Sustained attention and apical dendrite activity in recurrent circuits

Layer 6b controls brain state via apical dendrites and the higher-order thalamocortical system

The deepest layer of the cortex (layer 6b [L6b]) contains relatively few neurons, but it is the only cortical layer responsive to the potent wake-promoting neuropeptide orexin/hypocretin. Can these few neurons significantly influence brain state? Here, we show that L6b-photoactivation causes a surprisingly robust enhancement of attention-associated high-gamma oscillations and population spiking while abolishing slow waves in sleep-deprived mice. To explain this powerful impact on brain state, we investigated L6b's synaptic output using optogenetics, electrophysiology, and monoCaTChR ex vivo. We found powerful output in the higher-order thalamus and apical dendrites of L5 pyramidal neurons, via L1a and L5a, as well as in superior colliculus and L6 interneurons. L6b subpopulations with distinct morphologies and short- and long-term plasticities project to these diverse targets. The L1a-targeting subpopulation triggered powerful NMDA-receptor-dependent spikes that elicited burst firing in L5. We conclude that orexin/hypocretin-activated cortical neurons form a multifaceted, fine-tuned circuit for the sustained control of the higher-order thalamocortical system.

The neural hierarchy of consciousness

The chief undertaking in the studies of consciousness is that of unravelling "the minimal set of neural processes that are together sufficient for the conscious experience of a particular content - the neural correlates of consciousness". To this day, this crusade remains at an impasse, with a clash of two main theories: consciousness may arise either in a graded and cortically-localised fashion, or in an all-or-none and widespread one. In spite of the long-lasting theoretical debates, neurophysiological theories of consciousness have been mostly dissociated from them. Herein, a theoretical review will be put forth with the aim to change that. In its first half, we will cover the hard available evidence on the neurophysiology of consciousness, whereas in its second half we will weave a series of considerations on both theories and substantiate a novel take on conscious awareness: the levels of processing approach, partitioning the conscious architecture into lower- and higher-order, graded and nonlinear.

Semantic priming and neurobiology in schizophrenia: A theoretical review

In this theoretical review we bridge the cognitive and neurobiological sciences to shed light on the neurocognitive foundations of the semantic priming effect in schizophrenia. We review and theoretically evaluate the neurotransmitter systems (dopaminergic, GABAergic and glutamatergic) and neurobiological underpinnings of behavioural and electrophysiological (N400) semantic priming in the pathology, and the main hypotheses on their geneses: a disinhibition of the semantic spread of activation, a disorganised semantic storage or noisy lexical-semantic associations, a psychomotor artefact, an artefact of relatedness proportions, or an inability to mobilise contextual information. We further assess the literature on the endophenotype of Formal Thought Disorder from multiple standpoints, ranging from neurophysiology to cognition: considerations are weaved on neuronal (PV basket cell, SST, VIP) and receptor deficits (DRD1, NMDA), neurotransmitter imbalances (dopamine), cortical and dopaminergic lateralisation, inter alia. In conclusion, we put forth novel postulates on the underlying causes of controlled hypopriming, automatic hyperpriming, N400 reversals (larger amplitudes for close associations), indirect versus direct hyperpriming, and the endophenotype of lexical-semantic disturbances in schizophrenia.

Pharmacological rescue of cognitive function in a mouse model of chemobrain

BACKGROUND

After chemotherapy, many cancer survivors suffer from long-lasting cognitive impairment, colloquially known as "chemobrain." However, the trajectories of cognitive changes and the underlying mechanisms remain unclear. We previously established paclitaxel-induced inositol trisphosphate receptor (InsP3R)-dependent calcium oscillations as a mechanism for peripheral neuropathy, which was prevented by lithium pretreatment. Here, we investigated if a similar mechanism also underlay paclitaxel-induced chemobrain.

METHOD

Mice were injected with 4 doses of 20 mg/kg paclitaxel every other day to induced cognitive impairment. Memory acquisition was assessed with the displaced object recognition test. The morphology of neurons in the prefrontal cortex and the hippocampus was analyzed using Golgi-Cox staining, followed by Sholl analyses. Changes in protein expression were measured by Western blot.

RESULTS

Mice receiving paclitaxel showed impaired short-term spatial memory acquisition both acutely 5 days post injection and chronically 23 days post injection. Dendritic length and complexity were reduced in the hippocampus and the prefrontal cortex after paclitaxel injection. Concurrently, the expression of protein kinase C α (PKCα), an effector in the InsP3R pathway, was increased. Treatment with lithium before or shortly after paclitaxel injection rescued the behavioral, cellular, and molecular deficits observed. Similarly, memory and morphological deficits could be rescued by pretreatment with chelerythrine, a PKC inhibitor.

CONCLUSION

We establish the InsP3R calcium pathway and impaired neuronal morphology as mechanisms for paclitaxel-induced cognitive impairment. Our findings suggest lithium and PKC inhibitors as candidate agents for preventing chemotherapy-induced cognitive impairment.

An Attempt at a Unified Theory of the Neocortical Microcircuit in Sensory Cortex

The neocortex performs a wide range of functions, including working memory, sensory perception, and motor planning. Despite this diversity in function, evidence suggests that the neocortex is made up of repeating subunits ("macrocolumns"), each of which is largely identical in circuitry. As such, the specific computations performed by these macrocolumns are of great interest to neuroscientists and AI researchers. Leading theories of this microcircuit include models of predictive coding, hierarchical temporal memory (HTM), and Adaptive Resonance Theory (ART). However, these models have not yet explained: (1) how microcircuits learn sequences input with delay (i.e., working memory); (2) how networks of columns coordinate processing on precise timescales; or (3) how top-down attention modulates sensory processing. I provide a theory of the neocortical microcircuit that extends prior models in all three ways. Additionally, this theory provides a novel working memory circuit that extends prior models to support simultaneous multi-item storage without disrupting ongoing sensory processing. I then use this theory to explain the functional origin of a diverse set of experimental findings, such as cortical oscillations.

Neuroelectric Tuning of Cortical Oscillations by Apical Dendrites in Loop Circuits

Bundles of relatively long apical dendrites dominate the neurons that make up the thickness of the cerebral cortex. It is proposed that a major function of the apical dendrite is to produce sustained oscillations at a specific frequency that can serve as a common timing unit for the processing of information in circuits connected to that apical dendrite. Many layer 5 and 6 pyramidal neurons are connected to thalamic neurons in loop circuits. A model of the apical dendrites of these pyramidal neurons has been used to simulate the electric activity of the apical dendrite. The results of that simulation demonstrated that subthreshold electric pulses in these apical dendrites can be tuned to specific frequencies and also can be fine-tuned to narrow bandwidths of less than one Hertz (1 Hz). Synchronous pulse outputs from the circuit loops containing apical dendrites can tune subthreshold membrane oscillations of neurons they contact. When the pulse outputs are finely tuned, they function as a local “clock,” which enables the contacted neurons to synchronously communicate with each other. Thus, a shared tuning frequency can select neurons for membership in a circuit. Unlike layer 6 apical dendrites, layer 5 apical dendrites can produce burst firing in many of their neurons, which increases the amplitude of signals in the neurons they contact. This difference in amplitude of signals serves as basis of selecting a sub-circuit for specialized processing (e.g., sustained attention) within the typically larger layer 6-based circuit. After examining the sustaining of oscillations in loop circuits and the processing of spikes in network circuits, we propose that cortical functioning can be globally viewed as two systems: a loop system and a network system. The loop system oscillations influence the network system’s timing and amplitude of pulse signals, both of which can select circuits that are momentarily dominant in cortical activity.

Reduced density of dendritic spines in pyramidal neurons of rats exposed to alcohol during early postnatal life

Dendritic spines are the main postsynaptic sites of excitatory connections of neocortical pyramidal neurons. Alterations of spine shape, number, and density can be observed in different mental diseases, including those caused by developmental alcohol exposure. Pyramidal neurons of layer 2/3 are the most abundant cells of the neocortex and represent the main source of associative cortico-cortical connections. These neurons are essential for higher functions mediated by the cortex such as feature selection and perceptual grouping. Furthermore, their connections have been shown to be altered in experimental models of fetal alcohol spectrum disorders. Here, we used a Golgi-like tracing method to study the spine density of layer 2/3 associative pyramidal neurons in the somatosensory cortex of adult rats exposed to alcohol during the first postnatal week. The main result of the present study is represented by the decreased spine density in the apical dendrite of alcohol-treated rats, as compared to controls. As to the basal dendritic tree, there were no significant differences between the experimental and the control group. A decreased density of dendritic spines in the apical dendrite may impair the excitatory input onto pyramidal neurons, thus resulting in a widespread alteration of the cortical information flow.

The cognitive significance of resonating neurons in the cerebral cortex

Most neural fibers of the cerebral cortex engage in electric signaling, but one particular fiber, the apical dendrite of the pyramidal neuron, specializes in electric resonating. This dendrite extends upward from somas of pyramidal neurons, the most numerous neurons of the cortex. The apical dendrite is embedded in a recurrent corticothalamic circuit that induces surges of electric current to move repeatedly down the dendrite. Narrow bandwidths of surge frequency (resonating) enable cortical circuits to use specific carrier frequencies, which isolate the processing of those circuits from other circuits. Resonating greatly enhances the intensity and duration of electrical activity of a neuron over a narrow frequency range, which underlies attention in its various modes. Within the minicolumn, separation of the central resonating circuit from the surrounding signal processing network separates "having" subjective impressions from "thinking about" them. Resonating neurons in the insular cortex apparently underlie cognitive impressions of feelings.

Multichannel linear descriptors analysis for sustained attention-related electroencephalography

This study investigated the differences in brain functional state between sustained attention and ignoring task conditions using the electroencephalography in association with sustained attention to response task (SART) performance. Multichannel electroencephalography data were obtained from 10 male healthy volunteers while performing the SART. Three multichannel linear descriptors, that is spatial complexity (Ω), field strength (Σ), and frequency of field changes (Φ), were applied to analyze three frequency bands (θ, α, and β) for sustained attention and ignoring task conditions. The experimental results showed that participants had a significantly lower Ω value in the θ and α band in the SART state. The Σ value was significantly higher in each frequency band of interest in almost all region of interest areas during SART performance. In addition, the Φ value was significantly lower in the θ band and significantly higher in the β band during the sustained attention condition. The results indicated that multichannel linear descriptors could show the differences in brain functions between sustained attention and ignoring task conditions, and might be used to evaluate disorders with an attentional dysfunction.

Modulating the focus of attention for spoken words at encoding affects frontoparietal activation for incidental verbal memory

Attention is crucial for encoding information into memory, and current dual-process models seek to explain the roles of attention in both recollection memory and incidental-perceptual memory processes. The present study combined an incidental memory paradigm with event-related functional MRI to examine the effect of attention at encoding on the subsequent neural activation associated with unintended perceptual memory for spoken words. At encoding, we systematically varied attention levels as listeners heard a list of single English nouns. We then presented these words again in the context of a recognition task and assessed the effect of modulating attention at encoding on the BOLD responses to words that were either attended strongly, weakly, or not heard previously. MRI revealed activity in right-lateralized inferior parietal and prefrontal regions, and positive BOLD signals varied with the relative level of attention present at encoding. Temporal analysis of hemodynamic responses further showed that the time course of BOLD activity was modulated differentially by unintentionally encoded words compared to novel items. Our findings largely support current models of memory consolidation and retrieval, but they also provide fresh evidence for hemispheric differences and functional subdivisions in right frontoparietal attention networks that help shape auditory episodic recall.

Theory of electric resonance in the neocortical apical dendrite

Pyramidal neurons of the neocortex display a wide range of synchronous EEG rhythms, which arise from electric activity along the apical dendrites of neocortical pyramidal neurons. Here we present a theoretical description of oscillation frequency profiles along apical dendrites which exhibit resonance frequencies in the range of 10 to 100 Hz. The apical dendrite is modeled as a leaky coaxial cable coated with a dielectric, in which a series of compartments act as coupled electric circuits that gradually narrow the resonance profile. The tuning of the peak frequency is assumed to be controlled by the average amplitude of voltage-gated outward currents, which in turn are regulated by the subthreshold noise in the thousands of synaptic spines that are continuously bombarded by local circuits. The results of simulations confirmed the ability of the model both to tune the peak frequency in the 10–100 Hz range and to gradually narrow the resonance profile. Considerable additional narrowing of the resonance profile is provided by repeated looping through the apical dendrite via the corticothalamocortical circuit, which reduced the width of each resonance curve (at half-maximum) to approximately 1 Hz. Synaptic noise in the neural circuit is discussed in relation to the ways it can influence the narrowing process.

How negative affect influences neural control processes underlying the resolution of cognitive interference: an event-related fMRI study

In this event-related fMRI study, we sought to investigate the influence of negative affect on the processing of two kinds of cognitive interference: Stroop-interference and oddball interference. For our purpose, we adopted an oddball variant of the Stroop task in which Stroop-interference and oddball interference conditions were created by presenting incongruent and rarely occurring word meanings, respectively. Immediately preceding the target stimuli, we presented pictures of the International Affective Picture System which were either emotionally negative and arousing or emotionally neutral, providing two affective conditions under which the cognitive task was administered. Both the behavioral and the neuroimaging data exhibited an interaction effect between emotional and cognitive condition. First, the emotion induction selectively impaired behavioral performance on interference trials while behavioral measures on non-interference trials were roughly identical in both emotional conditions. Second, in the negative emotional condition there was incremental interference-related activation in control-related regions (fronto-parietal cortices). Taken together, findings suggest that negative affect specifically disturbs the neural control processes that in a neutral affective state allow to select task-relevant information and to shield its processing from task-irrelevant distraction. Accordingly, agents in a negative affective state have to exert enhanced control efforts to resolve cognitive interference. Additional connectivity analyses revealed that a negative coupling between lateral PFC on the one hand and amygdala and OFC on the other is related to enhanced interference resolution which can be tentatively interpreted as evidence that emotional regulation is an integrated part of an agent's efforts to preserve cognitive performance in affective situations.

Analysis of propagation of multi-channel EEG in the test of sustained attention

the psychological construct 'sustained attention' describes a basic component of attention characterized by the subject's readiness to detect rarely and unpredictably occurring signals over prolonged periods of time. In this study, six healthy volunteers underwent a sustained attention to response task (SART), while their electroencephalographic (EEG) were recorded contemporarily. Directed Transfer Function (DTF) was used as estimator for direction of propagation of EEG function coupling. The results of DTF showed that the information flux within EEG functional coupling changed when attention condition changed from inattention state to sustained attention state, principally at alpha and beta rhythms. The DTF could be used to evaluate sustained attention condition and they might be used for research on damage of attention mechanisms of ADHD and TBI diseases in future.

Attentional modulation of alpha oscillations in macaque inferotemporal cortex

Recent work reported the observation of alpha frequency oscillations (8-12 Hz) in several regions of macaque visual cortex, including V2, V4, and inferotemporal cortex (IT). While alpha-related physiology in V2 and V4 appears consistent with a role in attention-related suppression, in IT, alpha reactivity appears conflicted with such a role. We addressed this issue directly by analyzing laminar profiles of local field potentials and multiunit activities from the IT of macaque monkeys during performance of an intermodal selective attention task (visual versus auditory). We found that (1) before visual stimulus onset (-200 to 0 ms), attention to visual input increased ongoing alpha power in IT relative to attention to auditory input, and (2) in contrast to the prevailing view of alpha inhibition, the increased ongoing alpha activity is accompanied by increased concurrent multiunit firing and facilitates visual stimulus processing. These results suggest that ongoing alpha oscillations in IT play a different functional role than that in the occipital cortex and may be part of the neuronal mechanism representing task-relevant information.

Receptor-receptor interactions: A novel concept in brain integration

A brief historical presentation of the hypothesis on receptor-receptor interactions as an important integrative mechanism taking place at plasma membrane level is given. Some concepts derived from this integrative mechanism especially the possible assemblage of receptors in receptor mosaics (high-order receptor oligomers) and their relevance for the molecular networks associated with the plasma membrane are discussed. In particular, the Rodbell's disaggregation theory for G-proteins is revisited in the frame of receptor mosaic model. The paper also presents some new indirect evidence on A2A;D2 receptor interactions obtained by means of Atomic Force Microscopy on immunogold preparations of A2A and D2 receptors in CHO cells. These findings support previous data obtained by means of computer-assisted confocal laser microscopy. The allosteric control of G-protein coupled receptors is examined in the light of the new views on allosterism and recent data on a homocysteine analogue capable of modulating D2 receptors are shown. Finally, the hypothesis is introduced on the existence of check-points along the amino acid pathways connecting allosteric and orthosteric binding sites of a receptor and their potential importance for drug development.

Towards a mathematical theory of cortical micro-circuits

The theoretical setting of hierarchical Bayesian inference is gaining acceptance as a framework for understanding cortical computation. In this paper, we describe how Bayesian belief propagation in a spatio-temporal hierarchical model, called Hierarchical Temporal Memory (HTM), can lead to a mathematical model for cortical circuits. An HTM node is abstracted using a coincidence detector and a mixture of Markov chains. Bayesian belief propagation equations for such an HTM node define a set of functional constraints for a neuronal implementation. Anatomical data provide a contrasting set of organizational constraints. The combination of these two constraints suggests a theoretically derived interpretation for many anatomical and physiological features and predicts several others. We describe the pattern recognition capabilities of HTM networks and demonstrate the application of the derived circuits for modeling the subjective contour effect. We also discuss how the theory and the circuit can be extended to explain cortical features that are not explained by the current model and describe testable predictions that can be derived from the model.

Cortical and subcortical contributions to the attentive processing of speech

The neuroanatomical correlates of attentive listening were investigated with functional magnetic resonance imaging and an attention task in which listeners responded only to words that combined two specific attributes of voice and semantic content. This task was performed under two different attentive listening conditions: (i) diotically, with words presented sequentially, and (ii) dichotically, with male and female voices presented simultaneously but segregated to different ears. For both conditions, functional magnetic resonance imaging revealed bihemispheric but right-lateralized activity patterns in mid-prefrontal, anterior cingulate, and inferior parietal areas, as well as significant anterior insular and subcortical activation. Manipulating attentional demands under different listening conditions revealed an important role for right anterior insula, striatum, and thalamus in the regulation of attentive listening to spoken language.

Functional brain abnormalities in psychiatric disorders: neural mechanisms to detect and resolve cognitive conflict and interference

In the present article, we review functional neuroimaging studies on interference processing and performance monitoring in three groups of psychiatric disorders, (1) mood disorders, (2) schizophrenia, and (3) obsessive-compulsive disorder (OCD). Ad (1) Behavioral performance measures suggest an impaired interference resolution capability in symptomatic bipolar disorder patients. A series of neuroimaging analyses found alterations in the ACC-DLPFC system in mood disorder (unipolar depressed and bipolar) patients, putatively reflective of an abnormal interplay of monitoring and executive neurocognitive functions. Other studies of euthymic bipolar patients showed relatively decreased interference-related activation in rostroventral PFC which conceivably underlies defective inhibitory control. Ad (2) Behavioral Stroop studies revealed a specific performance pattern of schizophrenia patients (normal RT interference but increased error interference and RT facilitation) suggestive of a deficit in ignoring irrelevant (word) information. Moreover, reduced/absent behavioral post-error and post-conflict adaptation effects suggest alterations in performance monitoring and/or adjustment capability in these patients. Neuroimaging findings converge to suggest a disorder-related abnormal neurophysiology in ACC which consistently showed conflict- and error-related hypoactivation that, however, appeared to be modulated by different factors. Moreover, studies suggest a specific deficit in context processing in schizophrenia, evidently related to activation reduction in DLPFC. Ad (3) Behavioral findings provide evidence for impaired interference resolution in OCD. Neuroimaging results consistently showed conflict- and error-related ACC hyperactivation which--conforming OCD pathogenesis models--can be conclusively interpreted as reflecting overactive performance monitoring. Taken together, interference resolution and performance monitoring appeared to be fruitful concepts in the investigation of neurocognitive deficits in psychiatric disorders.

The apical dendrite theory of consciousness

The neural basis of consciousness is theorized here to be the elevated activity of the apical dendrite within a thalamocortical circuit. Both the anatomical and functional properties of these two brain structures are examined within the general context of the cortical minicolumn, which is regarded as the functional unit of the cerebral cortex. Two main circuits of the minicolumn are described: the axis circuit, which sustains activity for extended durations and produces our sensory impressions, and the shell circuit, which performs input-output processing and produces identifications, categorizations, and ideas. The apical dendrite operates within the axis circuit to stabilize neural activity, which enables conscious impressions to be steady and to be sustained over long periods of time. In an attempt to understand how the conscious aspect of subjective impressions may be related to apical dendrite activity, we examine the characteristics of the electric and magnetic fields during the movement of charges along the apical dendrite. The physical correlate of consciousness is regarded here as the relatively intense electromagnetic field that is located along the inside and the outside close to the surface of the active apical dendrite.

Oddball and incongruity effects during Stroop task performance: a comparative fMRI study on selective attention

The aim of this fMRI study was to investigate and compare the neural mechanisms of selective attention during two different operationalizations of competition between task-relevant and task-irrelevant information: Stroop-incongruity and oddballs. For this purpose, we employed a Stroop-like oddball task in which subjects responded to the font size of presented word stimuli. Stroop-incongruity was created by (response-)incongruent word information while oddballs comprised low-frequency events in a task-irrelevant, unattended dimension. Thereby, in order to elucidate the influence of processing domain from which competition emanates, oddball conditions were created in two different attribute dimensions, color and word meaning. Either oddball condition was expected to evoke an orienting response, which participants would have to override in order to maintain adequate performance. Incongruent Stroop trials were expected to produce Stroop-interference so that subjects would have to override the predominant tendency to read and respond to word meaning. All competition conditions exhibited significantly prolonged reaction times compared to control trials, demonstrating that our experimental manipulation was indeed effective. fMRI data analyses delineated two discriminative components of competition: one component mainly related to motor preparation and another, primarily attentional component. Regarding the first, Stroop-interference increased activation mainly in regions implicated in motor control or response preparation. Regarding the second, Word-oddballs increased activation in a frontoparietal "attention network". Furthermore, Word-oddballs and Color-oddballs exhibited striking activation overlap mainly in prefrontal regions but also in posterior processing areas. Here, the data emphasized a prominent role of posterior lateral PFC in implementing top-down attentional control.

Different dendrite and dendritic spine alterations in basal and apical arbors in mutant human amyloid precursor protein transgenic mice

The extracellular deposition of amyloid-beta peptide (Abeta) in brain parenchyma is one of the characteristic features of Alzheimer's disease and is suggested to induce reactive and degenerative changes in neuronal cell bodies, axons and dendritic processes. In particular, within and in close proximity to amyloid plaques, distinctive morphological alterations have been observed, including changes in neurite trajectory and decreases in dendritic diameter and in spine density. Apart from these plaque-associated focal aberrations, little is known regarding modifications of the global dendritic morphology including the detailed and comparative quantitative analysis of apical and basal arbors. The objective of the present study was to investigate the effects of amyloid plaque deposition and elevated soluble Abeta on neuronal morphology in mutant human amyloid precursor protein (hAPP) transgenic mice (line Tg2576; [K. Hsiao, P. Chapman, S. Nilsen, C. Eckman, Y. Harigaya, S. Younkin, F. Yang, G. Cole, Correlative memory deficits, Abeta elevation, and amyloid plaques in transgenic mice, Science 274 (1996) 99-102]). Retrogradelly labeled callosal-projecting pyramidal cells in the primary somatosensory cortex were three-dimensionally analyzed. Although basal dendrites remained unaffected, analysis of apical trees revealed a number of unambiguous morphological changes. Thus, in TG2576 mice, the apical arbors were shortened in total length and less branched. Furthermore, the diameter of proximal dendritic segments was increased whereas that of distal segments was reduced. Analysis of spine numbers and distribution on basal and apical trees demonstrated a significant reduction in spine densities along the whole course of dendrites. The findings suggest that Abeta-related pathology induces morphological aberrations in basal and apical arbors to different degrees which are unrelated to direct plaque-associated changes.

Apical dendrite activity in cognition and consciousness

The ongoing steady nature of consciousness in everyday life implies that the underlying neural activity possesses a high level of stability. The prolonged cognitive events of sustained attention, imagery, and working memory also imply high stability of underlying neural activity. This paper proposes that stabilization of neural activity is produced by apical dendrite activity in pyramidal neurons within recurrent corticothalamic circuits, and proposes that the wave activities of apical dendrites that stabilize ongoing activity constitute the subjective impressions of an attended object and the entire sensory background. The cortical minicolumn, as the functional unit of the cortex, is separated into an axis consisting of layer 5 pyramidal neurons and a surrounding shell consisting of layer 2/3 pyramidal neurons. It is proposed that apical dendrites of the axis generate sensory impressions, and basal dendrites of the shell process the brief-lasting input-output identifications of objects that give rise to ideas.