This was the first study to look at the ability to recognize facial expressions in young children through to adults in both typically developing individuals and individuals with autism. In contrast to prior research, this study controlled for the subtlety of the expressions and required participants make judgments of only briefly presented dynamic expressions. As mentioned earlier, such manipulations are extremely important. To date, the majority of research with both typically developing individuals and with individuals with autism has presented only prototypic expressions and allowed the participants to the view the expressions for as long as necessary before identifying them. Under such conditions, both developmental changes and potential differences between populations may be masked by what are likely ceiling effects in performance. This was clearly evident in our results of no group differences in the pretests.
When considering the results from the typically developing individuals, it appears that with development, these individuals become more proficient at recognizing subtle facial expressions; there was continuing improvement in the ability to recognize facial expressions from the child group through to the adult group. Although our paradigm was different, these findings are similar to those of Thomas, et al. (2007)
, who also found an improvement into adulthood. Thus, our results provide further evidence that many previous findings of no improvement in emotion recognition after the age of 10 (e.g., Bruce, et al., 2000
; Mondloch, et al., 2003
) may have resulted from these studies using methods that lead to performance ceiling effects. That is, by showing participants prototypical expressions and allowing unlimited time to respond, developmental changes that may be occurring past middle childhood have been masked. Given that there does appear to be improvement through to adulthood, at issue is what is changing with development.
Behavioral studies of face processing suggest that typically developing individuals slowly shift from a predominant reliance on more featural processing of faces in young childhood to ultimately having adult expertise in configural processing of faces (Mondloch, Dobson, Parsons, & Maurer, 2004
; Mondloch, Le Grand, & Maurer, 2002
; Schwarzer, 2000
). Configural processing is typically seen as the ability to perceive the spatial distances or relationships among aspects of the faces (e.g., eye separation) as opposed to attending to single non-spatial features such as the nose (Maurer, Le Grand, & Mondloch, 2002
). For emotion recognition, however, this distinction between configural and featural aspects of the face may not be as straightforward as is usually depicted in the facial recognition literature. Spatial distances can also vary in how easily they can be discriminated. For example, categorizing or perceiving whether a face is demonstrating fear or surprise requires the ability to pick up muscle movement difference in the eye region (Ekman, 2003
). When these expressions are prototypic or exaggerated, the amount of spatial movement from neutral to the full expression is much larger then when these expressions are subtle. Thus, in order to discriminate subtle facial expressions, individuals must be able to perceive very small motoric spatial differences of the muscles. We are suggesting that with development, not only do typical individuals rely more on configural information, but they also get better
at making subtle configural discriminations; thus, they may be getting better at picking up very small changes in muscle movements that occur during more subtle displays of emotion.
There is also evidence that with development, typically developing individuals become better at recognizing and categorizing faces (e.g., Carey, Diamond, & Woods, 1980
). Most cognitive models of face recognition in adults assume that individuals compare the faces they are viewing to either stored exemplars or to a multidimensional prototype that contains the mean of all of the dimensions that typically vary in faces (e.g., Valentine & Endo, 1992
). Valentine and others have called this multidimensional representation the “face space.” It is assumed that the center of this “face space” represents the central tendency of all the features and configurations that vary in faces. As an individual gains experience with faces in their environment, the faces are represented in this face-space according to the values of their features. More typical values (and therefore more typical faces) lie closer to the center of this framework, while less typical faces fall along the outer edges of the framework (Valentine, 1991
). The development of a face-space framework depends on experience and necessitates that over time, individuals be able to abstract both central tendencies and the range of values of features from their experience with faces. In essence, as children get older they continue to develop a more refined prototype. Essentially, the “face space” represents the totality of an individual’s experiences with faces and can be viewed as the knowledge base that drives a “top-down” perceptual learning processes that allow individuals to make more subtle facial discriminations as experience and learning increases.
While the face-space model has not previously been applied to recognizing emotional expressions, it is likely that emotional expressions are also discriminated by comparing multiple aspects of the to-be-categorized expression (e.g., the movement of the eyes region, mouth, or eyebrows) to separate prototypes of each of the expressions. Essentially, recognizing facial expressions can be considered a type of categorization task (Humphreys et al., 2007
). As with faces, facial expression categories may have “typicality structures” where individual instances of an expression vary in typicality. It is likely that prototypical expressions are easiest to categorize, and as expressions become more subtle or less prototypical, comparisons to prototypes becomes more difficult thus the expression is more difficult to categorize. Similar to the development of face recognition, with development and consequently experience, this comparison process may become more sophisticated and precise as individuals develop proficiency at discriminating expressions. Over development, children and adolescents are not necessarily developing new or different underlying processes, but rather they are getting more efficient at processing subtle configural information and comparing the information to a central representation (McKone & Boyer, 2006
The results from the individuals with autism found that while young children with autism did not recognize expressions as well as the typically developing controls, these group differences were not seen with the older children. However, by adulthood, individuals with autism were again performing worse than the typically developing controls. In essence, the adults with autism never seem to attain the levels of proficiency at emotion recognition of typically developing adults. These findings seem to indicate, as suggested earlier, that the lack of a developmental approach and a failure to consider task demands may be contributing to the equivocal findings regarding emotion recognition skills in this population.
Some previous research has shown that individuals with autism have impairments only in recognizing certain emotions. While this has varied from “surprise” (Baron-Cohen, Spitz, & Cross, 1993
), to “anger” (Teunisse & De Gelder, 2001
), the somewhat more consistent finding has been that they exhibit a specific deficit in the recognition of “fear” (Howard, et al., 2000
; Humphreys et al., 2007
; Pelphrey et al, 2002
). Our findings did not show any specific deficit for one
particular emotion. In the youngest age group, the children with autism performed worst on “afraid” and “angry,” but the mean score for “angry” was no lower than that for “afraid.” In all three of our older age groups, there was no one emotion that caused specific difficulty over and above the others for the individuals with autism. It is not clear how to interpret our results in light of the previous findings, since the current study was the only one to simultaneously vary expression intensity, use motion, and limit how long participants had to view the expression. Additionally, when only one exemplar of each expression is used (as was the case in the current study as well as others), results may be idiosyncratic to the particular exemplar used for each expression. The current study was not designed to carefully compare performance on different expressions, and future research designed to do so will require multiple exemplars of each expression and intensity.
At issue, then, is why
the adults with autism never reach the levels of proficiency at emotion recognition of typically developing adults. Research on the face recognition abilities of individuals with autism has suggested that they have a tendency to process faces and, hence, probably facial expressions, based more on featural than configural information (see reviews by Dawson, Webb, & McPartland, 2005
; Sasson, 2006
). It is unclear from this research whether individuals with autism are unable
to process configural information or whether they are just less efficient at processing this information. McKone and colleagues (Gilchrist & McKone, 2003
; McKone & Boyer, 2006
) have found that in typically developing individuals, even young children can process configural facial information provided that information is obvious (e.g., a large degree of eye separation). They suggest that with development, there is not a shift in processing strategies from features to configurations, but rather there is a developing ability to process subtle configural information. Similarly, it may be that individuals with autism are able to process spatial information, but they do not achieve the level of proficiency needed to perceive subtle facial expressions as well as typically developing adults can.
A second contributory factor may be that individuals with autism have poor mental representations of the basic emotions. As discussed, to develop a mature representation of faces (e.g., a face-space) it is necessary abstract prototypic representations of facial information. With respect to emotion, it is critical to have stored prototypic representations or exemplars of the basic emotional expressions. Research with individuals who have autism suggests that they may have difficulties in abstracting prototypical representations of categorical information (e.g., Gastgeb, Strauss, & Minshew, 2006
; Klinger & Dawson, 2001
). Thus, it might be expected that individuals with autism would have difficulty comparing a subtle facial expression to their inadequately formed stored prototypic representations of the different basic expressions. Interestingly, recent results have suggested that individuals with autism have difficulty not only with face categories, but seem to have a general difficulty processing and categorizing complex, subordinate stimuli (Behrmann, et al., 2006
; Gastgeb, et al., 2006
). Thus, the difficulty exhibited with subtle facial expressions may be more general in nature.
While the present study was not designed to test these various possibilities, it is the first study to demonstrate how the ability to recognize emotional expression compares in individuals with autism and typically developing individuals at different ages. More importantly, it demonstrates that while even young children with autism are able to perceive prototypic facial expressions, individuals with autism never achieve a level of proficiency that is comparable to typically developing adults. As the underlying cause of these differences are explored in future research, it will be critically important for future studies to carefully control the difficulty of the emotion expression task to with respect to both the subtlety of the expressions and the amount of time individuals have to process this information.