Autobiographical Memory Increases Pupil Dilation

Abstract

Background
Pupil activity has been widely considered as a “summed index” of physiological activities during cognitive processing.

Methodology
We investigated pupil dilation during retrieval of autobiographical memory and compared pupil diameter with a control condition in which participants had to count aloud. We also measured pupil diameters retrieval of free (i.e., first memory that comes to mind), positive, and negative memories (memories associated, respectively, with the words “happy” and “sad”).

Results
Analyses demonstrated larger pupil diameters during the free, positive, and negative autobiographical memory retrieval than during the control task. Analyses also demonstrated no significant differences in pupil diameters across the three autobiographical memory conditions.

Conclusion
These outcomes demonstrate that, compared with counting, autobiographical retrieval results in a larger pupil size. However, the emotional valence of memories yields non-significant effect on pupil diameters. Our findings demonstrate how autobiographical memory retrieval yields pupil dilation.

Full text

280
Translational Neuroscience
Research Article • DOI: 10.1515/tnsci-2019-0044 • Translational Neuroscience • 10 • 2019 • 280-287
Autobiographical memory
increases pupil dilation
This paper investigates whether retrieval
of autobiographical memory (i.e., memory
regarding personal information) increases pupil
dilation. Given this objective, we briey the
describe physiological bases of pupil dilation.
We then describe research demonstrating how
memory can modulate pupil dilation. As we
will highlight, there is a large body of research
demonstrating how memory can shape pupil
dilation, but there is a lack of studies on pupil
dilation during the retrieval of autobiographical
memory.
To begin with the physiological basis of
pupil dilation, the pupil is the opening area of
the iris that allows light to enter the eye and
reach the retina. The pupil is controlled by two
sets of smooth muscles in the iris, namely the
sphincter muscles and dilator muscles [1, 2].
Whereas the sphincter muscles decrease the
diameter of pupil, the dilator muscles increase
it. These muscles serve to optimize vision by
modulating the amount of light that reaches
the retina; whereas the pupil dilates in darker
conditions, it constricts in brighter conditions.
Pupil diameter typically varies from 1.5 to nine
mm; pupil reacts to stimulation in about 200
ms and, in standard light conditions, pupil
diameter is about three mm [3].
Pupil dilation is mediated by simultaneous
activation of the sympathetic system and
inhibition of the parasympathetic system,
more precisely, cortical inhibition of the
parasympathetic oculomotor nucleus [4]. Pupil
dilation has been shown to be mediated by
activity of the locus coeruleus–noradrenaline
system [5], which plays an important role
in cognitive processes [6]. More precisely,
pupil dilation is mediated by activation of
neurons in the locus coeruleus which supply
noradrenaline to the eyes and brain; in the
eye, the noradrenaline regulates pupil dilation,
and, in the brain, it regulates attention [7,
8]. Pupil dilation is not solely mediated by
neurophysiological processes but also by
cognitive and emotional processes.
In a seminal work, Hess and Polt [9]
reported that pupil was larger in response to
positive images and smaller in response to
negative images. Subsequent research has
demonstrated that pupil typically dilates when
participants are in conditions of increased
emotion [10, 11] or attention [12]. This dilation
has been attributed to the cognitive load of the
task [13]. The eect of cognitive load on pupil
dilation has been also observed on memory
tests. Kahneman and Beatty [13] reported
increased pupil dilation in response to the
increased diculty of a working memory task.
Similar ndings were reported by subsequent
studies using the span tasks, on which
participants are typically invited to repeat a
string of numbers. The studies reported that
pupil dilation increases with each digit retained
in digit span tasks until the length of the digits
exceeds the capacity of working memory, at
which pupil diameter begins to plateau or even
diminish [14-18].
Pupil dilation has been also studied in
recognition memory [19, 20]. In recognition
memory, participants typically make old/
new judgments on previously studied and
new information. Using these procedures,
Gardner, Philp [21] reported increased pupil
AUTOBIOGRAPHICAL MEMORY
INCREASES PUPIL DILATION
1
Nantes Université, Univ Angers, Laboratoire de
Psychologie des Pays de la Loire (LPPL - EA 4638),
F-44000 Nantes, France
2
Unité de Gériatrie, Centre Hospitalier de
Tourcoing, Tourcoing, France
3 Institut Universitaire de France, Paris, France
4
The University of Nottingham–Malaysia Campus,
Semenyih, Malaysia
5
Univ. Lille, CNRS, CHU Lille, UMR 9193 - SCALab -
Sciences Cognitives et
Sciences Aectives, F-59000 Lille, France
Mohamad El Haj1,2,3*
Steve M. J. Janssen4
Karim Gallouj2
Quentin Lenoble5
Abstract
Background: Pupil activity has been widely considered as a “summed index” of physiological activities during
cognitive processing.
Methodology: We investigated pupil dilation during retrieval of autobiographical memory and compared pupil
diameter with a control condition in which participants had to count aloud. We also measured pupil diameters
retrieval of free (i.e., rst memory that comes to mind), positive, and negative memories (memories associated,
respectively, with the words “happy” and “sad”).
Results: Analyses demonstrated larger pupil diameters during the free, positive, and negative autobiographical
memory retrieval than during the control task. Analyses also demonstrated no signicant dierences in pupil
diameters across the three autobiographical memory conditions.
Conclusion: These outcomes demonstrate that, compared with counting, autobiographical retrieval results in a
larger pupil size. However, the emotional valence of memories yields non-signicant eect on pupil diameters.
Our ndings demonstrate how autobiographical memory retrieval yields pupil dilation.
Received 22 July 2019
accepted 17 September 2019
Keywords
• autobiographical memory • pupil • pupil dilation • pupillometry
* E-mail: mohamad.elhaj@univ-nantes.fr
© 2019 Mohamad EL Haj et al. published by De Gruyter.
This work is licensed under the Creative Commons Attribution 4.0 Public License.

281
dilation when participants processed old
information during recognition memory tasks.
They suggested that pupil dilation mirrors
mental eort related to encoding and retrieval
of information from memory rather than the
general level of mental eort, as proposed by
the cognitive load theory of Kahneman and
Beatty [13]. In a similar vein, Vo, Jacobs [22]
attributed pupil dilation in recognition memory
tasks to the cognitive demands of recognizing
old information compared with rejecting
new information. According to Vo, Jacobs
[22], recognition of old information requires
retrieval of qualitative contextual information
about the encoding episode (e.g., when and
where the information was encoded), whereas
correct rejection does not require this eortful
retrieval. Also, Otero, Weekes [23] suggested
that pupil dilation for old information depends
on the strength of memory traces upon
which recognition judgment is made. These
suggestions can explain why, in recognition
memory, pupil dilates more for information
judged as old versus information judged as
new [20, 24-26].
The above-mentioned research suggests
that, on recognition memory tasks, pupil dilates
more when participants process old stimuli
compared with new stimuli. Inspired by this
research, studies consider pupil dilation as an
indicator of encoding and retrieval of long-term
memory as pupil dilation predicts the strength
of subsequent memory [27] and discriminating
between familiar and recollected information
[28]. The latter discrimination was evaluated
by Kafkas and Montaldi [28] who reported a
linear eect in which pupil dilation increased
linearly from new to familiar and recollected
information, with recollection producing the
highest levels of pupil dilation and novelty the
lowest, with familiarity falling somewhere in
between.
Pupil dilation, as observed during
recognition memory tasks, can be modulated
by expectations; an assumption proposed by
Mill, O’Connor [29] who reported increased
pupil dilation during recognition of unexpected
information compared to expected information.
The authors suggested that dilation responses
during memory recognition are mediated by
expectations; more precisely, they suggested
that expected information yields an acontextual
sense of recollection whereas unexpected
information yields a recollection of contextual
information, and consequently, pupil dilation.
This suggestion can be supported by the
dual processing model according to which
recollection-based decisions, compared to
familiarity-based memory decisions, yields
retrieval of contextual information about the
encoding episode [30].
Together, there is a substantial body of
research on the eects of memory on pupil
dilation. This research reported the increased
pupil dilation in response to increased diculty
of memory processing [13-18]. This research
also reported that, on recognition memory
tasks, pupil dilates more when participants
process old stimuli compared to new stimuli
[20, 24-26].
The Present Study
Although the previous research is useful
in understanding the eects of memory
processing on pupil dilation, there is, to the
very best of our knowledge, a lack of research
on pupil dilation during autobiographical
memory retrieval. This issue is important
because the study of autobiographical memory
is concerned with how people remember
personal events. Autobiographical memory
allows the recall of events that are relevant to
one’s identity and sense of self [31, 32].
To this aim, we compared pupil dilation
during autobiographical memory retrieval
and during a control task in which participants
had to count aloud. We also investigated
pupil dilation following the emotional
valence of memories for two reasons. First,
autobiographical memory has been intimately
associated with emotion [31]. Second, pupil
dilation has been found to be sensitive to
emotion [9-11]. Therefore, pupil dilation can be
inuenced by the emotional tone of memory.
As for hypotheses, we expected a larger pupil
diameter during autobiographical memory
retrieval than during the control task. We also
expected the large pupil diameter during
retrieval of emotional memories than during
the retrieval of free memories.
Method
Participants
The study included 36 graduate/undergraduate
students from the University of Nantes (19
females, M age = 24.21 years, SD = 6.14, M
education = 14.61 years, SD = 4.74). Participants
were native French speakers. Among the
original sample (N = 44), pupil data of two
participants were corrupted, three participants
were excluded owing to signal loss during
recording, and three participants were excluded
owing to previous psychiatric or neurological
disorders. This nal sample size was determined
a priori using G*Power [33]. The calculation was
conducted for repeated measures (four within-
subjects measurements) ANOVA tests, based on
95% power, an estimated probability of making
Type I error as .05, and a medium eect size of
0.25 [34]. In the nal sample size, no signicant
dierences were observed regarding gender [X2
(1, N = 36) = .87, p = .50].
Informed consent: Informed consent has been
obtained from all individuals included in this
study.
Ethical approval: The research related to
human use has been complied with all the
relevant national regulations, institutional
policies and in accordance the tenets of the
Helsinki Declaration, and has been approved
by the authors’ institutional review board or
equivalent committee.
Procedures and Materials
Generally speaking, procedures consisted of
four conditions (i.e., free autobiographical
recall, positive autobiographical recall, and
negative autobiographical recall, as well as
counting as a control condition). During these
conditions, the participants wore eye-tracking
glasses and faced a white wall (see Figure 1).
Participants were tested individually and were
informed that the experiment was concerned
with memory. However, in order not to
inuence their performance, the participants
were not provided with further details about
autobiographical memory or pupil dilation.
In the autobiographical conditions,
participants were invited to verbally generate
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282
three autobiographical events. Prior to each
autobiographical event, participants were
instructed to remember in detail an event
related to the cue. They were also instructed
that the event had to be personally experienced
in the past and that the description had to be
precise and specic (e.g., when and where the
event occurred, what they were doing during
it, who was present, what their feelings were).
The participants were invited to retrieve one
free memory, one positive memory, and one
negative memory. For free memory, they were
instructed to verbally describe the rst memory
that comes to mind. For the positive memory,
they were instructed to verbally describe a
memory associated with the word “happy”. For
the negative memory, they were instructed to
verbally describe a memory associated with
the word “sad”. One minute was allocated to
generate each autobiographical memory,
and the duration was made clear beforehand
so that participants could structure their
memories accordingly.
On the control condition, participants were
invited to count aloud to count from 1 “in their
own time” until the experimenter said “Stop”.
The latter signal was provided after one minute
of counting. This control condition was chosen,
because, like autobiographical memory, it relies
on verbal behaviour, therefore, any potential
dierences in pupil dilation between the two
conditions would not be the result of verbal
behaviour.
Autobiographical memory retrieval and
counting occurred while pupil dilation was
recorded. Recording was stopped directly after
memory retrieval and counting. Participants
wore eye-tracking glasses. These glasses (Pupil
Lab) are a remote pupil-tracking system that
uses infrared illumination with a gaze position
accuracy of < 0.1° and a 200 Hz sampling
rate. Recording was processed with the Pupil
Capture software. Prior to each condition (i.e.,
free, positive, and negative memories, as well as
for counting) calibration was made by inviting
participants to xate on a black cross (a 5 x 5
cm cross, printed on an A4 white paper xated
at the wall center) and the cross was dened
as a calibration reference, needless to say, that
the cross was withdrawn after calibration. The
experiment occurred in a quiet room at the
psychology department of the University of
Nantes. Blinds were closed and the lightness
of the room (60-watt uorescent lamp) was
the same in the two conditions to ensure that
dierences in pupil dilation were not caused by
dierences in retinal illumination. Participants
were seated in front of a white wall and the
distance between the subjects and wall was
approximately 30 to 50 cm. Participants were
invited not to look outside the wall but were
free to explore all parts of it. The wall displayed
no visual stimuli (e.g., drawings, windows). We
dened pupil dilation as the average dilation
during each trial; this interval was chosen
to encompass the full retrieval of memories,
as well as the full counting in the control
condition.
Results
We compared pupil dilation across counting
and the three autobiographical trials (i.e., the
“free”, “positive”, and “negative” memories) with
repeated measures ANOVA, followed up by
t-tests pair-wise comparisons. For t-tests, we
provided eect sizes by using Cohen’s d [34]:
0.20 = small, 0.50 = medium, 0.80 = large. For
all tests, the level of signicance was set as p ≤
0.05.
The pupil diameter data is provided
in Figure 2. Analysis showed signicant
dierences between pupil diameters across
the four trials (i.e., counting, “free”, “positive”,
and “negative” memories), F(3, 105) = 6.14,
p = .001, η2 = .15. This eect was solely caused
Figure 1. During the four conditions (i.e., free autobiographical recall, positive autobiographical recall, and nega-
tive autobiographical recall, as well as counting as a control condition), participants wore eye-tracking glasses
and faced a white wall
Figure 2. Means of pupil diameters during counting and during retrieval of “free” (i.e., rst memory that comes
to mind), and “positive” and “negative” memories (memories associated, respectively, with the words “happy” and
“sad”). Error bars are 95% within-subjects condence intervals.
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283
by the dierence between the counting
and the three autobiographical retrieval
conditions. Wilcoxon tests showed larger
pupil diameter during free autobiographical
retrieval than during counting [t(35) = 2.76, p
= .009, Cohen’s d = .56], larger pupil diameter
during positive autobiographical retrieval than
during counting [t(35) = 4.31, p < .001, Cohen’s
d = .66], and larger pupil diameter during
negative autobiographical retrieval than
during counting [t(35) = 3.52, p = .001, Cohen’s
d = .59]. However, no signicant dierences in
pupil diameter were observed between free
and positive autobiographical retrieval [t(35) =
.76, p > .10, Cohen’s d = .17], between free and
negative autobiographical retrieval [t(35) = .89,
p > .10, Cohen’s d = .18], and between positive
and negative autobiographical retrieval ([t(35)
= .35, p > .10, Cohen’s d = .09].
Discussion
We compared pupil diameter during the
retrieval of three autobiographical memories
and a control task, in which participants had
to count. Analyses demonstrated larger pupil
diameters during free, positive, and negative
autobiographical memory retrieval than during
the control task. Analyses also demonstrated
no signicant dierences in pupil diameter
across the three autobiographical memory
conditions (i.e., free, positive, and negative).
These outcomes demonstrate that, compared
to counting, autobiographical memory
retrieval results in larger pupil size. However,
the emotional valence of the recalled memories
yields no signicant eect on pupil diameter.
The main nding is the larger pupil diameter
during autobiographical memory retrieval. This
study is the rst one to demonstrate that the
retrieval of personal information yields dilation
of the pupils. More specically, it demonstrates
that pupil dilation, as observed in the literature
on the memory of general information (e.g.,
memory for words), can also be extended to
self-related information (i.e., autobiographical
memory).
Eects of Memory on Pupil Dilation
To begin with the contribution of our study
to the literature on the eects of memory
on pupil dilation (and as mentioned in the
introduction), there is a large body of research
reporting increased pupil dilation in response
to increased diculty of memory processing
[13-18]. This research also reported that,
on recognition memory tasks, pupil dilates
more when participants process old stimuli
compared to new stimuli [20, 24-26]. This body
of literature is, however, mainly concerned by
recognition memory and working memory.
Research on pupil dilation and working
memory has been mainly interested in how
updating in working memory yields pupillary
responses. In this research, updating has been
dened as the ability to integrate incoming
information with information that is currently
held in working memory [35]. Using span tasks,
research has shown increased pupil dilation
with the increased number of to-be-tracked
information [17, 18, 36]. Similar ndings are
reported by research using the n-back task. This
working memory task requires participants
to indicate whether the currently presented
information is the same as information
presented n trials back [35]. Using this task,
studies have demonstrated that pupil dilation
increases with increasing n [37-39]. This line of
research has attributed the pupil dilation to the
cognitive load of the tasks.
Interestingly, Hogervorst, Brouwer, and van
Erp (2014) have shown that pupil dilation can
be sucient to distinguish high cognitive load
from the low cognitive load. When compared
to other physiological measures, such as
electroencephalography, respiration, skin
conductance, or cardiac rhythms, pupil dilation
discriminates high and low cognitive load with
80% accuracy [40]. These ndings demonstrate
that pupil dilation is a reliable index of cognitive
load associated with memory.
Pupil dilation, as observed in our study, may
be attributed to the cognitive load associated
with autobiographical memory retrieval.
This attribution is supported by the above-
mentioned research, suggesting how pupil
dilation mirrors the cognitive load of working
memory tasks [40]. Our attribution is also
supported by the Self Memory System [31],
according to which autobiographical memory
retrieval requires a controlled reconstruction.
This controlled reconstruction is required to
retrieve not only the targeted information but
also the context in which this information was
encoded [30, 41-43]. In our view, pupil dilation
during autobiographical memory, compared
to counting, may be attributed to the general
cognitive load of memory retrieval and, more
specically, to the cognitive eort required to
construct the context in which the information
was previously encoded [22, 29]. Alternatively,
verbalizations were perhaps more complex
in the autobiographical memory conditions
than in the counting condition and this
complexity might, therefore, rather than the
autobiographical memory retrieval, drove the
pupil dilation.
To summarize, there is a large body of
literature on pupil dilation during memory
a recognition and working memory tasks.
Our ndings contribute to this literature by
demonstrating how the retrieval of self-related
information (i.e., autobiographical memory)
yields pupil activation as well. This activation
may be attributed to the cognitive eort as
required to reconstruct the memories and,
more specically, to retrieve the contextual
information about the encoding episode.
Pupil Dilation as Physiological
Measure
Our ndings demonstrate how pupil dilation
can be used as a physiological evaluation
of this retrieval. Physiological correlates of
autobiographical memory have been mainly
evaluated with regard to brain activity. In
this research, autobiographical memory has
been associated with a “core network” of brain
areas, including the hippocampus, medial
and ventrolateral prefrontal cortex, posterior
cingulate, and temporoparietal junction [44,
45].
Unlike the large body of research on brain
activity during autobiographical memory
retrieval, little research has attempted to
evaluate cardiovascular or electrodermal
activities during this retrieval. The latter
research has demonstrated signicant
variations in cardiovascular and electrodermal
activity during autobiographical memory
retrieval [46]. In a similar vein, Robertson,
Swickert [47] reported variations in blood
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284
pressure during autobiographical memory
retrieval. Considering aging, Labouvie-Vief,
Lumley [48] reported a lower heart rate in older
adults than in younger participants during
autobiographical memory retrieval.
Another physiological evaluation of
autobiographical memory is facial expressions.
Research has demonstrated variations of facial
expressions during autobiographical memory
retrieval [49-51]. This research has suggested
that emotional facial expressions may reect
the physiological states that were experienced
in the encoded events [52].
Autobiographical memory has not been
only associated with the brain, cardiovascular,
electrodermal, and facial expressions activities,
but also with eye movements [53]. Eye
movements during autobiographical memory
retrieval were evaluated in a study in which
participants had to retrieve autobiographical
memories and, as a control condition, count
aloud [54]. Results demonstrated a lower
number of xations but a higher number, larger
amplitude, and longer duration of saccades
in the autobiographical condition than in the
control condition. Eye movement activity was
attributed to the attempt of the visual system to
create and manipulate mental representations
of the memories [54]. A similar suggestion was
made in a study comparing eye movements
during retrieval of neutral memories and
emotional memories [55]. Another study
reported that autobiographical memory
retrieval and future episodic thinking were
accompanied by similar durations of xations
and saccades, as well as similar amplitudes of
saccades [56].
Together, physiological correlates of
autobiographical memory have been
evaluated with regard to facial expressions
and brain, cardiovascular, electrodermal,
and eye movement activity. Our study
contributes to this literature by demonstrating
that autobiographical memory can also be
evaluated with pupil dilation.
Eects of Emotion on Pupil Dilation
Although we have found that pupil diameter
was larger during autobiographical memory
retrieval than during the control task,
our ndings demonstrated no signicant
dierences between pupil dilations across
the retrieval of freely recalled, positive, and
negative memories. This nding seems to
contradict a body of research demonstrating
how pupil dilation is dependent on emotion.
Research has demonstrated that pupils tend
to dilate when participants view positive or
negative images relative to neutral images
[10]. This dilation has been replicated in many
studies in response to emotional images [57-
59], emotional video-clips [60], as well as facial
expressions [61, 62].
To resolve the apparent contradiction
between our ndings and the previous
literature on the pupil responsivity to
emotion, it would be of interest to highlight
dierences between our study and the studies
in the literature. Our study is concerned with
autobiographical memory, a memory system
that has been intimately associated with
emotion [31]. Except psychopathological
conditions [63], autobiographical memory
retrieval is emotionally loaded, the free retrieval,
therefore, might activate emotional material in
our participants. In other words, when asked to
describe the memory that comes to mind rst,
as was the case in the free autobiographical
condition, participants might have retrieved
emotional memories. This interpretation can
be supported by a previous study that reported
that the majority of autobiographical memories
cued by neutral words were emotional [51].
Because autobiographical memory typically
triggers emotional material [31, 63], the lack
of signicant dierences of pupil dilation
between the free autobiographical memory
retrieval compared with the positive and
negative retrieval can be attributed to the
assumption that all the three conditions might
triggeremotional material.
In our view, similar pupil dilation for freely
recalled, positive and negative memories may
be attributed to the fact that our study, and
that of Schaefer and Philippot [46] and Marci,
Glick [64], included subjects without aective
disorders. These subjects may succeed to
regulate the emotional load of memories. It
would be of interest therefore to replicate
our procedures with a clinical population. For
instance, because patients with depression
have demonstrated systematic bias that
favours negative memories [65, 66], it would be
of interest to assess pupil dilation for negative
memories in these patients.
Venues for Future Research
As previously mentioned, one venue for future
research would be to assess pupil dilation
during autobiographical memory retrieval in
pathological populations. The assessment of
pupil dilation would be especially valuable
in amnesia. For instance, patients with
Alzheimer’s disease have severe diculties to
retrieve autobiographical memories [67-69]
and the same thing can be said for patients
with Korsako’s syndrome [70-74]. It would be
of interest to investigate whether pupil dilation
would be observed despite autobiographical
memory retrieval in these patients.
Also, both Alzheimer’s disease and
Korsako’s syndrome are characterized by
false memories [75-86]. It would be of interest
to evaluate whether these memories would
activate pupil dilations because research
has demonstrated pupil dilation for false
memories in normal populations. For instance,
Montenese, Ambrosini [87] reported higher
pupil dilation for false alarms (i.e., items
erroneously judged as old) than misses (i.e.,
items erroneously judged as new).
We would also like to emphasize that pupil
dilation, as observed in our study, reects the
general physiological characteristics of this
dilation. As mentioned in the introduction,
pupil diameter typically varies from 1.5 to nine
mm, and, in standard light conditions, pupil
diameter is about 3 mm [3]. Also, the dierence
dierences on pupil dilation between the
autobiographical and control conditions
mirrors that observed in research on pupil
dilation and cognition; pupil typically dilates
around 0.5 mm to cognitive stimulation [2].
Conclusions
Pupil activity has been considered as a
“summed index” of brain activity during
cognitive processing. Cognitively relevant
pupil activity typically occurs following
inhibition of the parasympathetic nervous
system as controlled by the locus coeruleus–
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285
norepinephrine system, which plays a
key role in the regulation of cognition. By
demonstrating how autobiographical memory
can inuence pupil activity, our study shows
how pupillometry can be used as a measure
of physiological responses to retrieval of self-
related information.
Acknowledgments
The authors would like to thank Donia Ahmed
(in the Figure 1) who assisted in testing.
Conict of interest: Authors state no conict of
interest
Funding
MEH was supported by the Intstitut Universitaire
de France and the EU Interreg 2 Seas Programme
2014-2020 (co-funded by the European Regional
Development Fund). The funder has no other
role in the study design, data collection and
analysis or decision to publish.
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