Are Sensory Neurons in the Cortex Committed to Original Trigger Features?

Sensory cortices are inherently dynamic and exhibit plasticity in response to a variety of stimuli. Few studies have revealed that depending upon the nature of stimuli, excitation of the corresponding sensory region also evokes a response from other neighboring connected areas. It is even more striking, when somatosensory areas undergo reorganization as a result of an intentional disturbance and further explored as a paradigm to understand neuroplasticity. In addition, it has also been proved that drugs too can be used as a model to explore potential plasticity in sensory systems. To this aim, through electrophysiology in cats, we explored that visual neurons, throughout the cortical column, have a tendency to alter their inherent properties even when presented a non-visual stimulus. Furthermore, it was explored in mice, how the application of drugs (serotonin and ketamine) modulates potential plasticity within the visual system. Indeed, we found a shift in orientation tuning of neurons indicated by Gaussian tuning fits in both scenarios. These results together suggest that sensory cortices are capable of adapting to intense experiences by going through a recalibration of corresponding or neighboring sensory area(s) to redirect the sensory function and exhibit remarkable extent of neuroplasticity within the brain

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 ‘soloist’ differs from the firing pattern of a ‘chorister’. 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

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 “homeodynamic” manner which maintains the stability of cortical functional relationships after experience-dependent plasticity. Conclusions: Our results support the “homeostatic plasticity concept” 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

Investigating the comparative effects of adaptation on supra and infragranular layers with visual and acoustic stimulation in cat’s visual cortex

Dans le cortex visuel primaire (V1 ou l’aire 17) du chat, les neurones répondent aux orientations spécifiques des objets du monde extérieur et forment les colonnes d'orientation dans la zone V1. Un neurone répondant à une orientation horizontale sera excité par le contour horizontal d'un objet. Cette caractéristique de V1 appelée sélectivité d'orientation a été explorée pour étudier les effets de l'adaptation. Suivant un schéma d’entraînement (adaptation), le même neurone ayant initialement répondu à l’orientation horizontale répondra désormais à une orientation oblique. Dans cette thèse, nous étudions les propriétés d'ajustement d'orientation de neurones individuels dans des couches superficielles et plus profondes de V1 dans deux environnements d'adaptation. En raison de la grande interconnectivité entre les neurones de V1, nous émettons l'hypothèse que non seulement les neurones individuels sont affectés par l'adaptation, mais que tout le cortex est reprogrammé par l'adaptation. Des enregistrements extracellulaires ont été effectués sur des chats anesthésiés. Les activités neuronales ont été enregistrées simultanément aux couches 2/3 et à la 5/6 à l'aide d'une électrode de tungstène. Les neurones ont été adaptés à la fois par stimulation visuelle et son répétitif selon deux protocoles différents. Dans les deux cas, une plage stimulante constituée de sinusoïdes à défilement a été présentée pour évoquer les réponses dans V1 et générer des courbes de réglage d'activité multi-unités. La connectivité fonctionnelle entre les neurones enregistrés a été démontrée par un corrélogramme croisé entre les décharges cellulaires captées simultanément. En réponse à l'adaptation visuelle, les neurones des couches 2/3 et 5/6 ont montré des glissements attractifs et répulsifs classiques. En revanche en comparant le comportement des neurones de l'une et l'autre couche, on a observé une tendance équivalente. Les corrélogrammes croisés entre les trains de neurones des couches 2/3 et 5/6 ont révélé des décharges synchronisées entre les neurones. Durant l'adaptation au son, en l'absence totale de stimuli visuel, le glissement des courbes d’accord a été observés chez l'une et l'autre couche indiquant ainsi un changement de la sélectivité de l'orientation. Toutefois, il faut prendre note du fait que les cellules des deux couches ont un glissement aux directions opposées ce qui dénote un comportement indépendant. Nos résultats indiquent que les réponses des neurones du cortex V1 peuvent être évoqués par stimulation directe ou indirecte. La différence de réponses à différents environnements d'adaptation chez les neurones des couches 2/3 et 5/6 indiquent que les neurones de l'aire V1 peuvent choisir de se comporter de la même façon ou différemment lorsque confrontés à divers’ stimuli sensoriels. Ceci suggère que les réponses dans V1 sont dépendantes du stimulus environnemental. Aussi, les décharges synchronisées des neurones de la couche 2/3 et de la couche 5/6 démontre une connectivité fonctionnelle entre les paires de neurones. En définitive on pourrait affirmer que les neurones visuels subissent une altération de leur sélectivité en construisant de nouvelles cartes de sélectivité. À la lumière de nos résultats on pourrait concevoir que le cortex en entier serait multi sensoriel compte tenu de la plasticité entre les zones sensorielles.In the cat primary visual cortex (V1 or area17), neurons fundamentally respond to orientations of the objects in the outside world. Neurons responding to specific orientations form the orientation columns in V1. A neuron responding to a horizontal orientation will get optimally excited towards the outline of a horizontal object. This feature of the visual cortex known as orientation selectivity has been continuously explored to study the effects of adaptation. Following a training paradigm called adaptation, the same neuron that was inherently responding to the horizontal orientation will respond to an oblique orientation. In this thesis, we seek to examine the orientation tuning properties of individual neurons in superficial and deeper layers of V1 in different adaptation environments. Due to the extensive interconnectivity between V1 neurons, we hypothesize that not only do individual neurons get affected by adaptation paradigm, but the whole cortex is reprogramed. To this aim, extracellular recordings were performed in conventionally prepared anesthetized cats. Neural activities were recorded simultaneously from layer 2/3 and layer 5/6 using a tungsten multichannel electrode. Neurons were adapted with a visual adapter (visual adaptation) and a repetitive sound (sound adaptation) in two different settings. Both types of adaptations were performed uninterrupted for 12 minutes. In both settings, sine-wave drifting gratings were presented to evoke responses in V1 and generate tuning curves from the recorded multiunit activity. The functional connectivity between the recorded neurons was revealed by computing cross-correlation between individual neuron pairs. In response to visual adaptation, layer 2/3 and 5/6 neurons displayed classical attractive and repulsive shifts. On comparing the behaviour of the neurons in either layer, an equivalent tendency was observed. Cross-correlograms between the spike trains of neurons in layers 2/3 and 5/6 revealed synchronized firing between the neurons suggesting coordinated dynamics of the co-active neurons and their functional connections. During sound adaptation, where the visual adapter was completely absent, shifts in the tuning curves were observed in either layer indicating a novel orientation selectivity. However, it is noteworthy that cells in both layers shifted in opposite directions indicating independent behaviour. V1 neurons might have an additional role besides processing visual stimuli. The visual neurons may have demonstrated multisensory properties when stimulated indirectly through neighbouring sensory regions. Our results indicate that primary visual neurons can be evoked by direct or indirect stimulation. The difference in the responses of layer 2/3 and layer 5/6 neurons towards the different adaptation environments indicate that neurons in V1 may behave similar or different towards the different sensory stimulus. This suggests that V1 responses are stimulus dependent. Additionally, the synchronized firing of layer 2/3 and layer 5/6 neurons towards visual adapter signify an existence of functional connectivity between the neuron pairs. Together, it can be summarised that visual neurons undergo an alteration of selectivity by building new orientation maps that ultimately potentiates plasticity within sensory regions that are highly suggestive of entire cortex being multisensory