Each map corresponds to a single recording channel from F7—F8 to O1—O2 disposed on the scalp. There is a high probability, therefore, that the sustained Non-specific factors, such as noisy environment in the ISS, activity in the prefrontal cortex [34], initiated here by the stress, muscle artifacts and basic physiological factors brain and appearance of the 3D tunnel and playing the role of a driver in body blood circulation difference , seem unlikely to be the source the directional flow of EEG signals, is implicated in the navigation of the modifications to responses to the 3D image, given the task.
The neutral checkerboard stimulus, on the other hand, would preservation of the classical checkerboard VEP and the main- not recruit such pathways. This is also compatible with the new tained level of psychophysical performance in the navigation task view that reconciles top-down and bottom-up effects on attention [27]. The phase-locking contribution to the VEP [28] induced by where salience, current goals and behavioral history are integrated the presentation of 3D-tunnel on Earth was suppressed in in a functional map [36].
Navigational processes would normally be carried The major difference between the checkerboard and the 3D- out in a terrestrial gravitational frame of reference, which would tunnel tests is that in the latter situation the subject cognitively implicitly take part in the evoked response.
The unusual conditions processed the visual information in anticipation of a 3D navigation of weightlessness appear to alter the normal workings of the task [5].
As the presentation of the 3D tunnel was followed by a underlying neural circuitry. The repeated on whole body motion in visuospatial frame of navigation exposure to the 3D tunnel followed by the navigational task and [30,31,32]. In addition, the parieto-prefrontal and parieto- the related activation of working memory can influence the visual premotor pathways are respectively implicated in the top-down responses, as recently demonstrated in a target detection paradigm PLOS ONE www.
Comparison between checkerboard and 3D tunnel stimuli given at 1 Hz in control participants on Earth. The triggers vertical dashed lines were given at time zero. The stars indicate stronger ERS in the upper alpha band ,15 Hz followed by a stronger ERD at about ms in the upper alpha band ,15 Hz with respect to the checkerboard pattern.
Effect of microgravity on the imaginary part of the coherency for the 3D-tunnel presentation. Effect of microgravity on the imaginary part of the coherency for the checkerboard stimulation. Interestingly, the direction of information flow that areas V1 to higher-order areas; re-entrant feedback or top-down supports a frontal to occipital top-down action was altered and influences are critically involved in early-evoked responses replaced by two directional flows from central areas toward frontal [13,42,43,44,47,48,49,45].
The suppression of feedback or top- and occipital areas in weightlessness, providing an electrophysio- down mechanisms acting on the primary visual cortex [13,50,51] logical demonstration of a specific relocation of the driver along might therefore explain the effect of weightlessness on the 3D- the dorsal pathway [29]. This reflects functional reorganization of tunnel-evoked responses.
Interconnections between different frontal-central-occipital relationships to accommodate the absence networks related to visuospatial working memory and vestibular of actual graviception by repositioning the oscillatory neural input such as the cingulate cortex may contribute to top-down drivers and receivers.
Under this hypothesis, the top-down gravi- Some differences in the configuration of the evoked responses tational context would contribute to the channeling of visual argue in favour of the existence of specific neuronal populations information among the different possible neuronal populations, as activated by highly complex visual stimuli [40] or of perceptual recently demonstrated in prefrontal top-down modulation of early grouping of V1 neurons supported by an increase in the rate visual processing and working memory [13].
Indeed, it has been demonstrated in the macaque interaction characterized by the phase-slope index analysis of the that recognizable high-order stimuli induce larger activations in imaginary part of coherency between the frontal and occipital anterior visual and frontal areas while less meaningful stimuli cortex, while weightlessness may produce a basic interference in induce greater activations in posterior visual areas [46].
The the network dynamics. As the coherency between two EEG- critical role played by contextual cues in object-specific responses channels characterizes the linear relationship of the two time series can be applied to our virtual navigation task. In this environment, at a specific frequency, it essentially measures how the phases are the gravitational frame of reference may implicitly participate in coupled to each other. By using the imaginary part of this measure the visual perception and sensation of self-motion as an integral we avoid false positive results due to the problem of volume element of the general context [3,5,4].
Within this navigation conduction [16]. Figure 7. Effect of microgravity on directionality. The ith small circle is located at the ith electrode position and is a contour plot of the ith row of the matrix with elements y ij.
On Earth A , frontal areas are drivers and occipital areas are receivers. In weightlessness B flow is altered, splitting from the central area drivers into the frontal and occipital areas receivers. This reinforces the generators implicated in the fronto-occipital relationships, multiple idea that the specific alteration of the 3D-tunnel VEP was due to a equivalent dipole models were identified by Gramann et al.
Such 3D-tunnel followed by a navigation task, on the basis of the event gating has been found in patients with vestibulopathy where related spectral dynamics.
Their study demonstrated the existence cortical visual motion processing was suppressed [51]. These data the top-down processing. Conservation of the early power increase reinforce the results presented here about the existence of a phase in the same frequency band in the frontal region but not in the delay between occipital and frontal 10 Hz oscillations revealed by occipital region enhances the specificity of the alteration and the coherency and directionality analysis and corroborate a top- excludes a decrease in awareness during tunnel presentation.
As down modulation of the occipital cortex by the frontal one. The existence of the ERD and ERS during the arrest reaction increased when of a selective spatiotemporal coupling between dynamic motor the eyes were closed [59] rules out the existence of a general representations and neural structures involved in visual processing weakness in alpha rhythm generation in weightlessness. This process could also be present In conclusion, the present study shows that in weightlessness, in virtual navigation task.
In this environment, the gravitational although the classical checkerboard VEP were preserved, frame of reference may implicitly participate in the visual responses evoked by the image of a 3D tunnel image presented perception and sensation of navigation by the activation of at the start of a virtual navigation task were significantly altered. It was proposed that different by a marked reduction in the phase locking of theta-alpha cortical areas combine signals with different modalities into a oscillations and a reorganization of the fronto-occipital directional common spatial frame [55,56].
Depending on the functional flow of the 10 Hz oscillation that is present on Earth. Such effects context these multiple sensory inputs are dynamically re-weighted to maintain behavioral goals [55,12]. Phase coupling between demonstrate that a top-down modulation is exerted by gravity- different cortical and subcortical oscillations may provide the related sensory inputs on visual inputs involved in tasks of virtual physiological foundation for keeping the spatial frame into a stable 3D navigation.
The present results could be integrated in the concept of synchronized resonances. As described for 40 Hz oscillations in Acknowledgments the auditory domain [57] the phase coupling between the 10 Hz of We thank M. Lipshits for help with the experiments and fruitful the fronto-central and occipital areas may be viewed as a more discussions, and E. Toussaint, T. Hortmanns, and M. The authors would like to thank the stimuli along the visual pathway. The phase-locking of this rhythm cosmonauts who participated in this experiment, and the personnel at allows the placement of the 3D tunnel image in the temporal and ESA, CNES, Star City and TSUP who made this space experiment environmental context, taking into account the intrinsic functional possible, especially D.
Chaput, E. Lorigny, and V. Therefore, the specific effect of microgravity on the 3D tunnel-evoked responses Author Contributions may be explained by the suppression of a top-down mechanism Conceived and designed the experiments: GC MV A. Berthoz JM. Bengoetxea AC. Analyzed the natural gravity and acting on the primary visual cortex.
Bengoetxea AC BD. Nat Neurosci 4: — Nat Rev Neurosci 4: — Representation of visual gravitational motion in the human vestibular cortex. Cereb Science — But this depends on the ability of neurons to be independent from one another. The optimal information content would require zero or low noise correlations.
There is, however, a significant amount of noise correlation 88 - 91 , so a decrease in noise correlations induced by top-down influences would increase the amount of information encoded by the neuronal ensemble 92 - Decorrelation in the trial to trial variability of responses can allow groups of neurons to average out this variability and improve the signal to noise ratio.
This benefit depends on whether neurons are similarly tuned as noise correlation between differently tuned neurons can increase coding efficiency 92 , 93 , Attention and perceptual learning have been shown to reduce noise correlations, although this has been an area of some debate 97 - Even more task-specific effects are seen on noise correlations between cortical sites that are relevant to the task being performed, and these changes are larger than those associated with merely attending to the stimulus In area MT, noise correlations between a pair of neurons receiving identical visual stimuli can either increase or decrease depending on which of two orthogonal axes the monkey is cued to perform a motion detection task Top-down influences go well beyond specifying the locus of spatial attention and changing neuronal firing rates.
The recurrent pathways that convey these influences must be capable of conveying much more information than a locus to be attended. By the same token, top-down influences cause neurons at the antecedent stages in the cortical hierarchy to alter the nature of the information in their signals.
This is not simply a matter of gain control, but involves alterations in tuning that enable neurons to carry more information about stimulus components that are relevant to the task at hand, to take on selectivity for features that are components of expected objects, and to maintain a stable representation of the world in the face of continual eye movements. The increase in task relevant information is contributed in part by the changes in tuning of individual neurons and in part by the change in the structure of correlations across the neuronal ensemble.
Different forms of top-down influences have been documented in different cortical areas, and these effects are relevant to the functional properties of these areas. But all cortical areas, and even the thalamus, can exhibit profound top-down influences. Based on early findings on the lack of attentional effects in V1, along with findings of strong effects in V4 and MT, it has been suggested that attentional influences get progressively stronger along the visual pathway However, more recent findings, based on more complex stimuli and behavioral paradigms, have called this idea into question, and have suggested that all areas in the hierarchy are equally subject to top-down influences.
It is becoming increasingly evident that attention effects are seen early in the visual pathway 21 , 22 , 26 , 35 , 43 , 44 , 46 , The way in which these influences are manifest depends on the functional role of each cortical area: contour integration in V1, responses to movement direction in MT, modulation by eye position in parietal areas, and so on.
Many studies on top-down influences have focused on the enhancement or change in gain of responses induced by attention, which is equivalent to the stimulus being increased in contrast 50 , 52 , , Related to the idea of biased competition is a normalization model of attention, which involves a multiplicative scaling of responses to multiple stimuli in the receptive field, and attention affects the strength of the normalization , These models assume that attention does not affect the stimulus selectivity of neurons.
But top-down influences can alter the information carried in neuronal signals, which is distinct from a gain control. Attention can change stimulus selectivity in addition to changing gain of responses According to this model, although a neuron receives thousands of inputs from intrinsic connections, only a fraction of these are expressed under a particular behavioral context.
Interactions between reentrant connections from higher order cortical areas and intrinsic circuits enable the network to gate the connections that are appropriate for the task at hand, with different functional networks operating under different task conditions. As a consequence neurons multiplex their function in a state-dependent manner, and constitute adaptive processors running different operations under the instruction of feedback from higher order cortical areas The contextual influences that mediate higher order, complex receptive field properties in V1 involve lateral interactions across a topographically organized region, and they have the consequence of perceptual grouping, such as that involved with linking line segments to global contours.
The interactions follow precise geometric rules, showing facilitatory influences for neurons with receptive fields lying along collinear or cocircular contours. This is a general entity that has been identified in V1, but that is likely to have an analog in all cortical areas. The idea underlying the association field is a linkage between elements that are systematically represented, topographically, over each cortical area.
Lateral interactions between these elements allow perceptual linkage or association of pieces of information. The lateral interactions may be mediated by a plexus of long-range horizontal connections within V1. These connections are formed by pyramidal neurons, whose axons extend for long distances parallel to the cortical surface, and link neurons with widely separated receptive fields 11 , 14 , - Because of their extent and columnar specificity they connect neurons of similar orientation preference 11 , 14 , , , they are ideal conveyors of the contextual influences that enable contour integration Although the horizontal connections provide an anatomical framework for a range of contextual interactions, the observation that these interactions are subject to top-down control suggests that feedback signals can alter the effective connectivity of horizontal connections.
We have proposed that reentrant inputs dynamically modify intrinsic cortical connections, allowing the appropriate associations to be made under different behavioral contexts. A possible reason for the existence of horizontal connections is that they allow such changes in connectivity within the network, as opposed to each cell having a large receptive field generated by a fixed set of feedforward connections.
This idea has been implemented in models of cortical circuitry, in which changes in the gain of horizontal connections by feedback allows subsets of neuronal inputs to be selectively expressed It also accounts for the time course of contextual interactions, where delayed components of neuronal responses are due to the time required for the network to move from one stable state to another, rather than a function of the conduction time required to get information from a distant, more central source.
Finally, it provides a mechanism for contour integration and saliency The interaction between feedback and horizontal connections also suggests a mechanism for perceptual learning. During the encoding of learned information the recurrent input acquires the appropriate mapping to intrinsic connections, and during the recall of the learned information this relationship allows the appropriate inputs to be gated and the target neuron to assume the appropriate functional properties.
In V1 the association field mediates contour integration and saliency, and the top-down input allows for sub-components of the association field to be gated, leading to the manifestation of different shape selectivities. In other areas the association field would be defined by the properties and the kind of information that are topographically mapped in that area, and by the relationship between the long range horizontal connections and that map. Many of the task and expectation dependent effects described above can be explained by an input selection mechanism.
By selecting components of the association field, neurons can express contextual influences that are relevant to the task being performed. A contour detection task enhances collinear interactions and suppresses influences from non collinear elements in the background 5.
A shape discrimination task induces neurons to select collinear influences when the cue is a line and cocircular influences when the cue is a circle 8. By selection of components of the association field over multiple nodes in the horizontal network, neurons in V1 can take on selectivity for complex shapes, including wave like shapes with reversals in curvature.
The selective influence of parallel lines in a 3-line bisection task and collinear lines in a vernier discrimination task 20 can be mediated by changing the effective connectivity of task relevant inputs. This idea is supported by an experiment involving recording from an array of electrodes, where the interactions between cortical sites are measured by cross-correlation analysis based on the relative timing of spikes between pairs of neurons or coherence between local field potentials LFPs measured at different sites.
Changing the perceptual task with the identical visual stimulus strongly influences correlation strength. Perceptual grouping tasks enhance LFP coherence between parallel sites in 3-line bisection, and between collinear sites in contour detection.
Perceptual segregation decreases LFP coherence between collinear sites as seen in a vernier discrimination task Figure 5. This is similar to the expectation-dependent changes seen in noise correlations Although some measures of coherence suggest that attention decreases cortical interactions , the effect of top-down influences depends on the nature of the task and the way in which different cortical sites are engaged in the task.
Further support of this idea comes from fMRI measures of coupling between distant cortical sites representing separated stimuli in a task requiring integration of the two stimuli Task dependent changes in local field potential coherence and noise correlations in area V1. Neurons were recorded with a 96 electrode array in animals trained on the 3-line bisection or vernier discrimination tasks based on the 5 line stimulus a , b or on the contour detection task based on a series of collinear line segments embedded in a background of randomly positioned and oriented lines c.
The effective connectivity between cortical sites representing parallel flanks a and collinear flanks b was measured by calculating the coherence between local field potentials LFPs at different frequencies. The graphs in the center column represent LFP-LFP coherence during the response interval from to ms in the task relevant red and task irrelevant black conditions.
Operations involving grouping of parallel sites, 3-line bisection, or of collinear sites, contour detection, give stronger coherence in the task relevant condition.
Operations involving segregation of collinear sites, vernier discrimination, produces weaker coherence in the task relevant condition. The difference in coherence in the 3-line bisection and vernier tasks was seen not only during the entire response period but in the interval preceding stimulus presentation, indicating top-down setting of lateral cortical interactions in advance of the appearance of the stimulus. Calculated as Fisher information as a function of changes in stimulus bar position for the three task conditions black, attend-away, green, attention to the receptive field location, red, performing the relevant task at the receptive field location , the V1 network carried more information about the stimulus when the animal performed the task, and roughly equal contributions to the increase in information came from the changes in neuronal tuning dotted red line and from the changes in noise correlation solid red line.
Changes in effective connectivity mediated by top-down influences relates to the idea that neural synchrony is the neural code for perceptual grouping and segmentation - , although some studies have failed to confirm this idea , - It has been proposed that perceptual grouping is achieved by synchronizing the activity of neurons representing the grouped features , , and that neuronal synchrony plays important roles in sensorimotor integration - Synchrony in itself may be more a reflection of the dynamic connectivity leading to task dependent alterations in neural tuning rather than the information being carried by the relative timing of action potentials per se.
The two may in fact be related, with alterations in effective connectivity underlying the task-dependent changes in tuning. The role of attention in synchronization is seen in animals performing a color change detection task, in which there is gamma band synchronization between cortical sites encoding the behaviorally relevant stimulus Also, top-down influences can affect effective connectivity between cortical areas. Just as attention can increase gamma band synchronization within V4, it increases synchronization between the frontal eye fields and V4 , This idea is supported in human subjects by fMRI based correlations of BOLD background connectivity between cortical areas, which is specific to task and cortical area Signals that represent top-down influences are observed following the cue directing the task or expectation and before the stimulus presentation 99 , , , This view contrasts with the traditional idea that perception results from the activity of a small number of cells at the top of the visual hierarchy.
Instead, the percept arises from the global set of cortical states and task-specific interactions between multiple cortical areas. The connectivity mediating top-down instructions is likely to include cortico-cortical feedback connections. For area V1, for example, although the strongest feedback arises from area V2, there are a number of cortical areas that provide direct recurrent input to V1, including those in the ventral pathway, such as areas V4 and IT, and areas in the dorsal pathway, including area MT 14 , - The feedback projection from area MT to V1 has been implicated in visual awareness The ventral pathway inputs could provide information about object expectation and the dorsal pathway inputs could provide information about attentional locus or saliency maps.
In addition, other areas, such as prefrontal cortex, could provide executive control over a perceptual task, and the sites of transmission from prefrontal cortex to posterior areas depend on the nature of the task Though prefrontal cortex is not directly connected to V1, it could exert its influence by a cascade of connections descending through the parietal lobe.
Other non-cortical sources of recurrent input have been suggested, such as the pulvinar , Multiple sources are likely to be involved in top-down control, but as indicated above, they must be capable of carrying the richness of information involved in not only spatial attention but expectation and perceptual task. Figure 1. The existence of such a varied array of top-down influences and their profound effect on the functional properties of neurons as well as on their interactions within neuronal ensembles raises a host of questions for further investigation: What are the sources of the various types of top-down control and what are the pathways by which this control is exerted?
What is the nature of the signal that is conveyed along these recurrent pathways? What are the synaptic and network mechanisms by which feedforward, recurrent and intrinsic cortical connections interact to enable adaptive changes in neuronal function?
The challenge is to address these questions in the context of the intact, functioning system and to do so in behaving animals. By selecting different sets of inputs neurons take on different functions. The source of top-down influences can be widespread, either by direct connections from different cortical areas, or by a cascade of inputs originating from many more areas.
In effect a large part of the cerebral cortex can exert influences over individual neurons within a particular area, with multiple descending inputs interacting with intrinsic cortical connections Figure 6. As such, each neuron is a microcosm of the brain as a whole, with synapses carrying information originating from far flung brain regions.
This mode of operation has important implications for our understanding of the cortical mechanisms underlying all sensory modalities and behaviors, and its dysfunction may be the cause of behavioral disorders.
Top-down influences dynamically change effective connectivity within and between cortical areas, allowing neurons to select inputs, and take on functional properties, that are appropriate for the immediate behavioral context. As a result each cortical area and each neuron within that area is an adaptive processor, continuously changing its line label to serve different functions. Right, Long range horizontal connections link distant points in each cortical map, mediating an association field that provides a set of potential interactions.
The association field in V1 is represented by the gray cocircular and collinear lines and by the fields of oriented line segments on either side of the central black neuron.
The underlying circuit is represented by the long range horizontal connections formed by excitatory neurons triangles and disynaptic connections involving inhibitory neurons circles. Depending on the top-down instruction, different sets of inputs can be gated according to the state of feedback represented by the green connections coming from higher order cortical areas , so that under different tasks the black neuron may select either the red or blue inputs.
Because of the multiple sources of long range inputs coming from within the same cortical area and from many other cortical areas, and because these influences can cascade over multiple nodes, each neuron effectively becomes a microcosm of nearly the entire brain. Each cortical area is represented here as a 2-dimensional network, but because of their laminar structure different layers tend to be responsible for feedforward connections superficial cortical layers and feedback connections deep cortical layers.
McDonnell Foundation C. His work focuses on the brain mechanisms of visual perception and learning at the molecular, circuit and perceptual levels. He studies the way in which the brain analyzes visual images, how this analysis is shaped by higher order cognitive influences and by perceptual learning, and the circuit mechanisms of experience dependent plasticity of the adult visual cortex. He received his bachelor's degree in biophysics from University of Science and Technology of China, and his Ph.
He studies the neural mechanisms underlying visual perception using psychophysical and electrophysiological approaches. Online 'at-a-glance' summary. In contrast to the traditional idea that the processing of visual information consists of a sequence of feedforward operations, with neuronal functional properties taking on increasing complexity as one progresses through a hierarchy of cortical areas, increasing evidence points towards a reverse process, with higher order cognitive influences interacting with information coming from the retina.
As a consequence, one must incorporate the following principles in thinking of the brain mechanisms of information processing:. Rather than having a fixed functional role, neurons are adaptive processors, changing their function according to behavioral context. Vision is an active process, where higher order cognitive influences affect the operations performed by cortical neurons.
Visual pathways operate bidirectionally, with each feedforward connection matched by a feedback or reentrant connections going from higher to lower order cortical areas. Top-down influences include various forms of attention, including spatial, object oriented and feature oriented attention.
Top-down influences are not limited to attention but mediate a much broader range of functional roles, including perceptual task, object expectation, scene segmentation, efference copy, working memory, and the encoding and recall of learned information.
The effect of top-down influences is to change the information conveyed by neurons, both by alteration of the tuning of their responses to stimulus attributes and by changing the structure of correlations over neuronal ensembles. All areas of the visual pathway, except for the retina, are subject to top-down influences, including early cortical stages of visual processing such as the primary visual cortex and the lateral geniculate nucleus, and all areas along the dorsal and ventral visual cortical pathways.
Each area contains an association field of potential interactions, and expresses a subset of these interactions to execute different functions. The sources of top-down influences are widespread, with each area providing information reflecting the functional properties of that area.
As a consequence, even a single neuron can be viewed as a microcosm of activity occurring throughout the visual pathway..
We propose that the circuit mechanism of top-down control and adaptive processing involves a gating of intrinsic cortical circuits within an area mediated by long range feedback connections to that area. By selecting a subset of inputs, a neuron can express different components of its association field, and as a result take on different functional roles. National Center for Biotechnology Information , U.
Nat Rev Neurosci. Author manuscript; available in PMC Dec Charles D. Gilbert 1 and Wu Li 2. Author information Copyright and License information Disclaimer. Copyright notice. As for the task of segmentation, most traditional approaches operate in a bottom-up manner, whereby low-level information such as luminance and texture is used. More recently, Ullman and colleagues have proposed a top-down approach to segmentation [1].
Approach: We are using psychophysical methods to investigate the role of top-down processing in two common visual tasks: segmentation and stereopsis. Segmentation: Given a still image, humans can segment the image into different portions, each of which corresponds to an object or region with an easily understandable label, e.
We aim to demonstrate that, to the first approximation, segmentation requires top-down processing, whereas categorization does not require it. Since a full segmentation of an image is not a clearly-defined task, we construct a simplified segmentation task as a proxy for the full task. Preliminary results from psychophysical experiments are encouraging, and show that the time taken for segmentation is longer than for categorization.
Stereopsis: Random-dot stereograms [3] have convincingly demonstrated that there is a bottom-up component to stereopsis, since humans can binocularly fuse two seemingly random images without first recognizing the images that are being fused. However, what happens if the images do not match, or at least do not match perfectly?
We postulate that for ambiguous, occluded or degraded stimuli, top-down influences may play a role in aiding the binocular fusion of such stimuli. If true, it is quite likely that natural images that contain both low- level and high-level cues will engage both top-down and bottom-up stereo mechanisms.
Impact: Segmentation and stereopsis are two common visual tasks that are of interest to researchers from several fields, including computer vision and psychology. The human visual system is likely to utilize both top-down and bottom-up processing in achieving its remarkable capabilities in these tasks. Our work adds to the growing pool of research demonstrating the importance of top-down processing, in addition to the traditionally recognized bottom-up processing.
Future work: Building upon our preliminary results from segmentation, we intend to conduct follow-up experiments using another proxy to a full segmentation task, thereby validating earlier results. Save to Library Save. Create Alert Alert. Share This Paper. Background Citations. Methods Citations. Results Citations. Figures and Topics from this paper. Citation Type. Has PDF. Publication Type. More Filters. Coordination of top-down influence on V1 responses by interneurons and brain rhythms.
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