Light distribution and thermal effects in the rat brain under optogenetic stimulation

Optical brain stimulation gained a lot of attention in neuroscience due to its superior cell‐type specificity. In the design of illumination strategies, predicting the light propagation in a specific tissue is essential and requires knowledge of the optical properties of that tissue. We present the estimated absorption and reduced scattering in rodent brain tissue using non‐destructive contact spatially resolved spectroscopy (cSRS). The obtained absorption and scattering in the cortex, hippocampus and striatum are similar, but lower than in the thalamus, leading to a less deep but broader light penetration profile in the thalamus. Next, the light distribution was investigated for different stimulation protocols relevant for fiber‐optic based optogenetic experiments, using Monte Carlo simulation. A protocol specific analysis is proposed to evaluate the potential of thermally induced side effects. Investigation of brain mechanisms requires the ability to alter neural activity. Optogenetics is a novel, superb method for modulating neuron activity, requiring knowledge on the brain optical properties for precisely planning an experiment. Here, the absorption and reduced scattering coefficients measured in vitro in the rat brain are presented. Additionally, the influence of fiber properties and light power on photon propagation in brain tissue and on brain temperature was studied.