562 was safe and well tolerated, with no serious adverse events, severe adverse events, or adverse events leading to dose reduction or temporary discontinuation except for 1 permanent discontinuation due to increased orthostatic heart rate.

CONCLUSIONS

PF-06412562, in the dose range and patient population explored in this study, did not improve cognitive function or motivation/reward processing more than placebo over the 5- to 7-day treatment period.

CLINICALTRIALS.GOV IDENTIFIER

NCT02306876.

A literature review on the neurophysiological underpinnings and cognitive effects of transcutaneous vagus nerve stimulation: challenges and future directions

Brain stimulation approaches are important to gain causal mechanistic insights into the relevance of functional brain regions and/or neurophysiological systems for human cognitive functions. In recent years, transcutaneous vagus nerve stimulation (tVNS) has attracted considerable popularity. It is a noninvasive brain stimulation technique based on the stimulation of the vagus nerve. The stimulation of this nerve activates subcortical nuclei, such as the locus coeruleus and the nucleus of the solitary tract, and from there, the activation propagates to the cortex. Since tVNS is a novel stimulation technique, this literature review outlines a brief historical background of tVNS, before detailing underlying neurophysiological mechanisms of action, stimulation parameters, cognitive effects of tVNS on healthy humans, and, lastly, current challenges and future directions of tVNS research in cognitive functions. Although more research is needed, we conclude that tVNS, by increasing norepineprine (NE) and gamma-aminobutyric acid (GABA) levels, affects NE- and GABA-related cognitive performance. The review provides detailed background information how to use tVNS as a neuromodulatory tool in cognitive neuroscience and outlines important future leads of research on tVNS.

Properties of lower level processing modulate the actions of the norepinephrine system during response inhibition

We ask whether actions of the norepinephrine (NE) system during response inhibition are affected by properties of lower level sensory stimulus processing. We used a somato-sensory Go/Nogo task and combined ERP recordings with pupil diameter recordings as an index of NE system activity. The Go/Nogo task was designed to achieve processing of tactile stimuli predominantly over primary somatosensory (SI) and secondary somatosensory (SII) areas. The data show that response inhibition was better when stimuli were processed via SII, compared to SI areas. This was reflected by variations in the Nogo-N2/P3 associated with anterior cingulate structures. Correlations with the pupil diameter data, indicting modulations of the NE system during inhibitory control processes, were only evident when SI sensory areas were involved. These dissociable modulatory effects were associated with activations in the superior frontal gyrus. Actions of the NE system during response inhibition are modulated by properties of lower level processing.

Learning Experience Reverses Catecholaminergic Effects on Adaptive Behavior

BACKGROUND

Catecholamines are important for cognitive control and the ability to adapt behavior (e.g., after response errors). A prominent drug that modulates the catecholaminergic system is methylphenidate. On the basis of theoretical consideration, we propose that the effects of methylphenidate on behavioral adaptation depend on prior learning experience.

METHODS

In a double-blind, randomized, placebo-controlled crossover study design, we examined the effect of methylphenidate (0.25 mg/kg) on post error behavioral adaptation processes in a group of n = 43 healthy young adults. Behavioral adaptation processes were examined in a working memory, modulated response selection task. The focus of the analysis was on order effects within the crossover study design to evaluate effects of prior learning/task experience.

RESULTS

The effect of methylphenidate/placebo on post-error behavioral adaptation processes reverses depending on prior task experience. When there was no prior experience with the task, methylphenidate increased post-error slowing and thus intensified behavioral adaptation processes. However, when there was prior task experience, (i.e., when the placebo session was conducted first in the crossover design), methylphenidate even decreased post-error slowing and behavioral adaptation. Effect sizes were large and the power of the observed effects was higher than 95%.

CONCLUSIONS

The data suggest that catecholaminergic effects on cognitive control functions vary as a function of prior learning/task experience. The data establish a close link between learning/task familiarization and catecholaminergic effects for executive functions, which has not yet been studied, to our knowledge, but is of considerable clinical relevance. Theoretical implications are discussed.

Passive perceptual learning modulates motor inhibitory control in superior frontal regions

Response inhibition is of vital importance in the context of controlling inappropriate responses. The role of perceptual processes during inhibitory control has attracted increased interest. Yet, we are far from an understanding of the mechanisms. One candidate mechanism by which perceptual processes may affect response inhibition refers to “gain control” that is closely linked to the signal‐to‐noise ratio of incoming information. A means to modulate the signal‐to‐noise ratio and gain control mechanisms is perceptual learning. In the current study, we examine the impact of perceptual learning (i.e., passive repetitive sensory stimulation) on response inhibition combining EEG signal decomposition with source localization analyses. A tactile GO/NOGO paradigm was conducted to measure action restraint as one subcomponent of response inhibition. We show that passive perceptual learning modulates response inhibition processes. In particular, perceptual learning attenuates the detrimental effect of response automation during inhibitory control. Temporally decomposed EEG data show that stimulus‐related and not response selection processes during conflict monitoring are linked to these effects. The superior and middle frontal gyrus (BA6), as well as the motor cortex (BA4), are associated with the effects of perceptual learning on response inhibition. Reliable neurophysiological effects were not evident on the basis of standard ERPs, which has important methodological implications for perceptual learning research. The results detail how lower level sensory plasticity protocols affect higher‐order cognitive control functions in frontal cortical structures.

Dopamine Enhances Item Novelty Detection via Hippocampal and Associative Recall via Left Lateral Prefrontal Cortex Mechanisms

The involvement of fronto-striatal circuits in item and associative memory retrieval as well as in the stabilization of memories by retrieval practice suggests that both retrieval and re-encoding of stored memories might rely on dopaminergic mechanisms in humans. We tested these hypotheses in a placebo-controlled pharmacological fMRI study using 2 mg of the D2 antagonist haloperidol administered acutely before a cued associative recall task of previously encoded picture-word pairs in 53 healthy humans of both sexes. The cued associative recall was moreover repeated 3 d later outside the scanner without pharmacological intervention. Dopaminergic modulation significantly improved associative recall performance and recognition accuracy of verbal items. Moreover, we observed a significant dopamine effect on re-encoding in terms of increased specificity of associative memories from the first to the second cued associative recall. Better association memory under haloperidol was linked with higher activity in the left lateral prefrontal cortex and right parietal cortex, suggesting that dopamine facilitates associative retrieval through increased recruitment of frontoparietal monitoring processes. In contrast, improved recognition of verbal items under haloperidol was reflected by enhanced novelty detection in the hippocampus and increased activity in saliency networks. Together, these results show distinct but concomitant positive effects of dopamine on associative recall and item recognition and suggest that the specificity of associative recall through re-encoding mechanisms is likewise augmented by dopamine.SIGNIFICANCE STATEMENT Although the neurotransmitter dopamine has been linked with learning and memory for a long time, dopaminergic effects on item recognition in humans were demonstrated only recently. The involvement of fronto-striatal monitoring processes in association retrieval suggests that associative memory might be particularly affected by dopamine. Moreover, fronto-striatal dopaminergic signals have been hypothesized to determine the updating and re-encoding of previously retrieved memories. We here demonstrate clear facilitative effects of dopamine on associative recall and item recognition mediated by prefrontal and hippocampal mechanisms respectively. Additionally, effects on re-encoding were reflected by increased specificity of associative memories. These results augment our understanding of dopaminergic processes in episodic memory retrieval and offer new perspectives on memory impairments in dopamine-related disorders and their treatment.

A possible key role of vision in the development of schizophrenia

Based on a brief overview of the various aspects of schizophrenia reported by numerous studies, here we hypothesize that schizophrenia may originate (and in part be performed) from visual areas. In other words, it seems that a normal visual system or at least an evanescent visual perception may be an essential prerequisite for the development of schizophrenia as well as of various types of hallucinations. Our study focuses on auditory and visual hallucinations, as they are the most prominent features of schizophrenic hallucinations (and also the most studied types of hallucinations). Here, we evaluate the possible key role of the visual system in the development of schizophrenia.

The Presynaptic Regulation of Dopamine and Norepinephrine Synthesis Has Dissociable Effects on Different Kinds of Cognitive Conflicts

Goal-directed behavior requires the ability to resolve subliminally or consciously induced response conflicts, both of which may benefit from catecholamine-induced increases in gain control. We investigated the effects of presynaptic differences in dopamine and norepinephrine synthesis with the help of the tyrosine hydroxylase (TH) rs10770141 and the dopamine-β-hydroxylase (DBH) rs1611115, rs6271, and rs1611122 polymorphisms. Conscious and subliminal response conflicts were induced with flanker and prime distractors in (n = 207) healthy young participants while neurophysiological data (EEG) was recorded. The results demonstrated that the increased presynaptic catecholamine synthesis associated with the TH rs10770141 TT genotype improves cognitive control in case of consciously perceived (flanker) conflicts, but not in case of subliminally processed (prime) conflicts. Only norepinephrine seemed to also modulate subliminal conflict processing, as evidenced by better performance of the DBH rs1611122 CC genotype in case of high subliminal conflict load. Better performance was linked to larger conflict-induced modulations in post-response alpha band power arising from parietal and inferior frontal regions, which likely helps to suppress the processing of distracting information. In summary, presynaptic catecholamine synthesis benefits consciously perceived conflicts by improving the suppression of distracting information following a conflict. Subliminal conflicts were modulated via the same mechanism, but only by norepinephrine.

Neural Dedifferentiation in the Aging Brain

Many cognitive abilities decline with age even in the absence of detectable pathology. Recent evidence indicates that age-related neural dedifferentiation, operationalized in terms of neural selectivity, may contribute to this decline. We review here work exploring the relationship between neural dedifferentiation, cognition, and age. Compelling evidence for age effects on neural selectivity comes from both non-human animal and human research. However, current data suggest that age does not moderate the observed relationships between neural dedifferentiation and cognitive performance. We propose that functionally significant variance in measures of neural dedifferentiation reflects both age-dependent and age-independent factors. We further propose that the effects of age on neural dedifferentiation do not exclusively reflect detrimental consequences of aging.

Catecholaminergic effects on inhibitory control depend on the interplay of prior task experience and working memory demands

BACKGROUND

Catecholamines affect response inhibition, but the effects of methylphenidate on inhibitory control in healthy subjects are heterogenous. Theoretical considerations suggest that working memory demands and learning/familiarization processes are important factors to consider regarding catecholaminergic effects on response inhibition.

AIMS

The purpose of this study was to examine the role of working memory demands and familiarization for methylphenidate effects on response inhibition.

METHODS

Twenty-eight healthy adults received a single dose of methylphenidate (0.5 mg/kg) or placebo in a randomised, double-blind, crossover study design. The subjects were tested using a working memory-modulated response inhibition paradigm that combined a Go/Nogo task with a mental rotation task.

RESULTS

Methylphenidate effects were largest in the most challenging mental rotation condition. The direction of effects depended on the extent of the participants' task experience. When performing the task for the first time, methylphenidate impaired response inhibition performance in the most challenging mental rotation condition, as reflected by an increased false alarm rate. In sharp contrast to this, methylphenidate seemed to improve response execution performance in the most challenging condition when performing the task for the second time as reflected by reaction times on Go trials.

CONCLUSION

Effects of catecholamines on inhibitory control processes depend on the interplay of two factors: (a) working memory demands, and (b) learning or familiarization with a task. It seems that the net effect of increases in gain control and decreases in working memory processes determines the methylphenidate effect on response inhibition. Hence, crossover study designs likely underestimate methylphenidate effects on cognitive functions.

The Modulation of Neural Noise Underlies the Effectiveness of Methylphenidate Treatment in Attention-Deficit/Hyperactivity Disorder

BACKGROUND

Various lines of research suggest that the stability of neural processes is low in attention-deficit/hyperactivity disorder (ADHD). Considering overarching neural principles, this lack of stability relates to increased levels of neural noise. However, no study has directly examined neural noise in ADHD. Likewise, it is unknown whether the modulation of neural noise reflects a mechanistic link as to why methylphenidate (MPH) is effective in treating impulsivity in ADHD.

METHODS

We compared neural noise between 29 juvenile patients with ADHD and 32 healthy control subjects and examined the effects of MPH. We examined 1/f neural noise of electroencephalogram data collected while participants performed a response inhibition (Go/NoGo) task.

RESULTS

Specific during NoGo trials, children with ADHD showed more neural noise than healthy control subjects. This was especially the case with regard to the theta frequency band, which is very closely related to cognitive control. MPH treatment reduced neural noise in ADHD to the level of healthy control subjects. Correlational analyses showed a direct relationship between decreases in neural noise and increases in behavioral performance. Mechanistically, this can be explained by the MPH-induced increase in dopaminergic neurotransmission that enhances the signal-to-noise ratio in neural networks and thus reduces neural noise.

CONCLUSIONS

This study is the first to demonstrate increased (pink) neural noise in patients with ADHD and its reduction through MPH treatment. The study reveals an important mechanistic link as to why MPH is effective in treating impulsivity in ADHD.

On the interrelation of 1/f neural noise and norepinephrine system activity during motor response inhibition

Several lines of evidence suggest that there is a close interrelation between the degree of noise in neural circuits and the activity of the norepinephrine (NE) system, yet the precise nexus between these aspects is far from being understood during human information processing and cognitive control in particular. We examine this nexus during response inhibition in n = 47 healthy participants. Using high-density EEG recordings, we estimate neural noise by calculating "1/f noise" of those data and integrate these EEG parameters with pupil diameter data as an established indirect index of NE system activity. We show that neural noise is reduced when cognitive control processes to inhibit a prepotent/automated response are exerted. These neural noise variations were confined to the theta frequency band, which has also been shown to play a central role during response inhibition and cognitive control. There were strong positive correlations between the 1/f neural noise parameter and the pupil diameter data within the first 250 ms after the Nogo stimulus presentation at centro-parietal electrode sites. No such correlations were evident during automated responding on Go trials. Source localization analyses using standardized low-resolution brain electromagnetic tomography show that inferior parietal areas are activated in this time period in Nogo trials. The data suggest an interrelation of NE system activity and neural noise within early stages of information processing associated with inferior parietal areas when cognitive control processes are required. The data provide the first direct evidence for the nexus between NE system activity and the modulation of neural noise during inhibitory control in humans. NEW & NOTEWORTHY This is the first study showing that there is a nexus between norepinephrine system activity and the modulation of neural noise or scale-free neural activity during inhibitory control in humans. It does so by integrating pupil diameter data with analysis of EEG neural noise.

Dopamine is a double-edged sword: dopaminergic modulation enhances memory retrieval performance but impairs metacognition

While memory encoding and consolidation processes have been linked with dopaminergic signaling for a long time, the role of dopamine in episodic memory retrieval remained mostly unexplored. Based on previous observations of striatal activity during memory retrieval, we used pharmacological functional magnetic resonance imaging to investigate the effects of dopamine on retrieval performance and metacognitive memory confidence in healthy humans. Dopaminergic modulation by the D2 antagonist haloperidol administered acutely during the retrieval phase improved recognition accuracy of previously learned pictures significantly and was associated with increased activity in the substantia nigra/ventral tegmental area, locus coeruleus, hippocampus, and amygdala during retrieval. In contrast, confidence for new decisions was impaired by unsystematically increased activity of the striatum across confidence levels and restricted range of responsiveness in frontostriatal networks under haloperidol. These findings offer new insights into the mechanisms underlying memory retrieval and metacognition and provide a broader perspective on the presence of memory problems in dopamine-related diseases and the treatment of memory disorders.

Methamphetamine Users Show No Behavioral Deficits in Response Selection After Protracted Abstinence

Introduction: Chronic recreational methamphetamine use causes dopaminergic neurotoxicity, which has been linked to impairments in executive functioning. Within this functional domain, response selection and the resolution of associated conflicts have repeatedly been demonstrated to be strongly modulated by dopamine. Yet, it has never been investigated whether chronic methamphetamine use leads to general impairments in response selection (i.e., irrespective of consumption-associated behavior) after substance use is discontinued. Materials and Methods: We tested n = 24 abstinent methamphetamine users (on average 2.7 years of abstinence) and n = 24 individually matched controls in a cross-sectional design with a flanker task. Results: Compared to healthy controls, former methamphetamine consumers had significantly slower reaction times, but did not show differences in the size of the flanker or Gratton effect, or post-error slowing. Complementary Bayesian analyses further substantiated this lack of effects despite prior consumption for an average of 7.2 years. Discussion: The ability to select a correct response from a subset of conflicting alternatives, as well as the selective attention required for this seem to be largely preserved in case of prolonged abstinence. Likewise, the ability to take previous contextual information into account during response selection and to process errors seem to be largely preserved as well. Complementing previously published finding of worse inhibition/interference control in abstinent consumers, our results suggest that not all executive domains are (equally) impaired by methamphetamine, possibly because different cognitive processes require different levels of dopamine activity.

Catecholaminergic Modulation of Conflict Control Depends on the Source of Conflicts

Background

To display goal-directed behavior, we must be able to resolve response conflicts that arise from processing various distractors. Such conflicts may be triggered by different kinds of distractor stimuli (e.g., priming and flanker stimuli), but it has remained largely unclear whether the functional and neurobiological underpinnings of both conflict types differ. We therefore investigated the functional relevance of the catecholamines dopamine and norepinephrine, which have been shown to increase the signal-to-noise ratio in neuronal processing and should therefore modulate response conflicts.

Methods

In a double-blind, randomized, placebo-controlled study design, we examined the effect of methylphenidate (0.5 mg/kg) on both flanker-induced and priming-induced response conflicts in a group of n=25 healthy young adults. We used EEG recordings to examine event-related potentials in combination with source localization analyses to identify the cognitive-neurophysiological subprocesses and functional neuroanatomical structures modulated by methylphenidate.

Results

Compared with placebo, methylphenidate decreased flanker conflicts. This was matched by increased congruency effects in the fronto-central N2/P3 event-related potential complex and associated with modulations in the right inferior frontal gyrus. In contrast to this, methylphenidate did not modulate the size of prime-evoked conflicts.

Conclusions

Our results suggest that catecholamine-driven increases in signal-to-noise ratio and neural gain control do not equally benefit differently evoked conflicts. This supports the hypothesis of an at least partly different neurobiological basis for flanker- and prime-evoked response conflicts. As the right inferior frontal gyrus plays an important role in inhibition, the catecholaminergic system may reduce flanker conflicts by supporting the inhibition of distracting information.

The system-neurophysiological basis for how methylphenidate modulates perceptual-attentional conflicts during auditory processing

The ability to selectively perceive and flexibly attend to relevant sensory signals in the environment is essential for action control. Whereas neuromodulation of sensory or attentional processing is often investigated, neuromodulation of interactive effects between perception and attention, that is, high attentional control demand when the relevant sensory information is perceptually less salient than the irrelevant one, is not well understood. To fill this gap, this pharmacological-electroencephalogram (EEG) study applied an intensity-modulated, focused-attention dichotic listening paradigm together with temporal EEG signal decomposition and source localization analyses. We used a double-blind MPH/placebo crossover design to delineate the effects of methylphenidate (MPH)-a dopamine/norepinephrine transporter blocker-on the resolution of perceptual-attentional conflicts, when perceptual saliency and attentional focus favor opposing ears, in healthy young adults. We show that MPH increased behavioral performance specifically in the condition with the most pronounced conflict between perceptual saliency and attentional focus. On the neurophysiological level, MPH effects in line with the behavioral data were observed after accounting for intraindividual variability in the signal. More specifically, MPH did not show an effect on stimulus-related processes but modulated the onset latency of processes between stimulus evaluation and responding. These modulations were further shown to be associated with activation differences in the temporoparietal junction (BA40) and the superior parietal cortex (BA7) and may reflect neuronal gain modulation principles. The findings provide mechanistic insights into the role of modulated dopamine/norepinephrine transmitter systems for the interactions between perception and attention.

Neuromolecular basis of faded perception associated with unreality experience

Perceptual changes in shape, size, or color are observed in patients with derealization symptoms; however, the underlying neural and molecular mechanisms are not well understood. The current study explored the relationship between neural activity associated with altered colorfulness perception assessed by fMRI and striatal dopamine D₂ receptor availability measured by [¹¹C]raclopride PET in healthy participants. Inside an fMRI scanner, participants performed the saturation adaptation task, where they rated how much vivid/faded visual objects looked like real/unreal ones using a visual analog scale. We found that participants experienced greater unreality when they perceived fadedness than vividness despite physically identical saturation. The combined fMRI and PET analyses revealed that the faded perception-related activities of the dorsolateral prefrontal and parietal cortex were positively correlated with striatal D₂ receptor availability. This finding may help to understand the neuromolecular mechanisms of faded perception associated with feeling unreal in derealization symptoms.

Dopamine Modulates the Efficiency of Sensory Evidence Accumulation During Perceptual Decision Making

BACKGROUND

Perceptual decision making is the process through which available sensory information is gathered and processed to guide our choices. However, the neuropsychopharmacological basis of this important cognitive function is largely elusive. Yet, theoretical considerations suggest that the dopaminergic system may play an important role.

METHODS

In a double-blind, randomized, placebo-controlled study design, we examined the effect of methylphenidate in 2 dosages (0.25 mg/kg and 0.5 mg/kg body weight) in separate groups of healthy young adults. We used a moving dots task in which the coherency of the direction of moving dots stimuli was manipulated in 3 levels (5%, 15%, and 35%). Drift diffusion modelling was applied to behavioral data to capture subprocesses of perceptual decision making.

RESULTS

The findings show that only the drift rate (v), reflecting the efficiency of sensory evidence accumulation, but not the decision criterion threshold (a) or the duration of nondecisional processes (Ter), is affected by methylphenidate vs placebo administration. Compared with placebo, administering 0.25 mg/kg methylphenidate increased v, but only in the 35% coherence condition. Administering 0.5 mg/kg methylphenidate did not induce modulations.

CONCLUSIONS

The data suggest that dopamine selectively modulates the efficacy of evidence accumulation during perceptual decision making. This modulation depends on 2 factors: (1) the degree to which the dopaminergic system is modulated using methylphenidate (i.e., methylphenidate dosage) and (2) the signal-to-noise ratio of the visual information. Dopamine affects sensory evidence accumulation only when dopamine concentration is not shifted beyond an optimal level and the incoming information is less noisy.