On dichoptic reaction times

Measurements of reaction times to monocular visual stimuli are routine in many automated perimeters but appear to hold little useful information. Dichoptic stimulation, that is stimulation that unpredictably alternates between the left eye, the right eye, or both eyes, may be more informative, particularly in the setting of functional loss of vision.

Functional loss of vision is a challenging diagnosis because of its lack of objective corollaries. Clinical test results are characteristically exceedingly variable in both space and time and do not comply with current pathophysiological concepts. Examples of real test results can be viewed elsewhere on this site.

Quite unexpectedly, a recent study of simple reaction times in a dichoptic display revealed abnormally long reaction times in subjects diagnosed with functional loss of vision as compared to normal subjects and subjects with recognized causes of visual loss, hinting at the operation of a previously unknown pathophysiological factor [1]. This study used digital display goggles and purpose-written software to present test targets to the left eye alone, to the right eye alone, or to both eyes. Reaction times were recorded separately for each test condition.

The test presented here is an anaglyphic adaptation of the original video goggle test. The anaglyphic technique uses color targets and color filters to unobtrusively restrict stimulation to the left eye, to the right eye, or to both eyes. Red-cyan goggles of the type often used in simple 3D displays should work well. The cross-shaped test targets are presented under computer control in a pseudo-random sequence.

A full test comprises 10 presentations for each test condition. The test task is simply to click the mouse button whenever a target is seen. Once presented, the target remains on screen until a response is obtained. Any premature responses are recorded as errors. The next target will be presented after a randomly selected 2 - 4 s delay. Reaction times are recorded separately for each test condition and presented as means and standard deviations at the end of the test. There is also a prompt for placing the raw data on the clipboard for further analysis. Tap Ctrl-C and then Enter to effect the transfer.

The brightnesses of the test targets should be adjusted to suit the color filters actually used. The opening page presents a color panel (shown in miniature to the right) and two sliders for brightness adjustment, one for red and one for cyan. With the color filters in place before the eyes, close the left eye, and compare the brightnesses of the gray and cyan panels. If there is a difference, use the slider labelled cyan to obtain a match. Repeat the procedure for the left eye, using the red slider. Make a note of the slider settings for future reference. Next, explain the test task to the test subject. The task is simply to watch the target area all the time and to click the mouse whenever a cross-shaped figure appears in the target area. Hand the mouse to the subject and put the color filters in place. Start the test by tapping the keyboard space key. The test can be paused by tapping the same key again. Tap the ESC key to abort a test. Tap the function key F5 to obtain a reset.

The test should be used in a secluded area with subdued lighting, at a reading distance, with a proper ametropia correction. Use the monitor manufacturer's default settings for brightness and contrast. Turn off any computer background processes to avoid interference with response timing.

In the original video goggle study, normal and control subjects presented reaction times averaging 0.4 s whereas subjects with functional loss of vision averaged 1.1 s. Results may be different with the anaglyphic test, which differs from the goggle test in several respects. These include the use of colored instead of black-on-white targets, and the lack of in-depth cues for the target background. Note that poor-quality color filters may allow both eyes to see targets intended for single eyes.

Nowadays, video goggles have been largely replaced with so-called virtual-reality displays, which may form useful alternative platforms. An inexpensive solution is to contain a smartphone within a simple cardboard case. The main problem may be to arrange for response input.

Posted on January 16, 2016.