BIOSENSING & PERFORMANCE
An optical sensor pressed against your temple cannot tell you why the light coming back changed. A heartbeat may have pushed a fresh volume of blood through the artery underneath it. Or you may have leaned back in your chair.
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FREQUENTLY ASKED
An IMU, or inertial measurement unit, is a sensor package that measures a device's own motion. It contains an accelerometer measuring linear acceleration and a gyroscope measuring angular velocity, and sometimes a magnetometer measuring magnetic heading.
Acceleration along its axes, in metres per second squared or g, and rate of rotation about its axes, in degrees per second. Where a magnetometer is fitted, it also measures local magnetic field direction and strength.
Linear acceleration along each of its axes, including the constant acceleration due to gravity. That gravity component is why a stationary accelerometer reads about 1 g rather than zero, and why it can also sense tilt.
Angular velocity, meaning how fast the device is rotating about each axis, in degrees per second. It does not measure orientation directly, and it does not measure angular acceleration.
They measure different physical quantities. An accelerometer measures linear acceleration, including gravity. A gyroscope measures rate of rotation. They are commonly paired because each compensates for a weakness in the other, with accelerometer gravity readings correcting gyroscope drift about the horizontal axes.
For two separate reasons. Movement corrupts physiological sensor signals, so a motion channel records when corruption was likely. Movement also changes physiology in its own right, so it supplies context for interpreting a reading rather than only checking it.
It disturbs optical coupling between sensor and skin, alters contact pressure and therefore local perfusion, and changes blood volume at the measurement site independently of the pulse. Because motion artifacts overlap the pulse signal in frequency, fixed filters cannot remove them.
Not optically or electrically, but accelerometers can detect the body's mechanical response to the heartbeat under research conditions. Ear-worn accelerometer ballistocardiography has recorded head-level cardiac recoil where J-wave frequency corresponds to heart rate. Those are research systems requiring stillness. We read your cardiovascular state through PPG, not through motion.
No. An IMU measures physical motion. Stress is a physiological and psychological state that motion sensing has no access to. Any stress-related output, ours included, is derived from other sensors and represents an inference rather than a measurement.
No. Measuring cortical electrical activity requires electroencephalography with scalp electrodes. An IMU has no access to neural signals.
Because HRV depends on precise beat-to-beat timing, which makes it far more vulnerable to corruption than averaged heart rate. Published work found a single inserted artifact could raise RMSSD by 413%. Knowing whether you moved gives a system a basis for judging whether a variability figure should be trusted.
By supplying an independent record of the physical conditions during measurement. That record can serve as a reference signal for artifact reduction, as a quality flag for discarding unreliable windows, or as context for interpreting a physiological change that movement itself may have caused.
Yes. Our closed-loop system reads your physiological state in real time through PPG and IMU sensors, then adjusts tDCS and audio on what it finds. The optical read runs at 100 Hz at the superficial temporal artery; motion sensing runs alongside it as physical context.
In head-worn devices, motion data supports functions such as head-orientation-aware audio, wear detection, and physical context for any physiological sensing the device performs. In a device that acts on physiological readings rather than displaying them, the last of those carries the most weight.
Combining data from multiple sensors so the resulting estimate is better constrained than any single sensor could achieve alone. It improves confidence in quantities already being measured. It does not create measurements of quantities no sensor captures.
THE INSTRUMENT
Sychedelic combines everything described in this article into one 20-minute protocol.