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THE SENSOR
PPG is in every smartwatch. What makes Sychedelic's implementation different — and why placement and sampling rate are not marketing specifications.
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FREQUENTLY ASKED
PPG (photoplethysmography) measures blood volume changes with each heartbeat by shining light into the skin and detecting the reflected signal. HRV — RMSSD, LF/HF ratio, and pNN50 — is derived from beat-to-beat intervals and reflects autonomic nervous system balance. Sychedelic samples at 100 Hz at the superficial temporal artery, computing these metrics in a rolling 120-second window for real-time cognitive zone classification.
HRV (heart rate variability) measures the millisecond variation between heartbeats. Higher HRV indicates greater parasympathetic tone, better cognitive readiness, and greater stress resilience. Gbadeyan et al. (2019) demonstrated that tDCS effects on executive function are state-dependent — making HRV the optimal gate variable for determining when a neurostimulation session should begin.
A closed-loop wearable reads a biological signal and uses it to control stimulation. Sychedelic's PPG sensor reads HRV continuously the algorithm classifies cognitive zone tDCS gates on when the Focused zone is reached. This is hardware-enforced: the impedance gate also confirms electrode contact before stimulation begins. Open-loop devices stimulate on a schedule regardless of readiness.
PPG at 100 Hz reads HRV at the superficial temporal artery. The algorithm classifies cognitive zone every 120 seconds using RMSSD, LF/HF, and pNN50. When the Focused zone is reached, the impedance gate checks electrode contact (<10 kΩ required), then tDCS activates at 2.0 mA for 20 minutes at F3/F4 (DLPFC). Deep work begins in the 20–90 minute post-session window.
THE INSTRUMENT
Sychedelic combines everything described in this article into one 20-minute protocol.