Heat exposure following encoding can interfere with subsequent recognition memory

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Hea exposu e ollowing encoding
can in e e e wi h subsequen
ecogni ion memo y
Jesús Cudei o
1, Da id So o
2,3* & Emilio Gu ié ez
1,4*
Co ela ional s udies sugges ha high empe a u es may impai online cogni i e pe o mance
and lea ning p ocesses. He e, we es ed he hypo hesis ha hea exposu e blocks o line memo y
consolida ion. We epo wo s udies, including a p e- egis e ed eplica ion. Fi s , du ing a s udy
phase, pa icipan s we e amilia ized wi h neu al and nega i ely alenced images. One day la e ,
hal o he pa icipan s we e exposed o high empe a u es in a sauna session a 50 °C. Recogni ion
memo y was es ed 24 h la e . We ound ha pa icipan s exposed o high empe a u e showed an
impai men in ecogni ion memo y pe o mance, ela i e o a con ol g oup o pa icipan s ha
we e no exposed o hea o ha had a sauna a 28 °C. This occu ed o bo h emo ional and neu al
i ems. These esul s indica e ha hea exposu e impai s memo y consolida ion, he eby opening he
possibili y o using hea exposu e as an agen o he ea men o clinical men al diso de s.
In ecen yea s, se e al obse a ional s udies ha e shown ha high empe a u es (hea s ess) can a ec wo k and
school p oduc i i y1–3. Pa k e al., showed ha hea exposu e (> 26.7°C) du ing he lea ning pe iod o s uden s
could cause pe sis en dis up ions o he lea ning p ocess3. Howe e , empi ical esea ch in his subjec is e y
limi ed. In animal models, he e is some e idence ha hea may impai memo y consolida ion, bu his is limi ed
o ish species, wo ms and lies4–6. The e a e no expe imen al s udies in humans o da e ha di ec ly e alua e he
e ec o hea ollowing s udy on la e , o line memo y pe o mance.
Ou main goal he e was o add ess whe he consolida ed memo ies can be dis up ed by hea exposu e. To
do his, we used a a ian o he h ee days pa adigm om memo y econsolida ion s udies7–9. Pa icipan s i s
s udied a se ies o emo ional and neu al images. Then, 24h la e , he expe imen al pa icipan s a e exposed o
hea , while he con ol pa icipan s a e exposed o a simila en i onmen wi hou hea . To p oduce hea s ess
- he condi ion in which he human body canno id o excess hea and he co e empe a u e ise- we used a a -
in a ed sauna. This ype o sauna allows con ol o empe a u e apidly (in a ew minu es empe a u e can ise
o 60°C) and hea pene a es deeply in he body, p o oking hea s ess in less han 10 minu es10. Finally, 24h
la e , pa icipan s we e equi ed o pe o m a ecogni ion ask (see Fig.1). Ini ially, we conduc ed a pilo s udy
assessing he e ec o hea exposu e and i s in e ac ion wi h memo y eac i a ion on subsequen ecogni ion
pe o mance. E idence om animal models and humans indica es ha memo y is highly dynamic11–13. When
a memo y is e ie ed, i becomes labile o a pe iod o ime be o e being e-s abilized again h ough a econ-
solida ion p ocess14. Unde speci ic condi ions, e en consolida ed memo ies can be dis up ed. Animal s udies
showed ha econsolida ion can happen when wo condi ions a e p esen : (1) e ie al o he consolida ed
memo y, (2) blockade o p o ein syn hesis du ing o sho ly a e memo y e ie al14,15. This disco e y is used o
manipula e maladap i e ea memo ies in pa ien s wi h anxie y diso de s16. In his pilo s udy, di e en g oups
o pa icipan s we e equi ed (o no ) o eac i a e ( e ie e) he images ha had been s udied in he p e ious
day while hey we e exposed (o no ) o hea du ing he sauna ba h. We did no obse e any e ec o eac i a ion
on subsequen ecogni ion pe o mance in he pilo s udy bu he e was an e ec o hea exposu e, impai ing
on subsequen ecogni ion pe o mance ela i e o he con ol g oup. Then, in a p e- egis e ed eplica ion (p e-
egis a ion a ailable a h ps:// os . io/ 3bjg/), we sough o e i y he hypo hesis ha hea exposu e can impai
he consolida ed memo y ace o p e iously s udied i ems.
OPEN
1Depa men o Clinical Psychology and Psychobiology, Facul y o Psychology, Uni e si y o San iago de
Compos ela, San iago de Compos ela, Spain. 2Basque Cen e On Cogni ion, B ain and Language, San
Sebas ian, Spain. 3Ike basque Founda ion o Science, Bilbao, Spain. 4Unidade de Ven es Clínicos, Facul y o
Psychology, Uni e si y o San iago de Compos ela, San iago de Compos ela, Spain. *email: [email p o ec ed];
[email p o ec ed]
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Expe imen 1: a pilo s udy
Due o he lack o expe imen al e idence on he ole o hea exposu e on he malleabili y o consolida ed memo-
ies and subsequen ecogni ion pe o mance3, Expe imen 1 was an explo a o y s udy he ollowing aims (i)
o in es iga e whe he hea exposu e a ec s he memo y ace o images ha had been p e iously lea ned; (ii)
assess whe he any e ec o hea exposu e in e ac s wi h he emo ional alence o he s udy i ems and (ii) o
es whe he he e ec o hea in e ac s wi h he equi emen o eac i a e he memo y du ing he sauna. We
compa ed 4 expe imen al g oups: (1) Sauna—Reac i a ion condi ion, (2) Sauna—No eac i a ion condi ion and
(3) No sauna—Reac i a ion and (4) No sauna—No eac i a ion.
Me hods. Pa icipan s. 70 pa icipan s (39 emales, 31 males; age ange = 19–29yea s) we e ec ui ed om
Uni e si y o San iago de Compos ela ia ad e isemen s in he Facul y o Psychology. They we e andomly
assigned o one o he 4 expe imen al g oups: Sauna—Reac i a ion (n = 19), Sauna—No eac i a ion (n = 17), No
sauna—Reac i a ion (n = 18), No sauna—No eac i a ion (n = 16). All pa icipan s p o ided w i en in o med
consen p io o es ing. The consen included in o ma ion abou he dis essing na u e o pic u es, ecommen-
da ions, and isks abou he use o sauna (h ps:// sauna. i/ en/) and a eminde ha hey could end he expe i-
men a any ime wi hou any ype o penaliza ion. The s udy con o med o he Decla a ion o Helsinki. Pa ici-
pan s p o ided in o med consen p io o hei pa icipa ion. E hical app o al was ob ained om he Comi é de
Bioé ica o he Uni e si y o San iago de Compos ela.
Tasks and p ocedu e. S imuli. We used images aken om he In e na ional A ec i e Pic u e Sys em (IAPS)17.
We selec ed 100 pic u es—50 neu al and 50 unpleasan —based on s anda dized sco es o emo ional a ousal
and alence (see Supplemen a y Ma e ial). Addi ionally, 5 mo e pic u es we e selec ed o use as examples o
p ac icing he S udy and Recogni ion phases. To a oid pe o mance biases due o he images, hese we e subdi-
ided in o wo se s (i.e., A and B), each consis ing o 25 neu al and 25 unpleasan pic u es. We pe o med pai ed
es s o compa e he means o emo ional alence be ween neu al and emo ional images o each Se . The e we e
signi ican di e ences in se A [M neu al = 5.141, SD = 0.406; M emo ional = 2.067, SD = 0.593 [ (24) = 19.943,
p < 0.001, d = 3.99)] and in se B [M neu al = 4.990, SD = 0.609; M emo ional = 2.087, SD = 0.526 [ (24) = 19.331,
p < 0.001, d = 3.87]. Fu he mo e, we pe o med a 2 × 2 ANOVA wi h Image Se (A, B) × Valence (neu al, emo-
ional) as ac o s, and ound a signi ican e ec o Valence [F(1,48) = 771.43, p < 0.001, ηp2 = 0.88] bu no e ec
o Se s [F(1,48) = 0.367, p = 0.547] no in e ac ions be ween ac o s [F(1,48) = 0.633, p = 0.43]. Hal o he pa -
icipan s we e p esen ed wi h se A du ing s udy and hen p esen ed wi h se A (old) and B (new) du ing he
ecogni ion ask. The o he hal o he pa icipan s we e p esen ed wi h se B in he s udy phase and he se A
we e he new i ems in he es phase. Expe imen al asks we e p og ammed wi h Psychopy18.
S a e- ai anxie y in en o y (STAI)19. The STAI is a 40-i em sel - epo measu e designed o e alua e wo
independen concep s o anxie y: s a e anxie y (emo ional and ansi o y condi ion) and ai anxie y ( ela i ely
s able anxious p opensi y). S a e anxie y was measu ed in o de o es o possible di e ences ac oss he di e en
g oups. The STAI-S consis o a 20-i em sel - epo measu e and each i em is a ed on a 0–3-poin scale. Sco es
ange om 0 o 60. We used he Spanish adap a ion made by Vi ella e al.20.
Each pa icipan was andomized o one o he ou expe imen al g oups (sauna— eac i a ion g oup;
sauna—no eac i a ion; no sauna— eac i a ion and no sauna—no eac i a ion). All pa icipan s pe o med
h ee expe imen al sessions (S udy Phase, Sauna Phase and Recogni ion Phase) excep o he no sauna—no
eac i a ion g oup, which only pe o med he S udy and Recogni ion phases. These sessions we e conduc ed in
h ee consecu i e days and always a he same ime o each pa icipan . Bo h he S udy and Recogni ion phase
we e conduc ed in he same oom while he Sauna Phase ook place in a di e en oom equipped wi h a showe .
Figu e1. Gene al p ocedu e. In he S udy phase, pa icipan s lea ned a numbe o neu al and emo ional
images. 24h la e , hey had a 7min sauna session. In Expe imen 1 pa icipan s unde wen he sauna ba h a
50°C o a oom empe a u e (sauna o ) while being asked o eac i a e images om he p e ious day o no .
One g oup did no unde go he sauna session. In Expe imen 2 all pa icipan s had he sauna ba h a 50°C
(N = 30) o a 28°C (N = 30). Twen y- ou hou s la e , all pa icipan s comple ed a ecogni ion memo y ask.
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S udy Phase (Day 1). Pa icipan s p o ided w i en in o med consen , as well as he STAI-S. A e ha , each
pa icipan sa a a iewing dis ance o app oxima ely 70–80cm. They ecei ed he o al and w i en ins uc ion
“In his ask you will ha e o look ca e ully a a se ies o images. Be o e each image he e will be a whi e do in
he cen e o he sc een. Th ee example images a e shown below”. A e he p esen a ion o h ee example pic-
u es, ano he ins uc ion appea ed on he moni o : “when you a e eady p ess space ba ”. Each ial las ed 3s
(1s he ixa ion do , 2s he image). The p esen a ion o he 50 pic u es on he S udy phase las ed 3min. Du ing
he ask, he expe imen e emained in he same oom wi h a compu e connec ed o he pa icipan ´s moni o .
A e he S udy phase pa icipan s a ed how hey el “a his e y momen ” on a isual-analogue scale om
1 (no a all elaxed) o 10 ( e y elaxed). Be o e lea ing he oom, pa icipan s we e scheduled o he Sauna
session 24h la e .
Sauna phase (Day 2). In a ed Sauna (model in aspa TI1). One day a e he S udy phase, he same expe i-
men e ecei ed he pa icipan s in he sauna oom. This had wo compa men s sepa a ed by an opaque ba ie
wi h a doo , enabling e bal communica ion be ween bo h compa men s. The sauna compa men included
a d essing space, a showe and an in a ed sauna. We used his ype o sauna because i allows us o each a
maximum empe a u e o 60°C in a ew minu es and allows us o p og am wi h accu acy he empe a u e o
which he use s a e exposed. Once he e he expe imen e explained o each pa icipan he p o ocol o he
sauna session. This p o ocol included lea ing he clo hes in he d essing oom, aking o eye’s glasses, wa ches,
o jewel y, aking a small owel o si in o he sauna and inally aking a showe a e he sauna ba h o cool o
he body. Pa icipan s we e ad ised o a oid alcohol be o e and a e he sauna session. Sauna g oups ecei ed
a 7min sauna ba h a 50°C which p og essi ely inc eased o 54/55°C. This was ollowed by a showe o lowe
body empe a u e. The pa icipan s in he Sauna—Reac i a ion g oup we e explici ly asked o eac i a e he
memo y o he p e iously s udied i ems by means o he ollowing ins uc ion: “T y o emembe as p ecisely
as possible he images you ha e seen yes e day”. The memo y eac i a ion s a ed a minu e 3 o he sauna ba h
and ended a minu e 7. Pa icipan s in he sauna wi hou eac i a ion g oup ollowed he same p ocedu e, excep
ha hey we e no asked o eac i a e he memo y. The pa icipan s in he No sauna—Reac i a ion g oup en e ed
he sauna cabin d essed and a oom empe a u e (he e he sauna was u ned o ). They we e asked o eac i a e
memo y a minu e 3, and hey did no ake a showe a e he sauna ba h. The pa icipan s in he No sauna—no
eac i a ion g oup did no unde go he Sauna Phase.
Recogni ion phase (Day 3). Pa icipan s pe o med a ecogni ion memo y es in which pa icipan s we e
exposed ac oss ials o he wo se s o old and new images ( he se hey had s udied in Day 1 plus he o he se
o 50 images). In each ial pa icipan s had o decide whe he he image was old o new and also a e he con-
idence in he memo y decision using a scale anging om 1 (guess) o 4 (su e). Pa icipan s we e encou aged
o be as accu a e as possible wi hou ocusing on esponse ime. As in S udy Phase, each pic u e was p esen ed
o 2s a e a whi e ixa ion poin . Subsequen ly, he old/new decision sc een was p esen ed un il he esponse.
The expe imen e was he same in all ou g oups o all h ee days.
Resul s. We pe o med a mixed ANOVA wi h wo be ween subjec ac o s (i.e., sauna and memo y eac i a-
ion) and one wi hin-subjec , epea ed measu es ac o (i.e., emo ional alence o he images) on he p opo ion
o co ec ecogni ion esponses in he memo y es . Recogni ion accu acy was ope a ionalised he e as he sum
o he hi and co ec ejec ion a e in keeping wi h p e ious s udies21–23. Hi s we e de ined as esponding ‘old’
when an old i em was p esen ed in he ecogni ion es . Co ec ejec ions we e de ined as “new” esponses when
a “new” i em was p esen ed) The esul s showed no e ec o eac i a ion on subsequen ecogni ion pe o mance
in his pilo expe imen [F(1,66) = 2.78; p > 0.1], bu he e was a signi ican e ec o he sauna on day 2 on he
subsequen ecogni ion pe o mance on day 3 [F(1,66) = 4.721; p = 0.033; ηp2 = 0.067]. The e we e no signi ican
in e ac ions be ween ac o s, sugges ing ha he e ec was addi i e ac oss bo h neu al and nega i ely alenced
images. Figu e2 illus a es he sauna e ec on ecogni ion memo y as a unc ion o he emo ional alence, pool-
ing ac oss he eac i a ion and he no eac i a ion condi ion o he sauna and o he no sauna g oups. We did
no obse e an e ec o emo ional alence on ecogni ion memo y accu acy (p = 0.962), howe e an e ec o
emo ional alence was obse ed in he hi a e (neu al: M = 0.739, SD = 0.137; nega i e: M = 0.838, SD = 0.109)
[F(1,66) = 39.69; p < 0.001; ηp2 = 0.376], which is in keeping wi h p e ious obse a ions in he li e a u e. We also
obse ed an e ec o emo ional alence on alse ala ms a es [F(1,66) = 40.547; p < 0.001; ηp2 = 0.183]. The e we e
no u he signi ican in e ac ions be ween sauna, eac i a ion and emo ion ac o s on ecogni ion pe o mance
(p > 0.42). Fo desc ip i e s a is ics see Table1.
To con ol o po en ial c i e ion biases in he ecogni ion es , we e-analysed he da a using A′24, a bias- ee
index o memo y sensi i i y compu ed based on he hi and alse ala m a es (A′ = 0.5 + (0.25 * (((HR − FAR) *
(1 + HR − FAR))/(HR * (1 − FAR))). We obse ed a simila pa e n o ecogni ion accu acy, wi h highe A’ on
he no-sauna g oup (A′ = 0.903) compa ed o he sauna g oup (A′ = 0.886), al hough his e ec was ma ginally
signi ican [F(1,66) = 3.318, p = 0.073]. The e ec o memo y eac i a ion was no signi ican [F(1,66) = 2.235,
p = 0.14, ηp2 = 0.033].
Fu he analyses o he mean con idence o ecogni ion decisions did no e eal an e ec o sauna (p = 0.298),
alence (p = 0.486) o in e ac ions be ween ac o s (lowes p alue = 0.089). We only obse ed a main e ec o
memo y eac i a ion [F(1, 66) = 5.133, p = 0.027, ηp2 = 0.072], wi h lowe con idence in he eac i a ion g oup
(mean = 2.902) compa ed o he non- eac i a ion g oup (mean = 3.088). Finally, STAI-S sco es showed no di -
e ences in anxie y s a e ac oss he di e en g oups [F(1,66) = 1.95; p = 0.130].
Discussion. The esul s o Expe imen 1 showed ha ecogni ion o p e iously s udied i ems was impai ed
by he adminis a ion o a sauna one day a e he s udy phase and one day be o e he ecogni ion es . To he
bes o ou knowledge, his is he i s s udy assessing he e ec o hea in memo y e-p ocessing o p e iously
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s udied in o ma ion and p o ides en a i e causal e idence o he iew ha hea exposu e impai s lea ning and
pe o mance3.
Howe e , he esul s di e om p e ious s udies as eac i a ion did no p oduce any e ec on subsequen
memo y pe o mance. The e a e se e al ac o s ha may ha e con ibu ed o his, including he e ec s o hea
exposu e du ing he sauna o he p esence o he expe imen e as a eminding cue. One limi a ion o his s udy
was ha he sauna was u ned o o hose pa icipan s in he no sauna- eac i a ion g oup who also kep on
hei clo hes a he ime o en e ing he sauna and did no showe a e wa ds.
Fu he mo e, li e a u e has shown epea edly ha memo y o emo ional s imuli and e en s is mo e accu a e
han memo y o neu al s imuli and e en s25,26, and we did no obse e his memo y enhancemen o emo ional
s imuli di ec ly, al hough we did obse e i in he hi a e and alse ala m a e. Howe e , his la e e ec could
be media ed by decision biases and he e we we e in e es ed in memo y sensi i i y a he han bias27. Hence, we
a emp ed o eplica e his inding in Expe imen 2 in a mo e con olled se ing in which we manipula ed he
hea le el o he sauna in he expe imen al and con ol g oups, and we inc eased he numbe o images o a oid
possible biases in he s imuli selec ion.
Expe imen 2: A p e- egis e ed eplica ion
Expe imen 2 was speci ically designed o isola e he e ec o hea exposu e on ecogni ion memo y by di ec ly
compa ing wo g oups o pa icipan s unde going sauna a di e en empe a u es. Based on he esul s o Expe i-
men 1, we p edic ed ha pa icipan s unde going a sauna ba h a 50°C on day 2 will show a educ ion in ec-
ogni ion pe o mance on day 3, ela i e o he con ol unde going a sauna ba h a 28°C.
Me hods. Pa icipan s. 60 pa icipan s (44 emales, 16 males; age ange = 21–29yea s) we e ec ui ed om
Uni e si y o San iago de Compos ela ia ad e isemen s in he Facul y o Psychology. They we e andomly
assigned o one o he wo expe imen al g oups (n = 30): sauna 50°C, o sauna 28°C. All pa icipan s p o ided
w i en in o med consen p io o es ing. The s udy con o med o he Decla a ion o Helsinki. Pa icipan s p o-
ided in o med consen p io o hei pa icipa ion. E hical app o al was ob ained om he Comi é de Bioé ica
o he Uni e si y o San iago de Compos ela.
S imuli and asks: The expe imen al p ocedu e was simila o Expe imen 1, wi h he addi ion ha we selec ed
100 mo e images (a o al o 200: 100 neu al and 100 emo ional images o nega i e alence). S imuli we e
Figu e2. Recogni ion memo y accu acy. This was based on he likelihood o co ec esponses (i.e. ‘old’
esponses o old i ems and ‘new’ esponses o new i ems) in he ecogni ion memo y es . The g aph shows
da a o sauna condi ion (collapsed ac oss eac i a ion and no- eac i a ion g oups) and o no sauna condi ion
(collapsed ac oss eac i a ion and no- eac i a ion g oups).
Table 1. Desc ip i e s a is ics. Means and s anda d de ia ions o A′, hi and alse ala m a es.
No sauna Sauna 50°C
Neu al Nega i e Neu al Nega i e
A′.912 ± .044 .892 ± .056 .884 ± .044 .888 ± .051
Hi a e .799 ± .130 .874 ± .078 .682 ± .121 .803 ± .123
False ala m a e .107 ± .097 .214 ± .127 .081 ± .072 .171 ± .111
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selec ed om he IAPS, ollowing he basis o hei s anda d sco es o emo ional a ousal and alence. As in
Expe imen 1, images we e subdi ided in o wo se s (A and B), each consis ing o 50 neu al and 50 unpleasan
pic u es. Hal o he pa icipan s we e p esen ed wi h se A du ing s udy and hen p esen ed wi h se A (old) and
B (new) du ing he ecogni ion ask. We pe o med pai ed es s o compa e he means o emo ional alence
be ween neu al and emo ional images o each Se . The e we e signi ican di e ences in se A [M neu al = 5.140,
SD = 0.777; M emo ional = 2.188, SD = 0.529 [ (49) = 22.293, p < 0.001, d = 3.153)] and in se B [M neu al = 5.039,
SD = 0.71; M emo ional = 2.189, SD = 0.519 [ (49) = 22.591, p < 0.001, d = 3.195]. Fu he mo e, we pe o med
a 2 × 2 ANOVA wi h Image Se (A, B) × Valence (neu al, emo ional) as ac o s, and ound a signi ican e ec
o Valence [F(1,48) = 1006.492, p < 0.001, ηp2 = 0.84]. The e we e no signi ican in e ac ions be ween ac o s
[F(1,48) = 0.25, p = 0.616], and he e we e no signi ican di e ences be ween Se s [F(1,48) = 0.377, p = 0.541].
P ocedu e. Expe imen 2 aimed o isola e he e ec o hea exposu e on ecogni ion memo y. The eac i a ion
ac o om Expe imen 1 was no included, and we only compa ed wo g oups o pa icipan s: sauna 50°C
condi ion and sauna 28°C. In his s udy all pa icipan s en e ed he sauna wea ing swimsui s and hey ook a
showe a e wa ds. The empe a u e o 28°C (28.8°C ± 0.3°C) was selec ed because i is conside ed a alue in he
he moneu al ange o ambien empe a u e whe e he body main ains i s co e empe a u e28. The expe imen al
p ocedu e ac oss he 3days was like Expe imen 1, including (i) he S udy phase on day 1, (ii) he Sauna phase
on day 2 and (iii) he subsequen Recogni ion es on day 3.
Resul s. We pe o med an ANOVA wi h one be ween subjec s ac o (Sauna: 28°C, 50°C) and one wi hin-
subjec epea ed measu es ac o (Valence: neu al, emo ional) on he p opo ion o co ec esponses in he
ecogni ion answe s. In keeping wi h Expe imen 1, he esul s depic ed in Fig.3 eplica e he signi ican
e ec o sauna 50°C on ecogni ion pe o mance [F(1,58) = 6.438, p = 0.014, ηp2 = 0.1 including all he da a;
F(1,56) = 14.063, p < 0.001, ηp2 = 0.2, a e emo ing ou lie s]. The ou lie s we e iden i ied in JASP by using he
de aul alues in he Box Plo and label ou lie s unc ion which basically iden i ies ou lie s ha ha below he
Qua ile 1 − 1.5 * In e Qua ile Range (IQR) o abo e he Qua ile 3 + 1.5 * IQR. Based on his c i e ion we
emo ed 3 ou lie s in Expe imen 2. Howe e , o he sake o anspa ency, we epo he esul s bo h including
and excluding he ou lie s. In addi ion, pa icipan s showed a be e memo y o nega i ely alenced han o
neu al s imuli [F(1,58) = 21.108, p < 0.001; ηp2 = 0.267]. The e was no signi ican in e ac ion be ween sauna and
emo ional alence (p = 0.737). Fo desc ip i e s a is ics see Table2.
Finally, o con ol o po en ial c i e ion biases, we conduc ed he analysis using a bias- ee index o memo y
sensi i i y (A′). The e ec o hea exposu e was also obse ed in memo y sensi i i y (50°C g oup: A′ = 0.876;
28°C g oup: A′ = 0.9) [F(1,58) = 4.732, p = 0.034, ηp2 = 0.075, including all he da a; F(1,56) = 14.932, p < 0.001,
Figu e3. Recogni ion memo y accu acy as a unc ion o he sauna condi ion and he alence o he i ems.
Table 2. Desc ip i e s a is ics. Means and s anda d de ia ions o A′, hi and alse ala m a es.
Sauna 28°C Sauna 50°C
Neu al Nega i e Neu al Nega i e
A′.885 ± .067 .916 ± .053 .858 ± .043 .893 ± .050
Hi a e .773 ± .134 .840 ± .107 .742 ± .097 .827 ± .110
False ala m a e .143 ± .083 .125 ± .093 .191 ± .114 .179 ± .105

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ηp2 = 0.21, a e emo ing ou lie s]. STAI-S sco es showed no di e ences in anxie y s a e be ween sauna 50°C
g oup (M = 10.00) and sauna 28°C g oup (M = 10.50) [ (58) = − 0.255; p = 0.79].
Gene al Discussion
The goal o his s udy was o es he e ec o hea exposu e on memo y pe o mance o p e iously s udied in o -
ma ion. Expe imen 1 showed ha he high- empe a u e sauna manipula ion signi ican ly wo sened ecogni ion
memo y one day la e . The e ec o he hea exposu e did no depend on he emo ional alence o he i ems,
occu ing bo h o neu al and nega i ely alence images. A p e- egis e ed Expe imen 2 eplica ed he e ec o
hea exposu e on ecogni ion memo y. No ably, he e ec o hea exposu e on memo y pe o mance occu ed
o i ems ha had been s udied a day ea lie and hence should be al eady consolida ed in he memo y epe oi e.
These esul s a e in keeping wi h he iew ha hea exposu e can lead o impai men s in lea ning p ocesses1–3.
To he bes o ou knowledge, his is he i s causal e idence ha inducing an e ec o hea exposu e by means
o a sho sauna ba h (7min) can ha e an impac on he ( e-) consolida ion o episodic memo ies in humans.
While he esul s sugges ha he e ec o hea exposu e on he aces o consolida ed memo ies a e independ-
en o he memo y eac i a ion du ing hea exposu e, one may a gue ha he p esence o he expe imen e could
ha e ac ed as a con ex ual cue and igge an implici eac i a ion e en hough he e was no explici ins uc ion
du ing he sauna o e ie e he s imuli p e iously s udied on day 1. Howe e , his accoun is unlikely gi en ha
in Expe imen 1 he e was no e idence ha explici eac i a ion ins uc ions we e associa ed wi h be e memo y
pe o mance. Mo eo e , p e ious s udies ha e es ablished he need o a eminde ial in o de o ac i a e he
econsolida ion p ocess, and he e is e idence o ailu es o eplica e e idence o he dis up ion o memo y
econsolida ion p ocesses29,30. Dis up ing memo y depends on se e al ac o s, including he du a ion o he
eminde ial, cue speci ici y, memo y s eng h and he en i onmen al con ex in which memo y is acqui ed31–34.
Below we p o ide a en a i e neu obiological accoun o how he e ec o hea on memo y may come abou .
Sauna ba hs in ol e a conside able he mal load in he body ha lead o a massi e ho monal eac ion. The -
mal s ess ac i a es hypo halamic–pi ui a y–ad enal axis, inc easing he le els o ad enoco ico opic ho mone
(ACTH), co isol, epineph ine and no epineph ine35. I is well known ha glucoco icoid ho mones can modu-
la e consolida ion and econsolida ion o memo y36. Howe e , some e idence indica es ha glucoco icoid
e ec s on memo y ollow an in e ed U-shaped cu e, as mode a e doses imp o e memo y, while highe doses
may impai memo y37. Se e al s udies ha e shown ha adminis a ion o co isone in humans impai s memo y
e ie al38,39. In addi ion, i has been clea ly shown ha glucoco icoids educe hippocampal ac i a ion du ing
decla a i e memo y e ie al40, and ha co isol supp ession ( ia me y apone) a encoding41 and a e memo y
eac i a ion weakens emo ional memo y42,43. Thus, i is possible ha sauna exposu e impai s consolida ed memo-
ies due o he momen a y acu e s ess p oduced by hype he mia, leading o co isol elease and modula ion
o hippocampal dependen memo y ne wo ks. Howe e , addi ional wo k is needed o es his possibili y and
make u he de e mina ions.
In animal models, he blockade o econsolida ion is achie ed by he adminis a ion o p o ein-syn hesis
inhibi o s such as anisomycin8, al hough he use o his ype o d ugs can be dange ous in humans because o
hei oxici y. Fo una ely, he e a e al e na i e beha io al44–46 and pha macological7,9,47 agen s ha a e sa e o
in e e e wi h memo y econsolida ion and ha opened a window o oppo uni y o c ea e new app oaches o
ea men o men al diso de s ela ed o anxie y. A ecen me a-analysis indica ed ha beha io al ea men s ha e
a small e ec in anxie y diso de s in compa ison o pha macological ea men s48. Thus, an impo an challenge
o psychological ea men s is o de elop mo e e icacious p ocedu es o in e e ing memo y econsolida ion
in a mo e na u al way, wi hou he disad an ages o pha macological in e en ions bu wi h su icien e icacy o
allow signi ican clinical imp o emen s. Hea exposu e h ough a sauna may be a p omising ool in his ega d
bu he cu en pa adigm needs o be in es iga ed u he in o de o de elop p o ocols ha can isola e sauna
e ec s on nega i e, unwan ed memo ies only wi h ami ica ions in pos - auma ic s ess and obsessi e–com-
pulsi e diso de s.
Da a a ailabili y
The beha io al da a can be ound a h ps:// os . io/ kdh2s/. The p e- egis a ion o Expe imen 1 is also a ailable
a h ps:// os . io/ 3bjg/.
Recei ed: 15 Feb ua y 2023; Accep ed: 5 July 2023
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Acknowledgemen s
This esea ch ecei ed inancial suppo om he budge o he Unidad Ven es Clínicos (Uni e si y o San iago
de Compos ela). DS acknowledges suppo om he Basque Go e nmen h ough he BERC 2022-2025 p og am
and by he Spanish S a e Resea ch Agency h ough BCBL Se e o Ochoa excellence acc edi a ion CEX2020-
001010-S. The unding sou ces had no in ol emen in p epa a ion o he manusc ip .
Au ho con ibu ions
E.G. concei ed he idea; J.C. p og ammed he asks unde he supe ision o D.S.; J.C. un he expe imen s; J.C.
analysed he da a unde he supe ision o D.S.; J.C. w o e he i s d a o he pape and E.G. and D.S. edi ed
he pape and p epa ed he inal e sion.
Compe ing in e es s
The au ho s decla e no compe ing in e es s.
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