Data S1. Materials and Methods.

Fig. S1. Contribution of Na+ cannels to the light dependentspiking activity. (A) Schematic diagram of the experiment. TTX(100 μm, 0.2 μL) was applied to near the probe tipvia a glass pipette. (B) Typical effect of TTX on light elicitedactivity. Light dependent activities were recorded before (Control)and 5 min after drug applications (Saline, TTX). In manycases, light dependent activity was not detected after TTXtreatment (Left). Sometimes transient activity at lightonset was remained after TTX treatment (Right). Laser powerfor stimulation was 0.6 mW.

Fig. S2. Measurement of spatial specificity. (A) Light irradiation at the tip of the optical fiber bundle. Stimulating light was emitted from one core at the tip of the bundle. (B) Upper, Photostimulation of recorded cell with optical fiberbundle. a: Recording pipette, b: Optical fiber bundle.Lower, Stimulating light was emitted at the bundle’stip. (C) Whole-cell current clamp recordings (Upper) orcell-attach recordings (Lower) in response to 0.5 slight pulses of various light intensities. Laser power forphotostimulation was 1.2 mW at maximum light intensity(denoted as 512). Voltage traces during five repetition ofphotostimulation series were displayed. For whole-cell recording,membrane potential at rest was held around −70 mV byinjecting bias current.

Fig. S3. Spatial resolution of action potential generation.Relationships between light intensity and spike probability weremeasured at various photostimulation points. (A) Stimulation pointwas moved along the axial axis of the bundle. Values on the leftside of the graph indicate distance between recorded cell and thetip of the bundle. (B) Stimulation point was moved along a lineperpendicular to the bundle’s axial axis. Values on the leftside of the graph indicate distance between recorded cell and thetip of the bundle. Laser power for photostimulation was 1.2 mWat maximum light intensity (denoted as 512).

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