This app allows you to determine the ability of people to position a line horizontal (subjective visual horizontal = SVH) or to position a line vertical (subjective vertical = SVV). This is done by holding the smartphone in front of your eyes and turn it slow but steady clock or anti-clockwise to position the line horizontal (SVH) or vertical (SVV) as good as you can. Then confirm this optimal position by holding the smartphone carefully in the chosen position and tapping one time on the screen. If you make an error, you can try again by swiping with your finger downwards over the screen (downwards = when holding the smartphone in vertical position). The test is repeated 20 times (with 10 starting positions of the line tilted to the left and 10 starting positions of the line tilted to the right). After finishing the 20 positionings, the app will show you the mean position of the line, as well as the standard deviation and error. When you tap the screen again you will get a complete list of the original orientation of the line and to which orientation you have set the line (the deviation from zero). A negative value indicates an anti-clockwise tilt, a positive value indicates a clockwise deviation from the true horizontal or vertical.
To get reliable results the subject should not be able to see anything in the surroundings that might help him or her to know what is truly horizontal or vertical. So, the test is best done in complete darkness or when the subject is standing in front of a wall that lacks any items or structure that might help orientation.
By connecting your iphone to the PC using I-Tunes, you can modify the settings (SVV or SVH, number of test, angles of each line); however the current setting which can be used for SVH and SVV is validated and optimal for accurate use. The mean value in healthy subjects does not exceed +/- 2.3 degrees when the test is performed with binocular vision and +/- 2.5 degrees when the test is performed with monocular vision in healthy subjects.