28 research outputs found
Investigating the encoding of visual stimuli by forming neural circuits in the cat primary visual cortex
Contexte
La connectomique, ou la cartographie des connexions neuronales, est un champ de recherche des neurosciences Ă©voluant rapidement, promettant des avancĂ©es majeures en ce qui concerne la comprĂ©hension du fonctionnement cĂ©rĂ©bral. La formation de circuits neuronaux en rĂ©ponse Ă des stimuli environnementaux est une propriĂ©tĂ© Ă©mergente du cerveau. Cependant, la connaissance que nous avons de la nature prĂ©cise de ces rĂ©seaux est encore limitĂ©e. Au niveau du cortex visuel, qui est lâaire cĂ©rĂ©brale la plus Ă©tudiĂ©e, la maniĂšre dont les informations se transmettent de neurone en neurone est une question qui reste encore inexplorĂ©e. Cela nous invite Ă Ă©tudier lâĂ©mergence des microcircuits en rĂ©ponse aux stimuli visuels. Autrement dit, comment lâinteraction entre un stimulus et une assemblĂ©e cellulaire est-elle mise en place et modulĂ©e?
MĂ©thodes
En rĂ©ponse Ă la prĂ©sentation de grilles sinusoĂŻdales en mouvement, des ensembles neuronaux ont Ă©tĂ© enregistrĂ©s dans la couche II/III (aire 17) du cortex visuel primaire de chats anesthĂ©siĂ©s, Ă lâaide de multi-Ă©lectrodes en tungstĂšne. Des corrĂ©lations croisĂ©es ont Ă©tĂ© effectuĂ©es entre lâactivitĂ© de chacun des neurones enregistrĂ©s simultanĂ©ment pour mettre en Ă©vidence les liens fonctionnels de quasi-synchronie (fenĂȘtre de ± 5 ms sur les corrĂ©logrammes croisĂ©s corrigĂ©s). Ces liens fonctionnels dĂ©voilĂ©s indiquent des connexions synaptiques putatives entre les neurones. Par la suite, les histogrammes peri-stimulus (PSTH) des neurones ont Ă©tĂ© comparĂ©s afin de mettre en Ă©vidence la collaboration synergique temporelle dans les rĂ©seaux fonctionnels rĂ©vĂ©lĂ©s. Enfin, des spectrogrammes dĂ©pendants du taux de dĂ©charges entre neurones ou stimulus-dĂ©pendants ont Ă©tĂ© calculĂ©s pour observer les oscillations gamma dans les microcircuits Ă©mergents. Un indice de corrĂ©lation (Rsc) a Ă©galement Ă©tĂ© calculĂ© pour les neurones connectĂ©s et non connectĂ©s.
RĂ©sultats
Les neurones liĂ©s fonctionnellement ont une activitĂ© accrue durant une pĂ©riode de 50 ms contrairement aux neurones fonctionnellement non connectĂ©s. Cela suggĂšre que les connexions entre neurones mĂšnent Ă une synergie de leur inter-excitabilitĂ©. En outre, lâanalyse du spectrogramme dĂ©pendant du taux de dĂ©charge entre neurones rĂ©vĂšle que les neurones connectĂ©s ont une plus forte activitĂ© gamma que les neurones non connectĂ©s durant une fenĂȘtre dâopportunitĂ© de 50ms. LâactivitĂ© gamma de basse-frĂ©quence (20-40 Hz) a Ă©tĂ© associĂ©e aux neurones Ă dĂ©charge rĂ©guliĂšre (RS) et lâactivitĂ© de haute frĂ©quence (60-80 Hz) aux neurones Ă dĂ©charge rapide (FS). Aussi, les neurones fonctionnellement connectĂ©s ont systĂ©matiquement un Rsc plus Ă©levĂ© que les neurones non connectĂ©s. Finalement, lâanalyse des corrĂ©logrammes croisĂ©s rĂ©vĂšle que dans une assemblĂ©e neuronale, le rĂ©seau fonctionnel change selon lâorientation de la grille. Nous dĂ©montrons ainsi que lâintensitĂ© des relations fonctionnelles dĂ©pend de lâorientation de la grille sinusoĂŻdale. Cette relation nous a amenĂ© Ă proposer lâhypothĂšse suivante : outre la sĂ©lectivitĂ© des neurones aux caractĂšres spĂ©cifiques du stimulus, il y a aussi une sĂ©lectivitĂ© du connectome. En bref, les rĂ©seaux fonctionnels «signature » sont activĂ©s dans une assemblĂ©e qui est strictement associĂ©e Ă lâorientation prĂ©sentĂ©e et plus gĂ©nĂ©ralement aux propriĂ©tĂ©s des stimuli.
Conclusion
Cette Ă©tude souligne le fait que lâassemblĂ©e cellulaire, plutĂŽt que le neurone, est l'unitĂ© fonctionnelle fondamentale du cerveau. Cela dilue l'importance du travail isolĂ© de chaque neurone, câest Ă dire le paradigme classique du taux de dĂ©charge qui a Ă©tĂ© traditionnellement utilisĂ© pour Ă©tudier l'encodage des stimuli. Cette Ă©tude contribue aussi Ă faire avancer le dĂ©bat sur les oscillations gamma, en ce qu'elles surviennent systĂ©matiquement entre neurones connectĂ©s dans les assemblĂ©es, en consĂ©quence dâun ajout de cohĂ©rence. Bien que la taille des assemblĂ©es enregistrĂ©es soit relativement faible, cette Ă©tude suggĂšre nĂ©anmoins une intrigante spĂ©cificitĂ© fonctionnelle entre neurones interagissant dans une assemblĂ©e en rĂ©ponse Ă une stimulation visuelle. Cette Ă©tude peut ĂȘtre considĂ©rĂ©e comme une prĂ©misse Ă la modĂ©lisation informatique Ă grande Ă©chelle de connectomes fonctionnels.Background
âConnectomicsââ the mapping of neural connections, is a rapidly advancing field in neurosciences and it promises significant insights into the brain functioning. The formation of neuronal circuits in response to the sensory environment is an emergent property of the brain; however, the knowledge about the precise nature of these sub-networks is still limited. Even at the level of the visual cortex, which is the most studied area in the brain, how sensory inputs are processed between its neurons, is a question yet to be completely explored. Heuristically, this invites an investigation into the emergence of micro-circuits in response to a visual input â that is, how the intriguing interplay between a stimulus and a cell assembly is engineered and modulated?
Methods
Neuronal assemblies were recorded in response to randomly presented drifting sine-wave gratings in the layer II/III (area 17) of the primary visual cortex (V1) in anaesthetized cats using tungsten multi-electrodes. Cross-correlograms (CCGs) between simultaneously recorded neural activities were computed to reveal the functional links between neurons that were indicative of putative synaptic connections between them. Further, the peristimulus time histograms (PSTH) of neurons were compared to divulge the epochal synergistic collaboration in the revealed functional networks. Thereafter, perievent spectrograms were computed to observe the gamma oscillations in emergent microcircuits. Noise correlation (Rsc) was calculated for the connected and unconnected neurons within these microcircuits.
Results
The functionally linked neurons collaborate synergistically with augmented activity in a 50-ms window of opportunity compared with the functionally unconnected neurons suggesting that the connectivity between neurons leads to the added excitability between them. Further, the perievent spectrogram analysis revealed that the connected neurons had an augmented power of gamma activity compared with the unconnected neurons in the emergent 50-ms window of opportunity. The low-band (20-40 Hz) gamma activity was linked to the regular-spiking (RS) neurons, whereas the high-band (60-80 Hz) activity was related to the fast-spiking (FS) neurons. The functionally connected neurons systematically displayed higher Rsc compared with the unconnected neurons in emergent microcircuits. Finally, the CCG analysis revealed that there is an activation of a salient functional network in an assembly in relation to the presented orientation. Closely tuned neurons exhibited more connections than the distantly tuned neurons. Untuned assemblies did not display functional linkage. In short, a âsignatureâ functional network was formed between neurons comprising an assembly that was strictly related to the presented orientation.
Conclusion
Indeed, this study points to the fact that a cell-assembly is the fundamental functional unit of information processing in the brain, rather than the individual neurons. This dilutes the importance of a neuron working in isolation, that is, the classical firing rate paradigm that has been traditionally used to study the encoding of a stimulus. This study also helps to reconcile the debate on gamma oscillations in that they systematically originate between the connected neurons in assemblies. Though the size of the recorded assemblies in the current investigation was relatively small, nevertheless, this study shows the intriguing functional specificity of interacting neurons in an assembly in response to a visual input. One may form this study as a premise to computationally infer the functional connectomes on a larger scale
High noise correlation between the functionally connected neurons in emergent V1 microcircuits
Abstract : Neural correlations (noise correlations and cross-correlograms) are widely studied to infer functional connectivity between neurons. High noise correlations (Rsc) between neurons have been reported to increase the encoding accuracy of a neuronal population; however, low noise correlations have also been documented to play a critical role in cortical microcircuits. Therefore, the role of noise correlations in neural encoding is highly debated. To this aim, through multi-electrodes, we recorded neuronal ensembles in the primary visual cortex of anesthetized cats. By computing cross-correlograms (CCGs), we divulged the functional network (microcircuit) between neurons within an ensemble in relation to a specific orientation. We show that functionally connected neurons systematically exhibit higher noise correlations than functionally unconnected neurons in a microcircuit that is activated in response to a particular orientation. Furthermore, the mean strength of noise correlations for the connected neurons increases steeply than the unconnected neurons as a function of the resolution-window used to calculate noise correlations. We suggest that, neurons that display high noise correlations in emergent microcircuits feature functional connections which are inevitable for information encoding in the primary visual cortex
Electrophysiological and firing properties of neurons: categorizing soloists and choristers in primary visual cortex
Abstract: Visual processing in the cortex involves various aspects of neuronal properties such as morphological, electrophysiological and molecular. In particular, the neural firing pattern is an important indicator of dynamic circuitry within a neuronal population. Indeed, in microcircuits, neurons act as soloists or choristers wherein the characteristical activity of a âsoloistâ differs from the firing pattern of a âchoristerâ. Both cell types correlate their respective firing rate with the global populational activity in a unique way. In the present study, we sought to examine the relationship between the spike shape (thin spike neurons and broad spike neurons) of cortical neurons recorded from V1, their firing levels and their propensity to act as soloists or choristers. We found that thin spike neurons, which exhibited higher levels of firing, generally correlate their activity with the neuronal population (choristers). On the other hand, broad spike neurons showed lower levels of firing and demonstrated weak correlations with the assembly (soloists). A major consequence of the present study is: estimating the correlation of neural spike trains with their neighboring population is a predictive indicator of spike waveforms and firing level. Indeed, we found a continuum distribution of coupling strength ranging from weak correlation-strength (attributed to low-firing neurons) to high correlation-strength (attributed to high-firing neurons). The tendency to exhibit high- or low-firing is conducive to the spike shape of neurons. Our results offer new insights into visual processing by showing how high-firing rate neurons (mostly thin spike neurons) could modulate the neuronal responses within cell-assemblies
Network-selectivity and stimulus-discrimination in the primary visual cortex : cell-assembly dynamics
Abstract : Visual neurons coordinate their responses in relation to the stimulus; however, the complex
interplay between a stimulus and the functional dynamics of an assembly still eludes
neuroscientists. To this aim, we recorded cell assemblies from multi-electrodes in the primary
visual cortex of anaesthetized cats in response to randomly presented sine-wave drifting gratings
whose orientation tilted in 22.5° steps. Cross-correlograms divulged the functional connections
at all the tested orientations. We show that a cell-assembly discriminates between orientations by
recruiting a âsalientâ functional network at every presented orientation, wherein, the connections
and their strengths (peak-probabilities in the cross-correlogram) change from one orientation to
another. Within these assemblies, closely tuned neurons exhibited increased connectivity and
connection-strengths than differently tuned neurons. Minimal connectivity between untuned
neurons suggests the significance of neuronal selectivity in assemblies. This study reflects upon
the dynamics of functional connectivity, and brings to the fore the importance of a âsignatureâ
functional network in an assembly that is strictly related to a specific stimulus. Apparently, it
points to the fact that an assembly is the major âfunctional unitâ of information processing in
cortical circuits, rather than the individual neurons
Comparative effects of adaptation on layer II-III and layer V-VI neurons in cat V1
Abstract: V1 is fundamentally grouped into columns that descend from layers IIâIII to VâVI. Neurons inherent to visual cortex are capable of adapting to changes in the incoming stimuli that drive the cortical plasticity. A principle feature called orientation selectivity can be altered by the presentation of nonâoptimal stimulus called âadapterâ. When triggered, LGN cells impinge upon layer IV and further relay the information to deeper layers via layers IIâIII. Using different adaptation protocols, neuronal plasticity can be investigated. Superficial neurons in area V1 are well acknowledged to exhibit attraction and repulsion by shifting their tuning peaks when challenged by a nonâoptimal stimulus called âadapterâ. Layers VâVI neurons in spite of partnering layers IIâIII neurons in cortical computation have not been explored simultaneously toward adaptation. We believe that adaptation not only affects cells specific to a layer but modifies the entire column. In this study, through simultaneous multiunit recordings in anesthetized cats using a multichannel depth electrode, we show for the first time how layers VâVI neurons (1000â1200 ÎŒm) along with layers IIâIII neurons (300â500 ÎŒm) exhibit plasticity in response to adaptation. Our results demonstrate that superficial and deeper layer neurons react synonymously toward adapter by exhibiting similar behavioral properties. The neurons displayed similar amplitude of shift and maintained equivalent sharpness of Gaussian tuning peaks before and the following adaptation. It appears that a similar mechanism, belonging to all layers, is responsible for the analog outcome of the neuronsâ experience with adapter
Synergistic activity between primary visual neurons
Abstract : Cortical microcircuitry plays a pivotal role in encoding sensory information reaching the
cortex. However, the fundamental knowledge concerning the mechanisms that govern
feature-encoding by these sub-networks is still sparse. Here, we show through multi
electrode recordings in V1 of conventionally prepared anesthetized cats, that an avalanche
of synergistic neural activity occurs between functionally connected neurons in a cell
assembly in response to the presented stimulus. The results specifically show that once the
reference neuron spikes in a connected neuron-pair, it facilitates the response of its
companion (target) neuron for 50 ms and, thereafter, the excitability of the target neuron
declines. On the other hand, the functionally unconnected neurons do not facilitate each
otherâs activity within the 50 ms time-window. The added excitation (facilitation) of
connected neurons is almost four times the responsiveness of unconnected neurons. This
suggests that connectedness confers the added excitability to neurons; consequently leading to feature-encoding within the emergent 50 ms-period. Furthermore, the facilitation
significantly decreases as a function of orientation selectivity spread
Modulation of functional connectivity following visual adaptation: homeostasis in V1
Abstract: Sensory neurons exhibit remarkable adaptability in acquiring new optimal selectivity to unfamiliar features when a new stimulus becomes prevalent in the environment. In conventionally prepared adult anesthetized cats, we used visual adaptation to change the preferred orientation selectivity in V1 neurons. Cortical circuits are dominated by complex and intricate connections between neurons. Cross-correlation of cellular spike-trains discloses the putative functional connection between two neurons. We sought to investigate changes in these links following a twelve minute uninterrupted application of a specific, usually non-preferred, orientation. We report that visual adaptation, mimicking training, modulates the magnitude of cross-correlograms suggesting that the strength of inter-neuronal relationships is modified. While individual cell-pairs exhibit changes in their response correlation strength, the average correlation of the recorded cell cluster remains unchanged. Hence, visual adaptation induces plastic changes that impact the connectivity between neurons
Summation of connectivity strengths in the visual cortex reveals stability of neuronal microcircuits after plasticity
Abstract : Background: Within sensory systems, neurons are continuously affected by environmental stimulation. Recently, we showed that, on cell-pair basis, visual adaptation modulates the connectivity strength between similarly tuned neurons to orientation and we suggested that, on a larger scale, the connectivity strength between neurons forming sub-networks could be maintained after adaptation-induced-plasticity. In the present paper, based on the summation of the connectivity strengths, we sought to examine how, within cell-assemblies, functional connectivity is regulated during an exposure-based adaptation. Results: Using intrinsic optical imaging combined with electrophysiological recordings following the reconfiguration of the maps of the primary visual cortex by long stimulus exposure, we found that within functionally connected cells, the summed connectivity strengths remain almost equal although connections among individual pairs are modified. Neuronal selectivity appears to be strongly associated with neuronal connectivity in a âhomeodynamicâ manner which maintains the stability of cortical functional relationships after experience-dependent plasticity. Conclusions: Our results support the âhomeostatic plasticity conceptâ giving new perspectives on how the summation in visual cortex leads to the stability within labile neuronal ensembles, depending on the newly acquired properties by neurons
Integration of landmark and saccade target signals in macaque frontal cortex visual responses
Abstract Visual landmarks influence spatial cognition and behavior, but their influence on visual codes for action is poorly understood. Here, we test landmark influence on the visual response to saccade targets recorded from 312 frontal and 256 supplementary eye field neurons in rhesus macaques. Visual response fields are characterized by recording neural responses to various target-landmark combinations, and then we test against several candidate spatial models. Overall, frontal/supplementary eye fields response fields preferentially code either saccade targets (40%/40%) or landmarks (30%/4.5%) in gaze fixation-centered coordinates, but most cells show multiplexed target-landmark coding within intermediate reference frames (between fixation-centered and landmark-centered). Further, these coding schemes interact: neurons with near-equal target and landmark coding show the biggest shift from fixation-centered toward landmark-centered target coding. These data show that landmark information is preserved and influences target coding in prefrontal visual responses, likely to stabilize movement goals in the presence of noisy egocentric signals