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The value of neuroimaging techniques (and what those squiggly lines mean)

The media reg­u­lar­ly reports on find­ings based on neu­roimag­ing stud­ies, but rarely do they explain exact­ly what these tech­niques are, their ben­e­fits or what it’s like to actu­al­ly par­tic­i­pate in these types of stud­ies. Today I’ll describe what a par­tic­i­pant goes through when they vol­un­teer for a cog­ni­tive neu­ro­science exper­i­ment using a neu­roimag­ing tech­nique called elec­troen­cephalog­ra­phy (EEG). Unfor­tu­nate­ly, it is exceed­ing­ly com­mon for par­tic­i­pants to not under­stand how these tech­niques ben­e­fit pre­vi­ous behav­ioral find­ings. Sim­ply stat­ed, if I were a par­tic­i­pant, I’d like to know why I need­ed to wear a weird swim cap and how it ben­e­fits the research being done.

EEG is a tool reg­u­lar­ly used to view and record the changes in brain activ­i­ty involved in the var­i­ous types of cog­ni­tive func­tions while per­form­ing a task. Brain cells com­mu­ni­cate by pro­duc­ing tiny elec­tri­cal impuls­es, and the func­tion of EEG is to record these pat­terns of elec­tri­cal activ­i­ty (as illus­trat­ed in Pan­el A of the fig­ure below; like you’d see with a poly­graph machine, but from your head) and then use this data to inform spe­cif­ic behav­iors. This activ­i­ty is record­ed by elec­trodes (small devices that act like micro­phones lis­ten­ing in on the brain’s spon­ta­neous elec­tri­cal activ­i­ty).

Fig­ure leg­end. Pan­el A shows the EEG from a par­tic­i­pant at 64 dif­fer­ent elec­trodes (along the y‑axis) over 16 sec­onds (x‑axis). Pan­el B high­lights the elec­tri­cal activ­i­ty from one of the lat­er­al pos­te­ri­or elec­trodes (PO7) when dif­fer­ent col­ored shapes were pre­sent­ed. Pan­el C illus­trates the event-relat­ed poten­tials (ERPs) observed after aver­ag­ing all of the seg­ments of data asso­ci­at­ed with green cir­cles (the red line) and all oth­er shape/color types (black line). The y‑axis is indica­tive of the volt­age (pos­i­tive or neg­a­tive), while the x‑axis shows time in mil­lisec­onds (msec): from 200 msec before the col­ored shapes were pre­sent­ed to 800 msec after­wards.

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EEG can be par­tic­u­lar­ly illu­mi­nat­ing for researchers try­ing to bet­ter under­stand how the brain actu­al­ly changes (for exam­ple, after atten­tion­al train­ing). For exam­ple, a recent study by Eldar and Bar-Haim (2010) exam­ined which func­tions of atten­tion­al pro­cess­ing are affect­ed by atten­tion train­ing in anx­ious indi­vid­u­als. They found that train­ing helped these indi­vid­u­als change their behav­ior to divert their atten­tion from threat­en­ing stim­uli faster. This change could also be seen neu­ral­ly with changes in their EEG activ­i­ty over pre­frontal elec­trodes.

So what is it like to actu­al­ly par­tic­i­pate in an EEG study? Here’s the exam­ple of some­one who recent­ly par­tic­i­pat­ed in a study in our lab. Our par­tic­i­pant (let’s call her Jen­ny) had an elec­trode cap (looks like a swim cap with 64 strate­gi­cal­ly placed holes in it) placed on her head. A few mil­li­liters of con­duc­tive gel were placed at each of the 64 points on her scalp where each elec­trode was going to rest. This is so that the tip of each elec­trode would ‘swim’ in this gel and record the neur­al activ­i­ty at her scalp. Sur­pris­ing­ly, wear­ing the cap real­ly isn’t that uncom­fort­able; in fact, many par­tic­i­pants real­ly enjoy the ‘pseu­do-scalp mas­sage’ that comes along with adding gel to each elec­trode site.

For this exper­i­ment, we were inter­est­ed in see­ing how atten­tion-relat­ed neur­al activ­i­ty dif­fered when prim­ing one’s self to respond to a giv­en tar­get image. Jen­ny was instruct­ed to push a but­ton in response to green cir­cles appear­ing on a com­put­er screen in front of her, while ignor­ing all oth­er shapes & col­ors (see the shapes described at the bot­tom of Pan­el B). In the course of the exper­i­ment, she saw a total of 100 green cir­cles, and 100 oth­er irrel­e­vant stim­uli (the “x 100 events” in the fig­ure). We were par­tic­u­lar­ly inter­est­ed in neur­al record­ings from the lat­er­al occip­i­tal cor­tex (on the back­side of the head) where activ­i­ty relat­ed to visu­al dis­crim­i­na­tion (“ooooh, that’s a green cir­cle, hit the button….nope, that’s a green square, he’s obvi­ous­ly try­ing to trick me, not going to respond…”) is typ­i­cal­ly record­ed. The fig­ure in the mid­dle pan­el shows the EEG that was record­ed over the course of 15 sec­onds at one of the elec­trodes where this activ­i­ty was the great­est.

To see dif­fer­ences between neur­al activ­i­ty for green cir­cles vs. every­thing else, we took small seg­ments of the data around the onset of green cir­cles (and each oth­er shape; see the box­es in the 2nd pan­el for each tar­get type), and aver­aged those events togeth­er. This aver­ag­ing led to the fig­ure in Pan­el C: these are called event-relat­ed poten­tials (ERPs) which require the aver­ag­ing of many tri­als to see them (that’s why they are tough to see in the non-aver­aged activ­i­ty in the top two pan­els). Notice that the wave­form goes ‘up’ around 100 msec (this is called the P1: p for ‘pos­i­tive volt­age’ and 1 for ‘near 100’ msec) fol­lowed by a N1 com­po­nent (n for ‘neg­a­tive volt­age’…).

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So what good are these sig­nals for any­ways?  Well, cer­tain ERPs can reflect one’s atten­tive­ness: if you’re primed for a cer­tain thing to hap­pen, you can respond to it soon­er and more accu­rate­ly. Notice that the P1 ampli­tude and N1 ampli­tude is greater for the green cir­cles as opposed to the ‘non-tar­gets’ blue square. This is espe­cial­ly inter­est­ing and impor­tant as these mark­ers pro­vide infor­ma­tion before the par­tic­i­pant even respond­ed! Hav­ing a mea­sure that relates to one’s per­for­mance before they actu­al­ly ‘per­form’ can be espe­cial­ly infor­ma­tive in fig­ur­ing out all kinds of things. For Eldar and Bar-Haim, they saw sim­i­lar mark­ers changed with train­ing; for us, we use these mark­ers to bet­ter explain how par­tic­i­pants respond­ed quick­ly to green cir­cles. These types of find­ings are typ­i­cal of what an EEG cog­ni­tive neu­ro­science exper­i­ment would look for. And yes, they are also the rea­son that we are thrilled when par­tic­i­pants are will­ing to put up with wear­ing strange look­ing head gear for an hour or two.

Ref­er­ences: Eldar S & Bar-Haim Y. (2010). Neur­al plas­tic­i­ty in response to atten­tion train­ing in anx­i­ety. Psy­chol. Med., 40(4): 667–77.

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—- Joaquin A. Anguera is a Post­doc­tor­al Fel­low work­ing in Adam Gazzaley’s cog­ni­tive neu­ro­science lab­o­ra­to­ry at the Uni­ver­si­ty of Cal­i­for­nia, San Fran­cis­co (www.gazzlab.com). His research focus­es on how aging affects dif­fer­ent aspects of motor & sen­so­ry per­for­mance using both behav­ioral and neu­roimag­ing tech­niques.

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Learn more on neu­roimag­ing tech­niques (MRI,  PET, etc.):

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As seen in The New York Times, The Wall Street Journal, BBC News, CNN, Reuters,  SharpBrains is an independent market research firm tracking how brain science can improve our health and our lives.

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