FAQ: I had the wrong PHYSICAL.INI / screen config settings, will my data be affected?
#1
The settings in the PHYSICAL.INI file (EyeLink  II / EyeLink 1000) or Screen Configuration settings (EyeLink 1000 Plus / EyeLink Portable Duo) are used by the Host PC to calculate pixel-per-degree resolution and thus the calculation of velocity and saccade amplitude. See What is visual angle and how does it apply to eye tracking? for more background.

There is no need to panic if the PHYSICAL.INI settings do not match the experimental setup - the calibration routine ensures that the gaze data (which is in screen pixels) will still be accurate, but the online event parser will not be accurate. To report accurate velocity or saccade amplitude metrics it is very simple to recalculate these based on the correct measurements using simple trigonometry.

In order to ensure that future gaze data is correct it is important to update the PHYSICAL.INI (EyeLink II / EyeLink 1000) or Screen Settings (EyeLink 1000 Plus / EyeLink Portable Duo) on the Host PC.

EyeLink II & EyeLink 1000: 

From the host software press CTL+ALT+Q to exit the Host software and enter the DOS environment. From DOS, make sure you are in the C:/ELCL/EXE folder and type:

Code:
>> edit physical.ini

This will open the PHYSICAL.INI file in a text editor. From here enter the correct parameters it asks for:

Code:
## screen_phys_coords = <left>, <top>, <right>, <bottom>
;; Meaure the distance of the visible part of the display screen edge
;; relative to the center of the screen (measured in in millimeters).
;; <left>, <top>, <right>, <bottom>:
;; position of display area corners relative to display center
screen_phys_coords = -188.0, 146.0, 188.0, -146.0

## screen_pixel_coords = <left> <top> <right> <bottom>
;; Sets the gaze-position coordinate system, which is used for all
;; calibration target locations and drawing commands.  Usually set
;; to correspond to the pixel mapping of the participant display. 
;; Issue the calibration_type command after changing this to recompute
;; fixation target positions. 
;; You should also write a DISPLAY_COORDS message to the start of
;; the EDF file to record the display resolution.
;;            <left>: X coordinate of left of display area
;;            <top>: Y coordinate of top of display area
;;            <right>: X coordinate of right of display area
;;            <bottom>: Y coordinate of bottom of display area
screen_pixel_coords = 0.0, 0.0, 1024.0, 768.0

## screen_distance = <mm to center> | <mm to top> <mm to bottom>
;; Used for visual angle and velocity calculations. 
;; Providing <mm to top> <mm to bottom> parameters will give better
;; estimates than <mm to center>
;; <mm to center> = distance from display center to participant in millimeters.
;; <mm to top> = distance from display top to participant in millimeters.
;; <mm to bottom> = distance from display bottom to participant in millimeters.
screen_distance = 600 660

For the EyeLink II the marker coordinates also need to be edited:

Code:
## marker_phys_coords = <x, y for top left> <x, y for bottom left>
##                                      <x, y for top right> <x, y for bottom right>
;; Physical position of LED markers in millimeters
;; The 4 LED markers are mounted at the corners of the display. 
;; These are their physical locations, measured with right and up
;; from screen center being positive, and left and down being negative.
;; The marker coordinate pairs are entered in this order:
;;                                                      1            3
;;                                                      2            4           
;; Physical coords: negative if to the left of the center of screen,
;;                              negative if below the center of screen
marker_phys_coords = -220,145, -220,-145, 220,145, 220,-145

If using the EyeLink 1000 in Remote mode, the camera-to-screen distance will also need to be edited via the remote_camera_position function by adding the following command to the FINAL.INI file. Adjust the last parameter to accurately reflect the distance from the Display monitor to the base of the camera lens in mm (keep the minus sign).

Code:
Code:
## remote_camera_position <rh> <rv> <dx> <dy> <dz>
;; <rh>: 10;    // rotation of camera from screen (clockwise from top)
;;              i.e. how much the right edge of the camera is closer than left edge of camera
;;              i.e. 10 assumes right edge is closer than left edge
;; <rv>: 17;  // tilt of camera from screen (top toward screen)
;; <dx>: -80;  // bottom-center of display in cam coords
;; <dy>:  60;
;; <dz>: -90;
remote_camera_position 10 17 -80 60 -90


Eyelink 1000 Plus / EyeLink Portable Duo: 

From the Host Software, press CTRL+ALT+Q to exit to the File Manager. From the File Manager click the gear icon in the top left corner to access the Configurations Screen.

   

From the Configurations Screen select the screen icon to access the Screen Settings.

   

From this interface, follow the on screen directions and enter in the values for Screen Size, Default Display Resolution, Eye-to-Screen Distance, and Camera-to-Screen Distance. Once complete, click the save button and press the EyeLink logo icon to start the host software again.

If there is concern about the quality of the gaze data collected with the these invalid settings, please contact support@sr-research.com so we can discuss the possible implications. Please make sure to include a description of the experiment along with the original settings and the corrected settings in the PHYSICAL.INI.