In the Feedback Settings window you can choose the way the particles respond to physiological measurements (stress levels, mental state changes, etc.). For instance, speed could be linked to heart rate variability (HRV), so that as HRV increases the particles move faster. You can click the reverse box to reverse the link (when HRV increases speed decreases).
Go to "Settings->Feedback Settings..." to open the Feedback Settings window.
The following are the possible particle system feedback settings. When you are viewing the feedback settings window, you can hold your mouse over a measurement drop-down box to view a brief description of that particle system property.
NUMBER PER SECOND - Links a measurement to the number of new particles created each second
SPEED - Links a measurement to the movement speed of the particles
LIFETIME - Links a measurement to the length of time that particles exist before disappearing
START SIZE - Links a measurement to the starting size of each particle
END SIZE - Links a measurement to the ending size of each particle
RED VALUE - Links a measurement that adds to or subtracts red from the particle color
GREEN VALUE - Links a measurement that adds to or subtracts green from the particle color
BLUE VALUE - Links a measurement that adds to or subtracts blue from the particle color
WIDTH - Links a measurement to the width of possible starting locations of the particles (the horizontal size of an imaginary object that creates the particles)
HEIGHT - Links a measurement to the height of possible starting locations of the particles
DEPTH - Links a measurement to the depth of possible starting locations of the particles
HORIZONTAL LOCATION - Links a measurement to the horizontal (x) starting point of the particles
VERTICAL LOCATION - Links a measurement to the vertical (y) starting point of the particles
DISTANCE FROM CAMERA - Links a measurement to the depth (z) starting point of the particles (0 is away from you, 1.0 is towards you)
HORIZONTAL DIRECTION - Links a measurement to the horizontal direction of the particles
VERTICAL DIRECTION - Links a measurement to the vertical direction of the particles
DEPTH DIRECTION - Links a measurement to the depth direction of the particles (0 is away from you, 1.0 is towards you)
HORIZONTAL RANGE - Links a measurement to the range of possible horizontal directions of the particles
VERTICAL RANGE - Links a measurement to the range of possible vertical directions of the particles
DEPTH RANGE - Links a measurement to the range of possible depth directions of the particles