[visionlist] LCD / OLED monitors for vision science

[Lindsey Fraser]

*Crossposting my response from cvnet in case folks are interested:

Hello Rufin,

 

As a vision scientist working with display technology, I thought I could offer some insight into the problem you're experiencing. It is most likely caused by a hardware property of the specific LCD panels you tested. Since it may be relevant to others using high-frequency stimuli, I thought I would post my response here.

 

The liquid crystals in an LCD display are twisted and untwisted by a variable DC voltage. The degree of twist determines how much light passes through the screen to the viewer. The twists can be clockwise or counterclockwise.

 

Repeated twisting of liquid crystal in the same direction can, over time, cause the crystal's shape to become rigid or “stuck.” A consequence of this is a persistent afterimage like the one you describe, something like old-time CRT phosphor burn-in, as the pixel slowly returns to its normal state. Stuck pixels can also appear as dynamic flickering images.

 

Most LCD panels avoid “sticking” pixels by alternating the polarity of the DC current on even-odd frames of the display. This causes the twist to alternate between clockwise and counterclockwise. So if you have a white pixel driven by +5V, it is actually alternating between +5V on one frame and -5V on the next. 

 

This means if you are flickering a 120 Hz screen at 60 Hz (white -> black -> white) then your stimulus is synchronized to the alternating voltage (+5V -> 0V -> +5V) and you are always twisting the liquid crystal in the same direction. This can cause your pixels to stick.

 

Depending on the manufacturer, some LCD panels may balance the voltage every two frames instead of every other frame; this is especially useful for 3D displays where frames alternate between left and right eyes. 

 

I'm not sure what refresh rate you are running at, but I would bet that your 30 Hz stimulus has synchronized with your display's voltage balancing, whether it be even-odd frame (60 Hz refresh) or 2-frame alternation (120 Hz refresh) voltage balancing.

 

You should be able to fix this by switching your display settings/refresh rate, or using an LCD with another voltage balancing strategy that doesn't synchronize with your stimulus.

 

Hope this helps,

Lindsey

 

Dr. Lindsey Fraser

Staff Scientist,

VPixx Technologies



From: visionlist < <mailto:visionlist-bounces at visionscience.com> visionlist-bounces at visionscience.com> On Behalf Of Rufin Vogels
Sent: November 9, 2020 10:58 AM
To:  <mailto:visionlist at visionscience.com> visionlist at visionscience.com
Subject: [visionlist] LCD / OLED monitors for vision science

 

Hi, we are still using CRTs for stimulus presentation but want to switch to more modern technology. We have been testing some LCD displays (LG 27GN750 UltraGear (IPS panel) & iiyama G-Master Red Eagle GB2760QSU-B1 (TN) panel)) recently but when tested with an alternating black-white square ( e.g. at 30 Hz for several seconds) all show a dim flickering “afterimage” (physically present and measurable!) at the stimulated location for up to a minute. Does anyone know LCD (or OLED) monitors that have decent performance (for motion stimuli) and do not show such disturbing effects? 

Thanks,

Rufin

  <https://mandrillapp.com/track/open.php?u=30884370&id=6393c8d458544b6eb7877cdd3fe0e1eb> 



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