Brain states: top-down influences in sensory processing

All cortical and thalamic levels of sensory processing are subject to powerful top-down influences, the shaping of lower-level processes by more complex information. New findings on the diversity of top-down interactions show that cortical areas function as adaptive processors, being subject to attention, expectation, and perceptual task. Brain states are determined by the interactions between multiple cortical areas and the modulation of intrinsic circuits by feedback connections. In perceptual learning, both the encoding and recall of learned information involves a selection of the appropriate inputs that convey information about the stimulus being discriminated. Disruption of this interaction may lead to behavioral disorders, including schizophrenia.     

Neuromodulation by Glutamate and Acetylcholine can Change Circuit Dynamics by Regulating the Relative Influence of Afferent Input and Excitatory Feedback

Substances such as acetylcholine and glutamate act as both neurotransmitters and neuromodulators. As neuromodulators, they change neural information processing by regulating synaptic transmitter release, altering baseline membrane potential and spiking activity, and modifying long-term synaptic plasticity. Slice physiology research has demonstrated that many neuromodulators differentially modulate afferent, incoming information compared to intrinsic and recurrent processing in cortical structures such as piriform cortex, neocortex, and the hippocampus. The enhancement of afferent (external) pathways versus the suppression at recurrent (internal) pathways could cause cortical dynamics to switch between a predominant influence of external stimulation to a predominant influence of internal recall. Modulation of afferent versus intrinsic processing could contribute to the role of neuromodulators in regulating attention, learning, and memory effects in behavior.     

Electroencephalographic brain dynamics following manually responded visual targets

Scalp-recorded electroencephalographic (EEG) signals produced by partial synchronization of cortical field activity mix locally synchronous electrical activities of many cortical areas. Analysis of event-related EEG signals typically assumes that poststimulus potentials emerge out of a flat baseline. Signals associated with a particular type of cognitive event are then assessed by averaging data from each scalp channel across trials, producing averaged event-related potentials (ERPs). ERP averaging, however, filters out much of the information about cortical dynamics available in the unaveraged data trials. Here, we studied the dynamics of cortical electrical activity while subjects detected and manually responded to visual targets, viewing signals retained in ERP averages not as responses of an otherwise silent system but as resulting from event-related alterations in ongoing EEG processes. We applied infomax independent component analysis to parse the dynamics of the unaveraged 31-channel EEG signals into maximally independent processes, then clustered the resulting processes across subjects by similarities in their scalp maps and activity power spectra, identifying nine classes of EEG processes with distinct spatial distributions and event-related dynamics. Coupled two-cycle postmotor theta bursts followed button presses in frontal midline and somatomotor clusters, while the broad postmotor "P300" positivity summed distinct contributions from several classes of frontal, parietal, and occipital processes. The observed event-related changes in local field activities, within and between cortical areas, may serve to modulate the strength of spike-based communication between cortical areas to update attention, expectancy, memory, and motor preparation during and after target recognition and speeded responding.     

Cognitive functions of the basal forebrain.

Studies of the function of the basal forebrain have focused on cholinergic neurons that project to cortical and limbic structures critical for various cognitive abilities. Recent experiments suggest that these neurons serve a modulatory function in cognition, by optimizing cortical information processing and influencing attention.     

Covert shift of attention modulates the ongoing neural activity in a reaching area of the macaque dorsomedial visual stream

Background

Attention is used to enhance neural processing of selected parts of a visual scene. It increases neural responses to stimuli near target locations and is usually coupled to eye movements. Covert attention shifts, however, decouple the attentional focus from gaze, allowing to direct the attention to a peripheral location without moving the eyes. We tested whether covert attention shifts modulate ongoing neuronal activity in cortical area V6A, an area that provides a bridge between visual signals and arm-motor control.

Methodology/Principal Findings

We performed single cell recordings from 3 Macaca Fascicularis trained to fixate straight-head, while shifting attention outward to a peripheral cue and inward again to the fixation point. We found that neurons in V6A are influenced by spatial attention. The attentional modulation occurs without gaze shifts and cannot be explained by visual stimulations. Visual, motor, and attentional responses can occur in combination in single neurons.

Conclusions/Significance

This modulation in an area primarily involved in visuo-motor transformation for reaching may form a neural basis for coupling attention to the preparation of reaching movements. Our results show that cortical processes of attention are related not only to eye-movements, as many studies have shown, but also to arm movements, a finding that has been suggested by some previous behavioral findings. Therefore, the widely-held view that spatial attention is tightly intertwined with—and perhaps directly derived from—motor preparatory processes should be extended to a broader spectrum of motor processes than just eye movements.     

Neurophysiological mechanisms of voluntary attention: a review

The review is focused on attention as behavior-controlling process. Neurophysiological, electrophysiological and neuropsychological studies of different brain structures during voluntary attention are analyzed. These data show that selective voluntary attention modulates activity of sensory specific cortical zones involved in relevant signal processing. Fronto-thalamic system consisting of prefrontal cortex and thalamic mediodorsal nuclei is shown to be main source of top-down selective modulation of voluntary attention. The review proposed the hypothetical model of selective cortical activity modulation during voluntary attention based upon the available data and evidence of own electroencephalographic studies.     

Sensory gain control (amplification) as a mechanism of selective attention: electrophysiological and neuroimaging evidence.

Both physiological and behavioral studies have suggested that stimulus-driven neural activity in the sensory pathways can be modulated in amplitude during selective attention. Recordings of event-related brain potentials indicate that such sensory gain control or amplification processes play an important role in visual-spatial attention. Combined event-related brain potential and neuroimaging experiments provide strong evidence that attentional gain control operates at an early stage of visual processing in extrastriate cortical areas. These data support early selection theories of attention and provide a basis for distinguishing between separate mechanisms of attentional suppression (of unattended inputs) and attentional facilitation (of attended inputs).     

A model of cortical memory processing based on columnar organization

Recent neurophysiological experiments using mammalian brains indicated that some cortical neurons exhibit oscillatory activities which can be of functional importance in visual perception. These findings suggest that the oscillation is an ubiquitous feature of cortical information processing carried out by columns which are receiving growing attention as functional subdivisions of cortical circuitry. On the assumption that a basic functional unit is a column comprising excitatory and inhibitory neurons, a network model of cortical memory processing which can account for these oscillations is proposed. Numerical simulations revealed that for appropriately determined parameters the network can attain memory-pattern retrieval resulting from fixed-point behaviour despite the fact that columns have the characteristic of oscillators. Received: 19 March 1993/Accepted in revised form: 23 September 1993     

Electrophysiological evidence for spatiotemporal flexibility in the ventrolateral attention network

Successful completion of many everyday tasks depends on interactions between voluntary attention, which acts to maintain current goals, and reflexive attention, which enables responding to unexpected events by interrupting the current focus of attention. Past studies, which have mostly examined each attentional mechanism in isolation, indicate that volitional and reflexive orienting depend on two functionally specialized cortical networks in the human brain. Here we investigated how the interplay between these two cortical networks affects sensory processing and the resulting overt behavior. By combining measurements of human performance and electrocortical recordings with a novel analytical technique for estimating spatiotemporal activity in the human cortex, we found that the subregions that comprise the reflexive ventrolateral attention network dissociate both spatially and temporally as a function of the nature of the sensory information and current task demands. Moreover, we found that together with the magnitude of the early sensory gain, the spatiotemporal neural dynamics accounted for the high amount of the variance in the behavioral data. Collectively these data support the conclusion that the ventrolateral attention network is recruited flexibly to support complex behaviors.     

Structural basis of the cholinergic and serotonergic modulation of GABAergic neurons in the hippocampus.

Ascending subcortical pathways effectively modulate hippocampal information processing. Two components, the cholinergic and serotonergic pathways have been demonstrated to play an important role in the generation of behaviour-dependent hippocampal EEG patterns. Several findings suggest that the above projections influence the activity of hippocampal interneurons. Here we review the available data from physiological, pharmacological and receptor localization experiments, drawing attention to the crucial role of interneurons in the transfer and amplification of subcortical effects on cortical information processing. We hypothesize that, by exerting diverse actions on different subsets of interneurons, the cholinergic and serotonergic systems might change the balance of somatic and dendritic inhibition, and consequently change the integrative properties of hippocampal principal cells.     

Mechanisms of Selective Attention during Competitive Discrimination of Visual and Auditory Verbal Information: Positron Emission Tomography and Cortical Evoked Potential Studies

The mechanisms of selective verbal attention were studied under conditions of simultaneous delivery of speech signals via the visual and auditory channels. The investigation was based on the comparison and synthesis of data obtained by two methods: positron emission tomography (PET) and brain evoked potentials (EPs). A new approach was developed: complementary tasks were constructed in such a way that, despite principal methodological problems, the same phenomenon could be investigated in one paradigm in EP and PET studies. The results obtained by the two methods are in rather good agreement with respect to topography: the secondary and tertiary areas, as well as the associative brain areas, are involved in attention concentration, that is, selection of verbal information occurs at the level of cognitive processes. The combination of two complementary methods, PET and EP, allowed the processes of processing of sensory information and brain mechanisms of selective attention to be investigated much more completely. The PET studies contributed to further understanding of brain mechanisms evidencing where processing occurs and the EP method provided insight into the mechanism of how this information is processed inside the corresponding cortical areas. The finding that the activation of primary areas of the visual cortex is accompanied by the inhibition of visual information deserves attention. This conclusion can be considered highly significant because of the concordance of the two independent methods. How to interpret it is not yet clear. It is possible that, in the case of primary importance of verbal information and priority of the visual channel for the repression from consciousness of artificially irrelevant information, a safety mechanism is activated: the amplified signal enters the brain cortex, where it is retained in the short-term iconic memory. This enables a reaction to this stimulus (if necessary), in the presence of any additional sign involving selective attention.     

Cortical mechanisms of space-based and object-based attentional control

Visual attention, the mechanism by which observers select relevant or important information from scenes, can be deployed to locations in space or to spatially invariant object representations. Studies have examined both the modulatory effects of attention on the strength of extrastriate cortical representations, and the control of attention by parietal and frontal cortical circuits. Subregions of parietal and frontal cortex are transiently active when attention is voluntarily shifted between spatial locations or object representations. This transient activity may reflect an abrupt shift in the attentional set of the observer, complementing sustained signals that are thought to maintain a given attentive state.     

What makes the prefrontal cortex so appealing in the era of brain imaging? a network analytical perspective

It is thought that the prefrontal cortex (PFC) subserves cognitive control processes by coordinating the flow of information in the cerebral cortex. In the network of cortical areas the central position of the PFC makes difficult to dissociate processing and the cognitive function mapped to this region, especially when using whole brain imaging techniques, which can detect frequently activated regions. Accordingly, the present study showed particularly high rate of increase of published studies citing the PFC and imaging as compared to other fields of the neurosciences on the PubMed. Network measures used to characterize the role of the areas in signal flow indicated specialization of the different regions of the PFC in cortical processing. Notably, areas of the dorsolateral PFC and the anterior cingulate cortex, which received the highest number of citations, were identified as global convergence points in the network. These prefrontal regions also had central position in the dominant cluster consisted exclusively by the associational areas of the cortex. We also present findings relevant to models suggesting that control processes of the PFC are depended on serial processing, which results in bottleneck effects. The findings suggest that PFC is best understood via its role in cortical information processing.     

Functional implications of temporal structure in primate cortical information processing

Processing of information in the cerebral cortex of primates is characterized by distributed representations and processing in neuronal assemblies rather than by detector neurons, cardinal cells or command neurons. Responses of individual neurons in sensory cortical areas contain limited and ambiguous information on common features of the natural environment which is disambiguated by comparison with the responses of other, related neurons. Distributed representations are also capable to represent the enormous complexity and variability of the natural environment by the large number of possible combinations of neurons that can engage in the representation of a stimulus or other content. A critical problem of distributed representation and processing is the superposition of several assemblies activated at the same time since interpretation and processing of a population code requires that the responses related to a single representation can be identified and distinguished from other, related activity. A possible mechanism which tags related responses is the synchronization of neuronal responses of the same assembly with a precision in the millisecond range. This mechanism also supports the separate processing of distributed activity and dynamic assembly formation. Experimental evidence from electrophysiological investigations of non-human primates and human subjects shows that synchronous activity can be found in visual, auditory and motor areas of the cortex. Simultaneous recordings of neurons in the visual cortex indicate that individual neurons synchronize their activity with each other, if they respond to the same stimulus but not if they are part of different assemblies representing different contents. Furthermore, evidence for synchronous activity related to perception, expectation, memory, and attention has been observed.     

Four concurrent feedforward and feedback networks with different roles in the visual cortical hierarchy

Visual stimuli evoke fast-evolving activity patterns that are distributed across multiple cortical areas. These areas are hierarchically structured, as indicated by their anatomical projections, but how large-scale feedforward and feedback streams are functionally organized in this system remains an important missing clue to understanding cortical processing. By analyzing visual evoked responses in laminar recordings from 6 cortical areas in awake mice, we uncovered a dominant feedforward network with scale-free interactions in the time domain. In addition, we established the simultaneous presence of a gamma band feedforward and 2 low frequency feedback networks, each with a distinct laminar functional connectivity profile, frequency spectrum, temporal dynamics, and functional hierarchy. We could identify distinct roles for each of these 4 processing streams, by leveraging stimulus contrast effects, analyzing receptive field (RF) convergency along functional interactions, and determining relationships to spiking activity. Our results support a dynamic dual counterstream view of hierarchical processing and provide new insight into how separate functional streams can simultaneously and dynamically support visual processes.

Visual stimuli evoke fast-evolving activity patterns that are distributed across multiple cortical areas, but how large-scale feedforward and feedback streams are functionally organized in this system remains unclear. Visual evoked responses in laminar recordings from six cortical areas in awake mice reveal how layers and rhythms dynamically orchestrate functional streams in vision.     

Generation of Ponto-Geniculo-Occipital (PGO) waves as a possible reason of disturbances in visual perception in microsleep

A hypothesis is put forward that one of the reasons for disturbances in visual perception during microsleep could be a spontaneous generation of Ponto-Geniculo-Occipital (PGO) waves. If the PGO waves are generated in microsleep, they could propagate into different thalamic nuclei conveying visual infomation. Consequently, a propagation of visual infonnation from the retina (if the eyes are opened) to visual neocortical areas and to input basal ganglia nucleus, striatum could be impaired. According to previously proposed mechanism of visual processing, which includes visual attention, in absence of striatum activation by a visual stimulus, a disinhibition through the basal ganglia of superior colliculus that transfer visual information to dopaminergic structures becomes impossible. Due to absence of dopamine release in response to visual stimulus, the attention to this stimulus cannot start, and therefore its processing worsens in all visual cortical areas. The suggested hypothesis could be verified in experiments with artificially evoked microsleep using non-invasive methods for searching for the correlates of the PGO activity presence in the brain.     

Human umbilical cord blood cells restore brain damage induced changes in rat somatosensory cortex

Intraperitoneal transplantation of human umbilical cord blood (hUCB) cells has been shown to reduce sensorimotor deficits after hypoxic ischemic brain injury in neonatal rats. However, the neuronal correlate of the functional recovery and how such a treatment enforces plastic remodelling at the level of neural processing remains elusive. Here we show by in-vivo recordings that hUCB cells have the capability of ameliorating the injury-related impairment of neural processing in primary somatosensory cortex. Intact cortical processing depends on a delicate balance of inhibitory and excitatory transmission, which is disturbed after injury. We found that the dimensions of cortical maps and receptive fields, which are significantly altered after injury, were largely restored. Additionally, the lesion induced hyperexcitability was no longer observed in hUCB treated animals as indicated by a paired-pulse behaviour resembling that observed in control animals. The beneficial effects on cortical processing were reflected in an almost complete recovery of sensorimotor behaviour. Our results demonstrate that hUCB cells reinstall the way central neurons process information by normalizing inhibitory and excitatory processes. We propose that the intermediate level of cortical processing will become relevant as a new stage to investigate efficacy and mechanisms of cell therapy in the treatment of brain injury.     

The Primary Visual Cortex Is Differentially Modulated by Stimulus-Driven and Top-Down Attention

Selective attention can be focused either volitionally, by top-down signals derived from task demands, or automatically, by bottom-up signals from salient stimuli. Because the brain mechanisms that underlie these two attention processes are poorly understood, we recorded local field potentials (LFPs) from primary visual cortical areas of cats as they performed stimulus-driven and anticipatory discrimination tasks. Consistent with our previous observations, in both tasks, we found enhanced beta activity, which we have postulated may serve as an attention carrier. We characterized the functional organization of task-related beta activity by (i) cortical responses (EPs) evoked by electrical stimulation of the optic chiasm and (ii) intracortical LFP correlations. During the anticipatory task, peripheral stimulation that was preceded by high-amplitude beta oscillations evoked large-amplitude EPs compared with EPs that followed low-amplitude beta. In contrast, during the stimulus-driven task, cortical EPs preceded by high-amplitude beta oscillations were, on average, smaller than those preceded by low-amplitude beta. Analysis of the correlations between the different recording sites revealed that beta activation maps were heterogeneous during the bottom-up task and homogeneous for the top-down task. We conclude that bottom-up attention activates cortical visual areas in a mosaic-like pattern, whereas top-down attentional modulation results in spatially homogeneous excitation.     

Attention to surfaces modulates motion processing in extrastriate area MT

In the visual system, early atomized representations are grouped into higher-level entities through processes of perceptual organization. Here we present neurophysiological evidence that a representation of a simple object, a surface defined by color and motion, can be the unit of attentional selection at an early stage of visual processing. Monkeys were cued by the color of a fixation spot to attend to one of two transparent random-dot surfaces, one red and one green, which occupied the same region of space. Motion of the attended surface drove neurons in the middle temporal (MT) visual area more strongly than physically identical motion of the non-attended surface, even though both occurred within the spotlight of attention. Surface-based effects of attention persisted even without differential surface coloring, but attentional modulation was stronger with color. These results show that attention can select surface representations to modulate visual processing as early as cortical area MT.     

How does the brain control its own activity? A new function for the basal ganglia

It has long been a problem in neuroscience to known how the brain controls its own activity, how it is able to control the level of CNS excitability and how it is able to select and act on some information as opposed to some other information. In this paper I propose a new theory in which the basal ganglia play a role in selecting information ("selective attention") and in controlling the general level of excitability of the CNS ("state control"), the two processes being to some extent interdependent. The basal ganglia achieve these functions by actions on the thalamic-frontal cortical axis and on the brainstem mesencephalic reticular formation.