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

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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.):