Increased cognitive functioning in symptomatic Huntington's disease as revealed by behavioral and event-related potential indices of auditory sensory memory and attention

Auditory mismatch negativity in pre-manifest and manifest Huntington's disease

AIM

The aim of this study was to investigate the characteristics of the electrophysiological brain response elicited in a passive acoustic oddball paradigm, i.e. mismatch negativity (MMN), in patients with Huntington's disease (HD) in the premanifest (pHD) and manifest (mHD) phases. In this regard, we correlated the results of event-related potentials (ERP) with disease characteristics.

METHODS

This was an observational cross-sectional MMN study. In addition to the MMN recording of the passive oddball task, all subjects with first-degree inheritance for HD underwent genetic testing for mutant HTT, the Huntington's Disease Rating Scale, the Total Functional Capacity Scale, the Problem Behaviors Assessment short form, and the Mini-Mental State Examination.

RESULTS

We found that global field power (GFP) was reduced in the MMN time window in mHD patients compared to pHD and normal controls (NC). In the pHD group, MMN amplitude was only slightly and not significantly increased compared to mHD, while pHD patients showed increased theta coherence between trials compared to mHD. In the entire sample of HD gene carriers, the main MMN traits were not correlated with motor performance, cognitive impairment and functional disability.

CONCLUSION

These results suggest an initial and subtle deterioration of pre-attentive mechanisms in the presymptomatic phase of HD, with an increasing phase shift in the MMN time frame. This result could indicate initial functional changes with a possible compensatory effect.

SIGNIFICANCE

An initial and slight decrease in MMN associated with increased phase coherence in the corresponding EEG frequencies could indicate an early functional involvement of pre-attentive resources that could precede the clinical expression of HD.

Rethinking Movement Disorders

At present, clinical practice and research in movement disorders (MDs) focus on the "normalization" of altered movements. In this review, rather than concentrating on problems and burdens people with MDs undoubtedly have, we highlight their hidden potentials. Starting with current definitions of Parkinson's disease (PD), dystonia, chorea, and tics, we outline that solely conceiving these phenomena as signs of dysfunction falls short of their complex nature comprising both problems and potentials. Such potentials can be traced and understood in light of well-established cognitive neuroscience frameworks, particularly ideomotor principles, and their influential modern derivatives. Using these frameworks, the wealth of data on altered perception-action integration in the different MDs can be explained and systematized using the mechanism-oriented concept of perception-action binding. According to this concept, MDs can be understood as phenomena requiring and fostering flexible modifications of perception-action associations. Consequently, although conceived as being caught in a (trough) state of deficits, given their high flexibility, people with MDs also have high potential to switch to (adaptive) peak activity that can be conceptualized as hidden potentials. Currently, clinical practice and research in MDs are concerned with deficits and thus the "deep and wide troughs," whereas "scattered narrow peaks" reflecting hidden potentials are neglected. To better delineate and utilize the latter to alleviate the burden of affected people, and destigmatize their conditions, we suggest some measures, including computational modeling combined with neurophysiological methods and tailored treatment. © 2024 International Parkinson and Movement Disorder Society.

Gamma-Aminobutyric Acid and Glutamate Concentrations in the Striatum and Anterior Cingulate Cortex Not Found to Be Associated with Cognitive Flexibility

Behavioral flexibility and goal-directed behavior heavily depend on fronto-striatal networks. Within these circuits, gamma-aminobutyric acid (GABA) and glutamate play an important role in (motor) response inhibition, but it has remained largely unclear whether they are also relevant for cognitive inhibition. We hence investigated the functional role of these transmitters for cognitive inhibition during cognitive flexibility. Healthy young adults performed two paradigms assessing different aspects of cognitive flexibility. Magnetic resonance spectroscopy (MRS) was used to quantify GABA+ and total glutamate/glutamine (Glx) levels in the striatum and anterior cingulate cortex (ACC) referenced to N-acetylaspartate (NAA). We observed typical task switching and backward inhibition effects, but striatal and ACC concentrations of GABA+/NAA and Glx/NAA were not associated with cognitive flexibility in a functionally relevant manner. The assumption of null effects was underpinned by Bayesian testing. These findings suggest that behavioral and cognitive inhibition are functionally distinct faculties, that depend on (at least partly) different brain structures and neurotransmitter systems. While previous studies consistently demonstrated that motor response inhibition is modulated by ACC and striatal GABA levels, our results suggest that the functionally distinct cognitive inhibition required for successful switching is not, or at least to a much lesser degree, modulated by these factors.

An Expanded Polyproline Domain Maintains Mutant Huntingtin Soluble in vivo and During Aging

Huntington's disease is a dominantly inherited neurodegenerative disorder caused by the expansion of a CAG repeat, encoding for the amino acid glutamine (Q), present in the first exon of the protein huntingtin. Over the threshold of Q39 HTT exon 1 (HTTEx1) tends to misfold and aggregate into large intracellular structures, but whether these end-stage aggregates or their on-pathway intermediates are responsible for cytotoxicity is still debated. HTTEx1 can be separated into three domains: an N-terminal 17 amino acid region, the polyglutamine (polyQ) expansion and a C-terminal proline rich domain (PRD). Alongside the expanded polyQ, these flanking domains influence the aggregation propensity of HTTEx1: with the N17 initiating and promoting aggregation, and the PRD modulating it. In this study we focus on the first 11 amino acids of the PRD, a stretch of pure prolines, which are an evolutionary recent addition to the expanding polyQ region. We hypothesize that this proline region is expanding alongside the polyQ to counteract its ability to misfold and cause toxicity, and that expanding this proline region would be overall beneficial. We generated HTTEx1 mutants lacking both flanking domains singularly, missing the first 11 prolines of the PRD, or with this stretch of prolines expanded. We then followed their aggregation landscape in vitro with a battery of biochemical assays, and in vivo in novel models of C. elegans expressing the HTTEx1 mutants pan-neuronally. Employing fluorescence lifetime imaging we could observe the aggregation propensity of all HTTEx1 mutants during aging and correlate this with toxicity via various phenotypic assays. We found that the presence of an expanded proline stretch is beneficial in maintaining HTTEx1 soluble over time, regardless of polyQ length. However, the expanded prolines were only advantageous in promoting the survival and fitness of an organism carrying a pathogenic stretch of Q48 but were extremely deleterious to the nematode expressing a physiological stretch of Q23. Our results reveal the unique importance of the prolines which have and still are evolving alongside expanding glutamines to promote the function of HTTEx1 and avoid pathology.

Hearing Loss in Neurological Disorders

Sensorineural hearing loss (SNHL) affects approximately 466 million people worldwide, which is projected to reach 900 million by 2050. Its histological characteristics are lesions in cochlear hair cells, supporting cells, and auditory nerve endings. Neurological disorders cover a wide range of diseases affecting the nervous system, including Alzheimer's disease (AD), Parkinson's disease (PD), Huntington's disease (HD), autism spectrum disorder (ASD), etc. Many studies have revealed that neurological disorders manifest with hearing loss, in addition to typical nervous symptoms. The prevalence, manifestations, and neuropathological mechanisms underlying vary among different diseases. In this review, we discuss the relevant literature, from clinical trials to research mice models, to provide an overview of auditory dysfunctions in the most common neurological disorders, particularly those associated with hearing loss, and to explain their underlying pathological and molecular mechanisms.

A Basal Ganglia Computational Model to Explain the Paradoxical Sensorial Improvement in the Presence of Huntington's Disease

The basal ganglia (BG) represent a critical center of the nervous system for sensorial discrimination. Although it is known that Huntington's disease (HD) affects this brain area, it still remains unclear how HD patients achieve paradoxical improvement in sensorial discrimination tasks. This paper presents a computational model of the BG including the main nuclei and the typical firing properties of their neurons. The BG model has been embedded within an auditory signal detection task. We have emulated the effect that the altered levels of dopamine and the degree of HD affectation have in information processing at different layers of the BG, and how these aspects shape transient and steady states differently throughout the selection task. By extracting the independent components of the BG activity at different populations, it is evidenced that early and medium stages of HD affectation may enhance transient activity in the striatum and the substantia nigra pars reticulata. These results represent a possible explanation for the paradoxical improvement that HD patients present in discrimination task performance. Thus, this paper provides a novel understanding on how the fast dynamics of the BG network at different layers interact and enable transient states to emerge throughout the successive neuron populations.

Central auditory processing in adults with chronic stroke without hearing loss: A magnetoencephalography study

OBJECTIVE

Stroke lesions in non-auditory areas may affect higher-order central auditory processing. We sought to characterize auditory functions in chronic stroke survivors with unilateral arm/hand impairment using auditory evoked responses (AERs) with lesion and perception metrics.

METHODS

The AERs in 29 stroke survivors and 14 controls were recorded with single tones, active and passive frequency-oddballs, and a dual-oddball with pitch-contour and time-interval deviants. Performance in speech-in-noise, mistuning detection, and moving-sound detection was assessed. Relationships between AERs, behaviour, and lesion overlap with functional networks, were examined.

RESULTS

Despite their normal hearing, eight patients showed unilateral AER in the hemisphere ipsilateral to the affected hand with reduced amplitude compared to those with bilateral AERs. Both groups showed increasing attenuation of later components. Hemispheric asymmetry of AER sources was reduced in bilateral-AER patients. The N1 wave (100 ms latency) and P2 (200 ms) were delayed in individuals with lesions in the basal-ganglia and white-matter, while lesions in the attention network reduced the frequency-MMN (mismatch negativity) responses and increased the pitch-contour P3a response. Patients' impaired speech-in-noise perception was explained by AER measures and frequency-deviant detection performance with multiple regression.

CONCLUSION

AERs reflect disruption of auditory functions due to damage outside of temporal lobe, and further explain complexity of neural mechanisms underlying higher-order auditory perception.

SIGNIFICANCE

Stroke survivors without obvious hearing problems may benefit from rehabilitation for central auditory processing.

Auditory mismatch detection, distraction, and attentional reorientation (MMN-P3a-RON) in neurological and psychiatric disorders: A review

Involuntary attention allows for the detection and processing of novel and potentially relevant stimuli that lie outside of cognitive focus. These processes comprise change detection in sensory contexts, automatic orientation toward this change, and the selection of adaptive responses, including reorientation to the original goal in cases when the detected change is not relevant for task demands. These processes have been studied using the Event-Related Potential (ERP) technique and have been associated to the Mismatch Negativity (MMN), the P3a, and the Reorienting Negativity (RON) electrophysiological components, respectively. This has allowed for the objective evaluation of the impact of different neuropsychiatric pathologies on involuntary attention. Additionally, these ERP have been proposed as alternative measures for the early detection of disease and the tracking of its progression. The objective of this review was to integrate the results reported to date about MMN, P3a, and RON in different neurological and psychiatric disorders. We included experimental studies with clinical populations that reported at least two of these three components in the same experimental paradigm. Overall, involuntary attention seems to reflect the state of cognitive integrity in different pathologies in adults. However, if the main goal for these ERP is to consider them as biomarkers, more research about their pathophysiological specificity in each disorder is needed, as well as improvement in the general experimental conditions under which these components are elicited. Nevertheless, these ERP represent a valuable neurophysiological tool for early detection and follow-up of diverse clinical populations.

Predictive coding and adaptive behavior in patients with genetically determined cerebellar ataxia--A neurophysiology study

Genetically determined cerebellar ataxias (CA) are a heterogeneous group of disorders with progressive decline of cerebellar functions. The cerebellum influences internal forward models that play a role in cognitive control, but whether these processes are dysfunctional in CA is unclear. Here, we examined sensory predictive coding processes and response adaptation in CA and healthy controls (HC) using behavioral tests with concomitant EEG recordings. N = 23 patients and N = 29 age- and sex-matched HC were studied. Sensory prediction coding was tested with an auditory distraction paradigm and error-related behavioral adaptation with a visual flanker task. As neurophysiological markers we studied different event-related potentials: the P3a for orientation of attention; the N2 and the error-related negativity (ERN) for cognitive adaptation processes/consequences of response errors; error-related positivity (Pe) for error-awareness; the mismatch negativity (MMN) for sensory predictive coding; and reorientation negativity (RON) for reorientation after unexpected events. Overall reaction times were slower in patients compared to HC, but error rates did not differ. Both in patients and HC, P3a amplitudes were larger in distraction trials, but the P3a amplitude was smaller in patients compared to HC. The MMN as well as behavioral and EEG-correlates of response adaptation (ERN/N2) did not differ between groups, while the Pe was attenuated in patients. During sensory predictive coding, RON amplitudes were significantly larger in HC compared to patients. In HC, but not in patients, RON amplitudes were also larger in deviant compared to frequent trials. Processes generating internal forward models are largely intact in genetically determined CA, whereas updating of mental models and error awareness are disturbed in these patients.

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

BACKGROUND

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

METHODS

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

RESULTS

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

CONCLUSIONS

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

Sensory Prediction Errors Are Less Modulated by Global Context in Autism Spectrum Disorder

BACKGROUND

Recent predictive coding accounts of autism spectrum disorder (ASD) suggest that a key deficit in ASD concerns the inflexibility in modulating local prediction errors as a function of global top-down expectations. As a direct test of this central hypothesis, we used electroencephalography to investigate whether local prediction error processing was less modulated by global context (i.e., global stimulus frequency) in ASD.

METHODS

A group of 18 adults with ASD was compared with a group of 24 typically developed adults on a well-validated hierarchical auditory oddball task in which participants listened to short sequences of either five identical sounds (local standard) or four identical sounds and a fifth deviant sound (local deviant). The latter condition is known to generate the mismatch negativity (MMN) component, believed to reflect early sensory prediction error processing. Crucially, previous studies have shown that in blocks with a higher frequency of local deviant sequences, top-down expectations seem to attenuate the MMN. We predicted that this modulation by global context would be less pronounced in the ASD group.

RESULTS

Both groups showed an MMN that was modulated by global context. However, this effect was smaller in the ASD group as compared with the typically developed group. In contrast, the P3b, as an electroencephalographic marker of conscious expectation processes, did not differ across groups.

CONCLUSIONS

Our results demonstrate that people with ASD are less flexible in modulating their local predictions (reflected in MMN), thereby confirming the central hypothesis of contemporary predictive coding accounts of ASD.

Dysfunctions in striatal microstructure can enhance perceptual decision making through deficits in predictive coding

An important brain function is to predict upcoming events on the basis of extracted regularities of previous inputs. These predictive coding processes can disturb performance in concurrent perceptual decision-making and are known to depend on fronto-striatal circuits. However, it is unknown whether, and if so, to what extent striatal microstructural properties modulate these processes. We addressed this question in a human disease model of striosomal dysfunction, i.e. X-linked dystonia-parkinsonism (XDP), using high-density EEG recordings and source localization. The results show faster and more accurate perceptual decision-making performance during distraction in XDP patients compared to healthy controls. The electrophysiological data show that sensory memory and predictive coding processes reflected by the mismatch negativity related to lateral prefrontal brain regions were weakened in XDP patients and thus induced less cognitive conflict than in controls as reflected by the N2 event-related potential (ERP). Consequently, attentional shifting (P3a ERP) and reorientation processes (RON ERP) were less pronounced in the XDP group. Taken together, these results suggests that striosomal dysfunction is related to predictive coding deficits leading to a better performance in concomitant perceptual decision-making, probably because predictive coding does not interfere with perceptual decision-making processes. These effects may reflect striatal imbalances between the striosomes and the matrix compartment.

Problem solving, impulse control and planning in patients with early- and late-stage Huntington's disease

Sub-domains of executive functions, including problems with planning, accuracy, impulsivity, and inhibition, are core features of Huntington's disease. It is known that the decline of cognitive function in Huntington's disease is related to the anatomical progression of pathology in the basal ganglia. However, it remains to be determined whether the severity of executive dysfunction depends on the stage of the disease. To examine the severity of sub-domains of executive dysfunction in early- and late-stage Huntington's disease, we studied performance in the Tower of London task of two groups of Huntington's disease patients (Group 1: early, n = 23, and Group 2: late stage, n = 29), as well as a third group of age, education, and IQ matched healthy controls (n = 34). During the task, we measured the total number of problems solved, total planning time, and total number of breaks taken. One aspect of executive function indexed by the number of solved problems seems to progress in the course of the disease. Late-stage Huntington's disease patients scored significantly worse than early-stage patients and controls, and early-stage patients scored significantly worse than controls on this measure of accuracy. In contrast, late- and early-stage HD patients did not differ in terms of planning time and number of breaks. Early- and late-stage HD pathology has a different impact on executive sub-domains. While accuracy differs between early- and late-stage HD patients, other domains like planning time and number of breaks do not. Striatal degeneration, which is a characteristic feature of the disease, might not affect all aspects of executive function in HD.

Laser Evoked Potentials in Early and Presymptomatic Huntington's Disease

Pain was rarely studied in Huntington's disease (HD). We presently aimed to extend our previous study on pain pathways functions by laser evoked potentials (LEPs) to a larger cohort of early unmedicated HD patients and a small group of presymptomatic HD (PHD) subjects. Forty-two early HD patients, 10 PHD patients, and 64 controls were submitted to LEPs by right-hand stimulation. Two series of 30 laser stimuli were delivered, and artifact-free responses were averaged. The N1, N2, and P2 latencies were significantly increased and the N2P2 amplitude significantly reduced in HD patients compared to controls. In the HD group, the LEPs abnormalities correlated with functional decline. PHD subjects showed a slight and insignificant increase in LEPs latencies, which was inversely correlated with the possible age of HD clinical onset. Data of the present study seem to suggest that the functional state of nociceptive pathways as assessed by LEPs may be a potential biomarker of disease onset and progression. The assessment of pain symptoms in premanifest and manifest HD may also open a new scenario in terms of subtle disturbances of pain processing, which may have a role in the global burden of the disease.

Hearing and music in dementia

Music is a complex acoustic signal that relies on a number of different brain and cognitive processes to create the sensation of hearing. Changes in hearing function are generally not a major focus of concern for persons with a majority of neurodegenerative diseases associated with dementia, such as Alzheimer disease (AD). However, changes in the processing of sounds may be an early, and possibly preclinical, feature of AD and other neurodegenerative diseases. The aim of this chapter is to review the current state of knowledge concerning hearing and music perception in persons who have a dementia as a result of a neurodegenerative disease. The review focuses on both peripheral and central auditory processing in common neurodegenerative diseases, with a particular focus on the processing of music and other non-verbal sounds. The chapter also reviews music interventions used for persons with neurodegenerative diseases.

NPY2-receptor variation modulates iconic memory processes

Sensory memory systems are modality-specific buffers that comprise information about external stimuli, which represent the earliest stage of information processing. While these systems have been the subject of cognitive neuroscience research for decades, little is known about the neurobiological basis of sensory memory. However, accumulating evidence suggests that the glutamatergic system and systems influencing glutamatergic neural transmission are important. In the current study we examine if functional promoter variations in neuropeptide Y (NPY) and its receptor gene NPY2R affect iconic memory processes using a partial report paradigm. We found that iconic memory decayed much faster in individuals carrying the rare promoter NPY2R G allele which is associated with increased expression of the Y2 receptor. Possibly this effect is due to altered presynaptic inhibition of glutamate release, known to be modulated by Y2 receptors. Altogether, our results provide evidence that the functionally relevant single nucleotide polymorphism (SNP) in the NPY2R promoter gene affect circumscribed processes of early sensory processing, i.e. only the stability of information in sensory memory buffers. This leads us to suggest that especially the stability of information in sensory memory buffers depends on glutamatergic neural transmission and factors modulating glutamatergic turnover.

Striatal disorders dissociate mechanisms of enhanced and impaired response selection - Evidence from cognitive neurophysiology and computational modelling

Paradoxically enhanced cognitive processes in neurological disorders provide vital clues to understanding neural function. However, what determines whether the neurological damage is impairing or enhancing is unclear. Here we use the performance of patients with two disorders of the striatum to dissociate mechanisms underlying cognitive enhancement and impairment resulting from damage to the same system. In a two-choice decision task, Huntington's disease patients were faster and less error prone than controls, yet a patient with the rare condition of benign hereditary chorea (BHC) was both slower and more error prone. EEG recordings confirmed significant differences in neural processing between the groups. Analysis of a computational model revealed that the common loss of connectivity between striatal neurons in BHC and Huntington's disease impairs response selection, but the increased sensitivity of NMDA receptors in Huntington's disease potentially enhances response selection. Crucially the model shows that there is a critical threshold for increased sensitivity: below that threshold, impaired response selection results. Our data and model thus predict that specific striatal malfunctions can contribute to either impaired or enhanced selection, and provide clues to solving the paradox of how Huntington's disease can lead to both impaired and enhanced cognitive processes.

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Comparative study on well-defined neurological disorders

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Striatal disorders dissociate mechanisms of enhanced and impaired cognition.

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Neurophysiological data in patients is combined with computational modelling.

Evidence for divergent effects of neurodegeneration in Huntington's disease on attentional selection and neural plasticity: implications for excitotoxicity

It is generally assumed that neurodegeneration leads to declines in cognitive functions. However, there is evidence that neurodegenerative processes related to excitotoxicity can lead to paradoxical improvements in circumscribed cognitive functions, while at the same time other processes are compromised. An open question is how such paradoxical improvements reported in literature and declines usually associated with neurodegeneration relate to each other. Do paradoxical improvements only reflect a transient phenomenon, or do they intensify in the course of neurodegeneration? We examine this question using behavioral and neurophysiological (EEG) data in a human model of excitotoxic neurodegeneration (i.e., Huntington's disease, HD). The results show that attentional selection processes decline during pre-manifest disease progression. Importantly, the efficacy of protocols used to induce neural plasticity in processes underlying attentional selection processes also increases in course of ongoing neurodegeneration in pre-manifest HD. This was reflected in behavioral data and electrophysiological correlates of processes related to the allocation of attention. To conclude, our results suggest that circumscribed enhancements of specific cognitive functions are as much a result of the developmental process of neurodegeneration as the well-known detrimental effects. The results account for the divergent effects of neurodegenerative processes closely related to excitotoxicity on cognitive functions.

Cross sectional PET study of cerebral adenosine A₁ receptors in premanifest and manifest Huntington's disease

PURPOSE

To study cerebral adenosine receptors (AR) in premanifest and manifest stages of Huntington's disease (HD).

METHODS

We quantified the cerebral binding potential (BP ND) of the A₁AR in carriers of the HD CAG trinucleotide repeat expansion using the radioligand [(18) F]CPFPX and PET. Four groups were investigated: (i) premanifest individuals far (preHD-A; n = 7) or (ii) near (preHD-B; n = 6) to the predicted symptom onset, (iii) manifest HD patients (n = 8), and (iv) controls (n = 36).

RESULTS

Cerebral A₁AR values of preHD-A subjects were generally higher than those of controls (by up to 31%, p < .01, in the thalamus on average). Across stages a successive reduction of A₁AR BPND was observed to the levels of controls in preHD-B and undercutting controls in manifest HD by down to 25%, p < .01, in the caudatus and amygdala. There was a strong correlation between A₁AR BP ND and years to onset. Before onset of HD, the assumed annual rates of change of A₁AR density were -1.2% in the caudatus, -1.7% in the thalamus and -3.4% in the amygdala, while the corresponding volume losses amounted to 0.6%, 0.1% and 0.2%, respectively.

CONCLUSIONS

Adenosine receptors switch from supra to subnormal levels during phenoconversion of HD. This differential regulation may play a role in the pathophysiology of altered energy metabolism.

Transient and steady-state selection in the striatal microcircuit

Although the basal ganglia have been widely studied and implicated in signal processing and action selection, little information is known about the active role the striatal microcircuit plays in action selection in the basal ganglia-thalamo-cortical loops. To address this knowledge gap we use a large scale three dimensional spiking model of the striatum, combined with a rate coded model of the basal ganglia-thalamo-cortical loop, to asses the computational role the striatum plays in action selection. We identify a robust transient phenomena generated by the striatal microcircuit, which temporarily enhances the difference between two competing cortical inputs. We show that this transient is sufficient to modulate decision making in the basal ganglia-thalamo-cortical circuit. We also find that the transient selection originates from a novel adaptation effect in single striatal projection neurons, which is amenable to experimental testing. Finally, we compared transient selection with models implementing classical steady-state selection. We challenged both forms of model to account for recent reports of paradoxically enhanced response selection in Huntington's disease patients. We found that steady-state selection was uniformly impaired under all simulated Huntington's conditions, but transient selection was enhanced given a sufficient Huntington's-like increase in NMDA receptor sensitivity. Thus our models provide an intriguing hypothesis for the mechanisms underlying the paradoxical cognitive improvements in manifest Huntington's patients.

Faster perceptual learning through excitotoxic neurodegeneration

Glutamatergic neural transmission is involved in both neural plasticity and neurodegeneration. This combination of roles could result in ambivalent effects in which excitotoxic neurodegeneration augments neural plasticity in parallel. Neural plasticity can be induced by exposure-based learning (EBL) that resembles timing properties of long-term potentiation (LTP) protocols (i.e., LTP-like learning). Even though it has not been demonstrated so far in animal models that perceptual effects of such stimulation protocols are mediated by typical LTP mechanisms, it has been shown that exposure-based learning exerts strong effects on cognitive brain functioning and is modulated by glutamatergic neural transmission. We reveal that exposure-based perceptual learning is more efficient in a human model of excitotoxic neurodegeneration than in healthy participants. Premanifest Huntington's disease gene mutation carriers showed faster increases in perceptual sensitivities than controls. This in turn changed attentional processing in extrastriate visual areas objectified using electroencephalogram data. The emergence of faster learning correlated positively with genetic disease load. Our results confirm an ambivalent action of increased glutamatergic transmission, implying that the process of excitotoxic neurodegeneration is associated with enhanced perceptual learning, which can be used to improve attentional and behavioral control via the alteration of perceptual sensitivities.

BDNF Val66Met polymorphism and goal-directed behavior in healthy elderly - evidence from auditory distraction

Aging affects the ability to focus attention on a given task and to ignore distractors. These functions subserve response control processes for which fronto-striatal networks have been shown to play an important role. Within these networks, the brain-derived-neurotrophic-factor (BDNF), which is known to underlie aging effects, plays a pivotal role. We investigated how cognitive subprocesses constituting a cycle of distraction, orientation and refocusing of attention are affected by the functional BDNF Val66Met polymorphism using event-related potentials (ERPs) in 122 healthy elderly. Using an auditory distraction paradigm we found that the Val/Val genotype confers a disadvantage to its carriers. This disadvantage was partly compensated by intensified attentional shifting mechanisms. It could be based on response selection processes being more vulnerable against interference from distractors in this genotype group. Processes reflecting transient sensory memory processes, or the re-orientation of attention were not affected by the BDNF Val66Met polymorphism, suggesting a higher importance of BDNF for mechanisms related to response control, than stimulus processing. The results add on recent literature showing that the Met allele confers some benefit to its carriers. We suggest an account for unifying different results of BDNF Val66Met association studies in executive functions, based on the role of BDNF in fronto-striatal circuits.

Differential effects of ADORA2A gene variations in pre-attentive visual sensory memory subprocesses

The ADORA2A gene encodes the adenosine A(2A) receptor that is highly expressed in the striatum where it plays a role in modulating glutamatergic and dopaminergic transmission. Glutamatergic signaling has been suggested to play a pivotal role in cognitive functions related to the pre-attentive processing of external stimuli. Yet, the precise molecular mechanism of these processes is poorly understood. Therefore, we aimed to investigate whether ADORA2A gene variation has modulating effects on visual pre-attentive sensory memory processing. Studying two polymorphisms, rs5751876 and rs2298383, in 199 healthy control subjects who performed a partial-report paradigm, we find that ADORA2A variation is associated with differences in the efficiency of pre-attentive sensory memory sub-processes. We show that especially the initial visual availability of stimulus information is rendered more efficiently in the homozygous rare genotype groups. Processes related to the transfer of information into working memory and the duration of visual sensory (iconic) memory are compromised in the homozygous rare genotype groups. Our results show a differential genotype-dependent modulation of pre-attentive sensory memory sub-processes. Hence, we assume that this modulation may be due to differential effects of increased adenosine A(2A) receptor signaling on glutamatergic transmission and striatal medium spiny neuron (MSN) interaction.

Deficits in selective attention in symptomatic Huntington disease: assessment using an attentional blink paradigm

BACKGROUND

Impaired selective attention in Huntington disease (HD) may manifest as difficulty in identifying a single target embedded among a series of distractors in rapid serial visual presentation tasks.

METHOD

We used an attentional blink (AB) paradigm to examine whether attentional control is impaired in symptomatic HD. Fourteen HD patients and 13 age-matched healthy controls performed a rapid serial visual presentation task in which 2 targets (T1 and T2) and numerous distractors were presented in rapid succession. We assessed the accuracy of T1 identification and the AB (impaired T2 detection after the correct identification of T1).

RESULTS

Among the HD patients, identification of T1 was significantly impaired and AB was significantly larger but not longer. The HD patients also made significantly more random errors.

CONCLUSIONS

Frontostriatal or frontoparietal dysfunction is likely to compromise attentional control in HD, such that well-masked and rapidly presented target stimuli are difficult to detect and identify, especially as the difficulty level increases. Although we previously reported no AB deficits in presymptomatic HD, with manifest disease we found that the progressive frontoparietal cortical changes compromise attentional control mechanisms.

N-methyl-D-aspartate receptor 2B subunit (GRIN2B) gene variation is associated with alerting, but not with orienting and conflicting in the Attention Network Test

Appropriate attention levels are pivotal for cognitive processes, and individual differences in attentional functioning are related to variations in the interplay of neurotransmitters. The attention network theory reflects attention as a non-homogenous set of separate neural networks: alerting, orienting and conflicting. In the present study, the role of variations in GRIN2B, which encodes the NR2B subunit of N-methyl-d-aspartate (NMDA) receptors, was explored with regard to the regulation of arousal and attention by comparing the efficiency of the three attentional networks as measured with the Attention Network Test (ANT). Two synonymous SNPs in GRIN2B, rs1806201 (T888T) and rs1806191 (H1178H) were genotyped in 324 young Caucasian adults. Results revealed a highly specific modulatory influence of SNP rs1806201 on alerting processes with subjects homozygous for the frequent C allele displaying higher alerting network scores as compared to the other two genotype groups (CT and TT). This effect is due to the fact that in the no cue condition faster reaction times were evident in participants carrying at least one of the rare T alleles, possibly as a result of more effective glutamatergic neurotransmission. The results might be further explained by a dissociation between tonic and phasic alertness modulated by the GRIN2B genotype and by a ceiling effect, meaning that subjects cannot be phasicly alert in excess to a certain level. Altogether, the results show that variations in GRIN2B have to be taken into consideration when examining attentional processes.

The mismatch negativity (MMN)--a unique window to disturbed central auditory processing in ageing and different clinical conditions

In this article, we review clinical research using the mismatch negativity (MMN), a change-detection response of the brain elicited even in the absence of attention or behavioural task. In these studies, the MMN was usually elicited by employing occasional frequency, duration or speech-sound changes in repetitive background stimulation while the patient was reading or watching videos. It was found that in a large number of different neuropsychiatric, neurological and neurodevelopmental disorders, as well as in normal ageing, the MMN amplitude was attenuated and peak latency prolonged. Besides indexing decreased discrimination accuracy, these effects may also reflect, depending on the specific stimulus paradigm used, decreased sensory-memory duration, abnormal perception or attention control or, most importantly, cognitive decline. In fact, MMN deficiency appears to index cognitive decline irrespective of the specific symptomatologies and aetiologies of the different disorders involved.

Translating neurobehavioural endpoints of developmental neurotoxicity tests into in vitro assays and readouts

The developing nervous system is particularly vulnerable to chemical insults. Exposure to chemicals can result in neurobehavioural alterations, and these have been used as sensitive readouts to assess neurotoxicity in animals and man. Deconstructing neurobehaviour into relevant cellular and molecular components may allow for detection of specific neurotoxic effects in cell-based systems, which in turn may allow an easier examination of neurotoxic pathways and modes of actions and eventually inform the regulatory assessment of chemicals with potential developmental neurotoxicity. Here, current developments towards these goals are reviewed. Imaging genetics (CB) provides new insights into the neurobiological correlates of cognitive function that are being used to delineate neurotoxic mechanisms. The gaps between in vivo neurobehaviour and real-time in vitro measurements of neuronal function are being bridged by ex vivo measurements of synaptic plasticity (RW). An example of solvent neurotoxicity demonstrates how an in vivo neurological defect can be linked via the N-methyl-d-aspartate (NMDA)-glutamate receptor as a common target to in vitro readouts (AB). Axonal and dendritic morphology in vitro proved to be good correlates of neuronal connectivity and neurobehaviour in animals exposed to polychlorinated biphenyls and organophosphorus pesticides (PJL). Similarly, chemically induced changes in neuronal morphology affected the formation of neuronal networks on structured surfaces. Such network formation may become an important readout for developmental neurotoxicity in vitro (CvT), especially when combined with human neurons derived from embryonic stem cells (ML). We envision that future in vitro test systems for developmental neurotoxicity will combine the above approaches with exposure information, and we suggest a strategy for test system development and cell-based risk assessment.

Dysregulated brain creatine kinase is associated with hearing impairment in mouse models of Huntington disease

Huntington disease (HD) is a degenerative disorder caused by expanded CAG repeats in exon 1 of the huntingtin gene (HTT). Patients with late-stage HD are known to have abnormal auditory processing, but the peripheral auditory functions of HD patients have yet to be thoroughly assessed. In this study, 19 HD patients (aged 40-59 years) were assessed for hearing impairment using pure-tone audiometry and assessment of auditory brainstem responses (ABRs). PTA thresholds were markedly elevated in HD patients. Consistent with this, elevated ABR thresholds were also detected in two mouse models of HD. Hearing loss thus appears to be an authentic symptom of HD. Immunohistochemical analyses demonstrated the presence of mutant huntingtin that formed intranuclear inclusions in the organ of Corti of HD mice, which might interfere with normal auditory function. Quantitative RT-PCR and Western blot analyses further revealed reduced expression of brain creatine kinase (CKB), a major enzyme responsible for ATP regeneration via the phosphocreatine-creatine kinase (PCr-CK) system, in the cochlea of HD mice. Treatment with creatine supplements ameliorated the hearing impairment of HD mice, suggesting that the impaired PCr-CK system in the cochlea of HD mice may contribute to their hearing impairment. These data also suggest that creatine may be useful for treating the hearing abnormalities of patients with HD.

The functional BDNF Val66Met polymorphism affects functions of pre-attentive visual sensory memory processes

The brain-derived neurotrophic factor (BDNF), a member of the neurotrophin family, is involved in nerve growth and survival. Especially, a single nucleotide polymorphism (SNP) in the BDNF gene, Val66Met, has gained a lot of attention, because of its effect on activity-dependent BDNF secretion and its link to impaired memory processes. We hypothesize that the BDNF Val66Met polymorphism may have modulatory effects on the visual sensory (iconic) memory performance. Two hundred and eleven healthy German students (106 female and 105 male) were included in the data analysis. Since BDNF is also discussed to be involved in the pathogenesis of depression, we additionally tested for possible interactions with depressive mood. The BDNF Val66Met polymorphism significantly influenced iconic-memory performance, with the combined Val/Met-Met/Met genotype group revealing less time stability of information stored in iconic memory than the Val/Val group. Furthermore, this stability was positively correlated with depressive mood exclusively in the Val/Val genotype group. Thus, these results show that the BDNF Val66Met polymorphism has an effect on pre-attentive visual sensory memory processes.

Variations in the TNF-α Gene (TNF-α -308G→A) Affect Attention and Action Selection Mechanisms in a Dissociated Fashion

There is growing interest to understand the molecular basis of complex cognitive processes. While neurotransmitter systems have frequently been examined, other, for example neuroimmunological factors have attracted much less interest. Recent evidence suggests that the A allele of the tumor necrosis factor alpha (TNF-α) 308G→A single nucleotide polymorphism (SNP; rs1800629) enhances cognitive functions. However, it is also known that TNF-α exerts divergent, region-specific effects on neuronal functioning. Thus the finding that the A allele is associated with enhanced cognitive performance may be due to regionally specific effects of TNF-α. In this study, associations between the TNF-α −308G→A single nucleotide polymorphism (rs1800629) and cognitive function in an event-related potential (ERP) study in healthy participants ( n = 96) are investigated. We focus on subprocesses of stimulus-response compatibility that are known to be mediated by different brain systems. The results show a dissociative effect of the TNF- 308G→A SNP on ERPs reflecting attentional (N1) versus conflict and action selection processes [N2 and early-lateralized readiness potential (e-LRP)] between the AA/AG and the GG genotypes. Compared with the GG genotype group, attentional processes (N1) were enhanced in the combined AA/AG genotype group, while conflict processing functions (N2) and the selection of actions (LRP) were reduced. The results refine the picture of the effects of the TNF-α −308G→A SNP on cognitive functions and emphasize the known divergent effects of TNF-α on brain functions.