Does Bilingualism Enhance Cognitive Reserve and thus Protect Against Dementia?
Tweetaligheid: hét medicijn tegen alzheimer?
Ieder jaar ontwikkelen 10 miljoen mensen de ziekte van Alzheimer of een andere vorm van dementie. Een medicijn is nog niet gevonden, maar wat als je jezelf tegen dementie zou kunnen beschermen? Onderzoekers komen steeds meer tot de conclusie dat we zelf de diagnose en symptoomontwikkeling van dementie met meerdere jaren kunnen uitstellen of dementie zelfs geheel kunnen voorkomen. Hoe? Door een tweede taal te leren.
Het effect van tweetaligheid op dementie
Verschillende studies hebben een verband gevonden tussen de beheersing van twee talen en een kleinere kans op verschillende vormen van dementie, waaronder alzheimer. Daarnaast rapporteren meerdere onderzoekers dat tweetaligen die tóch dementie ontwikkelen cognitief minder snel achteruitgaan dan ééntaligen. Tweetalige patiënten kunnen sneller informatie verwerken, beter irrelevante informatie negeren en hebben een sterker geheugen dan patiënten die slechts één taal machtig zijn.
Een veelgeciteerd onderzoek stelt bovendien dat tweetalige dementiepatiënten ruim 4 jaar later symptomen ontwikkelen dan ééntalige patiënten. Latere studies hebben aangetoond dat personen die meer dan één taal spreken niet alleen vertraagd zijn in hun symptoomontwikkeling maar ook 4 à 5 jaar ouder zijn dan ééntaligen op het moment van hun dementiediagnose. Sommige onderzoeken suggereren zelfs dat mensen die meer dan één taal spreken een kleinere kans hebben om dementie te ontwikkelen, maar die bevindingen zijn veelal beperkt tot mensen die drie of meer talen beheersen.
Wisselen tussen talen
Tweetaligen wisselen vaak van taal: thuis spreken ze bijvoorbeeld Turks, maar op school of op het werk gebruiken ze Nederlands. Dit constante “switchen” van de ene naar de andere taal gaat echter niet zonder cognitieve inspanning: ze zouden niet alleen de relevante taal moeten activeren maar ook de irrelevante taal moeten onderdrukken. Dit zou nodig zijn om te voorkomen dat ze in een gesprek woorden van de irrelevante taal zouden gebruiken.
Trainen van executieve functies
Door het continue wisselen tussen twee talen zouden tweetaligen vaak hun executieve functies trainen. De term “executieve functies” verwijst naar de mentale processen die we gebruiken om beslissingen te nemen, om ons geheel op één taak te focussen en om – in het geval van tweetaligheid – de juiste taal te kiezen.
Onderzoek suggereert dat de executieve functies van tweetaligen beter getraind zijn dan die van mensen die maar één taal spreken. Dit leidt tot een betere vaardigheid in het oplossen van cognitieve conflicten. Wie meer dan één taal beheerst, heeft sneller door welke informatie relevant is en welke informatie genegeerd kan worden. Dit is onder andere aangetoond met tests waarin tweetaligen en ééntaligen een reeks gekleurde vormen (bv. een rode cirkel) te zien krijgen en ofwel de kleur, ofwel de vorm moeten benoemen. Proefpersonen weten niet op voorhand welk kenmerk van de volgende stimulus zij zullen moeten benoemen. Dezelfde vorm wordt gebruikt voor zowel de kleur- als de vormtaak. Als na een aantal kleurtaken opeens een vormtaak volgt, of omgekeerd, moet een testpersoon de irrelevante informatie (bv. de kleur “rood”) onderdrukken om het goede antwoord te geven (de vorm “cirkel”). In dit opzicht lijkt zo een taak op wat een tweetalige volgens veel onderzoekers vaak zou doen: de irrelevante informatie onderdrukken, bijvoorbeeld het woord uit een taal die op dat moment niet van belang is, om zo beter toegang te krijgen tot de relevante informatie.
Onderzoeksresultaten wijzen inderdaad uit dat tweetaligen doorgaans sneller zijn in het switchen van een vormtaak naar een kleurtaak, en omgekeerd, dan mensen die geen tweede taal spreken. Hun kortere reactietijden worden geïnterpreteerd als een bewijs voor verbeterde executieve functies. Dat zou leiden tot een grotere cognitieve reserve: het geheel aan mentale capaciteiten die we kunnen gebruiken om onverwachte problemen op te lossen. Een toename van deze cognitieve reserve zou op zijn beurt een vertragend effect hebben op de symptoomontwikkeling en de diagnose van dementie.
Vergroten van cognitieve reserve
De cognitieve reserve bepaalt in welke mate beschadiging aan de hersenen dementiesymptomen veroorzaakt. Een dementiepatiënt met een grote cognitieve reserve vertoont geen of minder symptomen dan iemand met een vergelijkbaar hersenletsel maar een kleinere cognitieve reserve. Wanneer de hersenen beschadiging oplopen, bijvoorbeeld door de ziekte van Alzheimer, werkt de cognitieve reserve als compensatiemechanisme. Hoe krachtiger die reserve, hoe sterker de compensatie voor de cognitieve problemen die bij dementie optreden. Mensen die hersenbeschadiging oplopen maar over een grote cognitieve reserve beschikken, zijn blijkbaar door hun krachtige cognitieve reserve in staat om andere breinnetwerken in te schakelen en zo optimaal te blijven functioneren. Dit compensatiemechanisme zou verklaren waarom tweetalige dementiepatiënten met ernstige hersenschade toch op hetzelfde niveau kunnen blijven functioneren als ééntalige patiënten met aanzienlijk minder hersenletsels.
Het toekomstbeeld
Verder onderzoekzal moeten uitwijzen of aan bepaalde voorwaarden moet worden voldaan om als tweetalige beschermd te zijn tegen dementie. Ben je alleen beschermd als je tweetalig werd opgevoed? Of geldt dit ook als je pas in de puberteit of op volwassen leeftijd een tweede taal hebt geleerd? Is tweetaligheid op zich verantwoordelijk voor dit beschermend effect, of doet het zich enkel voor in combinatie met bijvoorbeeld een hoog opleidingsniveau? Mocht dat laatste het geval zijn, is de echte oorzaak van het effect dan de tweetaligheid of de hoger ontwikkelde cognitieve vaardigheden? En ten slotte: kan een sterke cognitieve reserve niet alleen leiden tot een uitstel van symptomen en diagnose maar zelfs tot het voorkomen van dementie?
Het debat omtrent het effect van tweetaligheid op dementie is nog niet helemaal gevoerd. Het is nog niet duidelijk of het beschermend effect op dementie aan de tweetaligheid zelf toegeschreven kan worden, of eerder aan de cognitieve training die de tweetaligheid met zich meebrengt. Er zijn in ieder gevalvoldoende redenen om te geloven dat het leren van een nieuwe taal je beter zal wapenen tegen dementie. Dus of je nu voor Engels, Arabisch of Spaans kiest: leer een taal en hou je brein actief.
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