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Iconic faces are not real faces: enhanced emotion detection and altered neural processing as faces become more iconic

Iconic representations are ubiquitous; they fill children’s cartoons, add humor to newspapers, and bring emotional tone to online communication. Yet, the communicative function they serve remains unaddressed by cognitive psychology. Here, we examined the hypothesis that iconic representations communicate emotional information more efficiently than their realistic counterparts. In Experiment 1, we manipulated low-level features of emotional faces to create five sets of stimuli that ranged from photorealistic to fully iconic. Participants identified emotions on briefly presented faces. Results showed that, at short presentation times, accuracy for identifying emotion on more “cartoonized” images was enhanced. In addition, increasing contrast and decreasing featural complexity benefited accuracy. In Experiment 2, we examined an event-related potential component, the P1, which is sensitive to low-level visual stimulus features. Lower levels of contrast and complexity within schematic stimuli were also associated with lower P1 amplitudes. These findings support the hypothesis that iconic representations differ from realistic images in their ability to communicate specific information, including emotion, quickly and efficiently, and that this effect is driven by changes in low-level visual features in the stimuli. Electronic supplementary material The online version of this article (doi:10.1186/s41235-016-0021-8) contains supplementary material, which is available to authorized users.

There was also an interaction between level of schematization and expression [F(12,552)= 6.27, p<.001, η 2 p = .12]. Planned contrasts revealed this interaction was driven by faster identification of disgust faces at shorter presentation times.
Finally there was a three-way interaction between stimulus type, expression, and presentation time [F(36,1656)= 11.55, p<.001, η 2 p = .20]. This interaction was driven once again by an exaggerated profile for disgust compared to the other expressions (the difference in accuracy between the disgust and other stimulus categories was wider at the faster presentation times).

Experiment 2 Behavioral
For accuracy, there was a main effect of expression type [F(2.4, 66.64)= 9.08, p<.001, η 2 p = .25]. While the disgust expression was different from happy (p=.04), this effect was primarily driven by the shocked expression differing from both happy (p<.001) and neutral (p=.001). No other comparisons were significant (ps>.250). The shocked faces may have seemed more ambiguous, as at 92.9%, they were over 3% lower than happy (97.4%) and neutral (96.5%) faces.
There was also an interaction between stimulus type and expression type [F(4.39,122.90)= 6.74, p<.001, η 2 p = .194]. Follow up contrasts revealed that this interaction was driven by the relative flatness of cartoon images (all ps =1.0, with a range of 98.0 -98.5%) compared to the other stimulus categories. For the rotoscoped images, the shocked expressions were significantly lower (90.0%) than the happy (96.8%) and neutral images (95.2%) (ps= .001 and .01 respectively). Similarly, for photos, the shocked expressions (90.1%) were significantly lower than the happy (97.2%) and neutral (96.2%) images (ps<.001). However here the disgust faces (92.4 %) were also low enough compared to the happy faces to be significant (p=.048).
Altogether this pattern of accuracy to emotional expressions suggested that the shocked and disgusted faces were easier to mistake than the happy or neutral, or that, more likely, with more realistic faces there is simply a bias to responding happy or neutral versus shocked or disgusted.
For reaction times, there was also a main effect of expression type [F(3, 84)= 4.41, p=.006, η 2 p = .14]. Planned comparisons revealed that this was driven by happy images being responded to faster than disgust (p=.024) or shocked (p=.012) images.
There was no interaction between stimulus type and expression type (p=.475).

Expressions
For P1 amplitude at electrode Oz, there was also a main effect of expression [F(3,75)= 4.83, p<.003, η 2 p = .16]. Follow up comparisons revealed that this was driven by the unique pattern of disgust faces, which were significantly different from shocked faces (p=.02) and neutral faces (p=.01). No other comparisons were significant (ps>.37). There was also a significant interaction of expression type and stimulus type [F(4.66,116.48)= 2.58, p=.033, η 2 p = .09].
There were no significant expression effects on the P1 for latency at Oz, or for amplitude or latency at P9/P10 (ps>.05).

N170 -P9/P10
Amplitude Analysis of N170 amplitude revealed a main effect of stimulus type [F(1.63,42.29)= 8.88, p=.001, η 2 p = .25] (see Figure 5). Contrasts revealed that the N170 for the cartoon stimulus set was larger than both the rotoscoped and photo sets (p=.017 and p=.004, respectively), but that these latter two sets did not differ from each other (p=.167). Because the rotoscoped images and the cartoon images share contrast, this finding suggests that featural simplicity of the cartoon stimulus set rather than differences in contrast underlie differences in N170 amplitude, which matches the differences found in the P1 analyses.
There was also an interaction between electrode and stimulus set [F(2, 52)= 14.05, p=.03, η 2 p = .13]; Follow-up ANOVAs at each electrode site (i.e., one for P9 and one for P10) revealed that this is because, while the main effect of stimulus set was in the expected direction, it was not significant at P9 when that electrode was considered alone (p=.11); in contrast, the effect was robust at P10 [F(2, 56)= 11.19, p<.001, η 2 p = .29].
Pairwise comparisons revealed that the disgust images evoked a larger amplitude N170, which differed from that for all other expressions (ps>.05), which in turn did not differ from each other (all ps > .2).
There was an interaction between stimulus set and expression type [F(6, 156)= 5.07, p=.009, η 2 p =.10]. Planned contrasts revealed that the main effect of stimulus set was not significant for the neutral expression stimuli (p=.143) but was significant for all other expressions (all Fs > 4.0, all ps< .05), suggesting that the advantage of featural complexity may have a greater advantage in emotional (i.e., non-neutral) stimuli.

Latency
Analysis of N170 latency revealed a main effect of stimulus type [F(2, 52) = 59.46, p<.001, η 2 p = .70]. Contrasts revealed differences in latency between the N170 for photos and both rotoscoped and cartoon stimuli (ps <.001). The rotoscoped and cartoon stimuli evoked an N170 that was, on average, 13.9 ms faster than the photo stimuli.
Pairwise contrasts revealed that, although happy expressions were processed slightly faster than other expressions (on average 1.56 -2.66ms), there were no significant latency differences between pairs of expression type (ps>.05).
Pairwise comparisons revealed that this was driven by the disgust faces showing a larger difference than all other stimulus types (ps<.001). This further supports our disgust stimuli as being processed uniquely, but as this did not vary with stimulus type, this information is tangential to our hypothesis.
There was also peak to peak interaction of stimulus type and emotional expression, [F(6, 150)= 3.63, p=.002, η 2 p =.13]. However, in follow up analyses this interaction seemed to be driven solely by a significantly greater N170/P1 difference for happy faces only between rotoscoped and photo stimulus sets (p=.044).