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Scans might reveal even past brain activity

June 25, 2013
Courtesy of the Weizmann Institute
and World Science staff

New re­search hints that sci­en­tists could probe the brain and un­cov­er the his­to­ry of past ex­pe­ri­ences. 

Re­search­ers found that waves of nerve cell ac­ti­vity in the brain bear im­prints of ear­li­er events, ex­tend­ing at least a day in­to the past.

The day- af­ter ef­fect of brain ac­ti­va­tion: The brain im­age at the back pre­s­ents spon­ta­ne­ous (rest­ing state) pat­terns be­fore a neu­ro­feed­back train­ing ses­sion. The front brain im­age pre­s­ents spon­ta­ne­ous (rest­ing state) pat­terns a day af­ter the train­ing ses­sion, il­lus­trat­ing the long-term trace of the train­ing. (Cour­te­sy Weiz­mann Inst.)


The re­search stems from work by neuro­bi­ol­o­gist Rafi Ma­lach at Is­ra­el’s Weiz­mann In­sti­tute and oth­ers showing that the brain nev­er rests, even when its own­er is rest­ing. 

When a per­son is rests with closed eyes, nor­mal bursts of nerve cell ac­ti­vity as­so­ci­at­ed with in­com­ing in­forma­t­ion give way to ultra-slow pat­terns of ac­ti­vity. Such spon­ta­ne­ous or “rest­ing” waves move in an or­gan­ized, re­pro­duc­i­ble way through the brain’s out­er lay­er, the cor­tex, cre­at­ing com­plex, yet re­pet­i­tive and sym­met­ri­cal pat­terns.

Tal Harm­elech, a re­search stu­dent un­der Malach’s guid­ance, in­ves­t­i­gated these pat­terns with the idea that they may con­sti­tute “archives” of ear­li­er ex­pe­ri­ences. As we add new ex­pe­ri­ences, the ac­tiva­t­ion of our brain’s net­works lead to long-term changes in the links be­tween brain cells, a func­tion called plas­ti­city.

As our ex­pe­ri­ences be­come em­bed­ded in these con­nec­tions, they cre­ate “ex­pecta­t­ions” that come in­to play be­fore we per­form any type of men­tal task, en­a­bling us to an­ti­cipate the re­sult, the re­search­ers said. They hy­poth­e­sized that in­forma­t­ion about ear­li­er ex­pe­ri­ences would thus be in­cor­po­rat­ed in­to the links be­tween net­works of nerve cells, and these would show up in the brain’s wave pat­terns.

The in­ves­ti­ga­tors asked vol­un­teers to un­der­take a train­ing ex­er­cise that would strongly ac­tivate a well-de­fined net­work of nerve cells in the front­al lobes, the for­ward part of the brain. 

While un­der­go­ing brain scans through func­tional Mag­net­ic Res­o­nance Im­ag­ing, which maps blood flow in the brain, they were asked to im­ag­ine a situa­t­ion in which they had to make rap­id de­ci­sions. They re­ceived au­di­o feed­back in real time based on the in­forma­t­ion ob­tained from their front­al lobe in­di­cat­ing the lev­el of nerve cell ac­ti­vity.

This “neurofeed­back” strat­e­gy proved suc­cess­ful in ac­tivating the front­al net­work – a part of the brain no­to­ri­ously dif­fi­cult to ac­tivate un­der con­trolled con­di­tions, the in­ves­ti­ga­tors said.

To test wheth­er the con­nec­tions cre­ated dur­ing the task would leave traces in the rest­ing brain wave pat­terns, the re­search­ers per­formed scans on the rest­ing sub­jects be­fore the task, im­me­di­ately af­ter­ward, and 24 hours lat­er. Their find­ings, which ap­pear in the Jour­nal of Neu­ro­sci­ence, in­di­cat­ed that the ac­tiva­t­ion of the spe­cif­ic ar­eas in the cor­tex re­mod­eled the rest­ing brain wave pat­terns. 

Sur­pris­ing­ly, they said, the new pat­terns not only stayed the next day, they were strength­ened. These ob­serva­t­ions fit in with the clas­sic learn­ing prin­ci­ples pro­posed by Don­ald Hebb in the mid-20th cen­tu­ry, in which the co-ac­tiva­t­ion of two linked nerve cells leads to long term strength­en­ing of their link, while ac­ti­vity that’s not co­or­di­nated weak­ens this link. 

The rest­ing brain wave im­ages showed that brain ar­eas ac­tivated to­geth­er dur­ing the train­ing in­creased their func­tional link a day later—while ar­eas deac­tivated by the train­ing showed weak­ened con­nec­ti­vity, the researchers added.


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New research hints that scientists could probe the brain and uncover the history of past experiences. Researchers found that waves of nerve cell activity in the brain bear imprints of earlier events, extending at least a day into the past. The research stems from earlier findings by neurobiologist Rafi Malach at the Weizmann Institute in Israel and others that the brain never rests, even when its owner is resting. When a person is rests with closed eyes, normal bursts of nerve cell activity associated with incoming information give way to ultra-slow patterns of activity. Such spontaneous or “resting” waves travel in an organized, reproducible way through the brain’s outer layer – the cortex – creating complex, yet repetitive and symmetrical patterns. Tal Harmelech, a research student under Malach’s guidance, investigated these patterns with the idea that they may constitute “archives” of earlier experiences. As we add new experiences, the activation of our brain’s networks lead to long-term changes in the links between brain cells, a function called plasticity. As our experiences become embedded in these connections, they create “expectations” that come into play before we perform any type of mental task, enabling us to anticipate the result, the researchers said. They hypothesized that information about earlier experiences would thus be incorporated into the links between networks of nerve cells, and these would show up in the brain’s wave patterns. The investigators asked volunteers to undertake a training exercise that would strongly activate a well-defined network of nerve cells in the frontal lobes, the forward part of the brain. While undergoing brain scans through functional Magnetic Resonance Imaging, which maps blood flow in the brain, they were asked to imagine a situation in which they had to make rapid decisions. The subjects received audio feedback in real time, based on the information obtained directly from their frontal lobe, which indicated the level of neuronal activity in the trained network. This “neurofeedback” strategy proved successful in activating the frontal network – a part of the brain notoriously difficult to activate under controlled conditions, the investigators said. To test whether the connections created during the task would leave traces in the resting brain wave patterns, the researchers performed scans on the resting subjects before the task, immediately afterward, and 24 hours later. Their findings, which appear in the Journal of Neuroscience, indicated that the activation of the specific areas in the cortex remodeled the resting brain wave patterns. Surprisingly, they said, the new patterns not only stayed the next day, they were strengthened. These observations fit in with the classic learning principles proposed by Donald Hebb in the mid-20th century, in which the co-activation of two linked nerve cells leads to long term strengthening of their link, while activity that’s not coordinated weakens this link. The images of the resting brain waves showed that brain areas that were activated together during the training sessions exhibited an increase in their functional link a day after the training, while those areas that were deactivated by the training showed a weakened functional connectivity.