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Summary: Exogenous opsins allow for in vivo interrogation of brain circuits at unprecedented temporal and spatial precision. Here, we found that optical fiber laser stimulation at wavelengths of 637, 594, or 473 nm within the cortex of mice lacking expression of exogenous opsins resulted in a strong neuronal response in the contralateral visual cortex. Evoked responses were observed even at low laser intensities (fiber tip power 1 mW) and most pronounced at 637 nm. We took advantage of retinal light adaptation by using a dim external light source (20 lux) that abolished the 594 and 473 nm-evoked neuronal responses even at high laser intensities (15 mW). The prevention of 637 nm-evoked responses, however, could only be achieved for stimulation intensities ≤ 2.5 mW. This highlights the need for careful selection of light wavelengths and intensities for optogenetic experiments. Additionally, retinal light adaptation offers an effective solution to minimize unintended activation.