Functional ultrasound imaging of the spreading activity following optogenetic stimulation of the rat visual cortex.

Optogenetics has revolutionized neurosciences by allowing fine control of neuronal activity. An important aspect for this control is assessing the activation and/or adjusting the stimulation, which requires imaging the entire volume of optogenetically-induced neuronal activity. An ideal technique for this aim is fUS imaging, which allows one to generate brain-wide activation maps with submesoscopic spatial resolution. However, optical stimulation of the brain with blue light might lead to non-specific activations at high irradiances. fUS imaging of optogenetic activations can be obtained at these wavelengths using lower light power (< 2mW) but it limits the depth of directly activatable neurons from the cortical surface. Our main goal was to report that we can detect specific optogenetic activations in V1 even in deep layers following stimulation at the cortical surface. Here, we show the possibility to detect deep optogenetic activations in anesthetized rats expressing the red-shifted opsin ChrimsonR in V1 using fUS imaging. We demonstrate the optogenetic specificity of these activations and their neuronal origin with electrophysiological recordings. Finally, we show that the optogenetic response initiated in V1 spreads to downstream (LGN) and upstream (V2) visual areas.

Natural textures classification in area V4 of the macaque monkey.

Natural texture of an object is an important cue for recognition. In real conditions, the incidence angle of light on natural textures leads to a complex pattern of micro-shading that modifies 3D rendering of surfaces. Little is known about visual processing of material properties. The present work aims to study the coding of natural textures by the neurons of area V4 of the awake macaque monkey. We used patches of natural textures issued from the CURET database and illuminated with two or three different angles with their corresponding controls (scrambled Fourier phase). We recorded the responses of V4 neurons to stimuli flashed in their receptive fields (RFs) while the macaques performed a simple fixation task. We show that a large majority of V4 neurons responded to texture patches with a strong modulation across stimuli. The analysis of those responses indicate that V4 neurons integrate first and second order parameters in the image (mean luminance, SNR, and energy), which may be used to achieve texture clustering in a multidimensional space. This clustering was comparable to that of a pyramid of Gabor filters and was not affected by illumination angles. Altogether, these results suggest that the V4 neuronal population acts as a set of filters able to classify textures independently of illumination angle. We conclude that area V4 contains mechanisms that are sensitive to the aspect of textured surfaces, even in an environment where illumination changes continuously.