fMRI response to blue light delivery in the naïve brain: Implications for combined optogenetic fMRI studies

► Blue light delivered to the brain of naïve rats causes pseudo BOLD activations. ► Thermal imaging data shows that light causes brain heating ∝ laser power. ► Pseudo activations are very similar in vivo and ex vivo indicating a MR artefact. ► Temperature induced relaxation changes (T1 and T2*) play an important role in the pseudo-activations.