Lighting Res. Technol. 2013; 45: 124-132

Flicker can be perceived during at frequencies in excess of 1 kHz JE Roberts MSc and AJ Wilkins DPhil Department of Psychology, University of Essex, Colchester, UK

Received 30 November2011; Revised 31 December 2011; Accepted 1 January 2012

When driving at night, flickering automobile LED tail lights can appear as multiple images. The perception of a flickering source of light was therefore studied during rapid movements Isaccades) of 20-400 amplitude in an otherwise dark room !<1Iuxl. The temporal modulation appeared as a spatial pattern known as a 'phantom array' during the . The appearance of the pattern enabled the discrimination of flicker from steady light at frequencies that in 11 observers averaged 1.98 kHz. At a frequency of 120 Hz, the intrasaccadic pattern was perceptible when the contrast of the flicker exceeded 10%. It is possible that intrasaccadic stimulation interferes with ocular motor control.

1, Introduction fusion frequency is, however, usually measured with a spatially unstructured field. When driving at night behind a car with LED It is rarely measured during eye movements, tail lights, it is possible to experience a trail of and when it is, estimates of the critical lights with each rapid movement of the freq uency increase.4 When each flash of a (saccade). The trail occurs because the LED spatially structured light source is imaged on lamps are lit intermittently to control heat a different part of the during a rapid build-up. The frequency ofoperation is above , the light source forms a flicker fusion, varies from one manufacturer 'phantom array', and the above example of to another and may not be the same for both tail light flicker shows that the array is tail lamps, to judge from videos posted on the sometimes perceptible under everyday view­ web. I Each flash of the lamp is imaged on a ing conditions. different part of the retina during the sac­ The chat forums contain complaints of cades, forming a pattern dubbed a 'phantom discomfort from the 'phantom array' when array' by Hershberger and Jordan.2 driving at night. Some drivers are aware of Normally, flicker is not perceived at fre­ the phenomenon, others are not. In an initial quencies greater than the so-called critical small-scale survey, respondents (20 students flicker fusion frequency. 'Classic data from and staff at the University of Essex) were Kelly for modulation of large (30 0 radius) asked the following question verbatim: 'When visual fields at different li/fht levels from you are driving at night and the car in front is 0.03cd·m-2 to 5000cd·m-- showed that a new one with long red lights, if you change beyond lOO Hz, even flicker with 100% mod­ the direction you are looking does anything ulation was hardly ever directly visible, either happen?' Despite the deliberately non-specific centrally or peripherally.,3 Critical flicker wording of the question, three of the 20 respondents reported a 'trail of lights'. This paper shows that the perception of flicker as Address for correspondence: A Wilkins, Department or Psychology, University of Essex. Colchester C04 3SQ. UK intrasaccadic patterns is possible at frequen­ E-mail: [email protected] cies in the kilohertz range, more than 10 times

© The Chartered Institution of Building Services Engineers 2012 10.1177/1477153512436367 Flicker pal/erns above 1kHz during saccades 125 the typical estimates of critical flicker fusion during a saccade and have shown that visual frequency. contrast processing is largely unaltered during Normally, we are unaware of the retinal saccades. Some of the reduction observed by image during a saccade. Mechanisms of Volkman et alY may have occurred because saccadic suppression thought to be largely the saccade trajectory was not perfectly central and cortical in origins.6 prevent the aligned with the stripes ofthe grating, thereby processing of the image during the saccade. reducing the contrast of the retinal image The structured images before and after the during the saccade. Indeed, this might explain saccade normally mask the intrasaccadic why Volkmann et al. observed that the image. However, as the example of tail light reduction in sensitivity increased as the spatial flicker shows, there are circumstances in frequency of the gratings increased. Be that as which the pre- and post-saccadic masking it may, the physical reduction in contrast due fails because at night the image before and to poorly aligned eye trajectories is irrelevant after the saccade is spatially unstructured. It when flicker is perceived as an intrasaccadic may then be difficult to distinguish intrasac­ pattern. If there is a reduction in contrast cadic percepts such as the phantom array sensitivity during a saccade, it will reduce the from those percepts that occur before and maximum frequency at which flicker can be after the saccade. perceived. If contrast sensitivity is reduced by During steady gaze (with a factor of four, then, on the basis of the and drifts), spatially repetitive patterns can be classic data from Campbell and Robson,7 the distinguished at spatial frequencies as high as maximum perceptible spatial frequency 40-50 cycle·deg- and can most readily be would be reduced from 48 cycle.deg- I to 38 I detected at spatial frequencies of around 4 cycle·deg- , a relatively modest change. cycle.deg-17 During a saccade, the angular The second consideration that might make velocity of the eye reaches a peak that can be a 35 kHz limit unlikely is the integration time as high as 700 deg.s- I depending on the of the retinal cells. Retinal cells integrate their amplitude of the saccade. 8 It might therefore signals over a period of time during which be possible in principle to distinguish f1icker­ intensity and duration trade off against each induced intrasaccadic patterns at frequencies other. Bloch's law of temporal summation of up to 50 x 700 = 35 kHz. Two consider­ states that 1:,1 x I:,T=constant where 1:,1 and ations make this unlikely. First, contrast I:, T are the light pulse intensity and temporal sensitivity may be reduced to some extent duration, respectively. At low light energy during a saccade. Volkmann et al. 9 measured levels with large high-velocity saccades and the ability to perceive the contrast of a grating high-frequency flicker, there may be insuffi­ during saccades. The gratings were flashed for cient variation in energy during the flight of 10 ms to the steadily fixating eye and also the eye to stimulate the retina cells. However, during 6° horizontal saccades. Contour mask­ Sam Berman in a memorandum to the IEEE ing before and after the saccade was mini­ PARl789 committee dated 11 January 2011 mised by a diffuse unpatterned field of view. combined data from Hart II with that from Relative to contrast sensitivity during steady Fukuda4 and showed .that typical light fixation, contrast sensitivity during the sac­ sources can provide the luminance increment cade was reduced by a factor of more than of sufficient magnitude such that temporal four. More recently, however, Garcia-Perez summation would not occur until I:,T was and Peli 10 have investigated contrast sensitiv­ greater than approximately 'h msec, corre­ ity for spatial patterns that appear only sponding to 2 KHz flicker frequency.

Lighrillg Res. Tee/mol. 2011; 45: 124-132 126 IE Roberls and Al Wilkins

Most of the existing literature on the black screen (width 2 m) on which were two 'phantom array' has been concerned with white discs that served as fixation points. issues surrounding the location of the intra­ These were positioned 600 mm horizontally to saccadic percep,ts jn relation to the timing of the left and right of the line and were visible the saccade, _-I, although Hershberger as a result of the low ambient lighting of the el al. 'l reported that a phantom array was room « I lux). visible by most of their subjects even at 500 Hz, the maximum frequency they used. 2.1.2. Panicipal/ls This observation raises the possibility of (I) Participants, four males and seven females visibility of the intrasaccadic percept at fre­ aged 22-65 years, mean 34 years, with normal quencies greater than 500 Hz and (2) individ­ or corrected to normal visual acuity were ual differences in its perceptibility. The recruited from staff, students and acquain­ purpose of this study was, therefore, to tances of the authors at the University of investigate in a range of observers the upper Essex. Three wore corrective lenses during the frequency limits below which flicker can be testing. One female, aged 26, used a green perceived as a phantom array and above overlay when reading (but not for the which no such perception is possible. We experiment). chose viewing conditions likely to enhance intrasaccadic perception in order to provide worst-case estimates of use to engineers 2.1.3. Pracedl/re responsible for designing signal lights. Participants were seated 1.7 m from the display, which was at eye level. The line was illuminated for 3 s in two immediately succes­ 2. Experiment 1: 400 saccades with 1, 2, 3 sive trials, during which participants were and 5 kHz flicker required to make saccades back and forth between the targets, and then decide in which 2.1. Method of the two presentations, a pattern ofspatially 2.1.1. Apparatus periodic lines could be seen. On one of the An analogue oscilloscope (ISO-TECH trials selected at random, the function gener­ ISR620) was operated on its side so that ator modulated the brightness of the line; on when the time base was set at its maximum of the other trial, the line was not modulated. 21ls per sweep of the screen (0.5 MHz), a Ten such pairs of trials, separated by a 3 s vertical green line was visible (CrE chroma­ interval, were given at each frequency in a ticity x= 0.307, y =0.529). The z-input of the constant random order. oscilloscope controlled the brightness of the line and was connected to the output of a computer-controlled function generator 2.2. Results (Velleman PCSUIOOO 2 MHz). The function The average data are presented in Figure 1. generator was programmed to generate a sine A cumulative normal was fitted to the data wave at a frequency of I, 2, 3 or 5 kHz or a using least squares. The 75% threshold based steady signal with similar time-averaged volt­ on the fitted curve was 1.67 kHz (standard age and luminance. The luminance of the line deviation 0.52 kHz). A cumulative normal varied over time between a minimum of was also fitted to each individual participant's 2 2 0.02cd·m- and a maximum of 3l0cd·m- . data using least squares and the mean of the The oscilloscope display was visible through a individual thresholds was 1.98 kHz (standard circular aperture (diameter 50 mm) in a matt deviation 1.13 kHz).

Lighting Res. Tecllllol. 2011; 45: 124-132 Flicker pal/ems above 1kHz during saccades 127

100 the estimate of 2 cycle.deg-' based on (I) the temporal frequency of the line, (2) the ampli­ ~ 80 tude of the saccade 40°) and (3) the peak 13 velocity of 500 deg·s- for a saccade of this ~ 0 amplitude expected on the basis of the data 0 '6 C 60 quoted in Leigh and Zee. Halving the '"~ amplitude of the saccade to 20° would be a.'" • 1 40 expected to change the velocity to 400deg·s- and increase the spatial frequency of the intrasaccadic pattern from 2 cycle.deg- I to 20 2.5 cycle·deg- at I kHz. At 2 kHz, the change 0 2 3 4 5 6 I would be from 4 cycle·deg-I to 5 cycle.deg- Frequency (kHz) and at 3 kHz from 6 cycle·deg- I to 7.5 I Figure 1 Experiment 1: Percentage of trials in which cycle.deg- . On the basis that the contrast flicker was correctly detected. shown as a function of sensitivity function peaks at about 4 flicker frequency cycle.deg- I and the fact that the threshold was between I kHz and 2 kHz, one might anticipate that the threshold frequency at 3. Experiment 2: corroboration using which the intrasaccadic pattern is perceptible different apparatus would be little affected by reducing the saccade amplitude to 20°. On the other Light from a 12 V tungsten halogen lamp hand, the amount of saccadic suppression controlled by a stabilised DC supply was has been shown to increase monotonically directed via an optic fibre to the aperture of a with the size of a saccade. I? Experiment 3 was Princeton Applied Research (Oak Ridge, TN, therefore designed to determine the threshold USA) light chopper (model 187). The end of for a saccade of 20° amplitude. the optic fibre measured I mm wide by 7 mm high and was observed by the authors from a distance of I m through the rotating shutter of 5. Experiment 3: 20° saccades with 1,2,3 the chopper in an otherwise darkened room and 5 kHz flicker «I lux). When the shutter was operated at I kHz, the intrasaccadic pattern was invari­ 5.1. Method ably perceived; at 2 kHz, it was occasionally The method was the same as for perceived and at 3 kHz, the pattern could not Experiment I except that the fixation points be distinguished. were repositioned 300 mm horizontally to the left and right of the light. Three participants from Experiment I aged 25-38 took part. 4. Interim discussion 5.2. Results Using the apparatus from Experiment I, the Figure 2 shows mean data for the three authors estimated the spatial frequency of the participants. The 75% threshold based on the intrasaccadic pattern by counting the number means of the group was 2.35 kHz (standard of lines in a known angular displacement at deviation 0.42 kHz). The mean of the individ­ lOO Hz intervals between lOO Hz and 500 Hz. ual thresholds was 2.47 kHz (standard devia­ The data were used to estimate the spatial tion 0.57 kHz). The thresholds did not differ frequency at I kHz. The average estimate of consistently or significantly from those in 1.8 cycle.deg- I agreed reasonably well with Experiment I.

Lighting Res. Teclmol. 2011; 45: 124-132 128 JE Roberts and AJ Wilkins

100~- ~------, headache. If so, the conditions known to have these untoward effects (e.g. fluorescent 80 lighting) might be those that sustain the U ~ intrasaccadic percept and those not so asso­ 0 <.> ciated (e.g. incandescent lighting) be those C 60 that fail to sustain the intrasaccadic percept. \,>'" This possibility was investigated in the next 0- '" 40 study. • Both fluorescent and incandescent lighting flicker at twice the frequency of the electricity 20.J--~-~-~-~--~-----1 o 234 5 6 supply, but the modulation depth of the Frequency (kHz) variation in light from an incandescent lamp (defined as (Lmax - Lm;n)/(Lmax + Lm;n)) is Figure 2 Experiment 3: Percentage of trials in which usually less than 10%. The modulation from flicker was correctly detected, shown as a function of flicker frequency fluorescent lighting varies between about 5% and 100% depending on the control circuitry and the phosphors23 and was typically 6. Interim discussion between 30% and 50% until high-frequency electronic ballasts became widespread. The As anticipated on the basis of the spatial following experiment was designed to deter­ frequency of the intrasaccadic pattern, the mine the modulation depth at which the amplitude of the saccade in the range 20--40 0 intrasaccadic pattern from flicker at 120 Hz had little effect on the upper frequency limit becomes perceptible. at which the intrasaccadic pattern could be perceived. 7. Experiment 4: modulation thresholds Flicker at frequencies of 100 Hz and 120 Hz at 120Hz (above flicker fusion) is known to interfere with the control of eye movements. 18.19 7.1. Method Flicker from fluorescent lighting at 100 Hz The method was the same as in Experiment and 120 Hz is known to impair visual perfor­ 20 1 except that the z-modulation of the oscillo­ mance and cause headaches in some indi­ scope had a square-wave luminance profile 20 21 22 viduals . Brundrett showed that and a frequency of 120 Hz. The modulation individuals who complained of discomfort depth defined as (Lmax - Lm;n)/(Lmax + Lm;n) from fluorescent lighting had a higher critical was 5%, 10%, 20% or 40%. A total of 40 flicker fusion frequency than those who did trials were presented, 10 for each modulation not, and corroborated these observations depth, selected in random order. Four partic­ electrophysiologically. One ofthe participants ipants from Experiment 1 aged 25-65, mean in Experiment I used a green overlay to treat 40, took part. visual stress when reading. This participant had a threshold of 4.9 kHz, well above that of 7.2. Results the other observers. The intrasaccadic per­ The average data are presented in Figure 3. ception of flicker as an anomalous pattern in A cumulative normal was fitted to the data the form of a phantom array provides a using least squares. The 75% threshold based putative mechanism for the interference with on the fitted curve was 10% (standard devi­ eye movements, and the possibly consequent ation 7.2%). A cumulative normal was also disruption of visual performance and fitted to each individual participant's data

Lighfil1g Re.\". Tee/mol. 2011; 45: 124~132 Flicker palleflls above I kHz during saccades 129

100 lower modulation with a smaller and there­ • • fore slower saccade because the spatial fre­ 90 quency would then be higher and closer to the 13 peak of the contrast sensitivity function. The ID 580- smallest saccades are the involuntary micro­ <.> C saccades that have velocities of about ~70 50 deg.s- .'4 and the smallest voluntary sac­ ID ' "- cades are probably those that occur during 0 60· reading, which are typically 1_2 in amplitude with a velocity that ranges from 80 deg.s- l to 50.'---__- __-_-__-, 120deg·s- 1 (S. Jainte, personal communica­ o 5 10 15 20 25 30 35 40 tion). With velocities such as these, only Percent modulation about one cycle of a pattern would be

Figure 3 Experiment 4: Percentage of trials in which available during the saccade, and the visibility flicker at 120 Hz was correctly detected, shown as a of a grating is known to decrease when fewer function of the modulation of the flicker bars are present'S When the authors used a 10 saccade, they were unable to see any periodic intrasaccadic pattern at 120 Hz even using least squares and the mean of the when the modulation was 100%. individual thresholds ranged between 3% and 14% with a mean of 9% (standard deviation 4%). 9. General discussion

The neurological effects of saccadic suppres­ 8. Interim discussion sion appear in motion-sensitive neurons26 The flashed presentation during a saccade The average modulation depth at which might be expected to have little effect on any flicker at 120 Hz gave rise to a perceptible mechanisms of suppression that are due to intrasaccadic pattern was 10% and therefore image motion. This may be one reason why greater than the modulation depth of incan­ the intrasaccadic stimulation was clearly descent lighting and lower than the modula­ perceived. tion depth typical of gas discharge lighting The above experiments were undertaken and some LED lighting. The modulation was looking at a light source in a darkened room. square-wave. The abrupt change in brightness The measurements, therefore, apply to con­ may have made the pattern more perceptible ditions in which light sources such as signal than it would have been had the modulation lamps are visible in low ambient lighting. One been sinusoidal. Gas discharge lighting and example of such conditions is that with which LED lighting can have abrupt transitions this paper began: driving at night behind tail reminiscent of those in a square wave. lights of cars. These are conditions in which With a saccade of 40 0 amplitude and the apparent displacement of the location of velocity of about 500deg.s-', the spatial the lights due to intrasaccadic perception can frequency of the intrasaccadic pattern at be a distraction - one that mayor may not a frequency of 120 Hz is only about impact on road safety. 0.24 cycle.deg-' , well below the peak of Recently, it has been shown that the the contrast sensitivity function at ~4 horizontal spatial periodicity of words inter­ cycle·deg-17 It is therefore possible that the feres with reading by prolonffng the time intrasaccadic pattern would be perceived at taken for vergence movements.- It is possible

Lighting Res. Tee/mol. 2011; 45: 124-132 130 JE Roberts and AJ Wilkills that the intrasaccadic simulation of spatial flickering light from an LED luminaire and periodicity may interfere further with this made a 40° saccade between two distant process and help to explain the effect of high­ targets; 30% were able to detect flicker at frequency flicker in disturbing eye movement 300 Hz. In a subsequent study,2S participants ,s control, particularly during reading. .19 The were required to wave a white rod and report measurements obtained here reflect the con­ any stroboscopic effects. Under these condi­ scious perception of spatial periodicity, and tions, the upper frequency limit was still it is possible that noil-conscious effects occur higher. The participants in the original at parameter ranges beyond those explored study3 did not include those with migraine here. or epilepsy, and it is possible that the greater The perception of the phantom array cortical hyperexcitability in these partici­ would appear to be predictable from consid­ pants29 would render the flicker more visible. erations of the spatial frequency, contrast Our observation that an observer who used and extent (number of cycles) of the intra­ coloured overlays for reading was able to saccadic stimulation. With this in mind, it is perceive the intrasaccadic pattern at higher of interest that the contrast limit for percep­ frequencies than those for the remainder of tion of the array at twice the electricity the sample is of interest because coloured supply frequency (120 Hz) averaged 10%, filters have been shown to reduce cortical which is above the modulation typical of hyperactivation in migraine.3D incandescent lighting but below that typical In summary, temporally modulating for gas discharge and LED lighting. In other lighting can result in intrasaccadic percep­ words, even under conditions in which the tion of spatial periodicity for parameter intrasaccadic stimulation ('phantom array') ranges in which the lighting has been is maximally visible, as here, a modulation associated with impaired ocular motor depth of 10% is insufficient for its reliable control, impaired visual performance, dis­ perception. It remains to be seen whether the comfort and headaches. The upper fre­ complaints of discomfort from gas discharge quency limit of such effects is well above lighting can be attributed to the 'phantom the frequency at which the modulation can array' (perceptible or otherwise) and any be appreciated as flicker. resultant interference with saccadic control. The experimental conditions used in this study might be expected to enhance the perceptibility of the intrasaccadic pattern, Funding because after the retina was exposed to a flash, little further light reached the stimu­ This research received no specific grant from lated location on the retina. In a well-lit any funding agency in the public, commercial, room, the contrast of the image of each flash would presumably be reduced by the addi­ or not-for-profit sectors. tional light reaching the part of the retina stimulated by the flash later in the saccade. The perceptibility of the intrasaccadic pattern Acknowledgements might, therefore, be less when a dark line is viewed on a white background that flickers. We thank Sam Berman and Brad Lehman for 3 Bullough et 01. have reported the percepti­ ideas that contributed to this study and Nina bility of flicker when subjects observed a R Wolinski for conducting the survey con­ minimally contoured wall of a room lit with cerning car tail lights.

Lighting Res. T(·cJmo/. 2011; 45: 124-132 Flicker pat/erns abo!'e I kHz during saccades 131

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