ERP: N170 and its Role in Visual FacialProcessingWithin the field of perception research, oneof the most popular topics of conversation is the study of the human face andour perception of it. When we consider how vital it is to understand how weperceive human faces, it makes sense that it is so widely researched. We’ve allheard the expression “the eyes are the window to the soul”. Maybe this is true,only because a person’s eyes might tell you something about what they’rethinking or feeling, but even more accurate information can be gathered fromthe rest of a person’s face. In less than a few seconds of gazing at anotherperson’s face, you can identify who they are, possibly their sex, line ofattention, and even their mood (Dering, Martin, Moro, Pegna, & Thierry,2011).
There have been numerous studies done involving facial perception, butthe studies that this paper will be looking deeper into are those dealing withthe event-related potential (ERP) N170, first described by the research ofShlomo Bentin in 1996. The N170 is a neural marker for facial recognition,measured by using an electroencephalograph (EEG). It has a negative peakwaveform that occurs around 170ms after the facial stimulus is presented. Onthe EEG, the response is most prominent in the occipito-temporal region of thebrain, more so in the right hemisphere than the left, showing some lateralization(Botzel, Schulze, & Stodieck, 1995). According to Cao, Li, Gaspar, andJiang, the N170 may also be used as an indicator of base-level expertise ofpattern perception, and that the N170 not only shows selectivity to faces butto other different visual patterns as well (Cao, Li, Gaspar, & Jiang,2013). Other topics that this paper will touch on include the N170 in responseto emotionally expressive stimuli, task difficulty, and faces versus non-facestimuli.In astudy performed by Botzel, Schulze, & Stodieck in 1995, before the N170 hadbeen formally described, subjects were exposed to black and white photos ofdifferent stimuli (Botzel et al.
, 1995). These images included photos of humanfaces, flowers, and leaves. Subjects were monitored by EEG, where the now knownN170 showed a negative peak at 175ms for the human face stimuli but not for theother stimuli (Botzel et al., 1995). This study also found that the occipitaland lateral temporal areas of the brain were active during the recorded ERPs(Botzel et al., 1995).
This gave a baseline idea of where the N170 component isgenerated, and that there were specific brain structures responsible for facerecognition, but not general objects. There isa plethora of interesting research that involves the N170 ERP component, onetopic including how the N170 responds to emotional facial expressions. Some researchsuggests that the N170 is not only sensitive to the basic facial structure, butalso to the presumed emotional expression on the face (Blau, Maurer, Tottenham& McCandliss, 2007). In a study by Blau et al., subjects were monitored byEEG and their response to both fearful and neutral faces were recorded. Theirstudy drew upon the finding that the N170 is sensitive to images of eyesisolated from the rest of the face, suggesting that it is sensitive to morethan just the visual structure of a face (Blau et al.
, 2007). The results ofthis study concluded that the N170 response to facial stimuli was modified accordingto the emotional expression of the face, and that this may indicate thetime-course of emotional processing (Blau et al., 2007). A studydone by Dering, Martin, Moro, Pegna, and Thierry in 2011 also supports the findingthat the N170 responds to more than just the facial structure (Dering et al., 2011).
Their study used different visual stimuli to measure face processing. Theyperformed three different experiments, all of which contained varying levels ofvisual stimuli containing both faces and cars. The results of these threeexperiments state that the N170 ERP component is able to process perceptual aspectsother than categorization, such as familiarity of, identity, ethnicity, andemotional expression of a particular face or facial stimuli (Dering et al.
, 2011).These findings are important because it gives weight to the idea that the N170does more than just process the basic visual structure of a face.In responseto the work of Dering et al., 2011, Eimer wrote a commentary that deals withthe face-selectivity of the N170 ERP component (Eimer, 2011).
In thiscommentary, Eimer states that images cropped to remove internal or externalfeatures may result in greater amplitudes of N170. Eimer refers to this as a”cropping effect” (Eimer, 2011). A notable finding is that the same effect occurswhen exposed to facial stimuli as when exposed to other stimuli, in this casecars and butterflies. Eimer suggests that this may lead to misinterpretedevidence of face-sensitivity of the N170.
In this article Eimer stresses thatelementary visual features of facial stimuli must be carefully controlled sincethey are essential in order to link the N170 to face-sensitive brain mechanisms(Eimer, 2011).There isa study from 2013 by Cao, Gaspar, Li, and Jiang that states that both faces andwords may evoke the N170 ERP component (Cao et al, 2013). In their experiment,the link between faces and words is tested by using a cross-category modificationto the N170. A significant irregularity was found between the N170 adaptation provokedby faces and by words. This is the first instance of EEG results showing thatneural discrimination of faces involves neural selectivity of words (Cao et al,2013). The results of this study suggest that the N170 response to faces formsa neural indicator for fluctuating representations of familiar photographic stimuli(Cao et al.
, 2011).Accordingto the research, in terms of visual perception, the N170 is a part of anexpertise system that starts at the base level for object recognition. It isnot limited strictly to the surface features of the face, as referenced by Caoet al., 2013 and Dering et al.
, 2011. The N170 shows differential activation/activationtimes for emotional recognition, and that there is evidence of more abstractmoments of facial recognition, including gender, ethnicity, emotionalexpression, and more. There are still many opportunities for expansion on thistopic. Further research might take a look at the overlap with cognitive sciencein an expertise theory model. It would be interesting to see if there are anydifferences in the activation of N170 in infants, as they most likely have notdeveloped enough expertise as compared to an adult. Further, thedifferentiation between non-face objects in varying situations might be examinedas well, to see what else the N170 ERP component might be responsible forprocessing.