Levodopa: Faster and better word learning in normal humans

The role of the basal ganglia in learning and memory: insight from Parkinson's disease

It has long been known that memory is not a single process. Rather, there are different kinds of memory that are supported by distinct neural systems. This idea stemmed from early findings of dissociable patterns of memory impairments in patients with selective damage to different brain regions. These studies highlighted the role of the basal ganglia in non-declarative memory, such as procedural or habit learning, contrasting it with the known role of the medial temporal lobes in declarative memory. In recent years, major advances across multiple areas of neuroscience have revealed an important role for the basal ganglia in motivation and decision making. These findings have led to new discoveries about the role of the basal ganglia in learning and highlighted the essential role of dopamine in specific forms of learning. Here we review these recent advances with an emphasis on novel discoveries from studies of learning in patients with Parkinson's disease. We discuss how these findings promote the development of current theories away from accounts that emphasize the verbalizability of the contents of memory and towards a focus on the specific computations carried out by distinct brain regions. Finally, we discuss new challenges that arise in the face of accumulating evidence for dynamic and interconnected memory systems that jointly contribute to learning.

A neoHebbian framework for episodic memory; role of dopamine-dependent late LTP

According to the Hebb rule, the change in the strength of a synapse depends only on the local interaction of presynaptic and postsynaptic events. Studies at many types of synapses indicate that the early phase of long-term potentiation (LTP) has Hebbian properties. However, it is now clear that the Hebb rule does not account for late LTP; this requires an additional signal that is non-local. For novel information and motivational events such as rewards this signal at hippocampal CA1 synapses is mediated by the neuromodulator, dopamine. In this Review we discuss recent experimental findings that support the view that this 'neoHebbian' framework can account for memory behavior in a variety of learning situations.

Dose-dependent nonlinear effect of L-DOPA on paired associative stimulation-induced neuroplasticity in humans

Dopamine is one of the major neuromodulators in the CNS, which is involved in learning and memory processes. A nonlinear, inverted U-shaped dose-response curve of its effects on cognition has been observed in animal studies. The basis for this nonlinear effect might be a similar effect of dopamine on neuroplasticity. Whereas it has been shown that dopamine affects paired associative stimulation (PAS)-induced plasticity, which might reflect learning-related processes to a larger degree than other noninvasive plasticity induction protocols in the human motor cortex in principle, its dose-dependency has not been explored previously. We studied the effect of different dosages of the dopamine precursor L-DOPA on motor cortex plasticity induced by facilitatory and inhibitory PAS of the motor cortex in 12 healthy humans. They received 25, 100, or 200 mg of L-DOPA or placebo medication combined with either excitability-enhancing or -diminishing PAS. Cortical excitability level was monitored before and for up to 2 d after plasticity induction by assessment of transcranial magnetic stimulation-induced motor-evoked potentials. Low-dose L-DOPA abolished the aftereffects of PAS and medium-dose L-DOPA prolonged facilitatory plasticity. High-dose L-DOPA reversed the excitability enhancement accomplished by facilitatory PAS to diminution. Thus, the results show a clear nonlinear effect of L-DOPA dosage on associative plasticity, different from that on nonfocal plasticity. This might help to explain dopaminergic effect on cognition and could be relevant for understanding the pathophysiology and treatment of neuropsychiatric diseases accompanied by alterations of the dopaminergic system.

Ventromedial prefrontal cortex activation is associated with memory formation for predictable rewards

During reinforcement learning, dopamine release shifts from the moment of reward consumption to the time point when the reward can be predicted. Previous studies provide consistent evidence that reward-predicting cues enhance long-term memory (LTM) formation of these items via dopaminergic projections to the ventral striatum. However, it is less clear whether memory for items that do not precede a reward but are directly associated with reward consumption is also facilitated. Here, we investigated this question in an fMRI paradigm in which LTM for reward-predicting and neutral cues was compared to LTM for items presented during consumption of reliably predictable as compared to less predictable rewards. We observed activation of the ventral striatum and enhanced memory formation during reward anticipation. During processing of less predictable as compared to reliably predictable rewards, the ventral striatum was activated as well, but items associated with less predictable outcomes were remembered worse than items associated with reliably predictable outcomes. Processing of reliably predictable rewards activated the ventromedial prefrontal cortex (vmPFC), and vmPFC BOLD responses were associated with successful memory formation of these items. Taken together, these findings show that consumption of reliably predictable rewards facilitates LTM formation and is associated with activation of the vmPFC.

Cognition, dopamine and bioactive lipids in schizophrenia

Schizophrenia is a remarkably complex disorder with a multitude of behavioral and biological perturbations. Cognitive deficits are a core feature of this disorder, and involve abnormalities across multiple domains, including memory, attention, and perception. The complexity of this debilitating illness has led to a view that the key to unraveling its pathophysiology lies in deconstructing the clinically-defined syndrome into pathophysiologically distinct intermediate phenotypes. Accumulating evidence suggests that one of these intermediate phenotypes may involve phospholipid signaling abnormalities, particularly in relation to arachidonic acid (AA). Our data show relationships between levels of AA and performance on tests of cognition for schizophrenia patients, with defects in AA signaling associated with deficits in cognition. Moreover, dopamine may moderate these relationships between AA and cognition. Taken together, cognitive deficits, dopaminergic neurotransmission, and bioactive lipids have emerged as related features of schizophrenia. Existing treatment options for cognitive deficits in schizophrenia do not specifically target lipid-derived signaling pathways; understanding these processes could inform efforts to identify novel targets for treatment innovation.

Cognitive enhancement by drugs in health and disease

Attempts to improve cognitive function in patients with brain disorders have become the focus of intensive research efforts. A recent emerging trend is the use of so-called cognitive enhancers by healthy individuals. Here, we consider some of the effects - positive and negative - that current drugs have in neurological conditions and healthy people. We conclude that, to date, experimental and clinical studies have demonstrated relatively modest overall effects, most probably because of substantial variability in response both across and within individuals. We discuss biological factors that might account for such variability and highlight the need to improve testing methods and to extend our understanding of how drugs modulate specific cognitive processes at the systems or network level.

Amphetamine-related improvement in executive function in patients with chronic schizophrenia is modulated by practice effects

BACKGROUND

Amelioration of cognitive impairment is an important treatment goal for a broad range of neuropsychiatric disorders, including schizophrenia. One critical issue in clinical trial design is the extent to which repeated exposure to cognitive tests (i.e., practice effects) may lead to improvement in performance on the cognitive tests in the absence of any true treatment effect. The current study examined the extent to which practice effects on a measure of executive function may influence the sensitivity of that task to detecting the cognitive-enhancing effects of a single acute dose of d-amphetamine in individuals with chronic schizophrenia.

METHODS

Twenty-four men with chronic schizophrenia were randomized to receive a constant or random/matched alternate form version of a hidden maze learning measure of executive function (Groton Maze Learning Test; GMLT) on four separate occasions in one month. They also completed a measure of psychomotor speed. Using a double-blind, placebo-controlled, parallel groups design, cognitive function following administration of a single dose of d-amphetamine (20mg p.o) or placebo was then assessed.

RESULTS

The group who received the constant-pathway version of the GMLT showed a large practice effect (d = 2.05) over four practice sessions. Consequently, they did not evidence any improvement on the GMLT following d-amphetamine administration. In contrast, the group who received the random/matched alternate version of the GMLT showed a statistically significant and large effect size (d = .84) improvement on this measure. Both groups showed d-amphetamine-related improvement on a measure of psychomotor speed.

CONCLUSIONS

Results of this study suggest that practice effects associated with repeated exposure to a cognitive test could obscure the sensitivity of the test to detecting true treatment-related cognitive improvement.

The influence of tobacco consumption on the relationship between schizotypy and hemispheric asymmetry

Tobacco use is positively associated with severity of symptoms along the schizophrenia spectrum. Accordingly it could be argued that neuropsychological performance, formerly thought to be modulated by schizotypy, is actually modulated by drug use or an interaction of drug use and schizotypy. We tested whether habitual cigarette smokers as compared to non-smokers would show a neuropsychological profile similar to that observed along the schizophrenia spectrum and, if so, whether smoking status or nicotine dependence would be more significant modulators of behavior than schizotypy. Because hemispheric dominance has been found to be attenuated along the schizophrenia spectrum, 40 right-handed male students (20 non-smokers) performed lateralized left- (lexical decisions) and right- (facial decision task) hemisphere dominant tasks. All individuals completed self-report measures of schizotypy and nicotine dependence. Schizotypy predicted laterality in addition to smoking status: While positive schizotypy (Unusual Experiences) was unrelated to hemispheric performance, Cognitive Disorganization predicted reduced left hemisphere dominant language functions. These latter findings suggest that Cognitive Disorganization should be regarded separately as a potentially important mediator of thought disorganization and language processing. Additionally, increasing nicotine dependence among smokers predicted a right hemisphere shift of function in both tasks that supports the role of the right hemisphere in compulsive/impulsive behavior.

Crossover trial of subacute computerized aphasia therapy for anomia with the addition of either levodopa or placebo

BACKGROUND

The effect of levodopa on recovery from aphasia is controversial.

OBJECTIVE

To determine whether levodopa enhances the effect of intensive computer-assisted therapy (CAT) of anomia in the postacute stage of aphasia.

METHODS

Double-blind multiple case study with intrasubject crossover design comparing the effect of levodopa (100 mg) versus placebo, each given for 2 weeks.

SUBJECTS

Twelve patients with onset of aphasia from 2 to 9 weeks after stroke or traumatic brain injury were compared on naming performance on items trained and not trained with CAT. Subjects were randomized to either levodopa or placebo first, separated by a 1-week washout, and then switched to the other drug intervention for the second 2-week CAT intervention. The subjects also received routine aphasia therapies during these periods.

RESULTS

All patients improved their naming performance for items trained by CAT in both periods (P = .001). No significant difference was found between the placebo and levodopa phases.

CONCLUSION

Administration of levodopa for 2 weeks during the postacute stage of aphasia did not augment the positive effects of subacute intensive language treatment with CAT for a spoken naming task.

Dopamine, Paranormal Belief, and the Detection of Meaningful Stimuli

Dopamine (DA) is suggested to improve perceptual and cognitive decisions by increasing the signal-to-noise ratio. Somewhat paradoxically, a hyperdopaminergia (arguably more accentuated in the right hemisphere) has also been implied in the genesis of unusual experiences such as hallucinations and paranormal thought. To test these opposing assumptions, we used two lateralized decision tasks, one with lexical (tapping left-hemisphere functions), the other with facial stimuli (tapping right-hemisphere functions). Participants were 40 healthy right-handed men, of whom 20 reported unusual, “paranormal” experiences and beliefs (“believers”), whereas the remaining participants were unexperienced and critical (“skeptics”). In a between-subject design, levodopa (200 mg) or placebo administration was balanced between belief groups (double-blind procedure). For each task and visual field, we calculated sensitivity (d′) and response tendency (criterion) derived from signal detection theory. Results showed the typical right visual field advantage for the lexical decision task and a higher d′ for verbal than facial stimuli. For the skeptics, d′ was lower in the levodopa than in the placebo group. Criterion analyses revealed that believers favored false alarms over misses, whereas skeptics displayed the opposite preference. Unexpectedly, under levodopa, these decision preferences were lower in both groups. We thus infer that levodopa (1) decreases sensitivity in perceptual–cognitive decisions, but only in skeptics, and (2) makes skeptics less and believers slightly more conservative. These results stand at odd to the common view that DA generally improves signal-to-noise ratios. Paranormal ideation seems an important personality dimension and should be assessed in investigations on the detection of signals in noise.

Dosage-dependent non-linear effect of L-dopa on human motor cortex plasticity

The neuromodulator dopamine affects learning and memory formation and their likely physiological correlates, long-term depression and potentiation, in animals and humans. It is known from animal experiments that dopamine exerts a dosage-dependent, inverted U-shaped effect on these functions. However, this has not been explored in humans so far. In order to reveal a non-linear dose-dependent effect of dopamine on cortical plasticity in humans, we explored the impact of 25, 100 and 200 mg of L-dopa on transcranial direct current (tDCS)-induced plasticity in twelve healthy human subjects. The primary motor cortex served as a model system, and plasticity was monitored by motor evoked potential amplitudes elicited by transcranial magnetic stimulation. As compared to placebo medication, low and high dosages of L-dopa abolished facilitatory as well as inhibitory plasticity, whereas the medium dosage prolonged inhibitory plasticity, and turned facilitatory plasticity into inhibition. Thus the results show clear non-linear, dosage-dependent effects of dopamine on both facilitatory and inhibitory plasticity, and support the assumption of the importance of a specific dosage of dopamine optimally suited to improve plasticity. This might be important for the therapeutic application of dopaminergic agents, especially for rehabilitative purposes, and explain some opposing results in former studies.

Non-native phonemes in adult word learning: evidence from the N400m

Newborns are equipped with a large phonemic inventory that becomes tuned to one's native language early in life. We review and add new data about how learning of a non-native phoneme can be accomplished in adults and how the efficiency of word learning can be assessed by neurophysiological measures. For this purpose, we studied the acquisition of the voiceless, bilabial fricative /Phi/ via a statistical-learning paradigm. Phonemes were embedded in minimal pairs of pseudowords, differing only with respect to the fricative (/aPhio/ versus /afo/). During learning, pseudowords were combined with pictures of objects with some combinations of pseudowords and pictures occurring more frequently than others. Behavioural data and the N400m component, as an index of lexical activation/semantic access, showed that participants had learned to associate the pseudowords with the pictures. However, they could not discriminate within the minimal pairs. Importantly, before learning, the novel words with the sound /Phi/ showed smaller N400 amplitudes than those with native phonemes, evidencing their non-word status. Learning abolished this difference indicating that /Phi/ had become integrated into the native category /f/, instead of establishing a novel category. Our data and review demonstrate that native phonemic categories are powerful attractors hampering the mastery of non-native contrasts.

Physical activity and memory functions: an interventional study

Previous studies have suggested beneficial effects of physical activity on cognition. Here, we asked in an interventional approach if physical activity performed at different intensity levels would differentially affect episodic memory function. Additionally, we tried to identify mechanisms mediating these changes. Sixty-two healthy elderly individuals were assessed for level of physical activity, aerobic fitness, episodic memory score, neurotrophin and catecholamine levels, and received a magnetic resonance image of the brain at baseline and after a six months intervention of medium or low-intensity physical activity or control. Increase in total physical activity was positively associated with increase in memory score over the entire cohort, without significant differences between intensity groups. It was also positively associated with increases in local gray matter volume in prefrontal and cingulate cortex, and BDNF levels (trend). In conclusion, we showed that physical activity conveys the beneficial effects on memory function independently of its intensity, possibly mediated by local gray matter volume and neurotrophic factors. Our findings may carry significant implications for prevention of cognitive decline in the elderly.

Pharmacotherapy in stroke rehabilitation

BACKGROUND

Pharmacotherapy is commonly given to patients recovering from a stroke to prevent further complications (e.g. recurrent stroke, seizures) or enhance recovery. However, some drugs may have a negative impact on neuroplasticity.

OBJECTIVES

This review examines currently used drugs that are believed to promote recovery from motor and cognitive disturbances associated with stroke.

METHODS

Literature regarding the properties, efficacy, safety, and dosing of drugs used to promote recovery after stroke was reviewed.

RESULTS

The data on pharmacotherapy are insufficient to support a claim of significantly improved rehabilitation outcomes. Moreover, a growing body of evidence indicates that some agents can impair functional reorganization and slow the recovery process. However, a few chemicals are reported to be beneficial for stroke rehabilitation. The most promising are noradrenergic and dopaminergic agents, as well as several growth factors; these should be the future focus of extensive randomized clinical trials.

CONCLUSIONS

Currently there is no drug with proven efficacy in enhancing poststroke recovery.

Memory encoding and dopamine in the aging brain: a psychopharmacological neuroimaging study

Normal aging brings with it changes in dopaminergic and memory functions. However, little is known about how these 2 changes are related. In this study, we identify a link between dopamine, episodic memory networks, and aging, using pharmacological functional magnetic resonance imaging. Young and older adults received a D2-like agonist (Bromocriptine, 1.25 mg), a D2-like antagonist (Sulpiride, 400 mg), and Placebo, in a double-blind crossover procedure. We observed group differences, during memory encoding, in medial temporal, frontal, and striatal regions and moreover, these regions were differentially sensitive across groups to dopaminergic perturbation. These findings suggest that brain systems underlying memory show age-related changes and that dopaminergic function may be key in understanding these changes. That these changes have behavioral consequences was suggested by the observation that drug modulations were most pronounced in older subjects with poorer recognition memory. Our findings provide direct evidence linking ageing, memory, and dopaminergic change.

Randomized, placebo-controlled, double-blind study of ropinirole in chronic stroke

BACKGROUND AND PURPOSE

Evidence suggests the potential to improve motor status in patients with stroke by modifying brain catecholaminergic tone. The current study hypothesized that increased dopaminergic tone via the dopamine agonist ropinirole, when combined with physiotherapy (PT), would significantly and safely increase gait velocity.

METHODS

Patients with moderate motor deficits due to stroke 1 to 12 months prior were randomized (double blinded) to 9 weeks of immediate-release ropinirole or placebo, each with PT, and followed up for 3 additional weeks. Drug dose (0.25 to 4 mg once daily) was titrated weekly, as tolerated. The primary end point was gait velocity during the 12 weeks of study participation.

RESULTS

Patients in the ropinirole+PT group averaged 2.4 mg/d by end of week 9, although the target dose was at least 3 mg/d. Ropinirole+PT was generally safe and well tolerated, including no drug-related serious adverse events. Across all 33 enrollees, significant gains were found over time for gait velocity and for most secondary end points. However, gains did not differ by treatment assignment. PT and occupational therapy were commonly prescribed outside of the trial, although the extent of these was not correlated with study outcomes.

CONCLUSIONS

At doses achieved in this trial, increased dopaminergic tone via ropinirole+PT was generally well tolerated but did not show any improvement over and above the effects of PT alone.

Recovery of function in humans: cortical stimulation and pharmacological treatments after stroke

In this contribution, we first provide an overview of general principles of reorganisation in the human brain, and point out possible biomarkers of recovery. Subsequently, we expand on possibilities of adjuvant therapy in human rehabilitation using cortical stimulation and pharmacological treatments. Finally, we suggest future directions for research in this field.

Dose-dependent inverted U-shaped effect of dopamine (D2-like) receptor activation on focal and nonfocal plasticity in humans

The neuromodulator dopamine (DA) has multiple modes of action on neuroplasticity induction and modulation, depending on subreceptor specificity, concentration level, and the kind of stimulation-induced plasticity. To determine the dosage-dependent effects of D(2)-like receptor activation on nonfocal and focal neuroplasticity in the human motor cortex, different doses of ropinirole (0.125, 0.25, 0.5, and 1.0 mg), a D(2)/D(3) dopamine agonist, or placebo medication were combined with anodal and cathodal transcranial direct current stimulation (tDCS) protocols, which induce nonfocal plasticity, or paired associative stimulation (PAS, ISI of 10 or 25 ms), which generates focal plasticity, in healthy volunteers. D(2)-like receptor activation produced an inverted "U"-shaped dose-response curve on plasticity for facilitatory tDCS and PAS and for inhibitory tDCS. Here, high or low dosages of ropinirole impaired plasticity. However, no dose-dependent response effect of D(2)-like receptor activation was evident for focal inhibitory plasticity. In general, our study supports the assumption that modulation of D(2)-like receptor activity exerts dose-dependent inhibitory or facilitatory effects on neuroplasticity in the human motor cortex depending on the topographic specificity of plasticity.

D1-receptor impact on neuroplasticity in humans

Dopamine improves learning and memory formation. The neurophysiological basis for these effects might be a focusing effect of dopamine on neuroplasticity: Accordingly, in humans L-dopa prolongs focal facilitatory plasticity, but turns nonfocal facilitatory plasticity into inhibition. Here we explore the impact of D(1) receptors on plasticity. Nonfocal plasticity was induced by transcranial direct current stimulation (tDCS), and focal plasticity by paired associative stimulation (PAS). Subjects received sulpiride, a D(2) antagonist, to increase the relative contribution of D(1) receptors to dopaminergic activity, combined sulpiride and L-dopa, to increase the relation of D(1)/D(2) activity further, or placebo medication. Under placebo, anodal tDCS and excitatory PAS (ePAS) increased motor cortex excitability. Cathodal tDCS and inhibitory PAS (iPAS) reduced it. Sulpiride abolished iPAS-induced inhibition, but not ePAS-generated facilitation, underlining the importance of D(1)-receptor activity for focal facilitatory neuroplasticity. Combining sulpiride with L-dopa reestablished iPAS-induced inhibition, but did not affect ePAS-induced plasticity. tDCS-induced plasticity, which was abolished by sulpiride in a former study, also recovered. Thus enhancing D(1) activity further relative to D(2) activity is relevant for facilitatory and inhibitory plasticity. However, comparison with former results show that an appropriate balance of D(1) and D(2) activity seems necessary to (1) consolidate the respective excitability modifications and (2) to elicit a focusing effect.

New approach to the rehabilitation of post-stroke focal cognitive syndrome: effect of levodopa combined with speech and language therapy on functional recovery from aphasia

OBJECTIVE

Few studies confirm that pharmacological treatments support post-stroke recovery. The purpose of this study was to determine whether the combination of levodopa with language therapy improves aphasia rehabilitation.

METHODS

We did a prospective, randomized, placebo-controlled, double-blind study in which twenty patients received levodopa before each language therapy session, and an additional 19 received a placebo. Language training was provided during a 3-week period. The efficacy variables were changes from baseline in Boston Diagnostic Aphasia Examination (BDAE) scores.

RESULTS

Patients receiving levodopa experienced greater language improvement in verbal fluency and repetition, compared to patients receiving placebo. Improvement was particularly distinct in patients with anterior lesions.

CONCLUSIONS

Supplementing language therapy with levodopa may improve recovery from aphasia in patients with frontal lesions.

Better than normal: improved formation of long-term spatial memory in healthy rats treated with levodopa

Dopaminergic signaling modulates learning and memory. Consequently, treatment with the dopamine precursor levodopa ameliorates memory deficits in murine models of Alzheimer's disease. In healthy humans, administration of L-DOPA increases learning and memory. However, it is unknown whether dopamine-enhanced memory can also be modeled in normal animals. We here investigated if in healthy non-food-deprived rats low and high doses of levodopa (20 and 50 mg levodopa/kg bodyweight) increase spatial learning and long-term memory performance in a radial arm maze. After 4 months, rats treated with levodopa during training had significantly better memory of food rewarded arms than vehicle-treated animals. Interestingly, acute learning curves did not differ between levodopa and vehicle animals. This suggests that enhanced dopaminergic signaling may have predominantly acted on the cortical long-term consolidation of newly acquired spatial information.

Levodopa improves procedural motor learning in chronic stroke patients

OBJECTIVE

To test the hypothesis that administration of dopamine precursor levodopa improves procedural motor learning (defined as the ability to acquire novel movement patterns gradually through practice) in patients with residual motor deficits in the chronic phase after stroke (> or =1 y after stroke).

DESIGN

A double-blind, placebo-controlled, randomized crossover design.

SETTING

Neurology department in a German university.

PARTICIPANTS

Eighteen patients with chronic motor dysfunction because of stroke (13 men, 5 women; age range, 53-78 y; mean time poststroke +/- SD, 3.3+/-2.1 y).

INTERVENTION

Patients received 3 doses of levodopa (100mg of levodopa plus 25mg of carbidopa) or placebo before 1 session of procedural motor learning.

MAIN OUTCOME MEASURES

Procedural motor learning performed by using the paretic hand assessed by using a modified version of the serial reaction time task with a probabilistic sequence. The primary outcome measure was the difference in reaction times between random and sequential elements.

RESULTS

Levodopa significantly improved our primary outcome measure, procedural motor learning, compared with placebo (P<.05). Reaction times to random elements, analysis of error rates, psychophysical assessments, and performance in a simple motor task were comparable between conditions, indicating that better learning under levodopa was not caused by differences in response styles, arousal, mood, or motor reaction times but that levodopa modulated learning.

CONCLUSIONS

Our results show that levodopa may improve procedural motor learning in patients with chronic stroke, in line with our hypothesis. These findings suggest that this interventional strategy in combination with customary rehabilitative treatments could significantly improve the outcome of neurorehabilitation in the chronic stage after stroke. (Clinicaltrials.gov identifier NCT00126087.)

Noninvasive Brain Stimulation Improves Language Learning

Anodal transcranial direct current stimulation (tDCS) is a reliable technique to improve motor learning. We here wanted to test its potential to enhance associative verbal learning, a skill crucial for both acquiring new languages in healthy individuals and for language reacquisition after stroke-induced aphasia. We applied tDCS (20 min, 1 mA) over the posterior part of the left peri-sylvian area of 19 young right-handed individuals while subjects acquired a miniature lexicon of 30 novel object names. Every subject participated in one session of anodal tDCS, one session of cathodal tDCS, and one sham session in a randomized and double-blinded design with three parallel versions of the miniature lexicon. Outcome measures were learning speed and learning success at the end of each session, and the transfer to the subjects' native language after the respective stimulation. With anodal stimulation, subjects showed faster and better associative learning as compared to sham stimulation. Mood ratings, reaction times, and response styles were comparable between stimulation conditions. Our results demonstrate that anodal tDCS is a promising technique to enhance language learning in healthy adults and may also have the potential to improve language reacquisition after stroke.

Aphasia therapy on a neuroscience basis

BACKGROUND: Brain research has documented that the cortical mechanisms for language and action are tightly interwoven and, concurrently, new approaches to language therapy in neurological patients are being developed that implement language training in the context of relevant linguistic and non-linguistic actions, therefore taking advantage of the mutual connections of language and action systems in the brain. A further well-known neuroscience principle is that learning at the neuronal level is driven by correlation; consequently, new approaches to language therapy emphasise massed practice in a short time, thus maximising therapy quantity and frequency and, therefore, correlation at the behavioural and neuronal levels. Learned non-use of unsuccessful actions plays a major role in the chronification of neurological deficits, and behavioural approaches to therapy have therefore employed shaping and other learning techniques to counteract such non-use. AIMS: Advances in theoretical and experimental neuroscience have important implications for clinical practice. We exemplify this in the domain of aphasia rehabilitation. MAIN CONTRIBUTION: Whereas classical wisdom had been that aphasia cannot be significantly improved at a chronic stage, we here review evidence that one type of intensive language-action therapy (ILAT)-constraint-induced aphasia therapy-led to significant improvement of language performance in patients with chronic aphasia. We discuss perspectives for further improving speech-language therapy, including drug treatment that may be particularly fruitful when applied in conjunction with behavioural treatment. In a final section we highlight intensive and rapid therapy studies in chronic aphasia as a unique tool for exploring the cortical reorganisation of language. CONCLUSIONS: We conclude that intensive language action therapy is an efficient tool for improving language functions even at chronic stages of aphasia. Therapy studies using this technique can open new perspectives for research into the plasticity of human language circuits.

Levodopa improves skilled hand functions in the elderly

The endogenous dopamine system is a potent modulator of motor function and learning. Previous studies have demonstrated that, in the elderly, age-related degeneration of the nigrostriatal dopamine system may contribute to deficits in execution of skilled motor functions. The present double-blind, randomized cross-over study examined whether pharmacologically replenishing dopamine improves the execution of complex motor tasks. Twenty healthy young and 20 healthy elderly subjects were studied in two different sessions: (i) after three doses of levodopa (each 100 mg levodopa plus 25 mg carbidopa) and (ii) after three doses of placebo. For each session, subjects completed a functional motor test that reflects hand activities of daily living (Jebsen-Taylor test). In the elderly, but not in the young, Jebsen-Taylor test performance improved significantly (4%) with levodopa compared with placebo, particularly for fine motor functions. Attention to the task, level of fatigue, and positive and negative feelings were similar between sessions. These results demonstrate that increasing the dopaminergic drive pharmacologically may be helpful when the motor system is challenged in the ageing process.

Levodopa ameliorates learning and memory deficits in a murine model of Alzheimer's disease

Dopamine plays an important role in learning and memory processes. A deficit of this neurotransmitter as it is apparent in Alzheimer's disease (AD) may contribute to cognitive decline, a major symptom of AD patients. The aim of this study was to elucidate whether or not stimulation of the dopaminergic system leads to an improvement of cognitive function and reduction of non-cognitive behavioral alterations in a murine model of AD. Transgenic and wild type male mice of the TgCRND8 line were treated either with the dopamine precursor levodopa or vehicle and tested in two learning tasks, the object-recognition task and the Barnes maze test. Additionally 24 h spontaneous behavior in the home cage was analyzed. In both memory tasks wild type mice performed significantly better than transgenics. However, transgenics treated with levodopa showed a significant object recognition memory and improved acquisition of spatial memory in the Barnes maze compared to vehicle treated transgenics. Concerning spontaneous behavior transgenic mice performed much more stereotypies than wild types. However, there was a trend for reduced stereotypies in the levodopa group in the time the drug was active. Neurochemical analysis revealed elevated levels of dopamine in the neostriata and frontal cortices and reduced levels in the hippocampi of transgenic mice compared to wild types. Thus cognitive deficits and stereotypies may be due to changes in the dopaminergic system as they could be ameliorated by levodopa treatment, that might also have a therapeutic significance for AD.

Lack of improvement in odor identification by levodopa in humans

Several animal studies demonstrated that olfaction is modulated by dopamine (DA). We examined if such results could be replicated in humans. Thus, we tested olfaction in elderly healthy humans before and after external administration of levodopa in a double-blind, placebo-controlled, randomized cross-over study. Main outcome measure was the sum of correctly identified odors in an odor identification paradigm. In contrast to what had been demonstrated in rats, levodopa did not improve olfaction. We even noted a trend for the reverse. Blood pressure, mood ratings, as well as performance in a simple motor task were comparable between conditions, indicating that levodopa did not induce differences in general arousal, mood, attention, or reaction time. Our findings may be explained by differences in the distribution of dopaminergic receptor subtypes in the olfactory system in animals and humans, by relative differences in dosing regimes, or by subtle differences in the respective paradigms. These hypotheses have to be tested in future experiments, but our study demonstrates that results from animal studies cannot be directly transferred to the human situation.

Dexamphetamine enhances explicit new word learning for novel objects

Past research suggests that dexamphetamine (Dex) can facilitate learning and memory in healthy individuals and after a neurological lesion. This study investigated the effects of Dex on the learning of names for new objects in young healthy adults (n=37) within an explicit learning paradigm by using a double-blind, placebo-controlled between- subjects design. Participants received 10 mg Dex or a placebo each morning over five consecutive days before viewing 100 novel objects with non-word names plus matched fillers. Compared to the placebo, Dex enhanced both the rate of learning and the retention of the words 1 wk and 1 month later. The improved word learning correlated with baseline attention and memory scores for participants in the Dex group only. No correlations were observed between word-learning success and sustained attention, mood or cardiovascular arousal. It was concluded that the improved explicit word learning may have reflected dexamphetamine-induced changes in short-term memory and/or memory consolidation.

Dexamphetamine boosts naming treatment effects in chronic aphasia

To date, minimal research has investigated the effect of combining dexamphetamine with standard naming therapy after stroke. The present study used a double-blind, placebo-controlled, multiple baseline, crossover design with two individuals in the chronic stage of stroke recovery. Each individual attended two 4-week blocks of naming therapy (two to three treatment sessions per week). Dexamphetamine (10 mg) was administered at the start of each session during one therapy block, while a placebo was administered during the other therapy block. Therapy progress on treated and untreated items was assessed by a confrontation naming task during and after each therapy block. Both individuals showed greater progress in therapy and maintenance of therapy gains when behavioral treatment was combined with dexamphetamine rather than placebo, although this gain was only statistically significant in one individual. There was no significant improvement on a control task (nonword reading) in either individual. The results provide preliminary evidence that dexamphetamine paired with combined semantic and phonological therapy may be beneficial for the treatment of naming disorders in chronic aphasia.

Obesity in neurobiology

Obesity reflects an imbalance between energy uptake and expenditure that is mediated by behavior. Obesity is a growing epidemic and a major risk factor for neurobiological diseases like stroke, dementia, intracranial hypertension and sleep disorders. Conversely, obesity can also be induced by neurobiological disorders and drugs. The etiology of obesity is complex and includes biology, behavior and environment. Physicians are faced with the need to manage obesity while strategies for prevention and sustained weight reduction are limited. Present treatment options comprise lifestyle modification, diet, pharmacotherapy and bariatric surgery. Considerable headway has been made into elucidating the neurobiological underpinnings of obesogenic behavior. There is now a growing understanding of the metabolic, hormonal and behavioral circuitries that contribute to the complex and redundant system for energy balance. Changing the net balance of this system to prevent or reduce obesity requires multimodal and long-term interventions.

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.

Mesolimbic novelty processing in older adults

Normal aging is associated with neuronal loss in the dopaminergic midbrain (substantia nigra/ventral tegmental area, SN/VTA), a region that has recently been implicated in processing novel stimuli as part of a mesolimbic network including the hippocampus. Here, we quantified age-related structural degeneration of the mesolimbic system using magnetization transfer ratio (MTR) and correlated it with mesolimbic hemodynamic responses (HRs) to stimulus novelty. Twenty-one healthy older adults between 55 and 77 years performed a visual oddball paradigm allowing to distinguish mesolimbic HRs to novelty from rareness, negative emotional valence, and targetness using functional magnetic resonance imaging. The HRs in the right SN/VTA and the right hippocampus to novelty were positively correlated both with the SN/VTA MTR and hippocampus MTR but not amygdala MTR. However, the HR of the amygdala to negative emotional valence correlated with the amygdala MTR but not with the MTR in SN/VTA or the hippocampus. The results establish a structure-function relationship in support of a hippocampal-SN/VTA loop of mesolimbic novelty processing by showing that the hemodynamic activation in SN/VTA and hippocampus for novelty is selectively affected by age-related degeneration of these structures.

Dopaminergic influences on changes in human tactile acuity induced by tactile coactivation

As shown in animal experiments, dopaminergic mechanisms participate in N-methyl-D-aspartate (NMDA) receptor-dependent neuroplasticity. Dopamine is thought to play a similar role in humans, where it influences learning and memory. Here, we tested the dopaminergic action on learning in the tactile domain. To induce tactile non-associative learning, we applied a tactile coactivation protocol, which is known to improve tactile two-point discrimination of the stimulated finger. We studied the influence of a single oral dose of levodopa (25, 50, 100, 250 or 350 mg) administered preceding the coactivation protocol on changes in tactile performance in different groups of subjects. In addition, 3 x 100 mg levodopa was administered over a time period of 3 h in another group. Under placebo conditions, tactile two-point discrimination was improved on the coactivated index finger. Similar improvement was found when 25, 50 and 250 mg levodopa was applied. On the contrary, tactile improvement was completely eliminated by 1 x 100 and 3 x 100 mg levodopa. No drug effects were found on the left index finger indicating that the drug had no effect on performance per se. In contrast to previous findings in the motor and speech domain, we found that the administration of levodopa exerts either no or even negative effects on non-associative learning in the human somatosensory system. Whenever levodopa is used in neurorehabilitative context, it has to be kept in mind that beneficial effects in the motor or speech domain cannot be easily generalized to other systems.

Plasma homovanillic acid correlates inversely with history of learning problems in healthy volunteer and personality disordered subjects

Central dopaminergic activity is critical to the functioning of both motor and cognitive systems. Based on the therapeutic action of dopaminergic agents in treating attention deficit hyperactivity disorder (ADHD), ADHD symptoms may be related to a reduction in central dopaminergic activity. We tested the hypothesis that dopaminergic activity, as reflected by plasma homovanillic acid (pHVA), may be related to dimensional aspects of ADHD in adults. Subjects were 30 healthy volunteer and 39 personality disordered subjects, in whom morning basal pHVA concentration and a dimensional measure of childhood ADHD symptoms (Wender Utah Rating Scale: WURS) were obtained. A significant inverse correlation was found between WURS Total score and pHVA concentration in the total sample. Among WURS factor scores, a significant inverse relationship was noted between pHVA and history of "childhood learning problems". Consistent with the dopaminergic dysfunction hypothesis of ADHD and of cognitive function, pHVA concentrations were correlated with childhood history of ADHD symptoms in general and with history of "learning problems" in non-ADHD psychiatric patients and controls. Replication is needed in treated and untreated ADHD samples to confirm these initial results.

High impact running improves learning

Regular physical exercise improves cognitive functions and lowers the risk for age-related cognitive decline. Since little is known about the nature and the timing of the underlying mechanisms, we probed whether exercise also has immediate beneficial effects on cognition. Learning performance was assessed directly after high impact anaerobic sprints, low impact aerobic running, or a period of rest in 27 healthy subjects in a randomized cross-over design. Dependent variables comprised learning speed as well as immediate (1 week) and long-term (>8 months) overall success in acquiring a novel vocabulary. Peripheral levels of brain-derived neurotrophic factor (BDNF) and catecholamines (dopamine, epinephrine, norepinephrine) were assessed prior to and after the interventions as well as after learning. We found that vocabulary learning was 20 percent faster after intense physical exercise as compared to the other two conditions. This condition also elicited the strongest increases in BDNF and catecholamine levels. More sustained BDNF levels during learning after intense exercise were related to better short-term learning success, whereas absolute dopamine and epinephrine levels were related to better intermediate (dopamine) and long-term (epinephrine) retentions of the novel vocabulary. Thus, BDNF and two of the catecholamines seem to be mediators by which physical exercise improves learning.

Amnesia and the hippocampus

PURPOSE OF REVIEW

Long-term memory impairments have great medical significance and a considerable health and economic burden. Understanding their cognitive and neuroanatomical underpinnings is of crucial importance. Severe amnesia is usually observed following bilateral hippocampal pathology. This review addresses the precise role of the hippocampus and related medial temporal lobe structures in amnesia.

RECENT FINDINGS

Disagreements exist over whether, following selective hippocampal damage: retrograde amnesia for episodic memories is temporally limited or extensive and ungraded; anterograde amnesia involves both recollective and familiarity processes. It is accepted that material specific impairments follow unilateral medial temporal lobe damage, with verbal and nonverbal memory lateralized to left or right, respectively. Memory for unknown faces, however, may not depend on the hippocampus. Pharmacological studies in animals, with some extension to humans, highlight promising future therapeutic interventions targeting synaptic plasticity modulation.

SUMMARY

Despite considerable progress, some issues remain unresolved. The available evidence favours the view, however, that the hippocampus, in conjunction with other cortical areas, is critical for the retrieval of remote episodic memories and for both recollection and familiarity anterograde memory processes. There are as yet no effective pharmacological treatments for medial temporal lobe amnesia, but various rehabilitative techniques may be useful.

Levodopa increases memory encoding and dopamine release in the striatum in the elderly

Normal aging is associated with a decrease in dopaminergic function and a reduced ability to form new motor memories with training. This study examined the link between both phenomena. We hypothesized that levodopa would (a) ameliorate aging-dependent deficits in motor memory formation, and (b) increase dopamine availability at the dopamine type 2-like (D2) receptor during training in task-relevant brain structures. The effects of training plus levodopa (100mg, plus 25mg carbidopa) on motor memory formation and striatal dopamine availability were measured with [(11)C]raclopride (RAC) positron emission tomography (PET). We found that levodopa did not alter RAC-binding potential at rest but it enhanced training effects on motor memory formation as well as dopamine release in the dorsal caudate nucleus. Motor memory formation during training correlated with the increase of dopamine release in the caudate nucleus. These results demonstrate that levodopa may ameliorate dopamine deficiencies in the elderly by replenishing dopaminergic presynaptic stores, actively engaged in phasic dopamine release during motor training.

Tonic dopaminergic stimulation impairs associative learning in healthy subjects

Endogenous dopamine plays a central role in salience coding during associative learning. Administration of the dopamine precursor levodopa enhances learning in healthy subjects and stroke patients. Because levodopa increases both phasic and tonic dopaminergic neurotransmission, the critical mechanism mediating the enhancement of learning is unresolved. We here probed how selective tonic dopaminergic stimulation affects associative learning. Forty healthy subjects were trained in a novel vocabulary of 45 concrete nouns over the course of 5 consecutive training days in a prospective, randomized, double-blind, placebo-controlled design. Subjects received the tonically stimulating dopamine-receptor agonist pergolide (0.1 mg) vs placebo 120 min before training on each training day. The dopamine agonist significantly impaired novel word learning compared to placebo. This learning decrement persisted up to the last follow-up 4 weeks post-training. Subjects treated with pergolide also showed restricted emotional responses compared to the PLACEBO group. The extent of 'flattened' affect with pergolide was related to the degree of learning inhibition. These findings suggest that tonic occupation of dopamine receptors impairs learning by competition with phasic dopamine signals. Thus, phasic signaling seems to be the critical mechanism by which dopamine enhances associative learning in healthy subjects and stroke patients.

Absolute coding of stimulus novelty in the human substantia nigra/VTA

Novelty exploration can enhance hippocampal plasticity in animals through dopaminergic neuromodulation arising in the substantia nigra/ventral tegmental area (SN/VTA). This enhancement can outlast the exploration phase by several minutes. Currently, little is known about dopaminergic novelty processing and its relationship to hippocampal function in humans. In two functional magnetic resonance imaging (fMRI) studies, SN/VTA activations in humans were indeed driven by stimulus novelty rather than other forms of stimulus salience such as rareness, negative emotional valence, or targetness of familiar stimuli, whereas hippocampal responses were less selective. SN/VTA novelty responses were scaled according to absolute rather than relative novelty in a given context, unlike adaptive SN/VTA responses recently reported for reward outcome in animal studies. Finally, novelty enhanced learning and perirhinal/parahippocampal processing of familiar items presented in the same context. Thus, the human SN/VTA can code absolute stimulus novelty and might contribute to enhancing learning in the context of novelty.

Metacognition in stroke: bedside assessment and relation to location, size, and stroke severity

BACKGROUND

Frontal syndromes are poorly represented on stroke scales, yet may constitute an important component of the neurologic deficit impacting treatment and rehabilitation efforts.

AIMS

To validate an assessment of a frontal network syndrome score (FNSS) in stroke and to ascertain the relationship of the FNSS to lesion location, volume, and severity in young people with stroke.

METHODS

Accrual through a cognitive stroke registry of young, alert, nonaphasic, nonencephalopathic, nonsubstance abuse prone people who were tested with the 16 domain FNSS within 4 weeks of their stroke. Lesion location was determined by the Cerefy 3-dimensional, digital, coxial brain atlas identifying 6 regions of cerebral interest. Lesion severity was assessed by the National Institute of Health Stroke Score and infarct volume was measured in cubic centimeters.

RESULTS

From a sample of 456 cases in the registry after exclusions of encephalopathy, depression, and aphasia, cases with frontal network syndrome encompassed 132 of 341 persons (39%). Of these patients (n=80) with mean age 45.7 years [confidence interval (CI): 43.4, 48.1], educational level 13.1 years (CI:12.5, 13.6), mean National Institute of Health Stroke Score of 3.3 (CI: 2.6, 4.0), and mean lesion volume 30.3 mL (CI: 13.7, 46.9), had full testing with the FNSS battery yielding sensitivity (93%), specificity (74%), positive predictive value (79%), negative predictive value (90%), and a good interrater reliability (kappa=0.89, P=0.001). Construct validity testing with 15 frontal neuropsychologic tests revealed good to excellent significant Pearson r values in 14 of 15 of the tests. There was no correlation (Pearson's) for lesion volume (r=0.07, P=0.52) but significant correlation with stroke severity (r=0.23, P=0.03). Analysis of variance testing of the FNSS revealed significant differences for all 6 domains without intergroup significant differences for frontal network syndrome domains in the stroke group.

CONCLUSIONS

The FNSS is a valid, practical measure of frontal syndromes in stroke. Frontal syndromes are common in young people with stroke and may be present no matter where the lesion, are not dependent on size of stroke and have a low correlation with stroke severity.