The present study sought to better understand the nature of the neural representation of expression. Specifically, we sought to compare the coding of naturally occurring expressions with the dimensional representation of facial identity (face space). Individual frames depicting the naturalistic facial expressions of a single individual were analyzed and used to estimate the mean posture and image texture of a dynamic sequence. The dimensionality present within the optic flow variation was extracted through the application of principal component analysis (PCA). Pairs of static anti-expressions were subsequently created by reconstructing postures corresponding to ±2.15 standard deviations along the axes defined by the first and second principal components comprising the computed “expression space.” Using an adaptation procedure, we show that adapting to an expression selectively biases perception of subsequently viewed stimuli in the direction of its anti-expression, analogous to similar findings with identity, but does not bias perception in the orthogonal direction. These findings suggest that the representation of naturally occurring expressions can be modeled using the same kind of multidimensional framework as has been proposed for identity.

*subset*of facial expressions: Successful recognition of emotional expressions allows us to represent others' mental states, infer others' intentions, and predict behaviors, while impaired recognition of emotional expressions is often associated with social difficulties (Critchley et al., 2000; Hefter, Manoach, & Barton, 2005). However, basic emotional expressions represent only a small fraction of the facial expressions human adults are capable of. There are numerous expressions we encounter everyday, conveying, for example, confusion, boredom, or skepticism, which fall outside this classically defined set of prototypical emotions. Equally numerous subtle changes in facial posture mediate non-verbal communicative nuance. In addition, perceiving the postures of facial speech can support parallel auditory processing (McGurk & MacDonald, 1976).

*neutral*expression with the

*mean*expression.

*n*− 1 frames, flow fields that warped the facial features back to their position in the reference image, on a pixel-by-pixel basis, were calculated. By averaging these flow fields, the mean warp was calculated, which in turn was applied to the reference frame to derive the mean face shape. The flow fields were then adjusted so that they registered to the mean face shape rather than the original reference. The resulting fields describe the variation in face shape present within the sequence.

*x*and

*y*dimensions, and an RGB triplet, describing the texture variation for each point on the mean face shape, was derived. For each frame, these pixel vectors were concatenated to produce 500 frame vectors of length 5 ×

*image width*×

*image height,*to which PCA was then applied. A compressed movie file depicting the first eight principal components can be viewed in the Supplementary material accompanying this article.

*SD*s) either side of the sequence mean, along the dimension specified by the first principal component. The C and D pair represented corresponding points on the second principal component. The expressions within each pair are thus on diametrically opposite sides of the expression space (“anti-expressions”).

*SD*s from the mean (hereafter referred to as the AB and CD continua). Thus, the AB test stimuli appeared to morph steadily from 1.5

*SD*s of posture A toward 1.5

*SD*s of B in equidistant 0.5

*SD*steps (Figure 2). In contrast, the CD test stimuli morphed from 1.5

*SD*s in the C direction to 1.5

*SD*s toward D. The central stimulus in both continua corresponded to the sequence mean.

*SD*unit of the veridical mean). For the remaining participants, psychometric functions were modeled by fitting cumulative Gaussian functions. Adaptation is defined as a shift of the point of subjective equivalence (PSE) toward the adapting stimulus, indicating that neutral stimuli appear less like the adapting stimulus and more like the stimulus at the other pole of the continuum. While we had no

*a priori*reason to believe that discrimination sensitivity would vary as a function of the adapting condition, the standard deviations of the underlying Gaussian error distributions were also estimated for each subject; however, no significant effects were revealed.

*F*(4,48) = 13.9,

*p*< 0.001,

*η*

^{2}= 0.537], indicating that the effects of the adapting conditions differed across the test dimensions. To better understand this interaction, the effects within the AB and CD groups were considered in more detail.

*F*(2,12) = 25.9,

*p*< 0.001,

*η*

^{2}= 0.812]. The effect size observed represents a large effect (Cohen, 1988). Planned contrasts indicated that the PSEs were significantly shifted toward A following adaptation to A (

*M*= −0.48,

*SD*= 0.26) compared to the no adaptation baseline condition (

*M*= 0.00,

*SD*= 0.18) [

*t*(6) = 8.5;

*p*< 0.001 (two-tailed)]. Similarly, the PSEs showed a significant shift toward B following adaptation to B (

*M*= 0.34,

*SD*= 0.41) compared to the no adaptation baseline condition [

*t*(6) = 2.4;

*p*= 0.05 (two-tailed)]. However, a within-subjects ANOVA across orthogonal adapting conditions (adapt-C, no adaptation, adapt-D) failed to reveal a significant main effect of adapting condition [

*F*(2,12) = 1.87,

*p*= 0.20] indicating that the PSE did not vary as a function of adapting condition.

*F*(2,12) = 25.3,

*p*< 0.001,

*η*

^{2}= 0.808]. This effect size again represents a large effect (Cohen, 1988). Planned contrasts indicated that the PSE shifted significantly toward C following adaptation to C (

*M*= −0.33,

*SD*= 0.27) compared to the no adaptation baseline condition (

*M*= −0.04,

*SD*= 0.10) [

*t*(6) = 3.14;

*p*< 0.025 (two-tailed)]. Similarly, the PSE shifted significantly toward D following adaptation to D (

*M*= 0.40,

*SD*= 0.26) compared to the no adaptation baseline condition [

*t*(6) = 4.80;

*p*< 0.01 (two-tailed)]. Crucially, a within-subjects ANOVA across the orthogonal adapting conditions (adapt-A, no adaptation, adapt-B) revealed only a marginally significant effect of adapting condition [

*F*(2,12) = 4.05,

*p*= 0.082]. A paired

*t*-test confirmed that the mean congruent shift (difference between adapt-C and adapt-D) was significantly larger than the mean orthogonal shift (difference between adapt-A and adapt-B) [

*t*(6) = 2.5;

*p*< 0.05 (two-tailed)]. Thus, as with the AB group, PSEs shifted substantially less when the adapting condition was orthogonal to the test dimension.

*novel*expressions can produce perceptual aftereffects, comparable to the aftereffects observed with

*novel*identities. Whereas previous studies have used prototypical emotional expressions highly familiar to observers, the present study used statistically derived postures that were in all likelihood novel to observers. Traditionally, aftereffects have been taken as evidence of dedicated channels or populations coding for particular stimulus attributes; “if you can adapt it, it's there” (Mollon, 1974). However, the recent aftereffects produced by novel exemplars are hard to reconcile with this view (Thompson & Burr, 2009). After all, it is difficult to conceive of a dedicated representation for a facial posture never previously encountered. Adaptation to novel exemplars may instead suggest that a dimensionality extracted from an observer's previous experience of the natural variation may be used to represent novel instances using combinations of weights within a more permanent space. Nevertheless, the nature of the perceptual learning responsible for such a perceptual scaffold needs to be carefully considered. In particular, it is remains to be discovered how adaptation to specific descriptions interacts with adaptation to the representational systems on which these descriptions are based.

**Supplementary Movie 1.**Dynamic representation of the output of the PCA cycling sinusoidally from +2.15

*SD*s to - 2.15

*SD*s along each component. The adapting and test stimuli were still image reconstructions of points from the first and second principal components.