Investigation into the verification of the Synapsys

Transcription

International Journal of Audiology 2014; 53: 618–624
Technical Report
Investigation into the verification of the Synapsys
videonystagmography (VNG) Ulmer calibration system
Int J Audiol Downloaded from informahealthcare.com by University of Texas At Dallas on 08/18/14
For personal use only.
Natalie Corless & Susannah Goggins
Audiology Department, Betsi Cadwaladr University Health Board, Wrexham, UK
Abstract
Objective: To assess the accuracy and stability of the Synapsys VNG Ulmer calibration system when the goggles and infrared camera are
repositioned, and the accuracy of the equipment’s geometric calibration system. Design: Prospective data collection involved participants
conducting the system’s horizontal calibration test. Eye measurements were then recorded for a 60° deviation (looking from 30° right
to ⫺ 30° left) and repeated after the goggles and infrared camera were removed/repositioned. Participants’ eye measurements were also
recorded after the geometric calibration had been activated. Study sample: Twenty-two participants with no history of visual or vestibular
pathology were recruited for this study. Results: No significant differences were found when the goggles were removed and replaced
(p ⫽ 0.21); when the infrared camera was repositioned within the goggles (p ⫽ 0.50); or when the goggles were removed and the camera
repositioned (p ⫽ 0.18) after horizontal calibration. A significant difference was found during the assessment of geometric calibration
(p ⬍ 0.01). Conclusions: Calibration is not affected by removal or repositioning of the goggles and/or infrared camera within the goggles,
therefore suggesting recalibration may not be necessary. Caution should be exercised when using the equipment’s geometric calibration
and should only be used when the patient is unable to conduct the system’s horizontal calibration test.
Key Words: Calibration; videonystagmography/VNG
The vestibular test battery involves the assessment of the vestibuloocular system (Farkashidy, 1966). Routine vestibular assessment
involves the recording and analysing of the patient’s eye movement
and any type of nystagmus (Bhansali & Honrubi, 1999). Nystagmus can indicate a static imbalance between the resting outputs
of the two vestibular systems (Jacobson & Shepard, 2008), and is
comprised of a slow and fast phase. The direction of the nystagmus
is identified by the direction of the fast phase. The intensity of
the nystagmus is defined by the slow-phase velocity. Therefore it
is preferable to record the eye movements rather than observation
alone to enable quantification to the direction, speed, and amplitude of the nystagmus and to enable comparison between different
eye recordings. The recording of nystagmic responses using electronystagmography (ENG) or videonystagmography (VNG) is a
widely recognized method for investigating vestibular disorders
(Des Courtis et al, 2008).
Traditionally, ENG utilized the corneo-retinal potential (CRP) to
infer eye position and movements. ENG applies the CRP variation
principle during changes in eye position to assess the function of
the vestibulo-ocular reflex (Bhansali & Honrubi, 1999) and involves
recording voltage changes detected by the electrodes placed around
the eye resulting from a known eye movement and comparing the
voltages to those of known eye movements. Literature has documented that there can be large variations in the CRP during short
periods of time (Proctor et al, 1980; Hickson, 1983). The CRP is
affected by a change in pupil diameter and depolarization of the
retina when exposed to light. Any lighting variations that may
occur during ENG testing can result in a change of calibration due
to changes in the CRP. Therefore, frequent recalibration of ENG
systems is necessary to reduce calibration errors (Lightfoot, 2004).
VNG uses a computerized system that does not require the placement of electrodes to record eye movements. VNG employs an
algorithm to accurately measure any ocular movements from the
centre of the pupil (Eggert, 2007) using infrared sensors built in to
a set of goggles or mask which are placed over the patient’s eyes.
The algorithm enables a two-dimensional analysis of the eye movement; vertical and horizontal. VNG can assess eye deviations from
a central position to 30° horizontally and 20° vertically (Gananca
Correspondence: Natalie Corless, Audiology Department, Wrexham Maelor Hospital, Croesnewydd Road, Wrexham, LL13 7TD, UK. E-mail: [email protected]
(Received 12 December 2012; accepted 24 February 2014 )
ISSN 1499-2027 print/ISSN 1708-8186 online © 2014 British Society of Audiology, International Society of Audiology, and Nordic Audiological Society
DOI: 10.3109/14992027.2014.900195
Verification of Synapsys VNG Calibration System
Abbreviations
CRP
ENG
VNG
Corneo-retinal potential
Electronystagmography
Videonystagmography
et al, 2010). During VNG testing, it is also possible to visualize
the eye movements which is advantageous to identify any eye
movements that cannot be recorded, such as torsional nystagmus
(Ruckenstein & Shepard, 2000). VNG has become the preferred
method of assessing changes in eye position or movement due to
increased sensitivity (Jacobson & Shepard, 2008), an improvement in resolution for recording eye movements in the vertical
plane (Eckert & Gizzi, 1998) and increased stability of calibration
(Lightfoot, 2004). Unlike ENG, VNG testing can be conducted under
any lighting conditions without affecting the accuracy of the calibration. Some VNG systems can offer binocular recording (records
movement from both eyes individually) which enable the recording
of disconjugate eye movements (Gananca et al, 2010).
VNG calibration is conducted prior to ocular motor assessments
and requires the patient to follow a computer-generated moving
visual target. Calibration is the process of calculating a conversion
factor so that the parameters measured by the recording device can
be converted into measurement of the eye movements (Kang &
Malpeli, 2003). The type of calibration test can vary between manufacturers, some will require the patient to use fast eye movements
to look between fixed points (saccade test) and other systems will
require the patient to track a moving target (smooth pursuit) (Barin,
2006). VNG calibration systems also offer a default calibration mode
which does not require the patient to follow a moving target. It
is always recommended to follow the manufacturer’s specification
for calibration testing. An error or faulty calibration recording will
result in inaccuracy of the amplitude of eye movements and this may
impact upon the data obtained during further subtests of the ENG/
VNG test battery (Barin, 2006).
It is stated in the British Society of Audiology (BSA) recommended procedure for caloric irrigation (2010) that ENG/VNG
calibration should be conducted prior to testing and recalibrated if
the goggles are removed or repositioned, or if the infrared camera
within the goggles are repositioned with the goggles in their original
placement. This is further supported by Barin (2006) who states
recalibration if required if the VNG goggles are moved with respect
to the patient’s eyes. However, recalibration may not be always
clinically viable due to changes in test position (for example, caloric
irrigation) and clinical time restraints of appointment length.
Andrew and Meredith (2009) conducted research into the verification of horizontal VNG calibration using the ChartR equipment
and investigated the effect of removing and repositioning goggles on
calibration traces on ten participants with no history of peripheral vestibular disorder. The ChartR equipment’s default calibration was also
examined to determine whether the default calibration setting was
accurate for clinical use and found default calibration overestimated
eye deviations. The findings from the study suggest that removing
or repositioning the goggles had no significant clinical effect on the
initial horizontal calibration, indicating that recalibration is not necessary if the goggles were to be removed and repositioned.
However, the findings from Andrew and Meredith s (2009)
work cannot be applied to other VNG systems due to differences in
calibration systems and VNG goggle design. At Wrexham Maelor
619
Hospital, Wales, the current VNG system is the Synapsys VNG
Ulmer. The Synapsys VNG Ulmer system differs from the ChartR
system as it uses a saccade calibration test instead of smooth
pursuit and has VNG goggles with a detachable infrared camera
(Figure 1, a–c) as opposed to the ChartR VNG goggles which has
an inbuilt infrared camera. Also, the Synapsys VNG Ulmer uses a
different default calibration test to the ChartR system.
The default calibration setting on the Synapsys VNG Ulmer
system is also known as geometric calibration and will be referred
to as geometric calibration in this paper. The geometric calibration
consists of measuring the patient’s iris diameter as one of the parameters of the ocular image. This is conducted by measuring the horizontal width of the patient’s iris on the VNG software (Figure 2).
The measurement of the patient’s iris is compared to manufacturer
gathered data and this enables the eye deviation to be calculated
from any movement from the centre of the pupil. As the geometric
calibration is based upon approximations of the actual diameter of
the patient’s iris it is less accurate, therefore the Synapsys VNG
Ulmer manufacturers advise that geometric calibration should only
be activated if a patient is unable to conduct a visual calibration
(VNG Ulmer User Manual, version C4-12C).
Due to the differences between the ChartR and Synapsys VNG
Ulmer systems, the aim of this study was to develop the past work
by Andrew and Meredith (2009) using the Synapsys VNG Ulmer
equipment at the Wrexham Maelor Hospital, Wales. The study aim
was to assess the accuracy and stability of the patient conducted
Figure 1. The VNG goggles used for the recording of eye deviation.
(a) The VNG mask. (b) VNG camera in situ monitoring the
left eye with the right eye uncovered (recording condition).
(c) Light-excluding cover in situ over right eye. (d) Side profile of
VNG camera illustrating range of camera movement.
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N. Corless & S. Goggins
This system uses a monocular recording of eye movements. The
VNG goggles consist of a mask (Figure 1, a) with a detachable
camera (records eye movements) which is placed over one eye (Figure 1, b), and a detachable light-excluding cover to remove fixation
(Figure 1, c). Once the camera is in position, it provides a completely
dark environment for the test eye (eye monitored by infrared camera). As the camera is detachable, the test eye is usually selected to
be the eye with the poorer vision to ensure the patient can clearly
visualize the light target with the non-test eye.
The Synapsys VNG Ulmer system calibration test is a saccade test
over an eye deviation of ⫾ 10°. The equipment offers both horizontal
and vertical calibration tests. However, at Wrexham Maelor Hospital
routine clinical assessment usually involves horizontal calibration
only. This is as all horizontal nystagmus is measured to determine
the significance of the nystagmus (nystagmus ⱖ 3° is reported as significant) whereas the recording of any vertical nystagmus is reported
as significant.
Figure 2. Example of conducting geometric calibration. The width
of the iris is measured by the equipment user placing a cursor either
side of a static image of the eye.
horizontal and geometric calibration on the Synapsys VNG Ulmer
equipment and to investigate whether recalibration was necessary
when the goggles and/or camera are removed or repositioned within
a clinical setting.
Method
Prior to the commencement of this study, full ethical approval was
granted by the North Wales NHS Research Ethics Committee (REC).
Participants
A power analysis was calculated and determined a sample size of
22 was sufficient to detect a 1.2 degree difference between the test
conditions with 95% power and p ⬍ 0.01.
Participants were recruited from the Audiology department at
Wrexham Maelor Hospital. Twenty-two individuals (15 females and
7 males) with no history of vestibular pathology or significant eye
conditions (disconjugate eye movements, congenital nystagmus, eye
surgery within last three months, or any neurological condition that
may affect eye movements such as multiple sclerosis) which could
interfere with normal eye movements were recruited for this study.
Prior to testing, it was ensured that each participant was able to
clearly visualize the light target on the projector screen at the set distance (2.19 m) with monocular vision. No participant was excluded
from this study due to difficulty in visualizing the light target. The
mean age for this group was 33.6 years ⫾ 8.9 years at assessment.
Experiment 1: Assessment of calibration accuracy
BASELINE
CALIBRATION ASSESSMENT
Each participant underwent the system’s recommended calibration
test using horizontal saccade testing. The participant was seated in a
central position at a fixed distance from the projector screen (2.19 m)
with the light target at eye level. Each participant was instructed to
follow the light target on the projector screen with their eyes without
making head movements. During recordings, the test eye was alternated between participants (11 right test eyes and 11 left test eyes)
to eliminate any potential test eye bias.
After the horizontal calibration test had been completed, the horizontal calibration waveform was visually inspected by either author
to ensure no abnormalities, such as poor accuracy, latency, or velocity were present before testing continued. Horizontal gaze testing
was then performed to record the actual eye deviation at a known
angle of 60°. The horizontal gaze test involved the participant looking with their eyes ⫹ 30° (right) to ⫺ 30° (left) while maintaining
their head in a central position. These eye deviations were selected
as it is routine clinical practice to measure eye movements at ⫾ 30°
during gaze testing. If the angle was too large in gaze testing, end
point nystagmus may be produced and errors seen in normal participants, however a sufficient angle is required during gaze testing to
provide an effect in pathological cases. The recorded eye movements
from looking ⫹ 30° (right) to ⫺ 30° (left) provided the baseline measurements. Three non-randomized conditions were conducted and
analysed to assess their effect on the baseline calibration results. To
eliminate any bias from the previous condition, horizontal calibration was repeated after each condition to ensure that an accurate
comparison between the condition and the baseline measurement
was achieved.
CONDITION 1: GOGGLES REMOVED
Equipment
The set up of the Synapsys VNG Ulmer equipment at Wrexham
Maelor Hospital comprises of a projector screen positioned at a fixed
distance of 2.19 m from the patient seat. This distance was based
upon departmental normative data to ensure accurate eye measurement recordings for horizontal gaze testing at eye deviations 30°
right and ⫺ 30° left. The patient’s seat has an adjustable height to
ensure that the light target is at eye-level. The light target is a square
shape of 4 cm² in size.
AND REPOSITIONED
After conducting the baseline measurements (as described above),
the goggles were removed while preserving the angle of the infrared camera and the participant was asked to walk around the test
room. The participant was then asked to return to their initial seated
position and the goggles were again placed over the participant’s
eye. As this study wanted to investigate the effect of removing and
replacing the goggles within a clinical setting, no formal assessment was conducted to ensure the participant had returned to their
exact, initial seated position. Horizontal gaze testing was repeated
to compare the eye deviations and examine the effect of removing
and repositioning the goggles.
CONDITION 2: INFRARED CAMERA REPOSITIONED WITHIN
THE GOGGLES
The angle of the infrared camera can be adjusted to ensure a clear
recording of the test eye. The infrared camera angle can be rotated
360° on the horizontal axis and can be moved approximately 45° on
the vertical (Figure 1, d).
During this condition, the angle of the infrared camera within the
goggles was adjusted from the initial calibration test. The camera
was rotated approximately 180° horizontally and moved approximately 45° vertically. After the infrared camera had been adjusted,
the camera was repositioned to maintaining a suitable recording of
the test eye. Horizontal gaze testing was repeated to compare the
eye deviations and examine the effect of repositioning the infrared
camera within the goggles after conducting calibration.
CONDITION 3: INFRARED CAMERA AND GOGGLES REMOVED
AND REPOSITIONED
Condition 3 is a combination of the elements in conditions 1 and 2.
Following the baseline measurements of eye movement was made,
the patient was asked to walk around the room. Once the participant
returned to a central, fixed seated position the goggles were replaced
to the same position while the angle of the infrared camera had been
adjusted. Horizontal gaze testing was then repeated to compare the
eye deviations to the baseline measurement.
Experiment 2: Assessment of the geometric calibration
In accordance to the manufacturer’s recommendation (VNG Ulmer
User Manual, version C4-12C), the assessment of geometric calibration
was conducted by measuring the horizontal width of the participant’s
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iris. This was performed by the application of cursors to the outer
boundary of the iris on a captured static image of the participant’s
eye (Figure 2). This enables the equipment to estimate the eye deviations by comparing the movement of the pupil in comparison to
manufacturer stored normative data. Horizontal gaze testing was
then performed to record the actual eye deviations of a known angle
of 60°. The obtained results were compared to the baseline results
obtained in experiment 1.
Statistical analysis
The actual eye deviation results for each condition were compared to the baseline eye deviation results (calculated before each
condition) using one tailed paired sample T test. For experiments 1
and 2, a Bonferroni correction was applied, so that for experiment
1, test conditions 1 to 3, the p value used was p ⬍ 0.0167 and for
experiment 2, p ⬍ 0.025 was used.
Results
Experiment 1: The effect of goggle and infrared camera
movement on initial calibration testing
The mean eye deviation from the baseline measurement was 56.4°
(⫾ 1.4°). The individual results from the baseline measurement were
compared with the findings from each condition.
CONDITION 1: GOGGLES
REMOVED AND REPOSITIONED
Figure 3 displays the individually gathered data obtained from condition 1 (grey coloured bar) in comparison to the baseline measurements (white coloured bar). The last bar (titled ‘mean’) shows the
data averaged for all 22 participants from condition 1 and the baseline
measurements. The error bars represent the standard deviation. The
mean (standard deviation) eye deviation for condition 1 was 55.1°
(5.2). There was no significant difference found between the baseline
Figure 3. Individual and average baseline calibration results compared to recordings after the goggles had been removed and repositioned.
The error bars represent the standard deviation of the mean values.
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Figure 4. Individual and average baseline calibration results compared to recordings after the infrared camera within the goggles had been
repositioned. The error bars represent the standard deviation of the mean values.
eye deviation testing results and the eye deviation results recorded
after the goggles had been removed and repositioned (p ⫽ 0.21). This
suggests that moving the goggles does not have a significant effect
on the initial calibration. However, there is a relatively large standard
deviation of 5.2° (9.4%) for condition 1 in comparison to the baseline measurement standard deviation of 1.4° (2.5%). This indicates
that there is a greater variation in eye deviation measurements once
the goggles are repositioned.
CONDITION 2: INFRARED CAMERA
REPOSITIONED WITHIN GOGGLES
Figure 4 shows the results obtained from condition 2 in comparison to the baseline measurements. The mean bar on Figure 4 shows
the eye deviation for condition 2 was 56.6° (1.9). There was no
significant difference found between the baseline eye deviation testing results and the eye deviation results recorded after the infrared
camera angle within the goggles had been repositioned (p ⫽ 0.50).
This finding suggests that repositioning the infrared camera angle
Figure 5. Individual and average baseline calibration results compared to recordings after the goggles and infrared camera within the
goggles had been repositioned and replaced. The error bars represent the standard deviation of the mean values.
623
Figure 6. Individual and average eye deviation results after geometric calibration in comparison with the baseline measurement. The error
bars represent the standard deviation of the mean values.
during testing does not have a significant effect on the initial
calibration. The standard deviation for condition 2 of 1.9° (3.4%)
is similar to the standard deviation from the baseline condition 1.4°
(2.5%) indicating that there is not a greater variation in eye deviation
measurements once the infrared camera had been repositioned.
CONDITION 3: INFRARED CAMERA AND GOGGLES REPOSITIONED
The individual results obtained from condition 3 are shown in
Figure 5 in comparison to the results from the baseline measurement. For condition 3, the mean eye deviation was 55.6° (3.2).
No significant difference was found between the baseline eye deviation testing results and the eye deviation results recorded after the
goggles and infrared camera within the goggles had been removed
and the infrared camera angle repositioned (p ⫽ 0.18). This suggests
that moving the goggles and infrared camera does not have a significant effect on the initial calibration. The standard deviation for
condition 3 is displayed as an error bar on Figure 5. The standard
deviation for condition 3 of 3.2° (5.8%) is greater than the baseline
measurement standard deviation of 1.4° (2.5%). This indicates that
there is a greater variation in eye deviation measurements once the
goggles and infrared camera have been repositioned.
Experiment 2: The accuracy of geometric calibration
Figure 6 displays the eye deviation results obtained from all 22
participants after the geometric calibration had been activated. These
results were compared to the results obtained from the baseline measurement condition in experiment 1. The error bars represent the
standard deviation. The mean eye deviation for experiment 2 was
49.2° (3.3). A significant difference was found between the obtained
baseline recordings from experiment 1, t(21) ⫽ 9.755, p ⬍ 0.01
with the recordings obtained after geometric calibration was applied.
As shown in Figure 6, the geometric calibration was found to
underestimate the true eye movement for each participant except
for one participant (participant ID 3). Therefore, the geometric calibration underestimated the eye deviations in 95% of participants
and by an overall mean of 7.2° (13%). The standard deviation for
experiment 2 is 3.3° (6.7%) which is greater than the baseline measurement standard deviation of 1.4° (2.5%). This indicates that there
is a greater variation in eye deviation measurements when activating
the geometric calibration than conducting visual calibration. However, it should be noted that the baseline recordings were lower than
the expected eye deviation of 60° and this may contribute towards
the significant difference between the baseline recordings and the
eye deviation measurements made after the geometric calibration
was activated.
Discussion
The results from experiment 1 showed that removing and repositioning the goggles (condition 1), repositioning the infrared camera
angle (condition 2), or repositioning both the infrared camera and
goggles (condition 3) did not have any significant effect on the initial
visual calibration results. The finding from condition 1 is consistent with the finding reported by Andrew and Meredith (2009) that
removing and replacing the VNG goggles did not affect initial calibration on the ChartR system. The study by Andrew and Meredith
(2009) did not examine the effect of adjusting the camera angle on
calibration. The findings from this study suggest that recalibration
may not be necessary when the goggles and/or infrared camera have
been repositioned.
The main parameter that influences VNG calibration is the distance of the camera from the participant’s eye. VNG goggles are
designed to keep the distance between the camera and eye constant.
Therefore, a possible explanation to why the initial calibration was
maintained despite moving or repositioning the goggles and/or infrared camera is due to the camera to eye distance remaining constant.
This results in the initial calibration being maintained. The findings
reported from this study differ to those recommended by the BSA
(2010) caloric irrigation recommended procedure who state that the
VNG system should be recalibrated if the goggles and/or camera are
repositioned during testing. However, the BSA (2010) recommended
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procedures do not provide any reasoning to justify why recalibration
is required if the goggles/infrared camera were to be repositioned
during testing. Barin (2006) reports that recalibration is required
only when the goggles or the camera are moved in respect to the
patient’s eyes due to a change in the camera to eye distance. Therefore, as camera to eye distance will not change significantly to have
an effect the initial calibration due to the design of the VNG goggles,
the findings from the study support that repositioning the goggles
and/or camera during testing does not require recalibration.
The comparison between the eye deviation results obtained from
performing geometric calibration (Experiment 2) and the baseline
measurements obtained by conducting the system’s horizontal saccade test (to record the actual eye deviations of a known angle of
60°) revealed a significant difference (p ⬍ 0.01). This is supported
by Andrew and Meredith (2009) who concluded in their study using
the ChartR VNG system that the default calibration did not provide a
good approximation of the true eye deviation. During the geometric
calibration, the true eye deviation is estimated by measuring the iris
width in comparison to manufacturer data to calculate the degree of
eye movement from the centre of the pupil. This method of calibration will have limited accuracy as it is dependent on the precision of
measuring the iris width and the comparison to the manufacturer’s
normative data. This study found that the geometric calibration of
the Synapsys VNG Ulmer system underestimated the true eye deviation by a mean of 13% (7.2°). This may have implications during
clinical testing as it will increase the likelihood of not detecting a
significant nystagmus because the slow-phase velocity will be underestimated. This may affect diagnosis of nystagmus due to peripheral
or central causes in gaze testing, positional testing, and caloric testing. An example of when this may affect patient diagnosis is bilateral
vestibular hypofunction as the calibration error will underestimate
the nystagmus. Also, the underestimation of eye deviation through
geometric calibration could result in the underestimation of overshoots or undershoots in ocular motor testing.
Although the underestimation of eye movements in caloric irrigation results could underestimate overall vestibular function, this
calibration error should not impact upon the caloric canal paresis
and directional preponderance results. This is because both these
calculations take into account the peak nystagmus from each caloric
irrigation. Therefore each peak response would be underestimated
by the same amount, so the percentage difference between each
side (in canal paresis) and the direction of nystagmus (in directional
preponderance) would not be affected. Interestingly, Andrew and
Meredith reported an overestimation of eye deviations by 37% using
the default calibration mode on the ChartR VNG system.
It should also be noted that during result analysis, we became
aware that the mean baseline eye deviation recording was 56.4°
(1.4). This is lower than the expected eye deviation of 60°. The
equipment had initially been set up using trigonometry calculations
and departmental normative data to examine horizontal gaze testing
at eye deviations 30° right and ⫺ 30° left. Although this finding
should have limited impact upon the result documented as the comparisons were made against baseline recordings and not the expected
eye deviation of 60°, it may have implications for departmental testing. This is a known issue with the Synapsys VNG equipment as
it does not measure the distance between the patient and the target,
thus highlighting the importance of collecting sufficient departmental normative data when setting up VNG equipment. A possible way
to overcome this problem would be to measure the exact distance
between the light target and patient’s head for each patient to ensure
accurate recordings at ⫾ 30°.
Conclusion
Based upon these results, recalibration may not be required if the
VNG goggles and/or infrared camera are removed or repositioned
during testing. However, due to the individual variability noted in the
results, it may be advisable to adhere to the recommendations stated
by the BSA (2010) recommended procedures regarding recalibration
during VNG testing.
Caution needs to be exercised when using the geometric calibration within the Synapsys VNG Ulmer equipment and should be used
only if the patient is unable to perform the system’s recommended
saccade calibration test.
Acknowledgements
The authors would like to acknowledge Guy Lightfoot, Royal
Liverpool Hospital, and Fiona Barker, Windsor Audiology Centre
for their input and guidance. The authors would also like to thank
Sally Mora and Rhys Meredith at Singleton Hospital, Swansea for
their support during the study design.
Declaration of interest: No outside funding or grants in support
of this research were received. The authors report no conflicts
of interest. The authors alone are responsible for the content and
writing of this paper.
References
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Investigation into the verification of the Synapsys

Transcription

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