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.

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.