Comparison of cathode ray tube and liquid crystal display stimulators for use in multifocal VEP

To compare the modified signal-to-noise ratio (SNR*) of multifocal visual evoked potential (mfVEP) responses elicited by a cathode ray tube (CRT) and liquid crystal display (LCD) monitor in normal subjects.

An LCD monitor and CRT monitor were luminance and contrast matched. Luminance stability and the effect of viewing angle on luminance and contrast was measured for both screens. The SNR* of mfVEP responses from 15 normal subjects was compared between the stimulators using repeated measures analysis of variance.

The CRT monitor took 10 min from switch on to reach the desired luminance compared to 60 min for the LCD monitor. LCD luminance was sensitive to variations in ambient temperature, fluctuating by 10 cd/m(-2) over approximately 20-27 °C, whereas CRT luminance was stable. Luminance variation from the centre to the edge of the CRT screen was 8 % when viewed perpendicularly and 28 % when viewed at an angle of 25°, compared to 24 and 46 %, respectively, for the LCD screen. Contrast was >94 % and varied by <3 % across both monitors for both viewing conditions. There was no significant difference in SNR* between responses elicited by the two stimulators (p = 0.76).

CRT and LCD stimulators elicited mfVEP responses with similar SNR* in normal subjects. This study highlighted practical issues with the use of LCD monitors as visual stimulators, particularly with regard to warm-up time, luminance stability and luminance uniformity.