Skip to content

Laboratory of Neuropsychology

Calibrating Eye/Joystick Signals

Calibration is a process of converting and aligning voltages of input signals to coordinates of visual object space. To do so, you first need to set the physical dimensions of the visual space correctly. Measure the diagonal size of the subject monitor and the distance between the monitor and the subject and put them in the MonkeyLogic main menu as below. Their units should be the same (let’s all use centimeters, to avoid any confusion). MonkeyLogic then gives you a conversion factor, pixels per degree (91.717 in the figure shown below), which ML uses to calculate visual angles. You can use this number to convert the size of visual objects to visual angles. For example, a 320 x 240 movie is [320 240] / 91.717 = [3.4890 2.6167] degrees in visual angles.

NIMH MonkeyLogic currently provides options to use Raw Signal and two calibration methods, Origin & Gain and 2-D Spatial Transformation. The Origin & Gain requires calibration just for 2 fixation points and is much easier to use with untrained subjects. The 2-D Spatial Transformation requires sampling voltages for at least 4 fixation points but can do more complex mapping by a projective transform. Both methods come with a tool that you can conveniently manipulate with the mouse. If you click yellow squares on the control screen, a fixation point will be shown in the corresponding location on the subject screen.

There are a few shortcut keys that you can use during calibration. These shortcuts work during the task as well.

  • [C] key: Brings the current eye position to the center of the screen.
  • [U] key: Cancels the previous 'C' key. You can undo multiple times.
  • [R] key: Delivers a manual reward.
  • [-] key: Decreases the reward pulse length by 10 ms.
  • [+] key: Increases the reward pulse length by 10 ms.

Raw Signal (Pre-calibrated)

Choose this option when the signals are calibrated already or you want to get unaltered input. One volt in the calibrated signals should correspond to one degree’s displacement. If the signal is calibrated but the conversion ratio is not one to one, you can do further adjustment with the Origin & Gain method.

Origin & Gain

  1. Click the "Show Center" button. It will show a fixation point at the center of the subject screen. Then wait until the subject look at it.
  2. Click the "Set Origin" button while the subject is looking at the center. It will register the current voltage reading. Alternatively, you can click the fixation point (yellow square) at the center with the mouse left button and press the space bar (or 'C' key) to register the voltage.
  3. Click one of the fixation points on the periphery. Increase (or decrease) X & Y gains if the subject's saccade undershoots (or overshoots).
  4. Click the "Save" button. The "Cancel" button reverts any change that has been made.

2-D Spatial Transformation

  1. You can use only selected fixation points for calibration in this method. Selected points have numbers on them. To select/unselect fixation points, use the right mouse button. If you change the intervals of fixation points, then some numbers may not be on a fixation point any more. For those numbers, you can only unselect them.
  2. Once you pick up all fixation points you want, you can turn them on/off one by one with the keyboard (N key & P key) or the mouse left-clicks.
  3. Hit the space key while the subject is looking at the displayed fixation point. Then the voltage reading at that moment will be registered for the shown fixation point and its number will turn from red to blue.
  4. You can choose how you want to reward the fixation behavior. The default option is to deliver reward at the time of the space key press and move to the next fixation point.
  5. Repeat 2) and 3) for the entire selected fixation points a couple of times. Then click the "Save" button. Eye traces won't appear until there are at least 4 fixation points calibrated.