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 2^nd 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.):

• Look­ing inside the Brain: is my Brain Fit?

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