Laminar distribution of cortical projection neurons to the pulvinar: A comparative study in cats and mice.

The cortical processing of visual information is thought to follow a hierarchical framework. This framework of connections between visual areas is based on the laminar patterns of direct feedforward and feedback cortico-cortical projections. However, this view ignores the cortico-thalamo-cortical projections to the pulvinar nucleus in the thalamus, which provides an alternative transthalamic information transfer between cortical areas. It was proposed that corticothalamic (CT) pathways follow a similar hierarchical pattern as cortico-cortical connections. Two main types of CT projections have been recognized: drivers and modulators. Drivers originate mainly in Layer 5 whereas modulators are from Layer 6. Little is known about the laminar distribution of these projections to the pulvinar across visual cortical areas. Here, we analyzed the distribution of CT neurons projecting to the lateral posterior (LP) thalamus in two species: cats and mice. Injections of the retrograde tracer fragment B of cholera toxin (CTb) were performed in the LP. The morphology and cortical laminar distribution of CTb-labeled neurons was assessed. In cats, neurons were mostly found in Layer 6 except in Area 17, where they were mostly in Layer 5. In contrast, CT neurons in mice were mostly located in Layer 6 across all areas. Thus, our results demonstrate that CT projections in mice do not follow the same organization as cats suggesting that the transthalamic pathways play distinct roles in these species.

Pulvinar Modulates Contrast Responses in the Visual Cortex as a Function of Cortical Hierarchy.

The pulvinar is the largest extrageniculate visual nucleus in mammals. Given its extensive reciprocal connectivity with the visual cortex, it allows the cortico-thalamocortical transfer of visual information. Nonetheless, knowledge of the nature of the pulvinar inputs to the cortex remains elusive. We investigated the impact of silencing the pulvinar on the contrast response function of neurons in 2 distinct hierarchical cortical areas in the cat (areas 17 and 21a). Pulvinar inactivation altered the response gain in both areas, but with larger changes observed in area 21a. A theoretical model was proposed, simulating the pulvinar contribution to cortical contrast responses by modifying the excitation-inhibition balanced state of neurons across the cortical hierarchy. Our experimental and theoretical data showed that the pulvinar exerts a greater modulatory influence on neuronal activity in area 21a than in the primary visual cortex, indicating that the pulvinar impact on cortical visual neurons varies along the cortical hierarchy.

Spatial Frequency Selectivity Is Impaired in Dopamine D2 Receptor Knockout Mice.

Dopamine is a neurotransmitter implicated in several brain functions, including vision. In the present study, we investigated the impacts of the lack of D2 dopamine receptors on the structure and function of the primary visual cortex (V1) of D2-KO mice using optical imaging of intrinsic signals. Retinotopic maps were generated in order to measure anatomo-functional parameters such as V1 shape, cortical magnification factor, scatter, and ocular dominance. Contrast sensitivity and spatial frequency selectivity (SF) functions were computed from responses to drifting gratings. When compared to control mice, none of the parameters of the retinotopic maps were affected by D2 receptor loss of function. While the contrast sensitivity function of D2-KO mice did not differ from their wild-type counterparts, SF selectivity function was significantly affected as the optimal SF and the high cut-off frequency (p < 0.01) were higher in D2-KO than in WT mice. These findings show that the lack of function of D2 dopamine receptors had no influence on cortical structure whereas it had a significant impact on the spatial frequency selectivity and high cut-off. Taken together, our results suggest that D2 receptors play a specific role on the processing of spatial features in early visual cortex while they do not seem to participate in its development.

Correlation of hemodynamic and fluorescence signals under resting state conditions in mice's barrel field cortex.

Both neurons and astrocytes are known to affect local vascular response in the brain following neuronal activity. In order to differentiate the contributions of each cell type to the hemodynamic response during stimulation and resting state, intrinsic optical signal (IOI) was recorded synchronized with fluorescence imaging of calcium concentration sensitive dye Oregon Green BAPTA-1 AM. By changing the stimulation parameters (frequency and duration), it was possible to individually promote neuronal and glial responses and to compare them to levels of oxy (HbO), deoxy (HbR) and total (HbT) hemoglobin concentrations. Finally, resting state recordings were done to investigate the possible correlation between hemoglobin fluctuation and calcium transients, based on different frequency bands associated either with neuronal or glial activity.

Potencial visual evocado por padrão reverso com tabuleiro de xadrez em gatos domésticos adultos / Pattern reversal visual evoked potential using checkerboard stimulus in adult domestic cats

O potencial visual evocado por padrão reverso (PVE-PR) é um método objetivo, não invasivo e estabelecido para as investigações funcionais. Este trabalho teve como objetivo a obtenção de dados do PVE-PR com tabuleiro de xadrez em gatos domésticos adultos, além de verificar a influência de diferentes ângulos visuais sobre os picos de tempo implícito e amplitude das ondas N75 e P100. Foram utilizados 13 gatos adultos (2-4 anos de idade), de ambos os sexos. Cada animal foi posicionado 100cm de distância de um monitor de vídeo situado a 17,2º. O estímulo gerado para a formação dos PVEs foi uma onda "quadrada" com a frequência de 6 Hz, apresentados em cinco frequências espaciais (16', 32', 64', 130' e 260'de arco). Os valores médios dos tempos implícitos (ms) e amplitudes (µV) de P100 não foram diferentes estatisticamente e nem estavam correlacionados (r=0,067). Com a mudança de tamanho do ângulo de estímulo apresentado, os picos de tempo implícito e amplitude do P100 apresentaram uma discreta variação. Aos 32' de arco, a amplitude de P100 atingiu seu pico máximo, com 14,94±1,13µV (média±erro padrão da média). N75 não mostrou nenhuma variação de amplitude. Com 16' de arco, o tempo implícito de N75 diminuiu, não havendo correlação com os valores de amplitude (r=0,26). O ângulo visual que desencadeou um maior pico de amplitude e tempo implícito constante para P100, foi o de 32' de arco. Em gatos adultos, um PVE-PR pode ser obtido sem o uso de midriáticos e sedativos, com estímulos subtendidos entre ângulos grandes como 64' e pequenos como 16'.

The pattern reversal visual evoked potential (PR-VEP) is an objective method, stablished for noninvasive functional investigations. This research aimed to obtain data from the PR-VEP with checkerboard in adult domestic cats and the influence of different visual angles on the peak latency and implicit time of waves N75 and P100. It was used 13 adult cats (2-4 years old), of both sexes. Each animal was positioned 100 cm away from a video monitor located at 17.2º. The stimulus generated for the formation of wave VEPs was "square" with the frequency of 6Hz, presented in five spatial frequencies (16 ', 32', 64 ', 130' and 260 'of arc). Mean values of implicit times (ms) and amplitudes (µvolts) of P100 were not statistically different and were not correlated (r=0.067). With the change in size of the angle of stimulus presented, the peak implicit time and amplitude of P100 showed a slight variation. At 32 'of arc amplitude of P100 reached its peak with 14,94±1,13µV (mean±SEM). N75 showed no variation in amplitude. With 16'of arc the implicit time N75 decreased and no correlation with amplitude values was observed (r=0,26). The visual angle that triggered higher peak amplitude and implicit time constant for P100, was 32'of arc. In adult cats, PR-VEP can be obtained without mydriatic and sedatives, with stimuli subtending angles between large as 64' and small as 16 '.