Journal of Neuroscience Methods 141 (2005) 1–7
Cathode-ray-tube monitor artefacts in neurophysiology
Andrew J. Zele, Algis J. Vingrys∗
Department of Optometry and Vision Sciences, The University of Melbourne, Parkville, Vic. 3010, Australia
Received 26 February 2004; received in revised form 12 May 2004; accepted 12 May 2004
Abstract
We demonstrate that cathode-ray-tube (CRT) monitors commonly used as stimulus generators in visual neuroscience produce signal artefacts.
This arises from two factors, one being the finite time needed for the raster scan of the CRT to cross the receptive field being stimulated,
and the other being the restraint imposed by the impulse response of the phosphor itself. Together these factors result in smearing or blurring
that manifests as high frequency noise, distorting the desired signal applied by the investigator. Our analysis identifies those conditions that
promote these artefacts and we describe methods for their minimisation. We suggest that a monitor frame rate ≥100 Hz provides a reasonable
trade-off between refresh and the generators of high frequency noise.
© 2004 Elsevier B.V. All rights reserved.
Keywords: CRT monitor; Duty cycle; Flicker sensitivity; Photometry; Luminance
1. Introduction
The widespread adoption of cathode-ray-tubes (CRTs)
implies that they can, and do, serve as useful stimulus generators for vision related experiments (e.g. Chander and
Chichilnisky, 2001; Chichilnisky and Kalmar, 2003;
Douglass and Strausfeld, 1996; Groner et al., 1993;
Kreiter and Singer, 1996; Muller et al., 2003; Sperling,
1971a; Travis, 1991). Classic CRT calibration procedures
have been described elsewhere and need to be adopted to
ensure precise control of luminance and chromatic output
(e.g. Metha et al., 1993). In this work we consider an important issue, being the effect that the CRT raster scan and
discrete temporal refresh can have in characterizing the receptive field profile of a neuron. These discussions are also
directly applicable to behavioural experiments.
Most investigators will appreciate that CRT-monitors
form images by sequential activation of spatially discrete
pixel elements (e.g. 1024 × 768 pixels) in a raster scan
across the screen, where the raster scans from left to right,
and during each frame, from top to bottom (Sperling,
1971b; Travis, 1991). The time taken for a complete scan
of the screen determines the frame rate. This leads to an
∗ Corresponding author. Tel.: +61-3-8344-7006;
fax: +61-3-9349-7498.
E-mail address: algis@unimelb.edu.au (A.J. Vingrys).
0165-0270/$ – see front matter © 2004 Elsevier B.V. All rights reserved.
doi:10.1016/j.jneumeth.2004.05.005
important consideration for behavioural and visual neuroscientists who use CRTs. In that, the time taken by the CRT
to scan the perceptive or receptive field of the preparation
under study may impact on this response.
Another factor that can contribute to artefact generation is
the duty cycle of phosphor activation. The duty cycle can be
considered as the ratio of the ON-period to the desired activation. In CRT technology, each phosphor rises rapidly on
electrical stimulation and decays exponentially with passage
of the stimulating electron beam (Vingrys and King-Smith,
1986; Westheimer, 1993). This means that pixel luminescence occurs over a fraction of the dwell time (∼2 ms) created by the monitor refresh (50 Hz gives 20 ms), which is
called the duty cycle. The effect that this limited duty cycle
can have on neural activity is not trivial as evident in the
refresh-locked artefacts observed in vivo by Keating et al.
(2001) and in vitro by Chander and Chichilnisky (2001).
The significance of these artefacts can be considered in
terms of the window of visibility as proposed by Watson
et al. (1986). In its original application, the concept was defined using the well specified behavioural limits of the visual
system in terms of the perceptual window of visibility (e.g.
Crawford, 1947; de Lange, 1954; Robson, 1966; Smith and
Pokorny, 1975). Given that these behavioural limits are below the resolving capacity of many neurons in vivo (Berman
et al., 1991; Burns et al., 1992; MacLeod and He, 1993;
MacLeod et al., 1992; Viswanathan et al., 2002) or that re-
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ported in vitro by single cell recordings (Berry et al., 1997;
Chander and Chichilnisky, 2001; Kraft, 1988; Kreiter and
Singer, 1996; Reinagel and Reid, 2000; Smith et al., 2001),
this gives rise to a neural window of visibility. It is in terms
of these windows that we will consider the impact of CRT
refresh and pixel activation.
2. Methods
We determine the phosphor luminance profile for a single
activation and use this to simulate the multiple activations
typical of a visual experiment. The signals from such multiple activations were considered by determining the Fourier
power spectra of the stimuli and evaluated in terms of the
perceptual and neural windows of visibility. We demonstrate how a CRT can blur the signal across a receptive
field.
2.1. Stimuli and apparatus
For this analysis, we used a commonly available commercial high-resolution CRT operated under the viewing conditions often adopted in vision testing and research. The
CRT was a calibrated Hitachi Accuvue HMD-22471 RGB
monitor (P22 phosphor) driven by a high-resolution graphics card (Visual Stimulus Generator, VSG2/3; Cambridge
Research Systems) hosted in a PC (Compaq 486DX, maths
co-processor). The monitor was driven at a frame rate of
120 Hz (8.33 ms refresh) although higher rates were possible. Even though our measures are equipment specific we
believe that our comments are not equipment dependent and
can be applied to all similar CRT devices.
2.2. Phosphor luminescence profiles
It is important to understand the implications that different field stops used in capturing CRT activations can have in
predicting the response of cells with different receptive field
extents. This will be developed later but for our application,
phosphor activation was measured using two calibrated (Optical and Photometric Technology, Melbourne, Australia)
photometers having different field stops, one that could measure the luminance profile of a single triad (Pritchard: 2 arc)
and a second that could measure over an extended visual
angle (Silicon cell: 1◦ arc) typical of some neural receptive
fields. The optics of the Pritchard photospectroradiometer
were focused onto a single picture element, isolating the
record to one group of phosphors (R + G + B) or pixel triad.
In comparison, the entrance pupil (1◦ ) of the photosensor
captures light over an extended spatial region. Depending on
the application, this latter method of measurement may return an inappropriate representation for pixel activation and
decay, as will be shown later. In an attempt to capture limited regions of the screen, we modified the photosensor by
interposition of several pin-hole apertures (5–15 arc) to act
as field stops. Unfortunately, the field stop modification returned unreliable signals due to the low light levels and the
silicon cell was used in its standard configuration (1◦ ) when
making measurements.
The Pritchard photospectroradiometer (model 1980B)
with photo-multiplier was connected to an ADI bio-amplifier
(Advanced Digital Instruments, ML135 PowerLab) and
high-speed recorder. The ADI was run with a continuous acquisition rate (100 kHz) to simulate a storage
cathode-ray-oscilloscope (CRO). As the Pritchard is unlikely to be readily available to all users, we decided
to implement a commercially available CRT light-probe
adopting a modification of Sperling’s method (1971a). We
believe that the photosensor provides an accurate representation of the many commercial spot meters available to
neuroscientists for the calibration of CRT luminance. The
photosensor was a commercially available 0.44 cm2 silicon
cell (OptiCALTM , Cambridge Research Systems) that we
connected to the ADI bio-amplifier and high-speed recorder
in its storage CRO mode. It was fastened to the centre of
the CRT using a suction cap that also served as an ambient
light shade. The silicon photosensor was calibrated against
a Xenon flash to ensure that response saturation did not
affect outcomes (Brainard et al., 2002).
In comparison to many of the past investigators who have
reported the temporal luminance profiles of a single phosphor (R or G or B, e.g. Vingrys and King-Smith, 1986;
Westheimer, 1993), in our analysis we chose to measure the
“white” response (R + G + B) as it is typical of many applications where achromatic stimuli are used. In particular
it should reflect better the complex operating interactions
between phosphor guns, beams and shadow mask elements
(Brainard et al., 2002). We show how these two different
temporal luminance profiles can result in different signals
due to the raster scan and give power at different frequencies, that can easily be quantified.
2.3. Discrete Fourier transforms
Discrete Fourier transforms (4096-point DFT) were
calculated using a Microsoft-ExcelTM spreadsheet. Independent analyses using MathWorks-MATLABTM gave the
same outcomes and we adopted the Excel spreadsheet
for convenience. The activations recorded by the photospectroradiometer were used to model stimulus profiles of
the time-modulated probes by simulating different refresh
rates. In order to consider the changes in a way meaningful to the visual system, we consider the activations in
terms of time-modulated contrasts about a mean luminance.
By doing so, we acknowledge that although the photoreceptors modulate in relation to the total quantal flux, the
response of many neurons shows contrast dependency (e.g.
Enroth-Cugell and Robson, 1984) and for this analysis we
adopted the approach proposed by others in quantifying
the effects of colour changes (Brainard, 1996). As we will
show, this provides a useful method for our analysis.
A.J. Zele, A.J. Vingrys / Journal of Neuroscience Methods 141 (2005) 1–7
3
file can have in limiting the temporal capabilities of stimuli
generated by CRTs.
3. Results
3.1. Phosphor luminance profile
Fig. 1. Phosphor activation for a high-resolution RGB monitor determined
from an achromatic triad with a screen luminance of 60 cd·m−2 . Measurements were made with a Pritchard photospectroradiometer (model 1980B)
with photo-multiplier tube (solid line) using a 2 arc diameter aperture
focused in the plane of a single pixel triad and a 0.44 cm2 aperture (∼1◦ )
spot-photometer (dotted line).
The DFT assumes an analysis based on a continuous
repeating time series (Bach and Meigen, 1999). In many
visual experiments, this is not the case (e.g. Zele and
Vingrys, 2001) and a single stimulation (which we will
call an impulse) might be the norm. For such stimuli, we
have adopted linear interpolation or decimation (Lyons,
1987) to resample the waveform over a binary scale. This
method allows frequencies with an integer number of cycles within the sampling window to be cleanly extracted by
DFT without the artefacts caused by truncation (Bach and
Meigen, 1999; Lyons, 1987). In addition to the impulse, we
also considered Gabor (a sinewave windowed by a Gaussian) and Gaussian time series as these have common applications.
2.4. Measuring phosphor activation and decay
The equipment (CRT display and photometers) was allowed to warm-up (45–60 min) before use. Phosphor activation and decay was recorded for a white (1931 CIE x
= 0.238, y = 0.319) screen luminance of 60 ± 1.0 cd·m−2 .
The Pritchard photospectroradiometer with photomultiplier
was focused in the plane of a single pixel triad using a 2 arc
diameter aperture. The silicon photosensor captured screen
activation over 1◦ from a common region of the CRT.
The phosphor decay profile returned from the photospectroradiometer (Fig. 1, solid line) was used as a template
to simulate time-modulated stimuli for which the frequency
spectra were calculated. The analysis was conducted assuming some commonly used monitor frame rates (50, 75 and
120 Hz) corresponding to 20.0, 13.3 and 8.33 ms activation
periods, respectively. Comparison was made between theoretical waveforms (e.g. square, sine, Gabor and Gaussian
waveforms of short and long duration) that we believe are
commonly used in neuroscientific investigations. This provides a basis for the identification of the artefacts introduced
by the CRT raster scan and the duty cycle from the phosphor
impulse response. In the final analysis we considered the
significance that the monitor frame rate and activation pro-
The normalized luminance profile of the Hitachi CRT
white phosphor triad driven at 120 Hz is shown in Fig. 1
for the 2 arc aperture (solid line). This profile (white = red
+ green + blue) has a half height of 0.63 ms and decays
to noise (±5%) within 3.2 ms of its 8.33 ms activation window (∼8% duty cycle). Fig. 1 (dashed line) also shows the
normalised phosphor luminance profile acquired with the 1◦
photosensor. This has a slower build up, is broader and decays over a longer time-frame than that returned by a single
triad. The two profiles demonstrate that different receptive
field sizes will perceive different light durations providing
that they have the appropriate temporal attributes to integrate the discrete activations of the raster scan. Consistent
with the data reported by Keating et al. (2001) and Brainard
et al. (2002), the silicon cell returns a luminance profile with
a half height of 1.9 ms and a significant persistence of up to
5–6 ms (∼25% duty cycle).
3.2. Power spectra of waveforms generated using
non-discrete stimulators
Fig. 2A and B shows the power spectra for long (continuous) and short (impulse) duration waveforms (schematics,
right hand-side of the normalized power spectra). Fig. 2A
shows the expected luminance profile (vertically adjusted)
for three commonly used, 60 Hz stimulus waveforms; a continuous sinewave (lower, solid line), gabor (middle; dashed
line) and square wave (upper, dotted line), with their relative power spectra. We chose to plot this data on a linear ordinate to emphasise meaningful signals with amplitudes >1/100 of the normalized maximum (2 log units). As
would be expected, the normalised frequency spectra for
each waveform (sine, square and gabor) is tightly centred on
60 Hz, with the square wave introducing higher frequency
harmonics (180 Hz; panel A, dotted line) due to its temporal
profile.
Reducing the duration of the waveform widens the power
spectra (Fig. 2B). Although the power spectra remain centred on the modulation fundamental (60 Hz), the broadened
spectra (Fig. 2B) may limit precision in isolating a neuron’s
temporal response. A single cycle ON–OFF square wave
(Fig. 2B, dashed line) has the broadest frequency response
of the three waveforms considered and it is accompanied by
very low energy harmonics.
The signals shown in Fig. 2 cannot be realized on a CRT
due to the discrete phosphor refresh (signalling) returned in
Fig. 1. The CRT will act to change the duty cycle of the
waveform, and it follows, that the duty cycle will vary ac-
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Fig. 3. Effect of duty cycle on the signal power spectra. Normalised
power for a 60 Hz square wave with either a 50% duty cycles (dashed
line), 24% (solid line) or 12% duty cycle (dotted line). The fundamental
of the waveform is centred on 60 Hz with frame rate artefacts introduced
at 120 Hz, as are higher order harmonics (180 Hz). The duty cycles
approximate those of the signals shown in Fig. 1. The power spectra for
the 12% and 24% duty cycles were shifted horizontally for clarity, by
±5 Hz, respectively.
3.3. Power spectra of waveforms generated using discrete
stimulators: CRTs
Fig. 2. Power spectra for conventional waveforms (long and short duration)
used in neuroscience. (A) Normalised magnitude of a discrete Fourier
transform (power spectra) for sinusoidal, gabor and square wave (60 Hz)
continuous waveforms (vertically adjusted for clarity) as indicated by the
schematic time-modulated profiles given to the right of the figure. (B)
The power spectra for a single cycle (impulse) of the sinusoidal, gabor
and square waveforms (vertically adjusted) and demonstration of how
the reduced temporal duration broadens the frequency spectrum of the
stimulus.
cording to the size of the receptive field (or the entrance pupil
of the acquisition device; see Fig. 1). Fig. 3 demonstrates
the effect that altering the duty cycle (12 and 24%, approximately the duty cycles of the signals shown in Fig. 1) of a
60 Hz continuous square wave (Fig. 3, dashed grey line) can
have on the power spectra. Two effects can be observed as
the duty cycle decreases. Significant power is introduced at
the frame rate (e.g. 120 Hz) and at higher frequencies (e.g.
harmonic at 180 Hz), both of which accompany reductions
in the energy at the fundamental frequency (60 Hz). Calculation of the signal-to-artefact ratio (i.e. 60:120 Hz and
60:180 Hz) gives an estimate of the energy found at the harmonics (frame rate and higher order) relative to the fundamental. For high duty cycles (upper dashed line), the power
at the frame rate is 2.9× the fundamental, and 3.8× at the
harmonic. The power found at the higher frequencies (beyond the fundamental frequency) increases at lower duty cycles (middle solid line), having 3.8× the power at the frame
rate and 7.5× at the harmonics. Next we show, by analysis, the power spectra of a flickering waveform generated on
CRTs having different frame rates.
The stimulus window for a single cycle (∼5 Hz) bipolar
probe generated using 50, 75 and 120 Hz frame rates are
shown in the left panels and, their respective power spectra of the contrast modulation, in the right panels of Fig. 4.
As frame rate increases, the effective duty cycle increases
and this modifies the power spectrum of the time-modulated
probe (see Fig. 3). At each frame rate, the fundamental
corresponds to the modulation frequency of 5 Hz (Fig. 4,
right panels). The power spectra, however, contain significant peaks at both the frame rate (arrow) and higher harmonics due to the finite stimulus window and the discrete
phosphor sampling. If these 5 Hz stimuli were shown to a
neuron having a larger receptive field (as in Fig. 1), the effect would be to alter the ratio of the powers found at the
fundamental and frame rate (as per Fig. 3).
To consider the effect that these CRT artefacts can have
on the visual system, the data from Fig. 4B (50 Hz refresh,
5 Hz pulse) have been redrawn in Fig. 5 along with the
human perceptual temporal response (de Lange, 1954) and
the horizontal cell (H1) temporal response which represents
the output of neurons early in the visual process (Smith et al.,
2001). This defines two windows of visibility as detailed in
the introduction. It can be seen that the visual system has
differential sensitivity to energy at high frequencies, which
may influence outcomes. In particular, Fig. 5 demonstrates
how the harmonics introduced by the frame rate will become
detectable by neurons as frame rates decease. This effect
is more pronounced for impulses than for multiple-cycle
probes due to the temporal stimulus widow (see Fig. 2).
What is most compelling, is that Fig. 5 identifies a range
of temporal frequencies (60–80 Hz) where the artefact is
perceptually invisible but can elicit neural responses.
A.J. Zele, A.J. Vingrys / Journal of Neuroscience Methods 141 (2005) 1–7
5
Fig. 4. Stimuli (left) and normalised Fourier magnitudes (right) of a single cycle (∼5 Hz) of flicker generated using three commonly implemented monitor
frame rates: 50 Hz (A and B), 75 Hz (C and D) and 120 Hz (E and F). Note how substantial noise is created by the frame rate (arrow).
4. Discussion
4.1. Phosphor activation profile
The rise and decay seen in our phosphor triad (Fig. 1)
is consistent with previous estimates derived from single phosphor activations (Farrell, 1991; Sperling, 1971b;
Vingrys and King-Smith, 1986; Westheimer, 1993). The
fast decay means that there is no chance of luminous carry
over or phosphor persistence even with a frame rate of up
to 200 Hz (5 ms). However, this must be tempered by the
operating luminance of the screen: had we used a brighter
background, phosphor persistence would have increased.
So it is imperative that each experimenter quantify the time
course of phosphor activation under the operating conditions
common to their laboratory and in cases where phosphor
persistence is found, a number of methods are available to
describe and correct for these artefacts (e.g. Metha et al.,
1993; Vingrys and King-Smith, 1986; Westheimer, 1993).
We have shown how the receptive field of a neuron can
modify the intended temporal profile of a CRT phosphor
due to its duty cycle or raster scan (Fig. 1). The nature of
the phosphor signal is dependent on the screen area over
which the signal has been captured and an accurate description of a triad can only be obtained from a region of the
screen encompassing a single red, blue and green phosphor
(Fig. 1, solid line). Otherwise, the waveform returned with
the larger receptive field will represent the output of the
three phosphors integrated over the aperture of the neuron
during the time needed to complete a scan. The integration
of the CRT signal over an extended region causes smearing
of the response over space and time (Fig. 1, dotted line).
Depending on the size of the receptive field, this may return
an inaccurate representation of the timing of the underly-
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Fig. 5. The discrete Fourier transform for a 5 Hz flickering waveform
generated on a CRT with a 50 Hz refresh rate redrawn from Fig. 3 to
emphasise the scale ranging between 50 and 120 Hz. The psychophysical
temporal sensitivity function of de Lange (1954) and that of a horizontal cell (H1) from Smith et al. (2001) have been plotted by the black
and grey solid lines, respectively. Note how artefacts introduced by the
discrete sampling of the CRT phosphors produce low amplitude signals
(60–100 Hz) that are invisible to the observer but can be detected by the
neurone.
ing activation (Fig. 1, solid line) as observed by Chander
and Chichilnisky (2001). Indeed, broad temporal luminance
profiles have been reported in the literature (e.g. see Fig 7
of Brainard et al., 2002; Fig 3 of Keating et al., 2001) and
imply that an extended field stop may have been used when
defining the luminance profile in these studies.
Not surprisingly, the nature of phosphor persistence has
been the subject of significant debate (Di Lollo et al., 1994;
Irwin, 1994; Westheimer, 1993, 1994). We suggest that, apart
from the physical issues constraining phosphor decay (see
Groner et al., 1993), the significance of the physiological
manifestations of persistence must be evaluated in terms
of a mechanism’s receptive field or entrance pupil and we
consider this in the next section. As a corollary, we propose
that a spot-photometer should be used as a luminance meter
and only applied to assay phosphor profiles when its entrance
pupil matches that of the detecting mechanism.
4.2. Detecting cathode ray tube signals
As we have alluded before, the implications of these measures need to be considered in terms of the window of visibility (Watson et al., 1986) as the issue rests, not with phosphor activation alone, but with the nature of receptive field of
the detecting neurone/neurones (Gawne and Woods, 2003).
We have shown that these artefacts arise from the discrete
phosphor resampling (see Figs. 3 and 4) that occurs within
the stimulus window (Watson, 1986). If the preparation under study were to involve a cone dominated process then
the 2 aperture is most appropriate and the luminance profile
given by the solid line of Fig. 1 is accurate for this purpose.
However, preparations with larger receptive fields, such as
the rod pathway (Lennie and Fairchild, 1984), will be better represented by the 1◦ aperture. Indeed, the ability of the
eye to respond to high temporal frequencies appears to be
an attribute of the local retina as the multi-focal ERG shows
a response at monitor refresh (Keating et al., 2001).
It is hard to judge the significance of these artefacts because human perception has little sensitivity at high temporal frequencies (Fig. 5, solid black line). We propose that
detection mechanisms at the fundamental will be substantially more sensitive than mechanisms located at any higher
frequencies, as evident from Fig. 5. So these artefacts are
unlikely to influence threshold-related experiments involving low temporal frequencies. Keating et al. (2001) have
noted that they might be significant for non-linear processes,
however, our analysis did not consider this possibility. It
might be expected that these high frequency artefacts will become more significant with stimuli that are presented above
threshold, as is the case with an ERG obtained at high luminous energies. Consistent with these predictions, Keating
et al. (2001) isolated robust oscillatory artefacts in their
75 Hz CRT-generated multi-focal ERG signals that are not
present when obtained using the same stimulus presented
on a continuous display (liquid crystal display). Fig. 5 also
demonstrates the importance of adopting high frame rates
(>100 Hz) as these will have the effect of placing any artefacts at frequencies well beyond the window of visibility of
retinal elements.
The frequency spectrum of the discrete sampling used by
CRTs is dependent on monitor frame rate (Fig. 4) with our
analyses demonstrating that the resultant power spectrum is
modified by the time between subsequent activations (duty
cycle). As the number of discrete activations within a stimulus window increases at higher frame rates, these will act
to reduce the number of high frequency transients that may
be detected at suprathreshold levels. The implementation of
a monitor having a high frame rate will not only decrease
the likelihood of artefact detection, but will also provide the
user with a larger selection of integer options at high temporal frequencies (see Section 4.3).
4.3. Restrictions imposed by monitor sampling
The discrete nature of the video frame rate coupled with
the fact that a repetitive waveform comprises two components (ON and OFF) in its period, means that not all integer
frequencies can be achieved with a CRT. As a consequence,
the potential frequencies form a binary sequence given in
Hz = F/2n where, F is the frame rate, and n, is the number
of cycles (ON–OFF) presented in the stimulus. For example, a frame rate, F, of 100 Hz and a period comprising 1, 2,
3, 4, . . . , n cycles yields 50, 25, 12.5, 6.25, . . . , x Hz. Note
that significant under sampling is found in the high temporal
frequency domain and this will become most marked with
lower frame rates.
Our analyses suggest that a monitor refresh ≥100 Hz provides a good trade-off between sampling (refresh) and the
visual response to act as a stimulus generator. Depending
on the particular application, the effects of phosphor persistence and frame rate may still prove bothersome so that they
A.J. Zele, A.J. Vingrys / Journal of Neuroscience Methods 141 (2005) 1–7
must be eliminated. In these cases implementation strategies (see previous) or alternative technologies, such as light
emitting diodes, liquid crystal display screens or digital light
projectors, which do not refresh line-by-line, may need to
be considered. However, these latter technologies all have
their own particular limitations (see Brainard et al., 2002;
Keating et al., 2001; Packer et al., 2001) that need to be fully
evaluated before implementation.
Acknowledgements
Dr George Smith and Dr Steven Jenkins provided guidance with phosphor luminance measurements. This work
is supported by an Australian Research Council Linkage
Project (LP0211474) grant.
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