Optical and functional characterization of parasitic light in a DLP-based optogenetic system for single-neuron control

Background: Digital light processor (DLP)–based patterned illumination enables targeted optogenetic stimulation with high speed and flexibility. While these systems are routinely characterized by optical contrast and spatial resolution, the functional impact of residual light leakage on biological samples remains frequently overlooked. Establishing thresholds and mitigation strategies for this unintended illumination is essential for ensuring the reliability and precision of targeted photostimulation assays. New method: We quantitatively characterized parasitic illumination in a standard DLP-based microscopy setup and evaluated its functional impact on ChR2-expressing neuronal cultures, ranging from subthreshold depolarization to downstream effects on network-level plasticity. Leakage intensities were measured across multiple system configurations and correlated with neuronal responses using calcium imaging and immunostaining of activity-dependent synaptic markers. Results: The intrinsic DLP black-level emission and back-reflections at the sample plane were identified as the dominant leakage sources, and mitigation strategies were suggested. Functional assays showed that leakages below 50 μW/mm2 induce detectable calcium responses without triggering synaptic strengthening, whereas higher intensities can drive spiking, alter network dynamics, and promote synaptic potentiation. Comparison with existing methods: Previous studies characterizing DLP-based optogenetic systems did not focus on the physiological effects induced by parasitic light with illumination intensities below the conventional ChR2 activation threshold. Conclusion: These findings establish thresholds for unintended optogenetic stimulation and provide a simple framework for mitigating parasitic illumination in DLP-based patterned-light optogenetic systems, thereby improving the precision and reliability of targeted photostimulation.