Neurochemical modulation of repetition suppression and novelty signals in the human brain

Novelty triggers time-dependent theta oscillatory dynamics in cortical-hippocampal-midbrain circuitry

Rapid adaptation to novel environments is crucial for survival, and this ability is impaired in many neuropsychiatric disorders. Understanding neural adaptation to novelty exposure therefore has therapeutic implications. Here, I found that novelty induces time-dependent theta (4-12Hz) oscillatory dynamics in brain circuits including the medial prefrontal cortex (mPFC), ventral hippocampus (vHPC), and ventral tegmental area (VTA), but not dorsal hippocampus (dHPC), as mice adapt to a novel environment. Local field potential (LFP) recordings were performed while mice were freely behaving in a novel or a familiar arena for 10 min. Initially, mice exhibited increased exploratory behavior upon exposure to novelty, which gradually decreased to levels observed in mice exposed to the familiar arena. Over the same time course, the mPFC, vHPC, and VTA displayed progressively increasing theta power through novelty exposure. Additionally, theta coherence and theta phase synchrony measures demonstrated that novelty weakened the connectivity between these areas, which then gradually strengthened to the level observed in the familiar group. Conversely, mice exposed to the familiar arena showed steady and consistent behavior as well as theta dynamics in all areas. Treatment with a dopamine D1-receptor (D1R) antagonist in the vHPC disrupted neurophysiological adaptation to novelty specifically in the vHPC-mPFC and vHPC-VTA circuits, without affecting behavior. Thus, novelty induces distinct theta dynamics that are not readily dictated by behavior in the mPFC, vHPC, and VTA circuits, a process mediated by D1Rs in the vHPC. The observed time-dependent circuit dynamics in the key learning and memory circuit would provide new insights for treating neuropsychiatric disorders that often show impaired novelty processing.

Reduction in the activity of VTA/SNc dopaminergic neurons underlies aging-related decline in novelty seeking

Curiosity, or novelty seeking, is a fundamental mechanism motivating animals to explore and exploit environments to improve survival, and is also positively associated with cognitive, intrapersonal and interpersonal well-being in humans. However, curiosity declines as humans age, and the decline even positively predicts the extent of cognitive decline in Alzheimer's disease patients. Therefore, determining the underlying mechanism, which is currently unknown, is an urgent task for the present aging society that is growing at an unprecedented rate. This study finds that seeking behaviors for both social and inanimate novelties are compromised in aged mice, suggesting that the aging-related decline in curiosity and novelty-seeking is a biological process. This study further identifies an aging-related reduction in the activity (manifesting as a reduction in spontaneous firing) of dopaminergic neurons in the ventral tegmental area (VTA) and substantia nigra pars compacta (SNc). Finally, this study establishes that this reduction in activity causally underlies the aging-related decline in novelty-seeking behaviors. This study potentially provides an interventional strategy for maintaining high curiosity in the aged population, i.e., compensating for the reduced activity of VTA/SNc dopaminergic neurons, enabling the aged population to cope more smoothly with the present growing aging society, physically, cognitively and socioeconomically.

Lack of source memory as a potential marker of early assimilation of novel items into current knowledge

In daily life, humans process a plethora of new information that can be either consistent (familiar) or inconsistent (novel) with prior knowledge. Over time, both types of information can integrate into our accumulated knowledge base via distinct pathways. However, the mnemonic processes supporting the integration of information that is inconsistent with prior knowledge remain under-characterized. In the current study, we used functional magnetic resonance imaging (fMRI) to examine the initial assimilation of novel items into the semantic network. Participants saw three repetitions of adjective-noun word pairs that were either consistent or inconsistent with prior knowledge. Twenty-four hours later, they were presented with the same stimuli again while undergoing fMRI scans. Outside the scanner, participants completed a surprise recognition test. We found that when the episodic context associated with initially inconsistent items was irretrievable, the neural signature of these items was indistinguishable from that of consistent items. In contrast, initially inconsistent items with accessible episodic contexts showed neural signatures that differed from those associated with consistent items. We suggest that, at least one day post encoding, items inconsistent with prior knowledge can show early assimilation into the semantic network only when their episodic contexts become inaccessible during retrieval, thus evoking a sense of familiarity.

Novelty selectively permits learning-associated plasticity in ventral tegmental-hippocampal-prefrontal circuitry

Modifying established behavior in novel situations is essential, and patients with neuropsychiatric disorders often lack this flexibility. Understanding how novelty affects behavioral flexibility therefore has therapeutic potential. Here, novelty differentially impacts connectivity within the ventral tegmental-hippocampal-medial prefrontal (VTA-HPC-mPFC) circuit, thereby enhancing the ability of mice to overcome established behavioral bias and adapt to new rules. Circuit connectivity was measured by local field potential (LFP) coherence. As mice exposed to novelty learned to overcome previously established spatial bias, the ventral HPC (vHPC) strengthens its coherence with the VTA and mPFC in theta frequency (4-8 Hz). Novelty or learning did not affect circuits involving the dorsal HPC (dHPC). Without novelty, however, mice continued following established spatial bias and connectivity strength remained stable in the VTA-HPC-mPFC circuit. Pharmacologically blocking dopamine D1-receptors (D1Rs) in the vHPC abolished the behavioral and physiological impacts of novelty. Thus, novelty promotes behavioral adaptation by permitting learning-associated plasticity in the vHPC-mPFC and VTA-vHPC circuit, a process mediated by D1Rs in the vHPC.

Brain Reactions to Opening and Closing the Eyes: Salivary Cortisol and Functional Connectivity

This study empirically assessed the strength and duration of short-term effects induced by brain reactions to closing/opening the eyes on a few well-known resting-state networks. We also examined the association between these reactions and subjects’ cortisol levels. A total of 55 young adults underwent 8-min resting-state fMRI (rs-fMRI) scans under 4-min eyes-closed and 4-min eyes-open conditions. Saliva samples were collected from 25 of the 55 subjects before and after the fMRI sessions and assayed for cortisol levels. Our empirical results indicate that when the subjects were relaxed with their eyes closed, the effect of opening the eyes on conventional resting-state networks (e.g., default-mode, frontal-parietal, and saliency networks) lasted for roughly 60-s, during which we observed a short-term increase in activity in rs-fMRI time courses. Moreover, brain reactions to opening the eyes had a pronounced effect on time courses in the temporo-parietal lobes and limbic structures, both of which presented a prolonged decrease in activity. After controlling for demographic factors, we observed a significantly positive correlation between pre-scan cortisol levels and connectivity in the limbic structures under both conditions. Under the eyes-closed condition, the temporo-parietal lobes presented significant connectivity to limbic structures and a significantly positive correlation with pre-scan cortisol levels. Future research on rs-fMRI could consider the eyes-closed condition when probing resting-state connectivity and its neuroendocrine correlates, such as cortisol levels. It also appears that abrupt instructions to open the eyes while the subject is resting quietly with eyes closed could be used to probe brain reactivity to aversive stimuli in the ventral hippocampus and other limbic structures.

Electrophysiological correlates of perceptual prediction error are attenuated in dyslexia

A perceptual adaptation deficit often accompanies reading difficulty in dyslexia, manifesting in poor perceptual learning of consistent stimuli and reduced neurophysiological adaptation to stimulus repetition. However, it is not known how adaptation deficits relate to differences in feedforward or feedback processes in the brain. Here we used electroencephalography (EEG) to interrogate the feedforward and feedback contributions to neural adaptation as adults with and without dyslexia viewed pairs of faces and words in a paradigm that manipulated whether there was a high probability of stimulus repetition versus a high probability of stimulus change. We measured three neural dependent variables: expectation (the difference between prestimulus EEG power with and without the expectation of stimulus repetition), feedforward repetition (the difference between event-related potentials (ERPs) evoked by an expected change and an unexpected repetition), and feedback-mediated prediction error (the difference between ERPs evoked by an unexpected change and an expected repetition). Expectation significantly modulated prestimulus theta- and alpha-band EEG in both groups. Unexpected repetitions of words, but not faces, also led to significant feedforward repetition effects in the ERPs of both groups. However, neural prediction error when an unexpected change occurred instead of an expected repetition was significantly weaker in dyslexia than the control group for both faces and words. These results suggest that the neural and perceptual adaptation deficits observed in dyslexia reflect the failure to effectively integrate perceptual predictions with feedforward sensory processing. In addition to reducing perceptual efficiency, the attenuation of neural prediction error signals would also be deleterious to the wide range of perceptual and procedural learning abilities that are critical for developing accurate and fluent reading skills.

Functional Interactions between Sensory and Memory Networks for Adaptive Behavior

The brain's capacity to adapt to sensory inputs is key for processing sensory information efficiently and interacting in new environments. Following repeated exposure to the same sensory input, brain activity in sensory areas is known to decrease as inputs become familiar, a process known as adaptation. Yet, the brain-wide mechanisms that mediate adaptive processing remain largely unknown. Here, we combine multimodal brain imaging (functional magnetic resonance imaging [fMRI], magnetic resonance spectroscopy) with behavioral measures of orientation-specific adaptation (i.e., tilt aftereffect) to investigate the functional and neurochemical mechanisms that support adaptive processing. Our results reveal two functional brain networks: 1) a sensory-adaptation network including occipital and dorsolateral prefrontal cortex regions that show decreased fMRI responses for repeated stimuli and 2) a perceptual-memory network including regions in the parietal memory network (PMN) and dorsomedial prefrontal cortex that relate to perceptual bias (i.e., tilt aftereffect). We demonstrate that adaptation relates to increased occipito-parietal connectivity, while decreased connectivity between sensory-adaptation and perceptual-memory networks relates to GABAergic inhibition in the PMN. Thus, our findings provide evidence that suppressive interactions between sensory-adaptation (i.e., occipito-parietal) and perceptual-memory (i.e., PMN) networks support adaptive processing and behavior, proposing a key role of memory systems in efficient sensory processing.

Examining the transition of novel information toward familiarity

Throughout their lives, humans encounter multiple instances of new information that can be inconsistent with prior knowledge (novel). Over time, the once-novel information becomes integrated into their established knowledge base, shifting from novelty to familiarity. In this study, we investigated the processes by which the first steps of this transition take place. We hypothesized that the neural representations of initially novel items gradually change over the course of repeated presentations, expressing a shift toward familiarity. We further assumed that this shift could be traced by examining neural patterns using fMRI. In two experiments, while being scanned, participants read noun-adjective word pairs that were either consistent or inconsistent with their prior knowledge. Stimuli were repeated 3-6 times within the scans. Employing mass univariate and multivariate similarity analyses, we showed that the neural representations associated with the initial presentation of familiar versus novel objects differed in lateral frontal and temporal regions, the medial prefrontal cortex, and the medial temporal lobe. Importantly, the neural representations of novel stimuli gradually changed throughout repetitions until they became indistinguishable from their respective familiar items. We interpret these findings as indicating that an early phase of familiarization can be completed within a few repetitions. This initial familiarization can then serve as the prerequisite to the integration of novel items into existing knowledge. Future empirical and theoretical works can build on the current findings to develop a comprehensive model of the transition from novelty to familiarity.

Dopaminergic modulation of novelty repetition in Parkinson's disease: A study of P3 event-related brain potentials

OBJECTIVE

Parkinson's Disease (PD) is a neurodegenerative disease caused by the loss of dopaminergic neurons. Cognitive impairments have been reported using the event-related potential (ERP) technique. Patients show reduced novelty P3 (nP3) amplitudes in oddball experiments, a response to infrequent, surprising stimuli, linked to the orienting response of the brain. The nP3 is thought to depend on dopaminergic neuronal pathways though the effect of dopaminergic medication in PD has not yet been investigated.

METHODS

Twenty-two patients with PD were examined "on" and "off" their regular dopaminergic medication in a novelty 3-stimulus-oddball task. Thirty-four healthy controls were also examined over two sessions, but received no medication. P3 amplitudes were compared throughout experimental conditions.

RESULTS

All participants showed sizeable novelty difference ERP effects, i.e. n_dP3 amplitudes, during both testing sessions. An interaction of diagnosis, medication and testing order was also found, indicating that dopaminergic medication modulated n_dP3 in patients with PD across the two testing sessions: We observed enhanced n_dP3 amplitudes from PD patients who were off medication on the second testing session.

CONCLUSION

Patients with PD 'off' medication showed ERP evidence for repetition-related enhancement of novelty responses. Dopamine depletion in neuronal pathways that are affected by mid-stage PD possibly accounts for this modulation of novelty processing.

SIGNIFICANCE

The data in this study potentially suggest that repetition effects on novelty processing in patients with PD are enhanced by dopaminergic depletion.

Midbrain circuits of novelty processing

Novelty triggers an increase in orienting behavior that is critical to evaluate the potential salience of unknown events. As novelty becomes familiar upon repeated encounters, this increase in response rapidly habituates as a form of behavioral adaptation underlying goal-directed behaviors. Many neurodevelopmental, psychiatric and neurodegenerative disorders are associated with abnormal responses to novelty and/or familiarity, although the neuronal circuits and cellular/molecular mechanisms underlying these natural behaviors in the healthy brain are largely unknown, as is the maladaptive processes that occur to induce impairment of novelty signaling in diseased brains. In rodents, the development of cutting-edge tools that allow for measurements of real time activity dynamics in selectively identified neuronal ensembles by gene expression signatures is beginning to provide advances in understanding the neural bases of the novelty response. Accumulating evidence indicate that midbrain circuits, the majority of which linked to dopamine transmission, promote exploratory assessments and guide approach/avoidance behaviors to different types of novelty via specific projection sites. The present review article focuses on midbrain circuit analysis relevant to novelty processing and habituation with familiarity.

Skill acquisition is enhanced by reducing trial-to-trial repetition

Developing approaches to improve motor skill learning is of considerable interest across multiple disciplines. Previous research has typically shown that repeating the same action on consecutive trials enhances short-term performance but has detrimental effects on longer term skill acquisition. However, most prior research has contrasted the effects of repetition only at the block level; in the current study we examined the effects of repeating individual trials embedded in a larger randomized block, a feature that is often overlooked when random trial orders are generated in learning tasks. With 4 days of practice, a "Minimal Repeats" group, who rarely experienced repeating stimuli on consecutive trials during training, improved to a greater extent than a "Frequent Repeats" group, who were frequently presented with repeating stimuli on consecutive trials during training. Our results extend the previous finding of the beneficial effects of random compared with blocked practice on performance, showing that reduced trial-to-trial repetition during training is favorable with regard to skill learning. This research highlights that limiting the number of repeats on consecutive trials is a simple behavioral manipulation that can enhance the process of skill learning. Data/analysis code and Supplemental Material are available at https://osf.io/p3278/.NEW & NOTEWORTHY Numerous studies have shown that performing different subtasks across consecutive blocks of trials enhances learning. We examined whether the same effect would occur on a trial-to-trial level. Our Minimal Repeats group, who primarily responded to different stimuli on consecutive trials, learned more than our Frequent Repeats group, who frequently responded to the same stimulus on consecutive trials. This shows that minimizing trial-to-trial repetition is a simple and easily applicable manipulation that can enhance learning.

Neural adaptation and cognitive inflexibility in repeated problem-solving behaviors

Repeated stimulus processing is often associated with a reduction in neural activity, known as neural adaptation. Therefore, people are more sensitive to novelty detection but likely lose flexibility in subsequent novelty processing after detection. To demonstrate the dynamic changes in neural adaption in repeated problem-solving behaviors and test its negative influence on subsequent nonrepetitive problem-solving behaviors, we adopted a Chinese character decomposition task in this fMRI study. Participants were asked to repeatedly perform 3-5 practice problems that could be solved by the same loose chunk decomposition (LCD) solution followed by a test problem that could be solved by a tight chunk decomposition (TCD) solution in the enhanced-set condition. The practice problem gradually elicited lower percent signal changes within the cuneus, superior parietal lobule (SPL), inferior frontal gyrus (IFG) and medial prefrontal cortex (mPFC) in serial positions -1, -2 and -3 of a set, implying that neural adaptation occurred in repeated practice. Both the test problem and the practice problem that following it recruited greater activation of the SPL and IFG in the enhanced-set condition than in the base-set condition when the practice problem and test problem alternately appeared, implying that the task switching cost from a more dominant task to a less dominant task and vice versa was increased after neural adaptation occurred. In other words, repeatedly solving a set of similar problems with the same solution likely leads to neural adaptation and cognitive inflexibility, which in turn have an undifferentiated impact on task switching. This finding expands existing knowledge about the neurocognitive mechanism underlying the formation of the mental set and sheds light on the influence of neural adaptation on subsequent processing.

GABAergic and cholinergic modulation of repetition suppression in inferior temporal cortex

Neurons in many brain areas of different species reduce their response when a stimulus is repeated. Such adaptation or repetition suppression is prevalent in inferior temporal (IT) cortex. The mechanisms underlying repetition suppression in IT are still poorly understood. Studies in rodents and in-vitro experiments suggest that acetylcholine and GABA can contribute to repetition suppression by interacting with fatigue-related or local adaptation mechanisms. Here, we examined the contribution of cholinergic and GABAergic mechanisms to repetition suppression in macaque IT, using an adaptation paradigm in which familiar images were presented successively with a short interstimulus interval. We found that intracortical local injections of acetylcholine and of the GABA_A receptor antagonist Gabazine both increased repetition suppression in awake macaque IT. The increased repetition suppression was observed for both spiking activity and local field potential power. The latter was present mainly for frequencies below 50 Hz, spectral bands that typically do not show consistent repetition suppression in IT. Although increased with drug application, repetition suppression remained stimulus selective. These findings agree with the hypothesis that repetition suppression of IT neurons mainly results from suppressed input from upstream and other IT neurons but depend less on intrinsic neuronal fatigue.

The neurochemical basis of the contextual interference effect

Efficient practice organization maximizes learning outcome. Although randomization of practice as compared to blocked practice damages training performance, it boosts retention performance, an effect called contextual interference. Motor learning modulates the GABAergic (gamma-aminobutyric acid) system within the sensorimotor cortex (SM); however, it is unclear whether different practice regimes differentially modulate this system and whether this is impacted by aging. Young and older participants were trained on 3 variations of a visuomotor task over 3 days, following either blocked or random practice schedule and retested 6 days later. Using magnetic resonance spectroscopy, SM and occipital cortex GABA+ levels were measured before and after training during the first and last training days. We found that (1) behavioral data confirmed the contextual interference effects, (2) within-day occipital cortex GABA+ levels decreased in random and increased in blocked group. This effect was more pronounced in older adults; and (3) baseline SM GABA+ levels predicted initial performance. These findings indicate a differential modulation of GABA levels across practice groups that is amplified by aging.

Brain regions that show repetition suppression and enhancement: A meta-analysis of 137 neuroimaging experiments

Repetition suppression and enhancement refer to the reduction and increase in the neural responses for repeated rather than novel stimuli, respectively. This study provides a meta-analysis of the effects of repetition suppression and enhancement, restricting the data used to that involving fMRI/PET, visual stimulus presentation, and healthy participants. The major findings were as follows. First, the global topography of the repetition suppression effects was strikingly similar to that of the "subsequent memory" effects, indicating that the mechanism for repetition suppression is the reduced engagement of an encoding system. The lateral frontal cortex effects involved the frontoparietal control network regions anteriorly and the dorsal attention network regions posteriorly. The left fusiform cortex effects predominantly involved the dorsal attention network regions, whereas the right fusiform cortex effects mainly involved the visual network regions. Second, the category-specific meta-analyses and their comparisons indicated that most parts of the alleged category-specific regions showed repetition suppression for more than one stimulus category. In this regard, these regions may not be "dedicated cortical modules," but are more likely parts of multiple overlapping large-scale maps of simple features. Finally, the global topography of the repetition enhancement effects was similar to that of the "retrieval success" effects, suggesting that the mechanism for repetition enhancement is voluntary or involuntary explicit retrieval during an implicit memory task. Taken together, these results clarify the network affiliations of the regions showing reliable repetition suppression and enhancement effects and contribute to the theoretical interpretations of the local and global topography of these two effects. Hum Brain Mapp 38:1894-1913, 2017. © 2017 Wiley Periodicals, Inc.