The un­sta­ble (adult) brain

MRI re­veals changes in early to mid-adult­hood brains

The McGill Daily - - Sci+Tech - Tai (Ritchie) Vinh Truong Sci+tech Writer

The brain is un­mis­tak­ably the most com­plex hu­man or­gan. De­spite hav­ing be­gin­ning in the 18th cen­tury, ef­forts to un­der­stand the hu­man brain through neu­ro­science have been im­peded due to many tech­ni­cal lim­i­ta­tions. The in­tri­cacy and del­i­cacy of the cen­tral ner­vous sys­tem ren­ders nu­mer­ous in­va­sive meth­ods im­plau­si­ble, forc­ing re­searchers and doc­tors to rely on re­stricted non-in­va­sive pro­ce­dures to study this struc­ture. Re­cent ad­vances in mag­netic res­o­nance imag­ing (MRI), a non-in­va­sive method, have al­lowed sci­en­tists to eval­u­ate mi­crostruc­tural changes dur­ing brain de­vel­op­ment and ag­ing. How­ever, the ma­jor­ity of the MRI stud­ies fo­cus on the early and late phases of brain mat­u­ra­tion, leav­ing the pe­riod be­tween early to mid-adult­hood rel­a­tively un­ex­plored. This pe­riod of brain de­vel­op­ment is thought to be un­fluc­tu­at­ing; the brain struc­tures do not un­dergo no­tice­able changes. How­ever, neu­ro­science re- searchers Lixia Tian and Lin Ma chal­lenged this no­tion by demon­strat­ing age-re­lated re­mod­el­ing of var­i­ous brain ar­eas.

In a newly pub­lished ar­ti­cle in the jour­nal Fron­tiers in Hu­man Neu­ro­sciences, re­searchers from Beijing Jiao­tong Univer­sity em­ployed data from Dif­fu­sion Ten­sion Imag­ing (DTI) to in­ves­ti­gate brain struc­tures of 111 adults aged 18-55 years. DTI is a spe­cific type of MRI that mea­sures the dif­fu­sion process of wa­ter mol­e­cules in par­tic­u­lar, and thus maps out the white mat­ter in dif­fer­ent brain re­gions. The DTI meth­ods pro­duce mul­ti­ple out­puts such as Frac­tional Anisotropy (FA), Ax­ial and Ra­dial Dif­fu­sion (AD & RD), and Mean Dif­fu­sion (MD). Fluc­tu­a­tions in th­ese mea­sure­ments can in­di­cate changes in var­i­ous brain struc­tures.

Tian and Ma ob­served an over­all re­duc­tion in FA with age, in­clud­ing the pe­riod be­tween early to mi­dadult­hood, which sug­gests that the in­tegrity of the white mat­ter is di­min­ish­ing in re­gions such as the bi­lat­eral cor­ti­cospinal tract (CST), the cor­pus cal­lo­sum (CC), the fornix, the left su­pe­rior lon­gi­tu­di­nal fas­ci­cu­lus (SLF), and the in­fe­rior lon­gi­tu­di­nal fas­ci­cu­lus (ILF). Ad­di­tion­ally, sig­nif­i­cant age-re­lated less­en­ing of AD and in­creas­ing RD, im­ply­ing ax­onal de­gen­er­a­tion and de­myeli­na­tion re­spec­tively, were de­tected in the CC and the CST. Based on th­ese find­ings, Tian and Ma were able to de­vise models to ac­cu­rately pre­dict an in­di­vid­ual’s age based on any one of the DTI out­puts. The ac­cu­racy of the age prediction models cor­rob­o­rates that mi­crostruc­tural changes in the brain be­tween early to mid-adult­hood are in fact sub­stan­tial.

Al­though it is un­equiv­o­cal that there are sig­nif­i­cant changes in brain struc­tures in the early and late stages of life, “the changes in brain struc­tures and func­tions from early to mi­dadult­hood were largely un­known,” noted Tian. Based on the po­ten­tially mis­taken as­sump­tion that brain struc­ture is sta­ble be­tween early to mid-adult­hood, many rel­e­vant stud­ies might have ne­glected the age ef­fect, which could lead to flawed re- sults. Tian and Ma’s find­ings alarmed sci­en­tists at the brain’s mea­sur­able mi­crostruc­tural changes over this pre­vi­ously untested de­vel­op­men­tal pe­riod. They also pro­vided the pos­si­bil­ity of valu­able in­sight into how dif­fer­ent struc­tures are in­volved in cog­ni­tive de­cline and when that de­cline hap­pens. The struc­tures which were found to un­dergo the ear­li­est changes, such as fornix, CC, and ILF, are cru­cial to mem­ory, learn­ing ca­pac­ity, and re­ac­tion time. “When does cog­ni­tive de­cline be­gin?” is still a con­sid­er­ably con­tentious ques­tion in the field of brain de­vel­op­ment. Many re­searchers ar­gue that cog­ni­tive de­cline sur­faces as early as 20 years of age, while oth­ers claim that the im­pair­ment is not pos­si­ble un­til af­ter the age of 60. Tian and Ma’s re­sults might even be use­ful for de­ter­min­ing the emer­gence of se­nil­ity, which could help de­sign both pre­ven­ta­tive and in­ter­ven­tional treat­ments for pa­tients. In ad­di­tion, Tian and Ma’s data sug­gests the pre­vi­ously men­tioned struc­tures po­ten­tially play a role in cog­ni­tion de­cline and the on­set of the Alzheimer’s disease.

De­spite the im­pli­ca­tions of this study in the field of brain de­vel­op­ment, the meth­ods em­ployed have sev­eral lim­i­ta­tions. The re­sults were ob­tained from a cross-sec­tional study, which ex­am­ined peo­ple of vary­ing ages. Cross-sec­tional stud­ies only pro­vide one mea­sure­ment per sub­ject. Th­ese stud­ies are not ideal be­cause they “could not rule out the pos­si­bil­ity that the cur­rent re­sults may, to some de­gree, be in­flu­enced by fac­tors such as co­hort ef­fects, which are in­evitable in stud­ies based on cross-sec­tional data,” ad­mit­ted Tian. Mcgill pro­fes­sor David Rudko shared his views with The Daily on the di­rec­tion of fu­ture re­search. He noted that, “[Tian and Ma’s] re­sults should be sup­ple­mented with fu­ture stud­ies us­ing lon­gi­tu­di­nal imag­ing.” The tech­nol­ogy of DTI is still at its in­fant stage; hence, es­ti­ma­tion er­rors are not neg­li­gi­ble. Rudko sug­gested that the sta­tis­ti­cally sig­nif­i­cant cor­re­la­tions be­tween age and mi­crostruc­tures should be fur­ther eval­u­ated us­ing more ad­vanced mi­crostruc­tural tech­niques to en­sure their va­lidi­ties.

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