Electrophysiological evidence for the existence of a posterior cortical-prefrontal-basal forebrain circuitry in modulating sensory responses in visual and somatosensory rat cortical areas

Cognitive Deficits in Aging Related to Changes in Basal Forebrain Neuronal Activity

Aging is a physiological process accompanied by a decline in cognitive performance. The cholinergic neurons of the basal forebrain provide projections to the cortex that are directly engaged in many cognitive processes in mammals. In addition, basal forebrain neurons contribute to the generation of different rhythms in the EEG along the sleep/wakefulness cycle. The aim of this review is to provide an overview of recent advances grouped around the changes in basal forebrain activity during healthy aging. Elucidating the underlying mechanisms of brain function and their decline is especially relevant in today's society as an increasingly aged population faces higher risks of developing neurodegenerative diseases such as Alzheimer's disease. The profound age-related cognitive deficits and neurodegenerative diseases associated with basal forebrain dysfunction highlight the importance of investigating the aging of this brain region.

The Neuromodulatory Role of the Noradrenergic and Cholinergic Systems and Their Interplay in Cognitive Functions: A Focused Review

The noradrenergic and cholinergic modulation of functionally distinct regions of the brain has become one of the primary organizational principles behind understanding the contribution of each system to the diversity of neural computation in the central nervous system. Decades of work has shown that a diverse family of receptors, stratified across different brain regions, and circuit-specific afferent and efferent projections play a critical role in helping such widespread neuromodulatory systems obtain substantial heterogeneity in neural information processing. This review briefly discusses the anatomical layout of both the noradrenergic and cholinergic systems, as well as the types and distributions of relevant receptors for each system. Previous work characterizing the direct and indirect interaction between these two systems is discussed, especially in the context of higher order cognitive functions such as attention, learning, and the decision-making process. Though a substantial amount of work has been done to characterize the role of each neuromodulator, a cohesive understanding of the region-specific cooperation of these two systems is not yet fully realized. For the field to progress, new experiments will need to be conducted that capitalize on the modular subdivisions of the brain and systematically explore the role of norepinephrine and acetylcholine in each of these subunits and across the full range of receptors expressed in different cell types in these regions.

Multisensory Integration: Is Medial Prefrontal Cortex Signaling Relevant for the Treatment of Higher-Order Visual Dysfunctions?

In the mammalian brain, information processing in sensory modalities and global mechanisms of multisensory integration facilitate perception. Emerging experimental evidence suggests that the contribution of multisensory integration to sensory perception is far more complex than previously expected. Here we revise how associative areas such as the prefrontal cortex, which receive and integrate inputs from diverse sensory modalities, can affect information processing in unisensory systems via processes of down-stream signaling. We focus our attention on the influence of the medial prefrontal cortex on the processing of information in the visual system and whether this phenomenon can be clinically used to treat higher-order visual dysfunctions. We propose that non-invasive and multisensory stimulation strategies such as environmental enrichment and/or attention-related tasks could be of clinical relevance to fight cerebral visual impairment.

Lower cholinergic basal forebrain volumes link with cognitive difficulties in schizophrenia

A potential pathophysiological mechanism of cognitive difficulties in schizophrenia is a dysregulated cholinergic system. Particularly, the cholinergic basal forebrain nuclei (BFCN), the source of cortical cholinergic innervation, support multiple cognitive functions, ranging from attention to decision-making. We hypothesized that BFCN structural integrity is altered in schizophrenia and associated with patients' attentional deficits. We assessed gray matter (GM) integrity of cytoarchitectonically defined BFCN region-of-interest in 72 patients with schizophrenia and 73 healthy controls, matched for age and gender, from the COBRE open-source database, via structural magnetic resonance imaging (MRI)-based volumetry. MRI-derived measures of GM integrity (i.e., volumes) were linked with performance on a symbol coding task (SCT), a paper-pencil-based metric that assesses attention, by correlation and mediation analysis. To assess the replicability of findings, we repeated the analyses in an independent dataset comprising 26 patients with schizophrenia and 24 matched healthy controls. BFCN volumes were lower in patients (t(139)=2.51, p = 0.01) and significantly associated with impaired SCT performance (r = 0.31, p = 0.01). Furthermore, lower BFCN volumes mediated the group difference in SCT performance. When including global GM volumes, which were lower in patients, as covariates-of-no-interest, these findings disappeared, indicating that schizophrenia did not have a specific effect on BFCN relative to other regional volume changes. We replicated these findings in the independent cohort, e.g., BFCN volumes were lower in patients and mediated patients' impaired SCT performance. Results demonstrate lower BFCN volumes in schizophrenia, which link with patients' attentional deficits. Data suggest that a dysregulated cholinergic system might contribute to cognitive difficulties in schizophrenia via impaired BFCN.

Reduced Insulin-Like Growth Factor-I Effects in the Basal Forebrain of Aging Mouse

It is known that aging is frequently accompanied by a decline in cognition. Furthermore, aging is associated with lower serum IGF-I levels that may contribute to this deterioration. We studied the effect of IGF-I in neurons of the horizontal diagonal band of Broca (HDB) of young (≤6 months old) and old (≥20-month-old) mice to determine if changes in the response of these neurons to IGF-I occur along with aging. Local injection of IGF-I in the HDB nucleus increased their neuronal activity and induced fast oscillatory activity in the electrocorticogram (ECoG). Furthermore, IGF-I facilitated tactile responses in the primary somatosensory cortex elicited by air-puffs delivered in the whiskers. These excitatory effects decreased in old mice. Immunohistochemistry showed that cholinergic HDB neurons express IGF-I receptors and that IGF-I injection increased the expression of c-fos in young, but not in old animals. IGF-I increased the activity of optogenetically-identified cholinergic neurons in young animals, suggesting that most of the IGF-I-induced excitatory effects were mediated by activation of these neurons. Effects of aging were partially ameliorated by chronic IGF-I treatment in old mice. The present findings suggest that reduced IGF-I activity in old animals participates in age-associated changes in cortical activity.

Symptomatologic pathomechanism of N-methyl D-aspartate receptor encephalitis

N-methyl D-aspartate receptor (NMDAR) encephalitis is a well-characterized clinical syndrome. The main molecular mechanism of NMDAR encephalitis is autoantibody-mediated NMDAR hypofunction in the neuronal synapse. Several pathomechanistic hypotheses might explain how NMDAR hypofunction causes the typical symptoms and prognosis of NMDAR encephalitis. Suppression of NMDAR-dependent gamma-aminobutyric acid interneurons provokes an accelerated activation of the positive feedback loops of the dorsolateral prefrontal cortex/subiculum-nucleus accumbens circuit in the striatum, the ventral tegmental area (VTA), and the nucleus reuniens in the thalamus-hippocampus-VTA loop. Dysregulated activation of the VTA and cortex via those positive feedback loops may explain the rapid clinical deterioration at acute stages of the disease and the well-characterized syndrome that includes limbic system dysfunction, intractable seizures, dyskinesia, coma, and the characteristic extreme delta brush. Progressive cerebellar atrophy is correlated with cumulative disease burden and is associated with worse long-term outcomes, which might be explained by the NMDAR-dependent pathways required to maintain neuronal survival. Those pathomechanistic hypotheses for NMDAR encephalitis support the rationale for the early introduction of combination immunotherapy and the use of adjuvant immunotherapy in patients with persisting symptoms in chronic disease phases.

Spectral plasticity in monkey primary auditory cortex limits performance generalization in a temporal discrimination task

Auditory experience and behavioral training can modify perceptual performance. However, the consequences of temporal perceptual learning for temporal and spectral neural processing remain unclear. Specifically, the attributes of neural plasticity that underlie task generalization in behavioral performance remain uncertain. To assess the relationship between behavioral and neural plasticity, we evaluated neuronal temporal processing and spectral tuning in primary auditory cortex (AI) of anesthetized owl monkeys trained to discriminate increases in the envelope frequency (e.g., 4-Hz standard vs. >5-Hz targets) of sinusoidally amplitude-modulated (SAM) 1-kHz or 2-kHz carriers. Behavioral and neuronal performance generalization was evaluated for carriers ranging from 0.5 kHz to 8 kHz. Psychophysical thresholds revealed high SAM discrimination acuity for carriers from one octave below to ∼0.6 octave above the trained carrier frequency. However, generalization of SAM discrimination learning progressively declined for carrier frequencies >0.6 octave above the trained carrier frequency. Neural responses in AI showed that SAM discrimination training resulted in 1) increases in temporal modulation preference, especially at carriers close to the trained frequency, 2) narrowing of spectral tuning for neurons with characteristic frequencies near the trained carrier frequency, potentially limiting spectral generalization of temporal training effects, and 3) enhancement of firing-rate contrast for rewarded versus nonrewarded SAM frequencies, providing a potential cue for behavioral temporal discrimination near the trained carrier frequency. Our findings suggest that temporal training at a specific spectral location sharpens local frequency tuning, thus, confining the training effects to a narrow frequency range and limiting generalization of temporal discrimination learning across a wider frequency range.NEW & NOTEWORTHY Monkeys' ability to generalize amplitude modulation discrimination to nontrained carriers was limited to one octave below and 0.6 octave above the trained carrier frequency. Asymmetric generalization was paralleled by sharpening in cortical spectral tuning and enhanced firing-rate contrast between rewarded and nonrewarded SAM stimuli at carriers near the trained frequency. The spectral content of the training stimulus specified spectral and temporal plasticity that may provide a neural substrate for limitations in generalization of temporal discrimination learning.

Acetylcholine-mediated top-down attention improves the response to bottom-up inputs by deformation of the attractor landscape

To understand the effect of attention on neuronal dynamics, we propose a multi-module network, with each module consisting of fully interconnected groups of excitatory and inhibitory neurons. This network shows transitive dynamics among quasi-attractors as its typical dynamics. When the release of acetylcholine onto the network is simulated by attention, the transitive dynamics change into stable dynamics in which the system converges to an attractor. We found that this network can reproduce three experimentally observed properties of attention-dependent response modulation, namely an increase in the firing rate, a decrease in the Fano factor of the firing rate, and a decrease in the correlation coefficients between the firing rates of pairs of neurons. Moreover, we also showed theoretically that the release of acetylcholine increases the sensitivity to bottom-up inputs by changing the response function.

Orbitofrontal Cortex Neurons Respond to Sound and Activate Primary Auditory Cortex Neurons

Sensory environments change over a wide dynamic range and sensory processing can change rapidly to facilitate stable perception. While rapid changes may occur throughout the sensory processing pathway, cortical changes are believed to profoundly influence perception. Prior stimulation studies showed that orbitofrontal cortex (OFC) can modify receptive fields and sensory coding in A1, but the engagement of OFC during listening and the pathways mediating OFC influences on A1 are unknown. We show in mice that OFC neurons respond to sounds consistent with a role of OFC in audition. We then show in vitro that OFC axons are present in A1 and excite pyramidal and GABAergic cells in all layers of A1 via glutamatergic synapses. Optogenetic stimulation of OFC terminals in A1 in vivo evokes short-latency neural activity in A1 and pairing activation of OFC projections in A1 with sounds alters sound-evoked A1 responses. Together, our results identify a direct connection from OFC to A1 that can excite A1 neurons at the earliest stage of cortical processing, and thereby sculpt A1 receptive fields. These results are consistent with a role for OFC in adjusting to changing behavioral relevance of sensory inputs and modulating A1 receptive fields to enhance sound processing.

The Cortical States of Wakefulness

Cortical neurons process information on a background of spontaneous, ongoing activity with distinct spatiotemporal profiles defining different cortical states. During wakefulness, cortical states alter constantly in relation to behavioral context, attentional level or general motor activity. In this review article, we will discuss our current understanding of cortical states in awake rodents, how they are controlled, their impact on sensory processing, and highlight areas for future research. A common observation in awake rodents is the rapid change in spontaneous cortical activity from high-amplitude, low-frequency (LF) fluctuations, when animals are quiet, to faster and smaller fluctuations when animals are active. This transition is typically thought of as a change in global brain state but recent work has shown variation in cortical states across regions, indicating the presence of a fine spatial scale control system. In sensory areas, the cortical state change is mediated by at least two convergent inputs, one from the thalamus and the other from cholinergic inputs in the basal forebrain. Cortical states have a major impact on the balance of activity between specific subtypes of neurons, on the synchronization between nearby neurons, as well as the functional coupling between distant cortical areas. This reorganization of the activity of cortical networks strongly affects sensory processing. Thus cortical states provide a dynamic control system for the moment-by-moment regulation of cortical processing.

Multiplexed oscillations and phase rate coding in the basal forebrain

Complex behaviors demand temporal coordination among functionally distinct brain regions. The basal forebrain's afferent and efferent structure suggests a capacity for mediating this coordination at a large scale. During performance of a spatial orientation task, synaptic activity in this region was dominated by four amplitude-independent oscillations temporally organized by the phase of the slowest, a theta-frequency rhythm. Oscillation amplitudes were also organized by task epoch and positively correlated to the task-related modulation of individual neuron firing rates. For many neurons, spiking was temporally organized through phase precession against theta band field potential oscillations. Theta phase precession advanced in parallel to task progression, rather than absolute spatial location or time. Together, the findings reveal a process by which associative brain regions can integrate independent oscillatory inputs and transform them into sequence-specific, rate-coded outputs that are adaptive to the pace with which organisms interact with their environment.

Function of Selective Neuromodulatory Projections in the Mammalian Cerebral Cortex: Comparison Between Cholinergic and Noradrenergic Systems

Cortical processing is dynamically modulated by different neuromodulators. Neuromodulation of the cerebral cortex is crucial for maintaining cognitive brain functions such as perception, attention and learning. However, we do not fully understand how neuromodulatory projections are organized in the cerebral cortex to exert various functions. The basal forebrain (BF) cholinergic projection and the locus coeruleus (LC) noradrenergic projection are well-known neuromodulatory projections to the cortex. Decades of studies have identified anatomical and physiological characteristics of these circuits. While both cholinergic and noradrenergic neurons widely project to the cortex, they exhibit different levels of selectivity. Here, we summarize their anatomical and physiological features, highlighting selectivity and specificity of these circuits to different cortical regions. We discuss the importance of selective modulation by comparing their functions in the cortex. We highlight key features in the input-output circuits and target selectivity of these neuromodulatory projections and their roles in controlling four major brain functions: attention, reinforcement, learning and memory, sleep and wakefulness.

Pharmacological Modulation of Long-Term Potentiation-Like Activity in the Dorsolateral Prefrontal Cortex

Background: Long-term potentiation (LTP) depends on glutamatergic neurotransmission and is modulated by cholinergic, dopaminergic and GABAergic inputs. Paired associative stimulation (PAS) is a neurostimulation paradigm that, when combined with electroencephalography (EEG), assesses LTP-like activity (PAS-induced LTP) in the dorsolateral prefrontal cortex (DLPFC). Thus, we conducted a study to assess the role of cholinergic, dopaminergic, GABAergic and glutamatergic neurotransmission on PAS-induced LTP in the DLPFC. We hypothesized that increasing the dopaminergic tone with L-DOPA and the cholinergic tone with rivastigmine will enhance PAS-induced LTP, while increasing the GABAergic tone with baclofen and inhibiting glutamatergic neurotransmission with dextromethorphan will reduce it compared to placebo. Methods: In this randomized controlled, double-blind cross-over within-subject study, 12 healthy participants received five sessions of PAS to the DLPFC in a random order, each preceded by the administration of placebo or one of the four active drugs. PAS-induced LTP was assessed after each drug administration and compared to PAS-induced LTP after placebo. Results: As predicted, L-DOPA and rivastigmine resulted in enhanced PAS-induced LTP in the DLPFC and dextromethorphan inhibited it compared to placebo. In contrast, baclofen did not significantly suppress PAS-induced LTP compared to placebo. Conclusions: This study provides a novel approach to study DLPFC neuroplasticity and its modulation in patients with brain disorders that are associated with abnormalities in these neurochemical systems. This study was based on a single dose administration of each drug. Given that these drugs are typically administered chronically, future studies should assess the effects of chronic administration.

Topographic Organization of Cholinergic Innervation From the Basal Forebrain to the Visual Cortex in the Rat

Acetylcholine is an important neurotransmitter for the regulation of visual attention, plasticity, and perceptual learning. It is released in the visual cortex predominantly by cholinergic projections from the basal forebrain, where stimulation may produce potentiation of visual processes. However, little is known about the fine organization of these corticopetal projections, such as whether basal forebrain neurons projecting to the primary and secondary visual cortical areas (V1 and V2, respectively) are organized retinotopically. The aim of this study was to map these basal forebrain-V1/V2 projections. Microinjections of the fluorescent retrograde tracer cholera toxin b fragment in different sites within V1 and V2 in Long–Evans rats were performed. Retrogradely labeled cell bodies in the horizontal and vertical limbs of the diagonal band of Broca (HDB and VDB, respectively), nucleus basalis magnocellularis, and substantia innominata (SI), were mapped ex vivo with a computer-assisted microscope stage controlled by stereological software. Choline acetyltranferase immunohistochemistry was used to identify cholinergic cells. Our results showed a predominance of cholinergic projections coming from the HDB. These projections were not retinotopically organized but projections to V1 arised from neurons located in the anterior HDB/SI whereas projections to V2 arised from neurons located throughout the whole extent of HDB/SI. The absence of a clear topography of these projections suggests that BF activation can stimulate visual cortices broadly.

Extent of Dorsolateral Prefrontal Cortex Plasticity and Its Association With Working Memory in Patients With Alzheimer Disease

IMPORTANCE

The extent of dorsolateral prefrontal cortex (DLPFC) plasticity in Alzheimer disease (AD) and its association with working memory are not known.

OBJECTIVES

To determine whether participants with AD had impaired DLPFC plasticity compared with healthy control participants, to compare working memory between participants with AD and controls, and to determine whether DLPFC plasticity was associated with working memory.

DESIGN, SETTING, AND PARTICIPANTS

This cross-sectional study included 32 participants with AD who were 65 years or older and met diagnostic criteria for dementia due to probable AD with a score of at least 17 on the Mini-Mental State Examination and 16 age-matched control participants. Participants were recruited from a university teaching hospital from May 2013 to October 2016.

MAIN OUTCOMES AND MEASURES

Plasticity of the DLPFC measured as potentiation of cortical-evoked activity using paired associative stimulation (a combination of peripheral nerve electrical stimulation and transcranial magnetic stimulation) combined with electroencephalography. Working memory was assessed with the n-back task (1- and 2-back) and measured using the A' statistic.

RESULTS

Among the 32 participants with AD, 17 were women and 15 were men (mean [SD] age, 76.3 [6.3] years); among the 16 controls, 8 were men and 8 were women (mean [SD] age, 76.4 [5.1] years). Participants with AD had impaired DLPFC plasticity (mean [SD] potentiation, 1.18 [0.25]) compared with controls (mean [SD] potentiation, 1.40 [0.35]; F1,44 = 5.90; P = .02; between-group comparison, Cohen d = 0.77; P = .01). Participants with AD also had impaired performances on the 1-back condition (mean [SD] A' = 0.47 [0.30]) compared with controls (mean [SD] A' = 0.96 [0.01]; Cohen d = 1.86; P < .001), with similar findings for participants with AD on the 2-back condition (mean [SD] A' = 0.29 [0.2]) compared with controls (mean [SD], A' = 0.85 [0.18]; Cohen d = 2.83; P < .001). Plasticity of DLPFC was positively associated with working memory performance on the 1-back A' (parameter estimate B [SE] = 0.32 [0.13]; standardized β = 0.29; P = .02) and 2-back A' (B [SE] = 0.43 [0.15]; β = 0.39; P = .006) across both groups after controlling for age, education, and attention.

CONCLUSIONS AND RELEVANCE

This study demonstrated impaired in vivo DLPFC plasticity in patients with AD. The findings support the use of DLPFC plasticity as a measure of DLPFC function and a potential treatment target to enhance DLPFC function and working memory in patients with AD.

Cholinergic shaping of neural correlations

A primary function of the brain is to form representations of the sensory world. Its capacity to do so depends on the relationship between signal correlations, associated with neuronal receptive fields, and noise correlations, associated with neuronal response variability. It was recently shown that the behavioral relevance of sensory stimuli can modify the relationship between signal and noise correlations, presumably increasing the encoding capacity of the brain. In this work, we use data from the visual cortex of the awake mouse watching naturalistic stimuli and show that a similar modification is observed under heightened cholinergic modulation. Increasing cholinergic levels in the cortex through optogenetic stimulation of basal forebrain cholinergic neurons decreases the dependency that is commonly observed between signal and noise correlations. Simulations of correlated neural networks with realistic firing statistics indicate that this change in the correlation structure increases the encoding capacity of the network.

Cholinergic Potentiation Improves Perceptual-Cognitive Training of Healthy Young Adults in Three Dimensional Multiple Object Tracking

A large body of literature supports cognitive enhancement as an effect of cholinergic potentiation. However, it remains elusive whether pharmacological manipulations of cholinergic neurotransmission enhance complex visual processing in healthy individuals. To test this hypothesis, we randomly administered either the cholinergic transmission enhancer donepezil (DPZ; 5 mg P.O.) or placebo (lactose) to young adults (n = 17) 3 h before each session of the three-dimensional (3D) multiple object tracking (3D-MOT) task. This multi-focal attention task evaluates perceptual-cognitive learning over five sessions conducted 7 days apart. A significant amount of learning was observed in the DPZ group but not the placebo group in the fourth session. In the fifth session, this learning effect was observed in both groups. Furthermore, preliminary results for a subgroup of participants (n = 9) 4–14 months later suggested the cholinergic enhancement effect was long lasting. On the other hand, DPZ had no effect on basic visual processing as measured by a motion and orientation discrimination task performed as an independent one-time, pre-post drug study without placebo control (n = 10). The results support the construct that cholinergic enhancement facilitates the encoding of a highly demanding perceptual-cognitive task although there were no significant drug effects on the performance levels compared to placebo.

The Input-Output Relationship of the Cholinergic Basal Forebrain

Basal forebrain cholinergic neurons influence cortical state, plasticity, learning, and attention. They collectively innervate the entire cerebral cortex, differentially controlling acetylcholine efflux across different cortical areas and timescales. Such control might be achieved by differential inputs driving separable cholinergic outputs, although no input-output relationship on a brain-wide level has ever been demonstrated. Here, we identify input neurons to cholinergic cells projecting to specific cortical regions by infecting cholinergic axon terminals with a monosynaptically restricted viral tracer. This approach revealed several circuit motifs, such as central amygdala neurons synapsing onto basolateral amygdala-projecting cholinergic neurons or strong somatosensory cortical input to motor cortex-projecting cholinergic neurons. The presence of input cells in the parasympathetic midbrain nuclei contacting frontally projecting cholinergic neurons suggest that the network regulating the inner eye muscles are additionally regulating cortical state via acetylcholine efflux. This dataset enables future circuit-level experiments to identify drivers of known cortical cholinergic functions.

Cell-specific modulation of plasticity and cortical state by cholinergic inputs to the visual cortex

Acetylcholine (ACh) modulates diverse vital brain functions. Cholinergic neurons from the basal forebrain innervate a wide range of cortical areas, including the primary visual cortex (V1), and multiple cortical cell types have been found to be responsive to ACh. Here we review how different cell types contribute to different cortical functions modulated by ACh. We specifically focus on two major cortical functions: plasticity and cortical state. In layer II/III of V1, ACh acting on astrocytes and somatostatin-expressing inhibitory neurons plays critical roles in these functions. Cell type specificity of cholinergic modulation points towards the growing understanding that even diffuse neurotransmitter systems can mediate specific functions through specific cell classes and receptors.

Modulation of Specific Sensory Cortical Areas by Segregated Basal Forebrain Cholinergic Neurons Demonstrated by Neuronal Tracing and Optogenetic Stimulation in Mice

Neocortical cholinergic activity plays a fundamental role in sensory processing and cognitive functions. Previous results have suggested a refined anatomical and functional topographical organization of basal forebrain (BF) projections that may control cortical sensory processing in a specific manner. We have used retrograde anatomical procedures to demonstrate the existence of specific neuronal groups in the BF involved in the control of specific sensory cortices. Fluoro-Gold (FlGo) and Fast Blue (FB) fluorescent retrograde tracers were deposited into the primary somatosensory (S1) and primary auditory (A1) cortices in mice. Our results revealed that the BF is a heterogeneous area in which neurons projecting to different cortical areas are segregated into different neuronal groups. Most of the neurons located in the horizontal limb of the diagonal band of Broca (HDB) projected to the S1 cortex, indicating that this area is specialized in the sensory processing of tactile stimuli. However, the nucleus basalis magnocellularis (B) nucleus shows a similar number of cells projecting to the S1 as to the A1 cortices. In addition, we analyzed the cholinergic effects on the S1 and A1 cortical sensory responses by optogenetic stimulation of the BF neurons in urethane-anesthetized transgenic mice. We used transgenic mice expressing the light-activated cation channel, channelrhodopsin-2, tagged with a fluorescent protein (ChR2-YFP) under the control of the choline-acetyl transferase promoter (ChAT). Cortical evoked potentials were induced by whisker deflections or by auditory clicks. According to the anatomical results, optogenetic HDB stimulation induced more extensive facilitation of tactile evoked potentials in S1 than auditory evoked potentials in A1, while optogenetic stimulation of the B nucleus facilitated either tactile or auditory evoked potentials equally. Consequently, our results suggest that cholinergic projections to the cortex are organized into segregated pools of neurons that may modulate specific cortical areas.

Molecular underpinnings of prefrontal cortex development in rodents provide insights into the etiology of neurodevelopmental disorders

The prefrontal cortex (PFC), seat of the highest-order cognitive functions, constitutes a conglomerate of highly specialized brain areas and has been implicated to have a role in the onset and installation of various neurodevelopmental disorders. The development of a properly functioning PFC is directed by transcription factors, guidance cues and other regulatory molecules and requires the intricate and temporal orchestration of a number of developmental processes. Disturbance or failure of any of these processes causing neurodevelopmental abnormalities within the PFC may contribute to several of the cognitive deficits seen in patients with neurodevelopmental disorders. In this review, we elaborate on the specific processes underlying prefrontal development, such as induction and patterning of the prefrontal area, proliferation, migration and axonal guidance of medial prefrontal progenitors, and their eventual efferent and afferent connections. We furthermore integrate for the first time the available knowledge from genome-wide studies that have revealed genes linked to neurodevelopmental disorders with experimental molecular evidence in rodents. The integrated data suggest that the pathogenic variants in the neurodevelopmental disorder-associated genes induce prefrontal cytoarchitectonical impairments. This enhances our understanding of the molecular mechanisms of prefrontal (mis)development underlying the four major neurodevelopmental disorders in humans, that is, intellectual disability, autism spectrum disorders, attention deficit hyperactivity disorder and schizophrenia, and may thus provide clues for the development of novel therapies.

Imperceptible somatosensory stimulation alters sensorimotor background rhythm and connectivity

Most sensory input to our body is not consciously perceived. Nevertheless, it may reach the cortex and influence our behavior. In this study, we investigated noninvasive neural signatures of unconscious cortical stimulus processing to understand mechanisms, which (1) prevent low-intensity somatosensory stimuli from getting access to conscious experience and which (2) can explain the associated impediment of conscious perception for additional stimuli. Stimulation of digit 2 in humans far below the detection threshold elicited a cortical evoked potential (P1) at 60 ms, but no further somatosensory evoked potential components. No event-related desynchronization was detected; rather, there was a transient synchronization in the alpha frequency range. Using the same stimulation during fMRI, a reduced centrality of contralateral primary somatosensory cortex (SI) was found, which appeared to be mainly driven by reduced functional connectivity to frontoparietal areas. We conclude that after subthreshold stimulation the (excitatory) feedforward sweep of bottom-up processing terminates in SI preventing access to conscious experience. We speculate that this interruption is due to a predominance of inhibitory processing in SI. The increase in alpha activity and the disconnection of SI from frontoparietal areas are likely correlates of an elevated perception threshold and may thus serve as a gating mechanism for the access to conscious experience.

Turning a Negative into a Positive: Ascending GABAergic Control of Cortical Activation and Arousal

Gamma-aminobutyric acid (GABA) is the main inhibitory neurotransmitter in the brain. Recent technological advances have illuminated the role of GABAergic neurons in control of cortical arousal and sleep. Sleep-promoting GABAergic neurons in the preoptic hypothalamus are well-known. Less well-appreciated are GABAergic projection neurons in the brainstem, midbrain, hypothalamus, and basal forebrain, which paradoxically promote arousal and fast electroencephalographic (EEG) rhythms. Thus, GABA is not purely a sleep-promoting neurotransmitter. GABAergic projection neurons in the brainstem nucleus incertus and ventral tegmental nucleus of Gudden promote theta (4-8 Hz) rhythms. Ventral tegmental area GABAergic neurons, neighboring midbrain dopamine neurons, project to the frontal cortex and nucleus accumbens. They discharge faster during cortical arousal and regulate reward. Thalamic reticular nucleus GABAergic neurons initiate sleep spindles in non-REM sleep. In addition, however, during wakefulness, they tonically regulate the activity of thalamocortical neurons. Other GABAergic inputs to the thalamus arising in the globus pallidus pars interna, substantia nigra pars reticulata, zona incerta, and basal forebrain regulate motor activity, arousal, attention, and sensory transmission. Several subpopulations of cortically projecting GABAergic neurons in the basal forebrain project to the thalamus and neocortex and preferentially promote cortical gamma-band (30-80 Hz) activity and wakefulness. Unlike sleep-active GABAergic neurons, these ascending GABAergic neurons are fast-firing neurons which disinhibit and synchronize the activity of their forebrain targets, promoting the fast EEG rhythms typical of conscious states. They are prominent targets of GABAergic hypnotic agents. Understanding the properties of ascending GABAergic neurons may lead to novel treatments for diseases involving disorders of cortical activation and wakefulness.

Acetylcholine mediates behavioral and neural post-error control

Humans often commit errors when they are distracted by irrelevant information and no longer focus on what is relevant to the task at hand. Adjustments following errors are essential for optimizing goal achievement. The posterior medial frontal cortex (pMFC), a key area for monitoring errors, has been shown to trigger such post-error adjustments by modulating activity in visual cortical areas. However, the mechanisms by which pMFC controls sensory cortices are unknown. We provide evidence for a mechanism based on pMFC-induced recruitment of cholinergic projections to task-relevant sensory areas. Using fMRI in healthy volunteers, we found that error-related pMFC activity predicted subsequent adjustments in task-relevant visual brain areas. In particular, following an error, activity increased in those visual cortical areas involved in processing task-relevant stimulus features, whereas activity decreased in areas representing irrelevant, distracting features. Following treatment with the muscarinic acetylcholine receptor antagonist biperiden, activity in visual areas was no longer under control of error-related pMFC activity. This was paralleled by abolished post-error behavioral adjustments under biperiden. Our results reveal a prominent role of acetylcholine in cognitive control that has not been recognized thus far. Regaining optimal performance after errors critically depends on top-down control of perception driven by the pMFC and mediated by acetylcholine. This may explain the lack of adaptivity in conditions with reduced availability of cortical acetylcholine, such as Alzheimer's disease.

Cell assemblies of the basal forebrain

The basal forebrain comprises several heterogeneous neuronal subgroupings having modular projection patterns to discrete sets of cortical subregions. Each cortical region forms recurrent projections, via prefrontal cortex, that reach the specific basal forebrain subgroups from which they receive afferents. This architecture enables the basal forebrain to selectively modulate cortical responsiveness according to current processing demands. Theoretically, optimal functioning of this distributed network would be enhanced by temporal coordination among coactive basal forebrain neurons, or the emergence of "cell assemblies." The present work demonstrates assembly formation in rat basal forebrain neuronal populations during a selective attention task. Neuron pairs exhibited coactivation patterns organized within beta-frequency time windows (55 ms), regardless of their membership within distinct bursting versus nonbursting basal forebrain subpopulations. Thus, the results reveal a specific temporal framework for integration of information within basal forebrain networks and for the modulation of cortical responsiveness.

Activation of the mouse primary visual cortex by medial prefrontal subregion stimulation is not mediated by cholinergic basalo-cortical projections

The medial prefrontal cortex (mPFC) exerts top-down control of primary visual cortex (V1) activity. As there is no direct neuronal projection from mPFC to V1, this functional connection may use an indirect route, i.e., via basalo-cortical cholinergic projections. The cholinergic projections to V1 originate from neurons in the horizontal limb of the diagonal band of Broca (HDB), which receive neuronal projections from the ventral part of the mPFC, composed of prelimbic (PrL) and infralimbic cortices (IL). Therefore, the objective of this study was to determine whether electrical stimulation of mice mPFC subregions activate (1) V1 neurons; and (2) HDB cholinergic neurons, suggesting that the HDB serves as a relay point in the mPFC-V1 interaction. Neuronal activation was quantified using c-Fos immunocytochemistry or thallium autometallography for each V1 layer using automated particle analysis tools and optical density measurement. Stimulation of IL and PrL induced significantly higher c-Fos expression or thallium labeling in layers II/III and V of V1 in the stimulated hemisphere only. A HDB cholinergic neuron-specific lesion by saporin administration reduced IL-induced c-Fos expression in layers II/III of V1 but not in layer V. However, there was no c-Fos expression or thallium labeling in the HDB neurons, suggesting that this area was not activated by IL stimulation. Stimulation of another mPFC subarea, the anterior cingulate cortex (AC), which is involved in attention and receives input from V1, activated neither V1 nor HDB. The present results indicate that IL and PrL, but not AC, stimulation activates V1 with the minor involvement of the HDB cholinergic projections. These results suggest a functional link between the ventral mPFC and V1, but this function is only marginally supported by HDB cholinergic neurons and may involve other brain regions.

Role of medio-dorsal frontal and posterior parietal neurons during auditory detection performance in rats

To further characterize the role of frontal and parietal cortices in rat cognition, we recorded action potentials simultaneously from multiple sites in the medio-dorsal frontal cortex and posterior parietal cortex of rats while they performed a two-choice auditory detection task. We quantified neural correlates of task performance, including response movements, perception of a target tone, and the differentiation between stimuli with distinct features (different pitches or durations). A minority of units--15% in frontal cortex, 23% in parietal cortex--significantly distinguished hit trials (successful detections, response movement to the right) from correct rejection trials (correct leftward response to the absence of the target tone). Estimating the contribution of movement-related activity to these responses suggested that more than half of these units were likely signaling correct perception of the auditory target, rather than merely movement direction. In addition, we found a smaller and mostly not overlapping population of units that differentiated stimuli based on task-irrelevant details. The detection-related spiking responses we observed suggest that correlates of perception in the rat are sparsely represented among neurons in the rat's frontal-parietal network, without being concentrated preferentially in frontal or parietal areas.

Substitution of natural sensory input by artificial neurostimulation of an amputated trigeminal nerve does not prevent the degeneration of basal forebrain cholinergic circuits projecting to the somatosensory cortex

Peripheral deafferentation downregulates acetylcholine (ACh) synthesis in sensory cortices. However, the responsible neural circuits and processes are not known. We irreversibly transected the rat infraorbital nerve and implanted neuroprosthetic microdevices for proximal stump stimulation, and assessed cytochrome-oxidase and choline- acetyl-transferase (ChAT) in somatosensory, auditory and visual cortices; estimated the number and density of ACh-neurons in the magnocellular basal nucleus (MBN); and localized down-regulated ACh-neurons in basal forebrain using retrograde labeling from deafferented cortices. Here we show that nerve transection, causes down regulation of MBN cholinergic neurons. Stimulation of the cut nerve reverses the metabolic decline but does not affect the decrease in cholinergic fibers in cortex or cholinergic neurons in basal forebrain. Artifical stimulation of the nerve also has no affect of ACh-innervation of other cortices. Cortical ChAT depletion is due to loss of corticopetal MBN ChAT-expressing neurons. MBN ChAT downregulation is not due to a decrease of afferent activity or to a failure of trophic support. Basalocortical ACh circuits are sensory specific, ACh is provided to each sensory cortex "on demand" by dedicated circuits. Our data support the existence of a modality-specific cortex-MBN-cortex circuit for cognitive information processing.

Adaptation to prolonged neuromodulation in cortical cultures: an invariable return to network synchrony

Background

Prolonged neuromodulatory regimes, such as those critically involved in promoting arousal and suppressing sleep-associated synchronous activity patterns, might be expected to trigger adaptation processes and, consequently, a decline in neuromodulator-driven effects. This possibility, however, has rarely been addressed.

Results

Using networks of cultured cortical neurons, acetylcholine microinjections and a novel closed-loop ‘synchrony-clamp’ system, we found that acetylcholine pulses strongly suppressed network synchrony. Over the course of many hours, however, synchrony invariably reemerged, even when feedback was used to compensate for declining cholinergic efficacy. Network synchrony also reemerged following its initial suppression by noradrenaline, but this did not occlude the suppression of synchrony or its gradual reemergence following subsequent cholinergic input. Importantly, cholinergic efficacy could be restored and preserved over extended time scales by periodically withdrawing cholinergic input.

Conclusions

These findings indicate that the capacity of neuromodulators to suppress network synchrony is constrained by slow-acting, reactive processes. A multiplicity of neuromodulators and ultimately neuromodulator withdrawal periods might thus be necessary to cope with an inevitable reemergence of network synchrony.

Electronic supplementary material

The online version of this article (doi:10.1186/s12915-014-0083-3) contains supplementary material, which is available to authorized users.

Task-phase-specific dynamics of basal forebrain neuronal ensembles

Cortically projecting basal forebrain neurons play a critical role in learning and attention, and their degeneration accompanies age-related impairments in cognition. Despite the impressive anatomical and cell-type complexity of this system, currently available data suggest that basal forebrain neurons lack complexity in their response fields, with activity primarily reflecting only macro-level brain states such as sleep and wake, onset of relevant stimuli and/or reward obtainment. The current study examined the spiking activity of basal forebrain neuron populations across multiple phases of a selective attention task, addressing, in particular, the issue of complexity in ensemble firing patterns across time. Clustering techniques applied to the full population revealed a large number of distinct categories of task-phase-specific activity patterns. Unique population firing-rate vectors defined each task phase and most categories of task-phase-specific firing had counterparts with opposing firing patterns. An analogous set of task-phase-specific firing patterns was also observed in a population of posterior parietal cortex neurons. Thus, consistent with the known anatomical complexity, basal forebrain population dynamics are capable of differentially modulating their cortical targets according to the unique sets of environmental stimuli, motor requirements, and cognitive processes associated with different task phases.

The cholinergic basal forebrain in the ferret and its inputs to the auditory cortex

Cholinergic inputs to the auditory cortex can modulate sensory processing and regulate stimulus-specific plasticity according to the behavioural state of the subject. In order to understand how acetylcholine achieves this, it is essential to elucidate the circuitry by which cholinergic inputs influence the cortex. In this study, we described the distribution of cholinergic neurons in the basal forebrain and their inputs to the auditory cortex of the ferret, a species used increasingly in studies of auditory learning and plasticity. Cholinergic neurons in the basal forebrain, visualized by choline acetyltransferase and p75 neurotrophin receptor immunocytochemistry, were distributed through the medial septum, diagonal band of Broca, and nucleus basalis magnocellularis. Epipial tracer deposits and injections of the immunotoxin ME20.4-SAP (monoclonal antibody specific for the p75 neurotrophin receptor conjugated to saporin) in the auditory cortex showed that cholinergic inputs originate almost exclusively in the ipsilateral nucleus basalis. Moreover, tracer injections in the nucleus basalis revealed a pattern of labelled fibres and terminal fields that resembled acetylcholinesterase fibre staining in the auditory cortex, with the heaviest labelling in layers II/III and in the infragranular layers. Labelled fibres with small en-passant varicosities and simple terminal swellings were observed throughout all auditory cortical regions. The widespread distribution of cholinergic inputs from the nucleus basalis to both primary and higher level areas of the auditory cortex suggests that acetylcholine is likely to be involved in modulating many aspects of auditory processing.

Posterior parietal cortex is critical for the encoding, consolidation, and retrieval of a memory that guides attention for learning

Within most contemporary learning theories, reinforcement prediction error, the difference between the obtained and expected reinforcer value, critically influences associative learning. In some theories, this prediction error determines the momentary effectiveness of the reinforcer itself, such that the same physical event produces more learning when its presentation is surprising than when it is expected. In other theories, prediction error enhances attention to potential cues for that reinforcer by adjusting cue-specific associability parameters, biasing the processing of those stimuli so that they more readily enter into new associations in the future. A unique feature of these latter theories is that such alterations in stimulus associability must be represented in memory in an enduring fashion. Indeed, considerable data indicate that altered associability may be expressed days after its induction. Previous research from our laboratory identified brain circuit elements critical to the enhancement of stimulus associability by the omission of an expected event, and to the subsequent expression of that altered associability in more rapid learning. Here, for the first time, we identified a brain region, the posterior parietal cortex, as a potential site for a memorial representation of altered stimulus associability. In three experiments using rats and a serial prediction task, we found that intact posterior parietal cortex function was essential during the encoding, consolidation, and retrieval of an associability memory enhanced by surprising omissions. We discuss these new results in the context of our previous findings and additional plausible frontoparietal and subcortical networks.

Frontal cortex activation causes rapid plasticity of auditory cortical processing

Neurons in the primary auditory cortex (A1) can show rapid changes in receptive fields when animals are engaged in sound detection and discrimination tasks. The source of a signal to A1 that triggers these changes is suspected to be in frontal cortical areas. How or whether activity in frontal areas can influence activity and sensory processing in A1 and the detailed changes occurring in A1 on the level of single neurons and in neuronal populations remain uncertain. Using electrophysiological techniques in mice, we found that pairing orbitofrontal cortex (OFC) stimulation with sound stimuli caused rapid changes in the sound-driven activity within A1 that are largely mediated by noncholinergic mechanisms. By integrating in vivo two-photon Ca(2+) imaging of A1 with OFC stimulation, we found that pairing OFC activity with sounds caused dynamic and selective changes in sensory responses of neural populations in A1. Further, analysis of changes in signal and noise correlation after OFC pairing revealed improvement in neural population-based discrimination performance within A1. This improvement was frequency specific and dependent on correlation changes. These OFC-induced influences on auditory responses resemble behavior-induced influences on auditory responses and demonstrate that OFC activity could underlie the coordination of rapid, dynamic changes in A1 to dynamic sensory environments.

PAS-induced potentiation of cortical-evoked activity in the dorsolateral prefrontal cortex

Neuroplasticity and long-term potentiation (LTP) in the dorsolateral prefrontal cortex (DLPFC) are considered important mechanisms in learning and memory, and their disruption may be related to the pathophysiology of several neuropsychiatric disorders. Paired associative stimulation (PAS) is a brain stimulation paradigm that produces enhanced activity in the human motor cortex that may be related to LTP. In a group of 15 healthy participants, we report on the potentiation of cortical-evoked activity in the human DLPFC using the combination of PAS and electroencephalography. In contrast, a PAS control condition did not result in potentiation in another group of nine healthy participants. We also demonstrate that PAS-induced potentiation of cortical-evoked activity is characterized by anatomical specificity that is largely confined to the site of stimulation. Finally, we show that PAS results in potentiation of θ- and γ-activity and θ-phase-γ-amplitude coupling. These neurophysiological indices may be related to working memory, an important function of the DLPFC. To our knowledge, this is the first report of potentiation of cortical-evoked activity in the DLPFC. As this potentiation may be related to LTP, our findings provide a model through which neuroplasticity in health and disease states in the frontal cortex can be studied.

The role of visual cortex acetylcholine in learning to discriminate temporally modulated visual stimuli

Cholinergic neurons in the basal forebrain innervate discrete regions of the cortical mantle, bestowing the cholinergic system with the potential to dynamically modulate sub-regions of the cortex according to behavioral demands. Cortical cholinergic activity has been shown to facilitate learning and modulate attention. Experiments addressing these issues have primarily focused on widespread cholinergic depletions, extending to areas involved in general cognitive processes and sleep cycle regulation, making a definitive interpretation of the behavioral role of cholinergic projections difficult. Furthermore, a review of the electrophysiological literature suggests that cholinergic modulation is particularly important in representing the fine temporal details of stimuli, an issue rarely addressed in behavioral experimentation. The goal of this work is to understand the role of cholinergic projections, specific to the sensory cortices, in learning to discriminate fine differences in the temporal structure of stimuli. A novel visual Go/No-Go task was developed to assess the ability of rats to learn to discriminate fine differences in the temporal structure of visual stimuli (lights flashing at various frequencies). The cholinergic contribution to this task was examined by selective reduction of acetylcholine projections to visual cortex (VCx) (using 192 IgG-saporin), either before or after discrimination training. We find that in the face of compromised cholinergic input to the VCx, the rats' ability to learn to perform fine discriminations is impaired, whereas their ability to perform previously learned discriminations remains unaffected. These results suggest that acetylcholine serves the role of facilitating plastic changes in the sensory cortices that are necessary for an animal to refine its sensitivity to the temporal characteristics of relevant stimuli.

Deformation of attractor landscape via cholinergic presynaptic modulations: a computational study using a phase neuron model

Corticopetal acetylcholine (ACh) is released transiently from the nucleus basalis of Meynert (NBM) into the cortical layers and is associated with top-down attention. Recent experimental data suggest that this release of ACh disinhibits layer 2/3 pyramidal neurons (PYRs) via muscarinic presynaptic effects on inhibitory synapses. Together with other possible presynaptic cholinergic effects on excitatory synapses, this may result in dynamic and temporal modifications of synapses associated with top-down attention. However, the system-level consequences and cognitive relevance of such disinhibitions are poorly understood. Herein, we propose a theoretical possibility that such transient modifications of connectivity associated with ACh release, in addition to top-down glutamatergic input, may provide a neural mechanism for the temporal reactivation of attractors as neural correlates of memories. With baseline levels of ACh, the brain returns to quasi-attractor states, exhibiting transitive dynamics between several intrinsic internal states. This suggests that top-down attention may cause the attention-induced deformations between two types of attractor landscapes: the quasi-attractor landscape (Q-landscape, present under low-ACh, non-attentional conditions) and the attractor landscape (A-landscape, present under high-ACh, top-down attentional conditions). We present a conceptual computational model based on experimental knowledge of the structure of PYRs and interneurons (INs) in cortical layers 1 and 2/3 and discuss the possible physiological implications of our results.

Leveraging the cortical cholinergic system to enhance attention

Attentional impairments are found in a range of neurodegenerative and neuropsychiatric disorders. However, the development of procognitive enhancers to alleviate these impairments has been hindered by a lack of comprehensive hypotheses regarding the circuitry mediating the targeted attentional functions. Here we discuss the role of the cortical cholinergic system in mediating cue detection and attentional control and propose two target mechanisms for cognition enhancers: stimulation of prefrontal α4β2* nicotinic acetylcholine receptors (nAChR) for the enhancement of cue detection and augmentation of tonic acetylcholine levels for the enhancement of attentional control. This article is part of a Special Issue entitled 'Cognitive Enhancers'.

Neuromodulation of brain states

Switches between different behavioral states of the animal are associated with prominent changes in global brain activity, between sleep and wakefulness or from inattentive to vigilant states. What mechanisms control brain states, and what are the functions of the different states? Here we summarize current understanding of the key neural circuits involved in regulating brain states, with a particular emphasis on the subcortical neuromodulatory systems. At the functional level, arousal and attention can greatly enhance sensory processing, whereas sleep and quiet wakefulness may facilitate learning and memory. Several new techniques developed over the past decade promise great advances in our understanding of the neural control and function of different brain states.

Fear conditioning induces guinea pig auditory cortex activation by foot shock alone

The present study used an optical imaging paradigm to investigate plastic changes in the auditory cortex induced by fear conditioning, in which a sound (conditioned stimulus, CS) was paired with an electric foot-shock (unconditioned stimulus, US). We report that, after conditioning, auditory information could be retrieved on the basis of an electric foot-shock alone. Before conditioning, the auditory cortex showed no response to a foot-shock presented in the absence of sound. In contrast, after conditioning, the mere presentation of a foot-shock without any sound succeeded in eliciting activity in the auditory cortex. Additionally, the magnitude of the optical response in the auditory cortex correlated with variation in the electrocardiogram (correlation coefficient: -0.68). The area activated in the auditory cortex, in response to the electric foot-shock, statistically significantly had a larger cross-correlation value for tone response to the CS sound (12 kHz) compared to the non-CS sounds in normal conditioning group. These results suggest that integration of different sensory modalities in the auditory cortex was established by fear conditioning.

Cholinergic enhancement reduces functional connectivity and BOLD variability in visual extrastriate cortex during selective attention

Enhancing cholinergic function improves performance on various cognitive tasks and alters neural responses in task specific brain regions. We have hypothesized that the changes in neural activity observed during increased cholinergic function reflect an increase in neural efficiency that leads to improved task performance. The current study tested this hypothesis by assessing neural efficiency based on cholinergically-mediated effects on regional brain connectivity and BOLD signal variability. Nine subjects participated in a double-blind, placebo-controlled crossover fMRI study. Following an infusion of physostigmine (1 mg/h) or placebo, echo-planar imaging (EPI) was conducted as participants performed a selective attention task. During the task, two images comprised of superimposed pictures of faces and houses were presented. Subjects were instructed periodically to shift their attention from one stimulus component to the other and to perform a matching task using hand held response buttons. A control condition included phase-scrambled images of superimposed faces and houses that were presented in the same temporal and spatial manner as the attention task; participants were instructed to perform a matching task. Cholinergic enhancement improved performance during the selective attention task, with no change during the control task. Functional connectivity analyses showed that the strength of connectivity between ventral visual processing areas and task-related occipital, parietal and prefrontal regions reduced significantly during cholinergic enhancement, exclusively during the selective attention task. Physostigmine administration also reduced BOLD signal temporal variability relative to placebo throughout temporal and occipital visual processing areas, again during the selective attention task only. Together with the observed behavioral improvement, the decreases in connectivity strength throughout task-relevant regions and BOLD variability within stimulus processing regions support the hypothesis that cholinergic augmentation results in enhanced neural efficiency. This article is part of a Special Issue entitled 'Cognitive Enhancers'.

Tuning shifts of the auditory system by corticocortical and corticofugal projections and conditioning

The central auditory system consists of the lemniscal and nonlemniscal systems. The thalamic lemniscal and nonlemniscal auditory nuclei are different from each other in response properties and neural connectivities. The cortical auditory areas receiving the projections from these thalamic nuclei interact with each other through corticocortical projections and project down to the subcortical auditory nuclei. This corticofugal (descending) system forms multiple feedback loops with the ascending system. The corticocortical and corticofugal projections modulate auditory signal processing and play an essential role in the plasticity of the auditory system. Focal electric stimulation - comparable to repetitive tonal stimulation - of the lemniscal system evokes three major types of changes in the physiological properties, such as the tuning to specific values of acoustic parameters of cortical and subcortical auditory neurons through different combinations of facilitation and inhibition. For such changes, a neuromodulator, acetylcholine, plays an essential role. Electric stimulation of the nonlemniscal system evokes changes in the lemniscal system that is different from those evoked by the lemniscal stimulation. Auditory signals ascending from the lemniscal and nonlemniscal thalamic nuclei to the cortical auditory areas appear to be selected or adjusted by a "differential" gating mechanism. Conditioning for associative learning and pseudo-conditioning for nonassociative learning respectively elicit tone-specific and nonspecific plastic changes. The lemniscal, corticofugal and cholinergic systems are involved in eliciting the former, but not the latter. The current article reviews the recent progress in the research of corticocortical and corticofugal modulations of the auditory system and its plasticity elicited by conditioning and pseudo-conditioning.

Axonal varicosity density as an index of local neuronal interactions

Diffuse transmission is an important non-synaptic communication mode in the cerebral neocortex, in which neurotransmitters released from en passant varicosities interact with surrounding cells. In a previous study we have shown that the cholinergic axonal segments which were in the microproximity with dopaminergic fibers possessed a greater density of en passant varicosities compared to more distant segments, suggesting an activity-dependent level of en passant varicosities in the axonal zone of interaction. To further evaluate this plastic relationship, the density of cholinergic varicosities was quantified on fiber segments within the microproximity of activated or non-activated pyramidal cells of the prefrontal cortex (mPFC). Repetitive 14 days patterned visual stimulation paired with an electrical stimulation of the cholinergic fibers projecting to the mPFC from the HDB was performed to induce persistent axonal plastic changes. The c-Fos early gene immunoreactivity was used as a neuronal activity marker of layer V pyramidal cells, labelled with anti-glutamate transporter EAAC1. Cholinergic fibers were labeled with anti-ChAT (choline acetyltransferase) immunostaining. The density of ChAT+ varicosities on and the length of fiber segments within the 3 µm microproximity of c-Fos positive/negative pyramidal cells were evaluated on confocal images. More than 50% of the pyramidal cells in the mPFC were c-Fos immunoreactive. Density of ChAT+ varicosities was significantly increased within 3 µm vicinity of activated pyramidal cells (0.50±0.01 per µm of ChAT+ fiber length) compared to non-activated cells in this group (0.34±0.001; p≤0.05) or control rats (0.32±0.02; p≤0.05). Different types of stimulation (visual, HDB or visual/HDB) induced similar increase of the density of ChAT+ varicosities within microproximity of activated pyramidal cells. This study demonstrated at the subcellular level an activity-dependent enrichment of ChAT+ varicosities in the axonal zone of interaction with other neuronal elements.

Cholinergic modulation of cognition: insights from human pharmacological functional neuroimaging

Evidence from lesion and cortical-slice studies implicate the neocortical cholinergic system in the modulation of sensory, attentional and memory processing. In this review we consider findings from sixty-three healthy human cholinergic functional neuroimaging studies that probe interactions of cholinergic drugs with brain activation profiles, and relate these to contemporary neurobiological models. Consistent patterns that emerge are: (1) the direction of cholinergic modulation of sensory cortex activations depends upon top-down influences; (2) cholinergic hyperstimulation reduces top-down selective modulation of sensory cortices; (3) cholinergic hyperstimulation interacts with task-specific frontoparietal activations according to one of several patterns, including: suppression of parietal-mediated reorienting; decreasing ‘effort’-associated activations in prefrontal regions; and deactivation of a ‘resting-state network’ in medial cortex, with reciprocal recruitment of dorsolateral frontoparietal regions during performance-challenging conditions; (4) encoding-related activations in both neocortical and hippocampal regions are disrupted by cholinergic blockade, or enhanced with cholinergic stimulation, while the opposite profile is observed during retrieval; (5) many examples exist of an ‘inverted-U shaped’ pattern of cholinergic influences by which the direction of functional neural activation (and performance) depends upon both task (e.g. relative difficulty) and subject (e.g. age) factors. Overall, human cholinergic functional neuroimaging studies both corroborate and extend physiological accounts of cholinergic function arising from other experimental contexts, while providing mechanistic insights into cholinergic-acting drugs and their potential clinical applications.

Emergent processes in cognitive-emotional interactions

Emotion and cognition have been viewed as largely separate entities in the brain. Within this framework, significant progress has been made in understanding specific aspects of behavior. Research in the past two decades, however, has started to paint a different picture of brain organization, one in which network interactions are key to understanding complex behaviors. From both basic and clinical perspectives, the characterization of cognitive-emotional interactions constitutes a fundamental issue in the investigation of the mind and brain. This review will highlight the interactive and integrative potential that exists in the brain to bring together the cognitive and emotional domains. First, anatomical evidence will be provided, focusing on structures such as hypothalamus, basal forebrain, amygdala, cingulate cortex, orbitofrontal cortex, and insula. Data on functional interactions will then be discussed, followed by a discussion of a dual competition framework, which describes cognitive-emotional interactions in terms of perceptual and cognitive competition mechanisms.

Auditory cortex spatial sensitivity sharpens during task performance

Activity in the primary auditory cortex (A1) is known to be essential for normal sound localization behavior, yet previous studies of the spatial sensitivity of neurons in A1 have found surprisingly broad spatial tuning. We tested the hypothesis that spatial tuning sharpens when an animal engages in an auditory task. Cats performed a task that required evaluation of the locations of sounds and another that required active listening but in which sound location was irrelevant. Some 26–44% of the units recorded in A1 showed significantly sharpened spatial tuning during the behavioral tasks compared to idle conditions, with greatest sharpening during the location-relevant task. Spatial sharpening occurred on a scale of tens of seconds and could be replicated multiple times within ~1.5-hr test sessions. Sharpening resulted primarily from increased suppression of responses to sounds at least-preferred locations. That and an observed increase in latencies suggest an important role of inhibitory mechanisms.