Figure 3.

Elimination of light-induced artifacts in recordings of the local field potential. (a) Direct exposure of metal electrodes to the laser beam causes large electrical artifacts. These examples are taken from recordings in a cortical site not transduced with AAV-DIO-ChR2-mCherry. A glass pipette with a nonreflective coating was placed in the superficial cortex ~350 μm away from a tungsten electrode. The shaft and exposed tip of the tungsten electrode were directly in the cone of blue light, as shown by the schematic to the right. Under these conditions, pulses of laser light (68 mW mm⁻²; blue trace) caused a large, repeated artifact in the LFP recording from the metal electrode (upper black trace). The artifact started at the onset of each light pulse and lasted at least 8 ms after the end of the light pulse (see inset). Simultaneous recordings from a glass pipette whose shaft was in the light cone under the optical fiber did not show any light-induced artifacts (lower black trace). (b) Similar artifacts were observed when the LFP was recorded from a cortical site containing interneurons transduced with AAV DIO ChR2-mCherry (red dots). In this case, the LFP recorded on the metal electrode shows a gamma oscillation induced by stimulating the fast-spiking interneurons at 40 Hz, but the oscillation signal is obscured by the light-induced artifact caused by the shaft of the metal electrode intersecting the laser beam (upper trace). In contrast, the simultaneous recording from the glass electrode shows the gamma oscillation in the absence of the artifact (middle trace), as highlighted by the filtered glass electrode LFP (lower trace). (c) Artifacts in the metal electrode recordings can be eliminated by changing the angle of the electrode so that the shaft does not intersect the laser beam, as shown in the schematic to the right. In this example, the signal on the angled metal electrode (upper traces) agrees well with the signal on the glass electrode (lower traces).