"Long before it's in the papers"
January 27, 2015


“False memories” planted in mice

July 25, 2013
Courtesy of Science
and World Science staff

Mem­o­ries can be un­re­li­able—a fact ex­em­pli­fied by the lim­it­ed im­por­tance of wit­ness tes­ti­mo­ny in many court sys­tems. Simply re­call­ing a mem­o­ry ren­ders it sus­cep­ti­ble to change.

But it’s been un­clear how men­tal rep­re­senta­t­ions be­come blurred by ex­ter­nal events, sci­en­tists say, to pro­duce brand new, seem­ingly ac­cu­rate—but com­pletely false—mem­o­ries. 

Sci­en­tists first put a mouse in one en­vi­ron­ment, il­lus­trat­ed as a blue box on the left, and la­beled the brain cells en­cod­ing for the mem­o­ry of this en­vi­ron­ment (white cir­cles). Then the sci­en­tists put the ro­dent in a dif­fer­ent en­vi­ron­ment, la­beled as the red box, and used light to ac­ti­vate the la­beled cells. This in­duced a re­call of the first en­vi­ron­ment. Mean­while the mouse re­ceived mild foot shocks. Fi­nal­ly, the mouse was placed back in­to the first en­vi­ron­ment, and showed signs of fear, sug­gest­ing it mis­tak­en­ly thought the shock had oc­curred there. (Im­age cour­te­sy Ev­an Won­dolowski/­Col­ Next)

In new re­search, Steve Ramirez of the Mas­sa­chu­setts In­sti­tute of Tech­nol­o­gy and col­leagues probed the is­sue by planting false mem­o­ries in­to mice, they said. 

This caused the ro­dents to re­call men­tal rep­re­senta­t­ions—which they had forged in one of four dis­tinct con­texts—in a com­pletely dif­fer­ent con­text al­to­geth­er, the sci­en­tists said.

Us­ing a technique called op­to­ge­netic ma­nipula­t­ion, the sci­en­tists stud­ied a popula­t­ion of brain cells called gran­ule cells, in a part of the brain known as the den­tate gy­rus. In so do­ing, they re­ported that they could cre­ate false as­socia­t­ions be­tween events and en­vi­ron­ments by ar­ti­fi­cially stim­u­lat­ing the cells.

Optogenetics is a new tech­nol­ogy that al­lows cells to be se­lect­ive­ly turned on or off us­ing light.

The re­search­ers not­ed which cells were ac­ti­vat­ed in the mice by mild foot shocks in one en­vi­ron­ment. They then moved the mice to an­oth­er, shock-free en­vi­ron­ment and stim­u­lat­ed those same cells with their tech­nique. 

This re­ac­tiva­t­ion of the cells that fired sig­nals when the mice were shocked caused the mice to freeze—a nat­u­ral re­sponse to the fear mem­o­ry—when there was no shock sup­plied, the sci­en­tists re­ported.

In fact, they said, the im­planted mem­o­ries were so ro­bust that the mice even­tu­ally be­came con­di­tioned to them, freez­ing up even when those gran­ule cells weren’t reac­ti­vat­ed from out­side. The find­ings pro­vide a mod­el for stu­dy­ing false mem­o­ry forma­t­ion in hu­mans, said the re­search­ers, re­port­ing their find­ings in the July 26 is­sue of the jour­nal Sci­ence.

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Memories can be unreliable—a fact exemplified by the limited importance of witness testimony in many court systems. Simply recalling a memory renders it susceptible to change. But it’s been unclear how mental representations become blurred by external events, scientists say, to produce brand new, seemingly accurate—but completely false—memories. In new research, Steve Ramirez of the Massachusetts Institute of Technology and colleagues probed the issue by planting false memories into mice, they said. This caused the rodents to recall mental representations—which they had forged in one of four distinct contexts—in a completely different context altogether, the scientists said. Using what they called optogenetic manipulations, the scientists studied a population of brain cells called granule cells, in a part of the brain known as the dentate gyrus. In so doing, they reported that they could create false associations between events and environments by artificially stimulating the cells. The researchers noted which cells were activated in the mice by foot shocks in a particular environment. They then moved the mice to another, shock-free environment and stimulated those same cells with their technique. This reactivation of the cells that fired signals when the mice were shocked caused the mice to freeze—a natural response to the fear memory—when there was no shock supplied, the scientists reported. In fact, they said, the implanted memories were so robust that the mice eventually became conditioned to them, freezing up even when those granule cells weren’t reactivated from outside. The findings provide a model for studying false memory formation in humans, said the researchers, reporting their findings in the July 26 issue of the journal Science.