Photoelectric Artifact

Mechanism

Light on the site metal changes the interfacial potential: photons release charge at the metal surface, the illuminated interface shifts its equilibrium potential, and the absorbed energy heats the interface and the electrolyte beside it. The amplifier records the sum as a transient on the channel. A larger uncoated metal area collects more light and a higher irradiance deposits more energy, so both enlarge the transient; it is largest at pulse onset and offset, when the illumination changes fastest, and a long pulse adds a slower thermal component between them (Kozai & Vazquez (2015)). Because the artifact and the evoked spikes occupy the same window, it contaminates exactly the milliseconds the experiment needs, and a fiber placed close to the sites drives it hardest (Cardin et al. (2010)).

How NeuroNexus addresses it

NeuroNexus removes the artifact at the electrode, before it reaches the amplifier, rather than subtracting it afterwards.

• Z-Coat site coating changes the site interface so the transient is suppressed, while the site keeps its footprint and its impedance stays low through repeated insertion. Z-Coat sites record cleanly through light pulses in combined optogenetic and recording experiments, and the coating is specified at order on catalog silicon probe designs (Z-Coat: Optimized Site Coating for Stimulation and Noise Reduction).
• Optrode packages place the fiber along the shank to minimize direct illumination of the sites, and the light pulse is timestamped with the neural data through GPIO so that onset and offset are known to the sample.

Typical values

The amplitude of the artifact depends on wavelength, irradiance at the site, pulse width, and the site's metal and exposed area; it is a property of the light and the interface, not of the probe design alone. NeuroNexus states the mechanism and the design remedy for every configuration, and the Z-Coat application note states where the coating applies. For a chronic recording, the suppression holds across sessions because the coating is mechanically stable through repeated insertion.