Cortical neuron calcium response to infrared neural stimulation depends on the spatiotemporal thermal gradient.

Infrared neural stimulation (INS) is a promising neuromodulation tool, yet its clinical translation is hindered by an incomplete understanding of how photothermal dynamics recruit specific physiological mechanisms. This study investigates how varying spatiotemporal thermal gradients influence calcium responses in primary rat cortical neurons by combining calcium imaging with numerical modeling and experimental validation. Laser dosimetry was performed by focusing a 1470 nm laser to a 15 µm diameter and varying the pulse energy for 0.5 ms, 5 ms, and 50 ms laser pulse durations. The thermal dynamics of the stimulus were simulated using a computational model, which was validated against thermal lensing experiments. Our results reveal a bimodal calcium response comprising high-frequency phasic spiking and low-frequency basal increases, which depend on the temporal thermal dynamics of the laser stimulus. This research provides a critical framework for designing spatially precise, effective, and safe INS technologies.