accurate stimulus presentation is vital to research in vision , and the biggest limitations are imposed by the device used to display stimuli ( krantz , 2000 ; bukhari and kurylo , 2008 ) . in early times ,
accurate timing of visual stimuli was achieved by specialized apparatus such as tachistoscopes ( mollon and polden , 1978 ) .
later , these expensive devices were replaced by cheaper and broadly available cathode - ray tube ( crt ) monitors , which could be controlled conveniently and flexibly via personal computers ( pcs ; brainard , 1989 ) . these systems allowed researchers to adjust a much larger number of stimulus parameters and are partly responsible for the expansion of the research in vision .
nevertheless , crt monitors are not perfect as they continue to pose several limitations for researchers .
first , the light is not presented continuously , but discretely in flashes . in crt monitors
each pixel of the screen changes from bright to dark with certain vertical refresh rate frequency ( 60  hz or higher ) , producing constant flicker .
this fast flicker is largely fused by our visual system ( kristofferson , 1967 ; andrews et al . , 1996 ; carmel et al . , 2006
) , but not necessarily completely , as it can affect electrophysiological recordings ( wollman and palmer , 1995 ; krolak - salmon et al . , 2003 ; williams et al . , 2004
this technology , based on non - continuous light sources , makes an exact calculation of the stimulus presentation time difficult ( bridgeman , 1998 ) .
for example , if an image is presented nominally for four screen frames with a 100-hz refresh rate , the total time span over which the screen is emitting light does not correspond to 4    10  =  40  ms , but to a shorter period of time , the exact duration of which depends on the duration of each flash , which in turn depends on the phosphor decay characteristics of a particular screen .
another disadvantage of the crt monitors is the lack of independence between neighboring pixels ( pelli , 1997 ) .
this can be illustrated by presenting the finest possible grating ( i.e. , one - pixel raster of black and white stripes ) with either horizontal or vertical orientation , in which cases the luminance can differ dramatically ( pelli , 1997 ; krantz , 2000 ) .
moreover , crt monitors produce electromagnetic radiation and hence often induce noise in electrophysiological recordings .
this noise is correlated to the stimulus and hence , can not be averaged out .
finally , crt monitors are becoming increasingly difficult to purchase due to the shrinkage of the market for such products .
there are several possible replacements of crt monitors in experiments , such as those based on digital light projection or mechanical shutters ( packer et al .
however , these solution need complex setups and the costs are considerable , which limits their application . more general solutions for visual stimulation may be based on liquid crystal display ( lcd ) technology .
lcd monitors present images continuously , i.e. , without flicker , generate the luminance of the neighboring pixels independently , produce negligible electromagnetic noise , and are broadly available .
unfortunately , so far , these monitors could not be used widely for research in vision because they lacked speed and accuracy mandatory for scientific investigation of the visual system .
these monitors were sluggish in responding to the changes dictated by the computer software and were unable to set the intensity of illumination with sufficient reliability ( wiens et al . , 2004 ) .
however , as the technology for lcd monitors has been improving steadily , one can expect lcd monitors to catch up eventually with crts also in respect to the timing and luminance accuracy .
we report here that at least one recently introduced lcd model has sufficient speed , and temporal and luminance accuracy adequate for most research purposes in vision .
the timing and intensity of luminance changes on the screen were controlled by general - purpose pc using specialized software activestim ( www.activestim.com ) , which enables creating simple scripts for visual stimulation in conventional programming languages such as labview , matlab , c++ , etc .
the pc was equipped with a modern , but inexpensive , display card ( nvidia quadro fx580 ) , which was used to drive , one at a time , all three different monitors that have been tested : ( i ) a new model of 22 120  hz lcd monitor ( samsung 2233rz ) , backlit by a cold cathode fluorescent lamp and developed for 3d gaming purposes , which was our candidate for a new lab monitor for visual stimulation , referred to as lcd-1 .
the performance of that monitor was compared to ( ii ) our current visual stimulation monitor , a 21 crt ( viewsonic p227f , crt ) , and ( iii ) an older lcd model ( dell 3007wfpt , lcd-2 ) , which we knew did not satisfy the needs for visual stimulation .
the settings of brightness , contrast and gamma mode for all monitors were kept constant during the test . in lcd-1 , these values were set to the 100 , 75 , and 1 , respectively . in crt brightness and contrast
in lcd-2 the default setup was used as there was no numeric indicator of the intensity of the luminance set manually .
the lcd monitors were always used with their native resolutions , 1680    1050 for lcd-1 and 2560    1600 for the lcd-2 .
the resolution of the crt was 1024    768 , unless specified otherwise .
a lux probe from a luxmeter ( voltcraft , lx-1108 ) was used to measure the luminance delivered by the monitors .
the sensor of the probe is shaped as a semi - hemisphere surrounded by a black plastic light shield ( see figure a1 in appendix ) .
the sensor was fixed to the screen by a sticky tape during measurements and always covered by a lid when no measurements took place . to measure average luminance produced by the images over multiple screen frames ,
the value from the sensor was read by an electronic readout device supplied with the probe , returning values directly in lux . to investigate frame - to - frame dynamic changes of the screen luminance
, we used our system for the acquisition of electrophysiological data to first amplify ( 30 ) the voltage generated by the sensor and then record the signal with high sampling frequency ( 10  khz ) .
the timing of stimuli was controlled by the phase change of a digital signal ( trigger ) sent by activestim via a digital io board ( national instrument , ni-6503 ) .
the pulses were sent at the beginning of each measurement , and were recorded by the acquisition system .
we also tested how the luminance was affected by changes in the viewing angles for a viewer positioned in the center of the screen and viewing either the center of the periphery of the screen .
all the generated images had square form and their size ( 7  cm    7  cm ) exceeded well the screen area sampled by the sensor . in the first test , the measurements of the luminance during sustained illumination of the screen , the used image was either uniform white or consisted of full contrast ( i.e. , white / black ) high - frequency grating with the widths of the stripes spanning a single pixel .
the lux values for white squares and gratings were measured at nine locations on the screen : the center , the four corners , and the four midpoints between the corners . on each location , the luminance produced by grating and uniform white images was measured twice , in the following order : vertical , horizontal , uniform , uniform , horizontal , and vertical .
these measurements were made with 60  hz refresh rate on all three monitors and with 120  hz on lcd-1 and crt ( lcd-2 does not support refresh rates higher than 60  hz ) .
in addition to the resolution 1024    768 , at 120  hz the crt monitor was tested also with another higher resolution ( 1280    1024 ) , closer to those of the two lcds .
to investigate the effect of a change in a viewing angle , the luminance of the screen was set constant and a chroma - meter ( konica minolta , model cs-100a ) was positioned at the distance of 57  cm facing the center of the monitor , which is a relatively small viewing distance for a typical experiment in vision and hence , it estimated the upper boundary of the magnitude of the effects of different viewing angles .
the luminance values were measured by rotating the chroma - meter along one horizontal and the vertical axis to point to five locations on the screen : the center of the screen and the four centers of the four edges of the screen .
the performance was compared between crt and lcd-1 at 120  hz . in lcd-1 and crt
the maximum luminance contrast was defined as the ratio between the difference and the sum of the maximum and the minimum luminance [ rgb values ( 255 255 255 ) and ( 0 0 0 ) , respectively ; i.e. , ( max    min)/(max  +  min ) ] . to investigate the resolution of the generated luminance , we attempted making luminance changes in the steps much finer than those provided by individual rgb values in the default mode . to this end
, we re - defined the color - lookup table to split three luminance levels ( rgb values , 100 , 101 , and 102 ) into 21 luminance levels .
the luminance levels generated by the two monitors were then measured for the white and separately , for each of the three color channels .
the color - lookup table was set back to default for all other tests . in the second test we measured accurately the time of rise and fall of illumination when an image was presented for a limited amount of time ( number of screen frames ) .
a white square ( rgb 255 , 255 , 255 ) appeared in the center of the display , which returned back to black ( rgb 0 , 0 , 0 ) after a duration of either 1 , 2 , 3 , 6 , 10 , 15 , 20 , or 30 frames of screen refresh .
each of these eight durations was presented in a block consisting of 100 repetitions , starting with a block of 1-frame and ending with a block of 30-frame long images .
lcd-1 and crt were tested at 60 , 100 , and 120  hz refresh rates , and lcd-2 , only at 60  hz . in the third test we investigated luminance delivered by the monitors when the luminance of the presented image changed randomly at each frame of screen
refresh . to this end , a square at the center of the screen took all the 256 different shades of gray ( from rgb 0 , 0 , 0 to rgb 255 , 255 , 255 ) in a random order , each shade lasting for only one frame .
lcd-1 and crt were tested at 60 and 100  hz ( lcd-2 was not tested ) .
finally , the second and the third test were conducted also for lcd-1 mimicking the flickering flashes of a crt . here
, lcd-1 was ran at 120  hz but every second screen frame was set always to full black , resembling the luminance flicker of a 60-hz crt monitor . to account for possible dc drifts of the recording system ,
the baseline luminance ( i.e. , full black ) in the second and third tests was subtracted from all signals prior to further analyses .
the baseline luminance was taken from the recordings 6010  ms prior to the occurrence of digital trigger .
since different monitors differed in absolute luminance generated , it was necessary to normalize the measured values to the maximum ( i.e. , full white ) for each monitor .
the maximum luminance for the normalization reference was determined as the peak luminance within a presentation period that lasted 30 frames ( at 60  hz ) and that was averaged across 100 repetitions .
the onset time of illumination was defined as the time at which the luminance rose to half of its maximum .
we defined the offset time in a similar way : the point at which the luminance reduced to half of its maximum in the last frame . to quantify the properties of the luminance rise and fall , we fit the following exponential function : to the regions of the measured luminance curves that preceded the onset times and that followed the offset times , respectively .
the smaller the time constant c , the steeper are the rising / falling slopes , and thus the monitor is closer to the ideal performance .
these measures are also related to the magnitude of the motion blur produced by the screen ( watson , 2010 ) .
the duration of illumination was defined then simply as the interval between the onset time and offset time . in the third test with random sequences of the shades of gray ,
the time period over which each frame was illuminated ( i.e. , the period between the onset and offset time ) was determined with methods similar to those in other tests , except that the luminance did not change from full black to a given level of gray but rather , in most cases , from other shades of gray .
hence , the duration of illumination could not be determined accurately for each level of gray , the dark ones being least accurate . as a result
, the illumination periods were first determined solely on the basis of the full white frames for which we used the darkest available shades of the preceding and succeeding frames . within this time window
we then calculated the mean luminance for all frames , including those with other levels of gray .
finally , for each level of gray ( i.e. , each target luminance ) , these individual averages were normalized relative to the gross average obtained across all 100 repetitions .
the timing and intensity of luminance changes on the screen were controlled by general - purpose pc using specialized software activestim ( www.activestim.com ) , which enables creating simple scripts for visual stimulation in conventional programming languages such as labview , matlab , c++ , etc .
the pc was equipped with a modern , but inexpensive , display card ( nvidia quadro fx580 ) , which was used to drive , one at a time , all three different monitors that have been tested : ( i ) a new model of 22 120  hz lcd monitor ( samsung 2233rz ) , backlit by a cold cathode fluorescent lamp and developed for 3d gaming purposes , which was our candidate for a new lab monitor for visual stimulation , referred to as lcd-1 .
the performance of that monitor was compared to ( ii ) our current visual stimulation monitor , a 21 crt ( viewsonic p227f , crt ) , and ( iii ) an older lcd model ( dell 3007wfpt , lcd-2 ) , which we knew did not satisfy the needs for visual stimulation .
the settings of brightness , contrast and gamma mode for all monitors were kept constant during the test . in lcd-1 , these values were set to the 100 , 75 , and 1 , respectively . in crt brightness and contrast
in lcd-2 the default setup was used as there was no numeric indicator of the intensity of the luminance set manually .
the lcd monitors were always used with their native resolutions , 1680    1050 for lcd-1 and 2560    1600 for the lcd-2 .
the resolution of the crt was 1024    768 , unless specified otherwise .
a lux probe from a luxmeter ( voltcraft , lx-1108 ) was used to measure the luminance delivered by the monitors .
the sensor of the probe is shaped as a semi - hemisphere surrounded by a black plastic light shield ( see figure a1 in appendix ) .
the sensor was fixed to the screen by a sticky tape during measurements and always covered by a lid when no measurements took place . to measure average luminance produced by the images over multiple screen frames ,
the value from the sensor was read by an electronic readout device supplied with the probe , returning values directly in lux . to investigate frame - to - frame dynamic changes of the screen luminance
, we used our system for the acquisition of electrophysiological data to first amplify ( 30 ) the voltage generated by the sensor and then record the signal with high sampling frequency ( 10  khz ) .
the timing of stimuli was controlled by the phase change of a digital signal ( trigger ) sent by activestim via a digital io board ( national instrument , ni-6503 ) .
the pulses were sent at the beginning of each measurement , and were recorded by the acquisition system .
we also tested how the luminance was affected by changes in the viewing angles for a viewer positioned in the center of the screen and viewing either the center of the periphery of the screen .
all the generated images had square form and their size ( 7  cm    7  cm ) exceeded well the screen area sampled by the sensor . in the first test , the measurements of the luminance during sustained illumination of the screen , the used image was either uniform white or consisted of full contrast ( i.e. , white / black ) high - frequency grating with the widths of the stripes spanning a single pixel .
the lux values for white squares and gratings were measured at nine locations on the screen : the center , the four corners , and the four midpoints between the corners . on each location ,
the luminance produced by grating and uniform white images was measured twice , in the following order : vertical , horizontal , uniform , uniform , horizontal , and vertical .
these measurements were made with 60  hz refresh rate on all three monitors and with 120  hz on lcd-1 and crt ( lcd-2 does not support refresh rates higher than 60  hz ) .
in addition to the resolution 1024    768 , at 120  hz the crt monitor was tested also with another higher resolution ( 1280    1024 ) , closer to those of the two lcds . to investigate the effect of a change in a viewing angle ,
the luminance of the screen was set constant and a chroma - meter ( konica minolta , model cs-100a ) was positioned at the distance of 57  cm facing the center of the monitor , which is a relatively small viewing distance for a typical experiment in vision and hence , it estimated the upper boundary of the magnitude of the effects of different viewing angles .
the luminance values were measured by rotating the chroma - meter along one horizontal and the vertical axis to point to five locations on the screen : the center of the screen and the four centers of the four edges of the screen .
the performance was compared between crt and lcd-1 at 120  hz . in lcd-1 and crt
the maximum luminance contrast was defined as the ratio between the difference and the sum of the maximum and the minimum luminance [ rgb values ( 255 255 255 ) and ( 0 0 0 ) , respectively ; i.e. , ( max    min)/(max  +  min ) ] . to investigate the resolution of the generated luminance ,
we attempted making luminance changes in the steps much finer than those provided by individual rgb values in the default mode . to this end , we re - defined the color - lookup table to split three luminance levels ( rgb values , 100 , 101 , and 102 ) into 21 luminance levels .
the luminance levels generated by the two monitors were then measured for the white and separately , for each of the three color channels .
the color - lookup table was set back to default for all other tests . in the second test we measured accurately the time of rise and fall of illumination when an image was presented for a limited amount of time ( number of screen frames ) .
a white square ( rgb 255 , 255 , 255 ) appeared in the center of the display , which returned back to black ( rgb 0 , 0 , 0 ) after a duration of either 1 , 2 , 3 , 6 , 10 , 15 , 20 , or 30 frames of screen refresh .
each of these eight durations was presented in a block consisting of 100 repetitions , starting with a block of 1-frame and ending with a block of 30-frame long images .
lcd-1 and crt were tested at 60 , 100 , and 120  hz refresh rates , and lcd-2 , only at 60  hz . in the third test we investigated luminance delivered by the monitors when the luminance of the presented image changed randomly at each frame of screen
refresh . to this end , a square at the center of the screen took all the 256 different shades of gray ( from rgb 0 , 0 , 0 to rgb 255 , 255 , 255 ) in a random order , each shade lasting for only one frame .
lcd-1 and crt were tested at 60 and 100  hz ( lcd-2 was not tested ) .
finally , the second and the third test were conducted also for lcd-1 mimicking the flickering flashes of a crt . here
, lcd-1 was ran at 120  hz but every second screen frame was set always to full black , resembling the luminance flicker of a 60-hz crt monitor .
to account for possible dc drifts of the recording system , the baseline luminance ( i.e. , full black ) in the second and third tests was subtracted from all signals prior to further analyses .
the baseline luminance was taken from the recordings 6010  ms prior to the occurrence of digital trigger .
since different monitors differed in absolute luminance generated , it was necessary to normalize the measured values to the maximum ( i.e. , full white ) for each monitor .
the maximum luminance for the normalization reference was determined as the peak luminance within a presentation period that lasted 30 frames ( at 60  hz ) and that was averaged across 100 repetitions .
the onset time of illumination was defined as the time at which the luminance rose to half of its maximum .
we defined the offset time in a similar way : the point at which the luminance reduced to half of its maximum in the last frame .
to quantify the properties of the luminance rise and fall , we fit the following exponential function : to the regions of the measured luminance curves that preceded the onset times and that followed the offset times , respectively .
the smaller the time constant c , the steeper are the rising / falling slopes , and thus the monitor is closer to the ideal performance .
these measures are also related to the magnitude of the motion blur produced by the screen ( watson , 2010 ) .
the duration of illumination was defined then simply as the interval between the onset time and offset time . in the third test with random sequences of the shades of gray , the time period over which each frame was illuminated ( i.e. , the period between the onset and offset time ) was determined with methods similar to those in other tests , except that the luminance did not change from full black to a given level of gray but rather , in most cases , from other shades of gray .
hence , the duration of illumination could not be determined accurately for each level of gray , the dark ones being least accurate . as a result
, the illumination periods were first determined solely on the basis of the full white frames for which we used the darkest available shades of the preceding and succeeding frames . within this time window
we then calculated the mean luminance for all frames , including those with other levels of gray .
finally , for each level of gray ( i.e. , each target luminance ) , these individual averages were normalized relative to the gross average obtained across all 100 repetitions .
the brightness of a full white statically presented image varied across locations in all three tested monitors . typically , luminance was highest at the center of the screen .
lcd-1 , our candidate for a new visual stimulation monitor , exhibited least variability of all tested monitors , while lcd-2 , an older model used for comparison , showed the largest variability ( figure 1a ) .
in addition , as would be expected for back - lit lcds , these monitors had the advantage of high constancy across different refresh rates , which was problematic for the crt monitor .
therefore , although lcd-1 does not illuminate the screen with perfect homogeneity , it is much closer to this goal than other tested monitors .
this disadvantage of the crt monitor was compensated by its better luminance stability as a function of changes in viewing angles . when luminance was measured from distance and under an angle by a chroma - meter ,
the drop of luminance in the corners of the screen was much larger for lcd than for crt .
consequently , the total drop of generated peripheral luminance about equaled for the two monitors , in one monitor being caused primarily by a reduction in the total amount of light generated ( crt ) and in the other by the viewing angle ( lcd ) .
diamonds : for each monitor and each setting ( x - axis ) , luminance measures on the screen shown for nine different locations ( eight along the borders and one in the center ) .
open circles : for crt and lcd-1 at 120  hz , the luminance measured from a viewing distance of 57  cm and from different angles , shown for five locations ( the four centers of the borders and one screen center ) . for easy comparison , the luminance at the center locations was always taken as a norm ( i.e. , it has been assigned the value of 1.0 ) , others being shown as proportions of the value at the center .
red : the new lcd model tested for the possible suitability for research in vision .
( b ) luminance produced by fine ( one - pixel wide stripes ) gratings of different orientations .
the luminance produced by gratings is shown relative to the expectation defined as 1/2 of the luminance produced by a white square ( indicated by the horizontal line ) .
( c ) test of fine changes in luminance for the levels of gray . to examine the luminance resolution of the monitors ,
the color - lookup table was modified such that the luminance steps between adjacent rgb values were reduced to 1/10 of those in the default color - lookup table , as shown by the solid line .
hence , the rgb range in the x - axis covered a very small luminance span with fine changes that corresponded to the span of only three rgb values ( 100102 ) of the original lookup table .
y - axis : the luminance normalized to the range measured in this test , which corresponds to about 2.4% of the total luminance range of the monitor .
horizontally oriented black / white gratings produced in all cases luminance that corresponded accurately to 1/2 of the luminance of a fully white square , as expected ( figure 1b ) .
however , when rendered on the crt , consistent with previous reports ( pelli , 1997 ; krantz , 2000 ) , the luminance was reduced relative to the horizontal grating .
this reduction became more severe with higher refresh rates and higher screen resolutions . in our most strenuous test ( 120  hz refresh rate , 1280    1024 resolution )
, the luminance of the vertical grating got reduced on the crt to 40% of that produced by the horizontal grating ( figure 1b ) .
thus , only lcd monitors , but not the crts , generate the luminance of the neighboring pixels independently .
theoretically , the highest possible value of luminance contrast is 1 , which would require the monitor to produce full black ( i.e. , zero luminance ) with the rgb values ( 0 0 0 ) . the minimal and maximal luminances for the crt monitor were 0.7  lx and 145.6  lx , respectively and for lcd-1 , 1.3  lx and 236.6  lx , respectively . thus the contrasts were 99.0 and 98.9% for crt and lcd-1 , respectively .
the test of luminance resolution indicated that lcd-1 generated a different value of luminance for each rgb value , but not finer . by modifying the color - lookup table
, we requested 21 levels of gray from the monitors within the very narrow range of luminance change corresponding to only three rgb levels of the original setup . in that setting ,
the monitors were still able to produce only about three levels ( figure 1c ) , as would be expected from 8-bit color depth screen .
the three groups of measured luminance values ( filled circles in figure 1c ) suggest that this limitation was imposed by digital technology .
the results were similar for the measurements of individual color channels ( figure a2 in appendix ) .
in contrast , crt monitor did not produce clear groups of luminance values as the values of luminance generated by this monitor deviated from the ideal in an irregular way ( open triangles in figure 1c ) .
this would be expected from analog technology . in conclusion , both monitors seem to generate luminance levels accurately enough to achieve 8-bit color depth , each achieving this goal with a different technical solution .
our second and more important set of tests was about precise temporally resolved measurements of the changes in luminance ( see methods ) .
repeated generations of an image enabled us to assess the reliability of the generated luminance .
lcd-1 and crt performed both excellently , as they produced identical luminance traces across repetitions . to illustrate this point , in figure 2a , the non - averaged luminance traces of each of the 100 repetitions
are shown superimposed for all tested monitors . as can be seen , no noticeable variability exists for lcd-1 and crt .
the traces had an oscillatory component of 200  hz which , at the maximum luminance , accounted for 11% of the total luminance .
this oscillatory component was not locked to the refresh rate of the monitor , resulting in a thick region of overlap in figure 2a .
( a ) overlay of 100 traces of luminance measurements for a presentation of a full white stimulus , shown for three actual monitors and one mimicked monitor ( mimicked crt ; every other frame of lcd-1 is set to full black ) .
the trace of lcd-1 is delayed relative to the crt mimicked on lcd-1 because in the former case the monitor ran at 60  hz while in the latter case its real underlying refresh rate was 120  hz . the time 0  ms is the moment at which a digital pulse was sent .
( b  e ) luminance traces averaged across 100 repetitions and produced by the different monitors at 60  hz when stimuli of different durations are presented ( 130 frames ; color coded ) .
consistent with the precisely overlapping traces in figure 2a for lcd-1 and crt , the quantitative measures of the precision ( i.e. , error defined by 95% confidence interval ) were also excellent for the onset time of the stimulus , for the time it needs to rise from the minimum to its maximum , and for the overall duration over which the stimulus was presented .
some of the advantages of the crt monitor came from its flickering nature , the return to full black at each screen frame contributing to , e.g. , the high consistency of the total generated luminance at each frame .
hence , when relevant , we also show the performance of lcd-1 when it mimics the flicker of the crt monitor by using 120  hz refresh rate to mimic a 60-hz flicker - based generation of an image .
these results are indicated as  mimicked crt  and are discussed at the end of the section  results .
 we also measured the stability of the peak intensity , i.e. , the maximum luminance within a single presentation , across 100 presentations of a stimulus .
lcd-2 , although not performing as well as the other two monitors , had nevertheless relatively low error of up to 0.11% of the maximum .
the monitors were also similarly consistent when we used the luminance traces to calculate the average intensity of luminance generated along each stimulus presentation .
the upper bound of the error across repeated presentations was around 0.07 , 0.02 , and 0.11% for crt , lcd-1 , and lcd-2 , respectively .
therefore , all three monitors can be described as generally reliable in producing luminance across repeated presentations , with the exception that lcd-2 had an oscillatory component not locked to the refresh rate , as described above .
unfortunately , the technology is not at that level and every monitor requires time to change from one luminance level to another , here referred to as rising and falling times . according to our measurement of the time constant of rising times , crt monitors outperformed both lcds ( 0.20.3  ms for crt vs. 1.1  ms for lcd-1 and 1.5  ms for lcd-2 , respectively ) .
crt was not generally better in changing the luminance , as the opposite was the case for the time constant of falling time , which was longer for the phosphor of crts ( 1.71.9  ms ) as compared to the modern liquid crystals ( 1.2  ms ; see figure 3a for details ) .
therefore , crts and lcds can be considered about even in this test , one being faster in building luminance , and the other in returning back to full black .
( a ) the rising and falling times obtained for different monitors by fitting exponential functions to luminance traces .
( b ) the duration over which stimuli are presented relative to the nominal duration specified by the number of screen frames , investigated at 60  hz refresh rate .
( c ) the amount of light delivered by the stimuli relative to the nominal amount computed from the number of screen frames , investigated at 120  hz refresh rate .
y - axis : the total amount of luminance generated per frame normalized relative to that obtained by the longest presentation ( 30 frames ) . rising and falling times determine also the total time window over which a stimulus is presented .
we compared how well the true duration of illumination corresponded to the nominal one computed by multiplying the number of presented frames with the nominal duration of the frame ( 1/refresh rate ) .
the true duration corresponded always better to the nominal ones for lcds than for the crt , the latter being especially inaccurate with short nominal presentation times ( one or two screen frames ; see figure 3b ) .
the main reason for the inaccuracy of the crt is that the effective time over which light has been generated is relatively short ( e.g. , only 3  ms ) .
an important related question is whether a monitor can reach 100% of its maximum intensity within a single frame of presentation .
the error was especially large for higher refresh rates and for the crt monitor , which , at 120  hz , delivered 4.28% smaller peak luminance at the first frame than at a later frame in which the maximum has been reached ( the traces are shown in figure a3 in appendix ) .
this should be compared to only 0.43% reduction in peak luminance at the first frame for lcd-1 running at 60  hz ( figure 2c ) .
fortunately , these problems seem relevant only for the first one or two frames . with longer stimulus durations , these errors vanished quickly for both lcd-1 and crt ( already with > 3 frames to < 0.20% ) .
as our recording progressed over the period of 8  min ( from 1-frame to 30-frame stimuli ) , the luminance produced by the light source gradually reduced in total by about 4% ( see zoom - in in figure 2b ) . therefore , the source of light of that monitor is not necessarily stable but one may have to ensure that the monitor is on for a certain period of time ( e.g. , several tens of minutes ) before it can be used for delivering visual stimuli . lcd-1 and crt did not suffer from this problem ( zoom - ins in figures 2c  e ) .
although lcd-1 was better than crt in reaching maximum intensity at the first frame , it was worse in the total amount of luminance generated at the first frame when compared to later frames ( figure 3c ) .
if the same image is shown for different numbers of frames , ideally , the amount of light delivered would be proportional to the number of frames , e.g. , the light energy of a one - frame presentation would be one - tenth of that of a ten - frame presentation .
a comparison between blocks with shortest duration ( only one frame ) and longest duration ( 30 frames ) , lead to a discrepancy of up to 20% in lcd-1 , compared to only 2% in crt .
our final set of questions was about possible temporal dependences between frames in the delivered luminance by displaying images of varying shades of gray sequentially one frame after another . for both lcd-1 and crt monitors ,
the luminance generated at the current screen frame depended to a degree on the luminance of the preceding frame .
high preceding luminance always increased the luminance of the current frame ( figures 4a , b ; also figure a4 in appendix ) and vice versa , low luminance in the preceding frame reduced the luminance of the current frame . in lcd , this was a carry - over effect due to the non - zero time needed to switch from one luminance level to the other , as can be seen in figure 4a .
in the crt , this carry - over effect ( figure 4b ) occurred perhaps due to insufficiently quick change in voltage in the emission guns .
note that in lcd-1 , toward the end of the frame also a small reversal of luminance occurred : high preceding luminance tended to reduce slightly the luminance generated at the very end of the current frame .
the coefficients of variation ( target luminance vs. actual luminance ) are shown for all monitors and all target luminance values in figure 4d .
as one can see , lcd-1 performed better than crt for lower target luminance ( rgb values < 80 , 80 , 80 ) , and this reversed for higher target luminance ( e.g. , rgb value > 120 , 120 , 120 ) , crt generating luminance more accurately than lcd-1 .
notice also that in most cases , with higher refresh rate the actual luminance varied also more .
( a  c ) traces of measured luminance for the specified rgb value ( 150 , 150 , 150 ) , obtained following either high luminance of the preceding frames ( green ) or low ( red ) .
gray box : the actual duration of illumination during the frame as defined in our analyses ( see methods ) .
( d ) the summary of the results for all different values of target luminance and for all monitors and all tested refresh rates , shown as coefficients of variation .
the target luminance is shown normalized relative to the maximum ( rgb 255 , 255 , 255 ) .
vertical dashed line : the target luminance used for plots in [ ( a  c ) rgb 150 ,  150 , 150 ] .
this monitor had very accurate onset , rising , and falling times , inherited from lcd-1 ( see figure 2e ) .
the same held for the actual duration of the stimulation , which was generated more accurately than by the crt monitor ( see figure 3b ) . also , this mimicked monitor had good average intensity of generated luminance due to the repeatable return to full black , a property of the crt ( figure 2e ) . in figures 4c ,
d one can see that this resulted in very low temporal dependence on luminance generated in the preceding frame , the coefficient of variation being especially favorable with low target luminance .
in general , a researcher in vision needs a monitor that creates stimuli as close as possible to their formal specification .
the duration and intensity of the stimuli should linearly scale with the values specified in the software and the values should remain constant across contextual and other variables , such as the immediately preceding stimuli , stimulus position on the screen , size , or orientation of the stimulus , etc .
our results indicate that a new model of lcd can satisfy most of these needs and that in many cases should be preferred over the crt monitor .
although the luminance of lcd decreases with an increase in viewing angle , lcd generates luminance more consistently over different screen locations , which makes lcd , as a net result , about even with crt .
. therefore , good reasons exist to prefer lcd over crt , in experiments in which these spatial variables are of concern . the temporal variables , i.e. , the speed and the temporal accuracy of the changes in luminance have improved significantly with the new model , which even leads in several categories . by not producing flicker in the frequency of the refresh rate of the monitor , the lcd monitor has the advantage of continuous illumination and has much better agreement between the actual duration of the stimulus and the nominal duration specified by the software .
the tested lcd monitor responds fast and is negligibly slower than crt in changing the luminance from zero to the maximum .
lcd is even faster than the crt in returning from the maximum back to zero .
the luminance dependency on the preceding frame in lcd may have undesirable carry - over effects in experiments with fast exchange of screen luminance .
an example is binocular rivalry induced using shutter glasses . in such a case , it may be advisable to use either a crt monitor or an lcd in a mimicked crt mode .
if the stimulus design requires only one- or two - frame long presentations ; the total amount of light generated does not scale linearly with the number of frames in lcd . in a single frame ,
this problem can be remedied on the lcd if the exact amount of delivered luminance can be measured , e.g. , as made in the present study . in the case
that the experimental design allows for low refresh rates ( 60  hz ) , another possibility exists : the flickering property of the crt can be mimicked on a fast lcd ( 120  hz ) by setting every second frame to full black . as our results show , this crt - mimicking approach offers even higher temporal accuracy than does crt , as the setup combines the respective advantages of crt and lcd monitors .
our perceptual experience with the use of the monitor was in agreement with the measurements .
for example , the perceived motion blur matched the measurements of rising and falling times .
crt seemed to produce less motion blur than lcd-1 when a dark object moved on a bright background , which was in agreement with the short rising time of the crt .
the opposite was the case when a bright object moved on a dark background . in that condition , the overall largest extent of motion blur
was perceived on the crt , which was in agreement with the unusually long falling time of that monitor . during our tests of lcd-1 , we detected no other artifacts despite having run the most demanding stimuli , such as the full contrast full - speed flickering and fast moving images .
one limitation for all monitors is that they draw each frame sequentially , starting from the upper left corner and proceeding to the right .
this has as consequence that different parts of the screen are illuminated at different times , delays being especially large along the vertical axis .
for example , in any task that measures response times ( physiological or behavioral ) to stimuli presented at different vertical locations on the screen , several milliseconds of differences ( sometimes up to 10  ms ) will be necessarily created by the monitor . in conclusion ,
as the technology of lcd monitors keeps improving , some of the other novel models may have even better performance . due to the variability of technical solutions no suitability for research should be assumed but each model should be tested thoroughly .
also , no suitability for all research questions in vision should be assumed for the presently evaluated model .
hence , we can not conclude that the tested monitor is suitable for all research problems in vision . nevertheless , the model that has excellent timing and generates stimulus intensities accurately , which makes it usable for many applications in vision research .
the authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest .
 until now , liquid crystal display ( lcd ) monitors have not been used widely for research in vision . despite their main advantages of continuous illumination and low electromagnetic emission , these monitors had problems with timing and reliability . here 
 we report that there is at least one new inexpensive 120  hz model , whose timing and stability is on a par with a benchmark cathode - ray tube monitor , or even better . 
 the onset time was stable across repetitions , 95% confidence interval ( the error ) of which was < 0.01  ms . 
 brightness was also delivered reliably across repeated presentations ( < 0.04% error ) and across blocks with different durations ( < 3% error ) . 
 the lcd monitor seems suitable for many applications in vision research , including the studies that require combined accuracy of timing and intensity of visual stimulation . 
Introduction
Methods
Experimental setup
Procedure
Analysis
Results
Discussion
Conflict of Interest Statement