Dopaminergic modulation of working memory for spatial but not object cues in normal humans

Effects of tyrosine/phenylalanine depletion on electrophysiological correlates of memory in healthy volunteers

Dopamine is well known for involvement in reinforcement, motor control and frontal lobe functions, such as attention and memory. Tyrosine/phenylalanine depletion (TPD) lowers dopamine synthesis and can therefore be used as a model to study the effects of low dopamine levels. This is the first study to assess the effect of TPD on memory performance and its electrophysiological correlates. In a double blind placebo (PLA)-controlled crossover design, 17 healthy volunteers (six males, 11 females) aged between 18 and 25 were tested after TPD and PLA. Working memory was assessed using a Sternberg memory scanning task (SMS) and episodic memory using the Visual Verbal Learning Test (VVLT). Simultaneously, event-related potentials (ERPs) were measured. The tyrosine and phenylalanine ratio was significantly reduced after TPD and increased after PLA. Working memory performance was not affected by TPD. However, ERP measures were affected by the treatment, indicating that TPD impaired stimulus processing during working memory performance. Episodic memory was not impaired after TPD. Again, alterations in ERP measures suggested adverse effects of TPD on memory-related processing. These results suggest that dopamine is involved in both working memory and episodic memory-related processing, although the effects are too small to be detected by performance measures.

Human cognitive flexibility depends on dopamine D2 receptor signaling

RATIONALE

Accumulating evidence indicates that the cognitive effects of dopamine depend on the subtype of dopamine receptor that is activated. In particular, recent work with animals as well as current theorizing has suggested that cognitive flexibility depends on dopamine D2 receptor signaling. However, there is no evidence for similar mechanisms in humans.

OBJECTIVES

We aim to demonstrate that optimal dopamine D2 receptor signaling is critical for human cognitive flexibility.

METHODS

To this end, a pharmacological pretreatment design was employed. This enabled us to investigate whether effects of the dopamine receptor agonist bromocriptine on task-set switching were abolished by pretreatment with the D2 receptor antagonist sulpiride. To account for individual (genetic) differences in baseline levels of dopamine, we made use of a common variable number of tandem repeat (VNTR) polymorphism in the 3'-untranslated region of the dopamine transporter gene, DAT1.

RESULTS

Bromocriptine improved cognitive flexibility relative to placebo, but only in subjects with genetically determined low levels of dopamine (n = 27). This beneficial effect of bromocriptine on cognitive flexibility was blocked by pretreatment with the selective dopamine D2 receptor antagonist sulpiride (n = 14).

CONCLUSIONS

These results provide strong evidence in favor of the hypothesis that human cognitive flexibility implicates dopamine D2 receptor signaling.

Impaired visuospatial associative memory and attention in obsessive compulsive disorder but no evidence for differential dopaminergic modulation

RATIONALE

Patients with obsessive compulsive disorder (OCD) demonstrate impaired cognition in some selected domains. Although serotoninergic dysfunction has been implicated in OCD, recent evidence suggests that dopamine may play a role as well.

OBJECTIVE

The aim of the study was to evaluate learning and working memory in OCD and to determine the effects of dopaminergic manipulations on these capacities.

METHODS

Visuospatial associative memory and spatial and verbal working memory were examined in 18 nondepressed patients with OCD and 18 matched healthy controls. The study further investigated whether acute administration of dopamine D2/D3 receptor agonist and antagonist would differentially modulate cognition in OCD. Each participant underwent the cognitive battery three times in a randomized double-blind, placebo-controlled crossover design.

RESULTS

Significant impairments in patients compared with controls were noted on the Cambridge Neuropsychological Test Automated Battery (CANTAB) paired associates learning (PAL) and a measure of sustained attention (rapid visual information processing, RVIP) that persisted across all sessions, with deficient strategy in the CANTAB spatial working memory task in the first session alone. Although the dopamine D2/D3 agonist, pramipexole, led to poorer performance on the PAL and RVIP tasks, no differential effects were noted between the two groups. No significant effects were noted for the D2/D3 antagonist, amisulpride.

CONCLUSIONS

The results are consistent with a specific associative memory deficit in OCD that remained robust despite possible practice effects and compensatory strategies and point to abnormal medial temporal lobe involvement in OCD in addition to the previously implicated frontostriatal loops, with no clear evidence of D2 receptor mediation.

Effect of D-amphetamine on inhibition and motor planning as a function of baseline performance

RATIONALE

Baseline performance has been reported to predict dopamine (DA) effects on working memory, following an inverted-U pattern. This pattern may hold true for other executive functions that are DA-sensitive.

OBJECTIVES

The objective of this study is to investigate the effect of D: -amphetamine, an indirect DA agonist, on two other putatively DA-sensitive executive functions, inhibition and motor planning, as a function of baseline performance.

METHODS

Participants with no prior stimulant exposure participated in a double-blind crossover study of a single dose of 0.3 mg/kg, p.o. of D: -amphetamine and placebo. Participants were divided into high and low groups, based on their performance on the antisaccade and predictive saccade tasks on the baseline day. Executive functions, mood states, heart rate and blood pressure were assessed before (T0) and after drug administration, at 1.5 (T1), 2.5 (T2) and 3.5 h (T3) post-drug.

RESULTS

Antisaccade errors decreased with D: -amphetamine irrespective of baseline performance (p = 0.025). For antisaccade latency, participants who generated short-latency antisaccades at baseline had longer latencies on D: -amphetamine than placebo, while those with long-latency antisaccades at baseline had shorter latencies on D: -amphetamine than placebo (drug x group, p = 0.04). D: -amphetamine did not affect motor planning. Ratings of mood improved on D: -amphetamine (p < 0.001). Magnitude of D: -amphetamine-induced changes in elation was related to baseline reaction time variability.

CONCLUSIONS

D: -amphetamine reduced antisaccade error rates in healthy controls, replicating and extending findings with DA agonists in clinical populations. D: -amphetamine had baseline-dependent effects on antisaccade latency, consistent with an inverted-U relationship between performance and DA activity.

Neurocognitive impairments in MDMA and other drug users: MDMA alone may not be a cognitive risk factor

BACKGROUND

MDMA (3,4-methylenedioxymethamphetamine; "Ecstasy") is an amphetamine derivative with mild hallucinogenic and stimulant qualities. MDMA leads to serotonin (5-hydroxytryptamine; 5-HT) neurotoxicity and has been linked to cognitive impairments. It remains unclear whether these impairments are due to MDMA versus other drug use.

METHOD

Neurocognitive functioning was measured in a sample of abstinent polydrug users (n = 52) with a range of MDMA use and healthy nondrug controls (n = 29). Participants completed a comprehensive neuropsychological battery and self-report measures of drug use.

RESULTS

Polydrug users performed worse than controls on spatial span and spatial working memory (ps < .05). Among polydrug users, lifetime marijuana use significantly predicted verbal learning and memory performance (p < .01), while MDMA use was not predictive of cognitive impairment.

CONCLUSIONS

This study and our previous report (Hanson, Luciana, & Sullwold, 2008) suggest that moderate MDMA use does not lead to persistent impairments above and beyond that associated with generally heavy drug use, but polydrug use may lead to dose-related temporal and frontoparietal dysfunction. Marijuana use may be particularly problematic. Cause-effect relations are unclear.

Simulating neurocognitive aging: effects of a dopaminergic antagonist on brain activity during working memory

BACKGROUND

Previous correlational studies have indirectly linked dysfunctional dopaminergic neurotransmission to age-related cognitive deficits and associated reductions in task-induced functional brain activity.

METHODS

We used an experimental-pharmacological functional magnetic resonance imaging (fMRI) approach to more directly examine the role of dopamine in neurocognitive aging. Twenty younger and 20 healthy older adults were included. During fMRI scanning, a spatial working memory (SWM) task was administered under two conditions, varying in cognitive load. Positron emission tomography measurements with the D1 receptor antagonist [(11)C]SCH23390 confirmed that a given experimental dose of unlabeled solution occupied 50% of D1 receptors in younger adults.

RESULTS

An age-related reduction in SWM performance was observed, and fMRI data revealed that, relative to younger adults under placebo conditions, elderly persons under-recruited load-sensitive fronto-parietal regions during SWM. Critically, in younger adults, the D1 antagonist resulted in a similar reduction in SWM performance and fMRI response.

CONCLUSIONS

These results suggest that depletion of dopamine, whether ontogenetically or pharmacologically, results in decreased SWM performance as well as reduced load-dependent modulation of the blood oxygen level dependent signal in fronto-parietal regions, possibly by decreasing the signal-to-noise ratio in relevant neural networks.

Neurobehavioral evidence for changes in dopamine system activity during adolescence

Human adolescence has been characterized by increases in risk-taking, emotional lability, and deficient patterns of behavioral regulation. These behaviors have often been attributed to changes in brain structure that occur during this developmental period, notably alterations in gray and white matter that impact synaptic architecture in frontal, limbic, and striatal regions. In this review, we provide a rationale for considering that these behaviors may be due to changes in dopamine system activity, particularly overactivity, during adolescence relative to either childhood or adulthood. This rationale relies on animal data due to limitations in assessing neurochemical activity more directly in juveniles. Accordingly, we also present a strategy that incorporates molecular genetic techniques to infer the status of the underlying tone of the dopamine system across developmental groups. Implications for the understanding of adolescent behavioral development are discussed.

Dopamine D(1) receptors and age differences in brain activation during working memory

In an fMRI study, 20 younger and 20 healthy older adults were scanned while performing a spatial working-memory task under two levels of load. On a separate occasion, the same subjects underwent PET measurements using the radioligand [(11)C] SCH23390 to determine dopamine D(1) receptor binding potential (BP) in caudate nucleus and dorsolateral prefrontal cortex (DLPFC). The fMRI study revealed a significant load modulation of brain activity (higher load>lower load) in frontal and parietal regions for younger, but not older, adults. The PET measurements showed marked age-related reductions of D(1) BP in caudate and DLPFC. Statistical control of caudate and DLPFC D(1) binding eliminated the age-related reduction in load-dependent BOLD signal in left frontal cortex, and attenuated greatly the reduction in right frontal and left parietal cortex. These findings suggest that age-related alterations in dopaminergic neurotransmission may contribute to underrecruitment of task-relevant brain regions during working-memory performance in old age.

Developmental changes in dopamine neurotransmission in adolescence: behavioral implications and issues in assessment

Adolescence is characterized by increased risk-taking, novelty-seeking, and locomotor activity, all of which suggest a heightened appetitive drive. The neurotransmitter dopamine is typically associated with behavioral activation and heightened forms of appetitive behavior in mammalian species, and this pattern of activation has been described in terms of a neurobehavioral system that underlies incentive-motivated behavior. Adolescence may be a time of elevated activity within this system. This review provides a summary of changes within cortical and subcortical dopaminergic systems that may account for changes in cognition and affect that characterize adolescent behavior. Because there is a dearth of information regarding neurochemical changes in human adolescents, models for assessing links between neurochemical activity and behavior in human adolescents will be described using molecular genetic techniques. Furthermore, we will suggest how these techniques can be combined with other methods such as pharmacology to measure the impact of dopamine activity on behavior and how this relation changes through the lifespan.

Striatal dopamine predicts outcome-specific reversal learning and its sensitivity to dopaminergic drug administration

Individual variability in reward-based learning has been ascribed to quantitative variation in baseline levels of striatal dopamine. However, direct evidence for this pervasive hypothesis has hitherto been unavailable. We demonstrate that individual differences in reward-based reversal learning reflect variation in baseline striatal dopamine synthesis capacity, as measured with neurochemical positron emission tomography. Subjects with high baseline dopamine synthesis in the striatum showed relatively better reversal learning from unexpected rewards than from unexpected punishments, whereas subjects with low baseline dopamine synthesis in the striatum showed the reverse pattern. In addition, baseline dopamine synthesis predicted the direction of dopaminergic drug effects. The D(2) receptor agonist bromocriptine improved reward-based relative to punishment-based reversal learning in subjects with low baseline dopamine synthesis capacity, while impairing it in subjects with high baseline dopamine synthesis capacity in the striatum. Finally, this pattern of drug effects was outcome-specific, and driven primarily by drug effects on punishment-, but not reward-based reversal learning. These data demonstrate that the effects of D(2) receptor stimulation on reversal learning in humans depend on task demands and baseline striatal dopamine synthesis capacity.

The dual-state theory of prefrontal cortex dopamine function with relevance to catechol-o-methyltransferase genotypes and schizophrenia

There is now general consensus that at least some of the cognitive deficits in schizophrenia are related to dysfunctions in the prefrontal cortex (PFC) dopamine (DA) system. At the cellular and synaptic level, the effects of DA in PFC via D1- and D2-class receptors are highly complex, often apparently opposing, and hence difficult to understand with regard to their functional implications. Biophysically realistic computational models have provided valuable insights into how the effects of DA on PFC neurons and synaptic currents as measured in vitro link up to the neural network and cognitive levels. They suggest the existence of two discrete dynamical regimes, a D1-dominated state characterized by a high energy barrier among different network patterns that favors robust online maintenance of information and a D2-dominated state characterized by a low energy barrier that is beneficial for flexible and fast switching among representational states. These predictions are consistent with a variety of electrophysiological, neuroimaging, and behavioral results in humans and nonhuman species. Moreover, these biophysically based models predict that imbalanced D1:D2 receptor activation causing extremely low or extremely high energy barriers among activity states could lead to the emergence of cognitive, positive, and negative symptoms observed in schizophrenia. Thus, combined experimental and computational approaches hold the promise of allowing a detailed mechanistic understanding of how DA alters information processing in normal and pathological conditions, thereby potentially providing new routes for the development of pharmacological treatments for schizophrenia.

Lack of dopaminergic modulation of cognitive flexibility

OBJECTIVE

To evaluate performance on a cognitive flexibility task after administration of a dopaminergic agonist.

BACKGROUND

L-3,4-dihydroxyphenylalanine, which is converted into dopamine and norepinephrine, results in a restriction of the semantic network in priming experiments. Recent evidence suggests that flexibility of access to semantic networks in problem solving is mediated by the noradrenergic system. We wished to determine if dopaminergic agonists also affect this type of cognitive flexibility.

METHODS

Eighty-four subjects were tested twice on an anagram task, once 1 hour after bromocriptine (dopamine agonist) and once 1 hour after placebo administration.

RESULTS

No difference was detected between placebo and bromocriptine on anagram performance.

CONCLUSIONS

Therefore, modulation of this type of cognitive flexibility does not seem to be mediated by the dopaminergic system. This suggests that the noradrenergic modulation of cognitive flexibility previously reported does not seem to extend to the dopaminergic system among catecholamines.

Tyrosine depletion alters cortical and limbic blood flow but does not modulate spatial working memory performance or task-related blood flow in humans

Dopamine appears critical in regulating spatial working memory (SWM) within the PFC of non-human primates; however findings in humans are less clear. Recent studies of the effects of global depletion of dopamine via acute tyrosine/phenylalanine depletion (TPD) on SWM task performance have yielded inconsistent results, which may be partly related to task differences. These previous studies do not address whether TPD can directly impair PFC functioning. The current study investigated the effects of TPD on (1) regional cerebral blood flow (rCBF) during a SWM n-back task using H(2) (15)O Positron Emission Tomography (PET), and (2) behavioural performance on three different SWM tasks. Ten healthy males were scanned twice: once following a placebo (balanced) amino acid mixture and once following an equivalent mixture deficient in tyrosine/phenylalanine (TPD condition). Participants completed two additional delayed-response tasks to examine whether differences in response demands influenced TPD effects on performance. TPD resulted in widespread increases in rCBF, with maximum increases in the region of the parahippocampal gyrus bilaterally, left inferior frontal gyrus, and the putamen. TPD related rCBF reductions were observed in the medial frontal gyrus bilaterally, right inferior temporal gyrus and the pons. Despite widespread changes in blood flow following TPD, no specific effects on SWM neural networks or task performance were observed. The use of three different SWM tasks suggests that task differences are unlikely to account for the lack of effects observed. These findings question the capacity of TPD to consistently modulate dopamine function and SWM neural networks in humans.

Dopamine D2 receptor occupancy levels of acute sulpiride challenges that produce working memory and learning impairments in healthy volunteers

RATIONALE

In humans, the effects of dopaminergic agents administered systemically are less clear-cut than studies in experimental animals where agents can be applied locally in the brain. DA receptor occupancy could clearly contribute to the variance in findings, although this is typically not known.

OBJECTIVES

The objective of the study was to measure the DA D2 receptor occupancy of sulpiride 200 and 400 mg and relate this to changes in task performance.

MATERIALS AND METHODS

Positron emission tomography scans were acquired in ten healthy volunteers with [11C]-raclopride. Striatal drug occupancy was calculated as the percentage change in binding potential between placebo and drug scans. All volunteers received placebo and sulpiride 400 mg, with four receiving 200 mg on a third session. Immediate post-scan neuropsychological assessment included working memory and learning tasks.

RESULTS

Striatal sulpiride occupancy was approximately 17% (200 mg) and approximately 28% (400 mg), with similar occupancy within the midbrain. Neuropsychological data analysis was restricted to the higher dose (n = 10). Accuracy on the spatial working memory and spatial learning tasks was impaired after the drug, and the former was inversely related to occupancy.

CONCLUSION

Doses of sulpiride typically used in human cognitive studies produced low levels of DA D2 receptor occupancy compared to that considered efficacious in the treatment of schizophrenia. The levels of occupancy were sufficient to replicate impairments on a spatial working memory task and impair spatial learning. The relationship between occupancy and working memory was suggestive of presynaptic effects, although the precise mechanism underlying the impairment will require studies of wider ranges of occupancy within and outside of the striatum.

Apomorphine effects on episodic memory in young healthy volunteers

RATIONALE

Dopamine (DA) modulates working memory. However, the relation between DA systems and episodic (declarative) memory is less established. Frontal lobe DA function may be involved. We were interested in assessing whether apomorphine (Apo), a drug used extensively in clinical research as a probe of DA function, has an effect on episodic memory test performance in healthy volunteers.

OBJECTIVE

To investigate the effect of a presynaptic dose of Apo on episodic memory tests and on other tests thought to be sensitive to frontal lobe functions.

METHODS

Twenty healthy subjects were treated with Apo HCl (5 microg/kg sc) or placebo (10 subjects/group) in a randomized, double blind parallel group design and performance on a battery of cognitive tests was assessed.

RESULTS

Apomorphine significantly impaired performance on tests of source recognition (d.f.=19, p=0.05) and item recognition memory (d.f.=19, p<0.05), and memory interference (d.f.=19, p<0.010). No significant change was found on other tests (Go/no-Go Test, Categorized Words, Stroop, Trail Making Test, and verbal fluency).

CONCLUSION

Findings in this small sample of subjects suggest that dopaminergic transmission affects episodic memory functions.

Boosting Focally-Induced Brain Plasticity by Dopamine

Dopamine (DA) simultaneously produces both excitation and inhibition in the human cortex. In order to shed light on the functional significance of these seemingly opposing effects, we administered the DA precursor levodopa (L-dopa) to healthy subjects in conjunction with 2 neuroplasticity-inducing motor cortex stimulation protocols. Transcranial direct current stimulation (tDCS) induces cortical excitability enhancement by anodal and depression by cathodal brain polarization, which is not restricted to specific subgroups of synapses. In contrast, paired associative stimulation (PAS) induces focal excitability enhancements of somatosensory and motor cortical neuronal synaptic connections. Here, we show that administering L-dopa turns the unspecific excitability enhancement caused by anodal tDCS into inhibition and prolongs the cathodal tDCS-induced excitability diminution. Conversely, it stabilizes the PAS-induced synapse-specific excitability increase. Most importantly, it prolongs all of these aftereffects by a factor of about 20. Hereby, DA focuses synapse-specific excitability-enhancing neuroplasticity in human cortical networks.

Iron treatment normalizes cognitive functioning in young women

BACKGROUND

Evidence suggests that brain iron deficiency at any time in life may disrupt metabolic processes and subsequently change cognitive and behavioral functioning. Women of reproductive age are among those most vulnerable to iron deficiency and may be at high risk for cognitive alterations due to iron deficiency.

OBJECTIVE

We aimed to examine the relation between iron status and cognitive abilities in young women.

DESIGN

A blinded, placebo-controlled, stratified intervention study was conducted in women aged 18-35 y of varied iron status who were randomly assigned to receive iron supplements or a placebo. Cognition was assessed by using 8 cognitive performance tasks (from Detterman's Cognitive Abilities Test) at baseline (n = 149) and after 16 wk of treatment (n = 113).

RESULTS

At baseline, the iron-sufficient women (n = 42) performed better on cognitive tasks (P = 0.011) and completed them faster (P = 0.038) than did the women with iron deficiency anemia (n = 34). Factors representing performance accuracy and the time needed to complete the tasks by the iron-deficient but nonanemic women (n = 73) were intermediate between the 2 extremes of iron status. After treatment, a significant improvement in serum ferritin was associated with a 5-7-fold improvement in cognitive performance, whereas a significant improvement in hemoglobin was related to improved speed in completing the cognitive tasks.

CONCLUSIONS

Iron status is a significant factor in cognitive performance in women of reproductive age. Severity of anemia primarily affects processing speed, and severity of iron deficiency affects accuracy of cognitive function over a broad range of tasks. Thus, the effects of iron deficiency on cognition are not limited to the developing brain.

Variations in the catechol O-methyltransferase polymorphism and prefrontally guided behaviors in adolescents

BACKGROUND

The catechol-O-methyltransferase (COMT) gene codes for an enzyme that degrades prefrontal cortex (PFC) synaptic dopamine. Of two identified alleles (Met and Val), the Met allele results in COMT activity that is up to 4 times less pronounced than that conferred by the Val allele, resulting in greater PFC dopamine concentrations. Met-Met homozygotes perform better than individuals who possess the Val allele on PFC-mediated cognitive tasks. These genotypic variations and their associations with executive functions have been described in adults and prepubescent children, but there is a paucity of research assessing these relations in adolescent samples.

METHODS

In this study, 70 children aged 9-17 were genotyped for COMT and completed measures of working memory, attention, fine motor coordination, and motor speed.

RESULTS

COMT genotype modulated all but the motor speed measures. The Val-Met genotype was optimal for performance in this adolescent sample.

CONCLUSIONS

Results are discussed within the context of developmental changes in the dopaminergic system during adolescence.

Effects of the dopamine agonist piribedil on prefrontal temporal cortical network function in normal aging as assessed by verbal fluency

Normal aging has been associated with impaired performance in verbal fluency suggesting a prefrontal temporal cortical network (PFTCN) deficiency. In this study, we investigated the effects of a 2-month treatment period with a dopaminergic agonist (DA) on PFTCN function. Forty healthy, elderly volunteers were assessed on semantic and phonemic verbal fluency after two months of a placebo or a DA treatment (i.e. piribedil 50 mg/day) in a double-blind crossover design. Protocols were scored considering clustering, (i.e. production of words within semantic or phonemic categories, depending on the integrity of temporal lobe), and switching (i.e. the ability to shift between clusters, depending on frontal lobe functioning). Results revealed no significant main effect of the DA treatment on either verbal fluency variables but showed a significant interaction with working memory capacities, with high-capacity span subjects improving phonemic switching on DA whereas low-capacity span subjects performed more poorly on the drug than off. These data are consistent with the literature and confirm the crucial link between working memory capacities and dopamine agonist effects. The present study also provides evidence that pharmacological remediation of age-related cognitive decline has to be taken into consideration.

Evidence for a dopaminergic link between working memory and agentic extraversion: An analysis of load-related changes in EEG alpha 1 activity

Several lines of research point to the possibility of a partially overlapping dopaminergic foundation of the trait of agentic extraversion and individual differences in working memory functioning. This study investigates interactive effects of agentic extraversion and dopamine on spectral EEG measures of working memory. Using EEG activity in the alpha 1 band (8-10.25 Hz) as a dependent variable, we tested in a randomized double-blind design the effects of the D2-dopamine antagonist sulpiride during the performance of four load-graded n-back working memory tasks in participants high versus low in agentic extraversion. We expected extraversion-related differences in the load-responsivity pattern to be reversed by sulpiride, and the alpha 1 anterior-posterior difference actually depicted this reversal effect. However, in contrast to our expectations this effect was largely due to parietal instead of frontal sites.

Mechanisms of working memory dysfunction after mild and moderate TBI: evidence from functional MRI and neurogenetics

Cognitive complaints are a frequent source of distress and disability after mild and moderate traumatic brain injury (TBI). While there are deficits in several cognitive domains, many aspects of these complaints and deficits suggest that problems in working memory (WM) play an important role. Functional imaging studies in healthy individuals have outlined the neural substrate of WM and have shown that regions important in WM circuitry overlap with regions commonly vulnerable to damage in TBI. Use of functional MRI (fMRI) in individuals with mild and moderate TBI suggests that they can have problems in the activation and allocation of WM, and several lines of evidence suggest that subtle alterations in central catecholaminergic sensitivity may underlie these problems. We review the evidence from fMRI and neurogenetic studies that support the role of catecholaminergic dysregulation in the etiology of WM complaints and deficits after mild and moderate TBI.

Affective bias and response modulation following tyrosine depletion in healthy adults

Acute phenylalanine/tyrosine depletion (ATPD) has been used to transiently lower central nervous system dopamine activity in animals and humans. Findings suggest that ATPD may impact dopamine transmission in limbic and striatal regions. Impact on cognitive functions has varied across studies, although several recent reports suggest that affective processing in the context of a go/no-go response control task may be impaired during ATPD. In this study, response control under affective vs nonaffective conditions was examined in healthy individuals who underwent either ATPD or a balanced condition in a between-subjects design. Effects of ATPD were validated through its effects on serum prolactin secretion. ATPD resulted in significantly increased prolactin levels relative to the balanced mixture. Although there were no differences in self-reported mood between the groups, individuals in the ATPD condition demonstrated diminished sensitivity to positively valenced words and seemingly enhanced sensitivity to negatively valenced words in an affective go/no-go task. They also showed difficulties in modulating ongoing behavior in a nonaffective go/no-go task when responses had to be intermittently inhibited then immediately restarted. Basic motor functions were not impacted. Findings are discussed in relation to dopamine's role in switching signals within neural networks that are important for response modulation and affective control.

Anxiety selectively disrupts visuospatial working memory

On the basis of a review of the extant literature describing emotion-cognition interactions, the authors propose 4 methodological desiderata for studying how task-irrelevant affect modulates cognition and present data from an experiment satisfying them. Consistent with accounts of the hemispheric asymmetries characterizing withdrawal-related negative affect and visuospatial working memory (WM) in prefrontal and parietal cortices, threat-induced anxiety selectively disrupted accuracy of spatial but not verbal WM performance. Furthermore, individual differences in physiological measures of anxiety statistically mediated the degree of disruption. A second experiment revealed that individuals characterized by high levels of behavioral inhibition exhibited more intense anxiety and relatively worse spatial WM performance in the absence of threat, solidifying the authors' inference that anxiety causally mediates disruption. These observations suggest a revision of extant models of how anxiety sculpts cognition and underscore the utility of the desiderata.

Bromocriptine in traumatic brain injury

The authors report the case of a 63-year-old patient with severe traumatic brain injury (TBI) associated with Parkinson's syndrome, whose performances were dramatically improved by bromocriptine therapy, with an improvement of the scores, not only on tests evaluating motor functions but also on tests evaluating the patient's cognitive functions. However, no improvement was observed with levodopa.

Agentic extraversion modulates the cardiovascular effects of the dopamine D2 agonist bromocriptine

A recent psychobiological theory postulates a dopaminergic basis for the agency facet of extraversion, leading to the prediction that this personality trait modulates the psychophysiological effects of dopaminergic drugs. A single dose of the dopamine D2 receptor agonist bromocriptine reduces blood pressure in healthy volunteers. However, it is currently unknown whether this hypotensive effect of bromocriptine is modulated by agentic extraversion. Therefore, we measured resting cardiovascular activation in groups of healthy male volunteers either high or low in agentic extraversion, either under bromocriptine (1.25 mg) or placebo. Focusing the analyses on activation components derived from 18 cardiovascular variables, we found that bromocriptine reduces alpha-adrenergic activation in the sample as a whole, whereas the effects on beta-adrenergic and cholinergic activation are modulated by agentic extraversion.

Dopaminergic control of working memory and its relevance to schizophrenia: A circuit dynamics perspective

This article argues how dopamine controls working memory and how the dysregulation of the dopaminergic system is related to schizophrenia. In the dorsolateral prefrontal cortex, which is the principal part of the working memory system, recurrent excitation is subtly balanced with intracortical inhibition. A potent controller of the dorsolateral prefrontal cortical circuit is the mesocortical dopaminergic system. To understand the characteristics of the dopaminergic control of working memory, the stability of the circuit dynamics under the influence of dopamine has been studied. Recent computational studies suggest that the hyperdopaminergic state is usually stable but the hypodopaminergic state tends to be unstable. The stability also depends on the efficacy of the glutamatergic transmission in the corticomesencephalic projections to dopamine neurons. When this cortical feedback is hypoglutamatergic, the circuit of the dorsolateral prefrontal cortex tends to be unstable, such that a slight increase in dopamine releasability causes a catastrophic jump of the dorsolateral prefrontal cortex activity from a low to a high level. This may account for the seemingly paradoxical overactivation of the dorsolateral prefrontal cortex observed in schizophrenic patients. Given that dopamine transmission is abnormal in the brains of patients with schizophrenia and working memory deficit is a core dysfunction in schizophrenia, the concept of circuit stability would be useful not only for understanding the mechanisms of working memory processing but for developing therapeutic strategies to enhance cognitive functions in schizophrenia.

A functional magnetic resonance imaging study of the effects of pergolide, a dopamine receptor agonist, on component processes of working memory

Working memory is an important cognitive process dependent on a network of prefrontal and posterior cortical regions. In this study we tested the effects of the mixed D1-D2 dopamine receptor agonist pergolide on component processes of human working memory using functional magnetic resonance imaging (fMRI). An event-related trial design allowed separation of the effects on encoding, maintenance, and retrieval processes. Subjects were tested with spatial and object memoranda to investigate modality-specific effects of dopaminergic stimulation. We also measured baseline working memory capacity as previous studies have shown that effects of dopamine agonists vary with working memory span. Pergolide improved reaction time for high-span subjects and impaired reaction time for low-span subjects. This span-dependent change in behavior was accompanied by span-dependent changes in delay-related activity in the premotor cortex. We also found evidence for modality-specific effects of pergolide only during the response period. Pergolide increased activity for spatial memoranda and decreased activity for object memoranda in task-related regions including the prefrontal and parietal cortices.

Influence of chronic treatment with olanzapine, clozapine and scopolamine on performance of a learned 8-arm radial maze task in rats

Cognitive deficit is a significant symptom in schizophrenic patients. Use of atypical antipsychotics has been demonstrated to improve some cognitive functions in schizophrenics, as well as in patients with dementia. However, side effects like sedation and muscarinic antagonism induced by these drugs have detracted from this improvement. We are interested in determining the behavioural effect of acute and chronic treatments with olanzapine and clozapine, two atypical antipsychotics, in a paradigm of working memory, and the influence on behavioural response of possible motor effects during test performance. Unspecific muscarinic antagonist scopolamine has been used for comparison. Male Wistar rats were trained on the 8-arm radial maze up to an accuracy level in choice of 80%. Distance travelled in the maze was also measured during test performance. Acute olanzapine, clozapine and scopolamine caused significant impairment of correct performance. Rats treated with olanzapine and clozapine presented a decrease in motor activity level at the same time. After the test at acute dosage, rats were chronically treated for 14 days with olanzapine, clozapine or scopolamine and 24 h after the last dose were again tested in the 8-arm radial maze. Under this procedure, chronic treatment with olanzapine, clozapine and scopolamine did not impair correct task performance and did not modify distance travelled. We concluded that the sedative effect masked a possible effect on working memory after acute administration of olanzapine and clozapine, whereas chronic treatment with olanzapine, clozapine and scopolamine did not adversely affect working memory performance. In the case of scopolamine, it suggests that chronic muscarinic antagonism does not induce memory impairment and for atypical antipsychotics, it suggests that chronic treatment induced a tolerance to acute motor effects of these drugs.

The correlative triad among aging, dopamine, and cognition: Current status and future prospects

The brain neuronal systems defined by the neurotransmitter dopamine (DA) have since long a recognized role in the regulation of motor functions. More recently, converging evidence from patient studies, animal research, pharmacological intervention, and molecular genetics indicates that DA is critically implicated also in higher-order cognitive functioning. Many cognitive functions and multiple markers of striatal and extrastriatal DA systems decline across adulthood and aging. Research examining the correlative triad among adult age, DA, and cognition has found strong support for the view that age-related DA losses are associated with age-related cognitive deficits. Future research strategies for examining the DA-cognitive aging link include assessing (a) the generality/specificity of the effects; (b) the relationship between neuromodulation and functional brain activation; and (c) the release of DA during actual task performance.

Combined D1/D2 receptor stimulation under conditions of dopamine depletion impairs spatial working memory performance in humans

RATIONALE

The mesocortical dopamine system is regarded as an important modulator of working memory. While it has been established that stimulation of the D1/D2 receptor in primates can improve spatial working memory performance, findings in humans are less consistent. Recent studies in humans suggest that global depletion of dopamine via tyrosine/phenylalanine depletion may impair spatial working memory performance, although these results are also inconsistent, and it has been suggested that task differences may partly underlie the inconsistent findings.

OBJECTIVES

This study had two aims: (1) to investigate the effects of acute tyrosine depletion (TPD) on a number of working memory tasks and (2) to examine whether stimulation of D1/D2 receptors under conditions of TPD can attenuate or "reverse" TPD-induced working memory impairments.

METHODS

Eighteen healthy male participants performed a spatial working memory delayed-recognition task, non-spatial working memory task and spatial n-back task on three separate occasions, after TPD, TPD and pergolide (D1/D2 agonist), and placebo.

RESULTS

TPD did not impair working memory performance on any of the tasks administered. However, stimulation of D1/D2 receptors under TPD conditions caused a subtle impairment in spatial working memory performance.

CONCLUSIONS

The finding that D1/D2 stimulation under TPD conditions impairs working memory highlights the complexity of functional effects of augmenting dopaminergic transmission within a dopamine-depleted state. The lack of TPD-related effects on a range of working memory tasks questions the reliability of TPD as a modulator of dopamine function and working memory performance in humans.

The development of nonverbal working memory and executive control processes in adolescents

The prefrontal cortex modulates executive control processes and structurally matures throughout adolescence. Consistent with these events, prefrontal functions that demand high levels of executive control may mature later than those that require working memory but decreased control. To test this hypothesis, adolescents (9 to 20 years old) completed nonverbal working memory tasks with varying levels of executive demands. Findings suggest that recall-guided action for single units of spatial information develops until 11 to 12 years. The ability to maintain and manipulate multiple spatial units develops until 13 to 15 years. Strategic self-organization develops until ages 16 to 17 years. Recognition memory did not appear to develop over this age range. Implications for prefrontal cortex organization by level of processing are discussed.

A functional MRI study of the effects of bromocriptine, a dopamine receptor agonist, on component processes of working memory

RATIONALE

Dopamine is abundant in the prefrontal cortex and striatum, regions implicated in working memory processes. Monkey studies suggest that subpopulations of prefrontal neurons are sensitive to component processes of working memory, and that dopaminergic actions at D1 and D2 receptors differentially affect these neurons. However, it is not known to what extent the effects of dopaminergic stimulation may differ in human subjects across the processing stages of working memory, and whether these effects are found throughout the network of task-related brain regions.

OBJECTIVE

In this study we tested the effects of the D2 dopamine agonist bromocriptine during the performance of a delayed recognition task using functional magnetic resonance imaging (fMRI).

METHODS

We measured blood oxygenation level dependent (BOLD) signals as subjects performed a spatial and object delayed recognition task. Subjects were scanned twice, once following 1.25 mg of bromocriptine and once following lactose placebo in a randomized double-blind design. Using an event-related design allowed for separate investigation of encoding, delay, and response period effects of dopaminergic stimulation.

RESULTS

A group analysis revealed that bromocriptine treatment decreased activity in the task network at encoding and increased activity at response. There was no clear pattern of change in the delay period network. Across subjects, these BOLD signal changes were accompanied by reductions in accuracy and increases in response time during delayed recognition for spatial and object information.

CONCLUSIONS

Decreased activity during encoding suggests that hyperdopaminergic stimulation may have reduced stimulus encoding processes, contributing to impaired performance.

The effects of acute tyrosine and phenylalanine depletion on spatial working memory and planning in healthy volunteers are predicted by changes in striatal dopamine levels

RATIONALE

Dopamine (DA) is considered important in the modulation of tasks of spatial working memory. However, the findings from studies in humans to date are mixed. While this may be due to the characteristics of the tasks used, it is also possible that these findings are explained by variable central effects of the manipulations used.

OBJECTIVE

To test the effects of acute tyrosine and phenylalanine depletion (TPD, which reduces synthesis and release of brain DA) on cognitive function and relate changes in performance accuracy to the central effects of TPD measured with [11C]raclopride positron emission tomography (PET).

METHODS

Fourteen participants were given tests of spatial working memory, planning, verbal memory span and trial-and-error learning after acute TPD, seven of whom also received PET scans to measure changes in striatal DA levels.

RESULTS

Although TPD produced a clear reduction in tyrosine and phenylalanine availability to the brain, no impairments on any of the cognitive tests were observed. However, changes in spatial working memory and planning accuracy after TPD showed a highly significant relationship with the changes in striatal DA levels.

CONCLUSIONS

Our findings suggest that the effects of TPD on spatial working memory and planning may be unreliable due to the variability of the changes in brain DA levels achieved with this manipulation.

Catechol O-Methyltransferase Val158Met Polymorphism is Associated with Cognitive Performance in Nondemented Adults

The catechol O-methyltransferase (COMT) gene is essential in the metabolic degradation of dopamine in the prefrontal cortex. In the present study, we examined the effect of a Val158Met polymorphism in the COMT gene on individual differences and changes in cognition (executive functions and visuospatial ability) in adulthood and old age. The participants were 292 nondemented men (initially aged 35-85 years) from a random sample of the population (i.e., the Betula study) tested at two occasions with a 5-year interval. Confirmatory factor analyses were used to test the underlying structure of three indicators of executive functions (verbal fluency, working memory, and Tower of Hanoi). Associations between COMT, age, executive functioning, and visuospatial (block design) tasks were examined using repeated-measures analyses of variance. Carriers of the Val allele (with higher enzyme activity) compared with carriers of the Met/Met genotype (with low enzyme activity) performed worse on executive functioning and visuo-spatial tasks. Individuals with the Val/Val genotype declined in executive functioning over the 5-year period, whereas carriers of the Met allele remained stable in performance. An Age COMT interaction for visuospatial ability located the effect for middle-aged men only. This COMT polymorphism is a plausible candidate gene for executive functioning and fluid intelligence in nondemented middle-aged and older adults.

The long-term effects of mild head injury on short-term memory for visual form, spatial location, and their conjunction in well-functioning university students

Research has suggested the presence of subtle long-term cognitive changes in otherwise well-functioning individuals who have previously sustained a mild head injury (MHI). This paper investigated the long-term effects of MHI on visual, spatial, and visual-spatial short-term memory in well-functioning university students. Sixteen students who reported having sustained a MHI were compared to 16 controls on tests of short-term memory (STM) for abstract polygons in haphazardly arranged locations. The three tests differed only in the requirements for recall (shapes for the visual task, locations for the spatial task, and the shapes in their respective locations for the visual-spatial task). MHI participants were selectively impaired on spatial memory, suggesting that tasks of spatial STM may be more sensitive, compared to tasks of visual STM, to the subtle long-term cognitive changes that may be present after a MHI.

The role of the striatal dopamine transporter in cognitive aging

We examined the relationship of age-related losses of striatal dopamine transporter (DAT) density to age-related deficits in episodic memory and executive functioning in a group of subjects (n = 12) ranging from 34 to 81 years of age. The radioligand [(11)C]beta-CIT-FE was used to determine DAT binding in caudate and putamen. Results showed clear age-related losses of striatal DAT binding from early to late adulthood, and a marked deterioration in episodic memory (word and figure recall, face recognition) and executive functioning (visual working memory, verbal fluency) with advancing age. Most importantly, the age-related cognitive deficits were mediated by reductions in DAT binding, whereas DAT binding added systematic cognitive variance after controlling for age. Further, interindividual differences in DAT binding were related to performance in a test of crystallized intelligence (the Information subtest from the Wechsler Adult Intelligence Scale-Revised) that showed no reliable age variation. These results suggest that DAT binding is a powerful mediator of age-related cognitive changes as well as of cognitive functioning in general. The findings were discussed relative to the view that the frontostriatal network is critically involved in multiple cognitive functions.

Sulpiride and mnemonic function: effects of a dopamine D2 receptor antagonist on working memory, emotional memory and long-term memory in healthy volunteers

There is now substantial evidence from animal studies showing modulation of cognitive performance after administration of dopaminergic agents. Previous studies have focused on cognitive functions such as working memory (WM), with particular reference to spatial processing. However, to date, studies in normal human volunteers have proved inconsistent. We have therefore tested the effects of the dopamine D2 receptor antagonist sulpiride (400 mg) on WM and learning tasks, including those using auditory, spatial or non-spatial stimuli. A further aim was to explore a broader role of the dopaminergic system in mnemonic function by examining long-term and emotional memory. Eighteen healthy male participants were given a battery of cognitive tests after oral sulpiride or placebo, using the cross-over design. WM was assessed using a spatial searching task, and a task of auditory counting with distraction. Tasks that did not emphasize WM were spatial and non-spatial trial-and-error learning, long-term spatial memory and emotional memory. After dopamine D2 receptor blockade, performance was not impaired on the spatial WM (SWM) task, but was impaired on the auditory counting task with distraction. Sulpiride did not impair, but rather appeared to enhance trial-and-error learning overall. Thus, we were unable to support the notion that dopaminergic modulation preferentially influences spatial over non-spatial processing during learning. In addition, recognition was impaired in the emotional memory task after encoding on drug compared to placebo. These findings question the precise role of dopamine D2 receptor modulation on WM, and highlight the need for sensitive tests to study dopaminergic modulation of emotional processing.

Cognitive and motor effects of dopaminergic medication withdrawal in Parkinson's disease

AIMS

Recent evidence points towards dissociable effects of dopaminergic medication on motor function and cognitive function mediated by different fronto-striatal neural circuits. This study aimed to clarify the role of dopaminergic medication in spatial working memory, and reinforcement-based associative learning in relation to clinical changes in motor function in early Parkinson's disease (PD).

METHOD

We tested 14 patients with mild to moderate PD on and off dopaminergic medication, on a spatial delayed-response working memory task, and on spatial and non-spatial (visual) trial-and-error learning tasks based on reinforcement, carefully matched for motor requirements. In addition, we explored relationships between the effects of withdrawal on motor symptom expression and performance on the cognitive tasks.

RESULTS

Withdrawal from dopaminergic medication significantly exacerbated motor symptoms. This was related to spatial learning, but not visual learning, or delayed response accuracy. Moreover, medication withdrawal led to dissociable effects of response latency on the spatial learning and spatial delayed response tasks, with patients becoming faster after spatial learning, but relatively slower on the delayed response task. These changes in response latency were unrelated to motor symptom impairment.

CONCLUSION

Our findings suggest dissociable effects of dopamine medication withdrawal on cognitive processes putatively mediated by dorsal and ventral striatal regions.

Cognitive skill learning in healthy older adults after 2 months of double-blind treatment with piribedil

RATIONALE

Cognitive skill learning, as assessed by repeated testing on the Tower of Toronto (TT) task, has been found to be impaired in normal aging. There is evidence that this impairment might be accounted for by the well-documented, age-dependent decline of dopamine D2-like receptor availability.

OBJECTIVES

This study was an investigation of whether the D2/D3 dopaminergic agonist piribedil improves cognitive skill learning in older adults.

METHODS

Healthy elderly volunteers were assessed using the TT paradigm. Subjects were evaluated at baseline, and after 2 months of a placebo or piribedil (50 mg daily) treatment in a double-blind, crossover design. Two components of cognitive skill learning were considered--the ability to learn to solve the puzzle and the acquisition of a problem-solving routine.

RESULTS

Piribedil showed a beneficial effect on the acquisition of a problem-solving routine, depending on the age of subjects, as revealed by solution reliability indexes measures. The effect of piribedil on the ability to learn to solve the puzzle was found to be dependent on the subject's working memory capacities.

CONCLUSIONS

The present results suggest that piribedil is likely to enhance cognitive skill learning in healthy older adults and give further evidence that age-related dopamine decline plays an important role in cognitive impairment occurring in normal aging.

Impaired set-shifting and dissociable effects on tests of spatial working memory following the dopamine D2 receptor antagonist sulpiride in human volunteers

RATIONALE

Dopamine (DA) D(2) receptor antagonists have been shown to produce similar impairments to those seen in Parkinson's disease. These include working memory and set-shifting deficits. Theories of DA function have predicted that distraction or impaired switching may be important determinants of such deficits.

OBJECTIVES

In order to test these hypotheses, we have followed up our previous findings with more refined tests (1) that allow measurement of spatial working memory (SWM) and distraction, (2) that allow separation of executive and mnemonic components of SWM and (3) that allow isolation of set-shifting from learning deficits.

METHODS

Thirty-six young healthy male volunteers were tested on two occasions after oral administration of either 400 mg sulpiride or placebo. All participants performed the delayed response task. Sixteen participants received task-irrelevant distractors during this task, and were also given a self-ordered SWM test. The remaining participants were given delayed response tasks with task-relevant distractors, and tests of attentional and task set-shifting.

RESULTS

Sulpiride impaired performance of the delayed-response task both without distraction and with task-relevant distraction. By contrast, the drug protected against deficits from task-irrelevant distraction seen in the placebo group. Task set-switching was also impaired by sulpiride, with participants being slower to respond on switch trials compared with non-switch trials. There was also a trend for attentional set-shifting to be impaired following sulpiride. In contrast, self-ordered SWM performance was enhanced by sulpiride on the second test session only.

CONCLUSIONS

These results support models of central DA function that postulate a role in switching behaviour, and in certain aspects of working memory.

Working memory deficits after traumatic brain injury: catecholaminergic mechanisms and prospects for treatment -- a review

PRIMARY OBJECTIVE

To review the neural circuitry and neurochemistry of working memory and outline the evidence for working memory deficits after traumatic brain injury, and the evidence for the use of catecholaminergic agents in the amelioration of these deficits. Current knowledge gaps and research needs are identified.

MAIN OUTCOMES AND RESULTS

Impairments in working memory are a core component of the cognitive deficits associated with traumatic brain injury. Recent progress in understanding the neural circuitry and neurochemistry of working memory suggests that catecholamines play a central role in the activation and regulation of working memory and thus lays a framework in which to consider the use of catecholaminergic agents (dopaminergic and alpha-2 adrenergic agonists) in the treatment of specific cognitive deficits after traumatic brain injury.

CONCLUSIONS

The combined methods of cognitive neuroscience, functional brain imaging and neuropharmacology are proposed as an excellent method for studying working memory deficits. A strong rationale exists for the targeted use of catecholaminergic agonists in the treatment of working memory deficits after traumatic brain injury.

Simultaneous depletion of serotonin and catecholamines impairs sustained attention in healthy female subjects without affecting learning and memory

Monoamine neurotransmitters, serotonin, noradrenaline and dopamine modulate many important cognitive processes such as attention, learning and memory. While the selective effects of serotonin and catecholamine depletion on such processes have been investigated, the effects of simultaneous depletion of these monoamines on cognition remain unclear. This is of particular interest given that multiple neurotransmitter abnormalities have been implicated in many psychiatric disorders. The aim of the current study was to examine the effects of lowered brain monoamine function on cognitive performance, using the technique of amino acid precursor depletion. The study was a double-blind, placebo-controlled design in which 20 healthy female subjects were tested under a combined monoamine depletion condition (CMD) and a balanced control condition (B). Cognitive testing was conducted at baseline and 5 h post-depletion. The CMD condition relative to the B condition resulted in deficits in digit vigilance (accuracy and reaction time), a measure of sustained attention. There were no effects on measures of learning and memory or psychomotor function. These findings suggest that simultaneously depleting the availability of brain serotonin and catecholamines in healthy female subjects selectively impairs sustained attention, without affecting other cognitive domains.

The effects of tyrosine depletion in normal healthy volunteers: implications for unipolar depression

RATIONALE

In recent years, there has been a growing interest in the role of dopamine (DA) both in the pathogenesis of unipolar depression and in motivated behaviour. The innovative technique of acute tyrosine depletion presents an opportunity to characterise further its function in these domains.

OBJECTIVE

The present study examined the physiological, subjective and cognitive effects of acute tyrosine depletion in healthy volunteers.

METHODS

A double-blind, placebo-controlled, parallel group design was employed. Half of the participants ingested a balanced amino-acid mixture (BAL) and the other half received an identical mixture except that tyrosine and phenylalanine were absent (TYR-free). Plasma amino acid concentrations and subjective ratings were monitored at both baseline (T(0)) and 5 h following consumption (T(5)) of the mixtures. A comprehensive neuropsychological test battery was also administered at T(5).

RESULTS

Relative to the BAL group, the reduction in TYR availability to the brain was more marked in the TYR-free group. Employment of psychological rating scales revealed that, compared with the BAL group, the TYR-free group became less content and more apathetic. For the affective go/no-go task, whilst the BAL group exhibited a happy latency bias, the TYR-free group demonstrated a sad latency bias. Furthermore, in the decision-making task, the rate at which the TYR-free group increased their bets in response to more likely outcomes was lower than that of the BAL group. Taken together, these neuropsychological findings strikingly paralleled those reported in previous investigations of unipolar depression. The experimental groups could not be differentiated on any of the other neuropsychological measures, including more classical assessments of fronto-executive function.

CONCLUSION

These findings are consistent with the hypothesis that dopaminergic factors are particularly involved in disrupted affect/reward-based processing characteristic of clinical depression.

Motor function under lower and higher controlled processing demands in early and continuously treated phenylketonuria

This study examined motor control in 61 early and continuously treated patients with phenylketonuria (PKU) and 69 control participants, aged 7 to 14 years. The pursuit task demanded concurrent planning and execution of unpredictable movements, whereas the tracking task required a highly automated circular movement that could be planned in advance. PKU patients showed significantly poorer motor control in both tasks compared with control participants. Deficits were particularly observed for younger patients (age < 11 years). Differences between control participants and PKU patients were significantly greater in the pursuit task compared with the tracking task, indicating more serious deficits when a higher level of controlled processing is required. Correlations with historical phenylalanine levels indicated a later maturation of the level of control required by the pursuit task compared with the tracking task.