In McCloud's Understanding Comics he proposed his theory of "cartoon identification" that cartoony* images are "identified" with better than realistic images. This study (pdf) tested McCloud's theory by using behavioral measures of a 7-point rating and EEG measures of the brain's electrical activity.
I've found that this theory of McCloud's was a bit ambiguous, since people have interpreted it in two different ways. It can either mean that people "identify" with cartoony images meaning...
1) They are perceived cognitively at a more "base" level.
2) That they empathize with the characters more.
Critics have usually tapped into the second reading, since it is close to a claim about how people "identify" with characters in a literary sense. However, I've always been more partial to the first interpretation (which I attempted to codify further in my book), though the two views aren't necessarily mutually exclusive. Indeed, cartoony images could evoke more empathy because they are more conceptually basic. This study aimed to examine the second version, strictly with a view of the degree of "empathy" styles create.
The experimenters contrasted comic strips that featured two opposing characters who were depicted in either realistic or cartoony styles (as in the example above) and put into different scenarios to evoke reader sympathy via who wins the confrontation (strips had varying endings, ex. winner is congratulated by a woman vs. loser is consoled by a woman). Participants only viewed one depiction, and only one option for the ending.
They then compared ratings in a behavioral study on a 7-point scale measuring empathy to both "winners" and "losers", and in a separate population, measured EEG brainwaves for the same stimuli.
Brain areas were activated that related to social perception, recognizing facial expressions, and seeing another person's pain**. They found both higher behavioral ratings of empathy and greater activation in the brain for areas for the cartoony characters than the realistic characters for both "winners" and "losers" (though different brain areas for different roles). They take these results to be support for McCloud's theory of identification that indeed, cartoony images do invoke greater empathy from a reader than realistic images.
*The authors, and McCloud, often use "iconic" to mean "cartoony" — I'm going to avoid this because it doesn't accurately convey what "iconic" means in a semiotic sense (i.e. meaning through resemblance). Technically, both cartoony and realistic images are "iconic."
** Just a caveat for those who actually follow the link to the pdf poster. The study shows nice pretty pictures of brains with activated regions to support its hypothesis, but these can be misleading given the actual methods used. Unlike a technique like fMRI, EEG does not give much information about where in the brain something occurs, and is much better at when it appears (i.e. the timing of processing). These brain images and results were gained using a "source localization" procedure which extrapolated from the data what brain regions were being used, a technique that is commonly employed, but often controversial. This isn't to say that the results are inaccurate, but they should be understood with this context.