Under the sun: adaptation effects to changes in facial complexion

BACKGROUND

Many Western people enjoy sunshine, and through the sun's stimulated increase in melanin, the skin tone or skin complexion will darken (and lighten again during winter). Although the initial salience of such a new look is remarkable - especially in the face - we seem to adapt to this new look relatively quickly. Research on face adaptation in general repeatedly showed that the inspection of manipulated versions of faces (so-called adaptor faces) leads to a change of the perception of subsequently presented faces. The present study investigates face adaptation to very natural changes in faces such as changes in complexion.

METHODS

During the adaptation phase in the present study, participants saw faces with either strongly increased or decreased complexion. After a pause of 5 min, participants had to identify the veridical (non-manipulated) face out of two faces (a face slightly manipulated in complexion combined with the non-manipulated face) during a test phase.

RESULTS

Results show strong adaptation effects to decreased complexion intensities.

DISCUSSION

It seems that we are updating our facial representations in memory quite quickly (i.e., optimizing our processing through adaptation) and seem to sustain those new representations over a certain timespan (at least 5 min). Our results demonstrate that changes in complexion draw our attention for deeper analysis (at least with decreased complexion). However, it loses its informative quality quickly via fast and relatively sustainable adaptation.

Changes in face category induce stronger duration distortion in the temporal oddball paradigm

A novel stimulus embedded in a sequence of repeated stimuli is often perceived to be longer in duration. Studies have indicated the involvement of repetition suppression in this duration distortion, but it remains unclear which processing stages are important. The present study examined whether high-level visual category processing contributes to the oddball's duration distortion. In Experiment 1, we presented a novel face image in either human, monkey, or cat category after a repetition of an identical human face image in the temporal oddball paradigm. We found that the duration distortion of the last stimulus increased when the face changed across different categories, than when it changed within the same category. However, the effect of category change disappeared when globally scrambled and locally scrambled face images were used in Experiments 2 and 3, respectively, suggesting that the difference in duration distortion cannot be attributed to low-level visual properties of the images. Furthermore, in Experiment 4, we again used intact face images and found that category changes can influence the duration distortion even when a series of different human faces was presented before the last stimulus. These findings indicate that high-level visual category processing plays an important role in the duration distortion of oddballs. This study supports the idea that visual processing at higher visual stages is involved in duration perception. (219 words).

Adults with autism spectrum disorder show evidence of figural aftereffects with male and female faces

The norm-based coding model of face perception posits that face perception involves an implicit comparison of observed faces to a representation of an average face (prototype) that is shaped by experience. Using some methods, observers with autism spectrum disorder (ASD) have shown atypical face perception, but other methods suggest preserved face perception. Here, we used a figural aftereffects paradigm to test whether adults with ASD showed evidence of norm-based coding of faces, and whether they encode separate prototypes for male and female faces, as typical observers do. Following prolonged exposure to distorted faces that differ from their stored prototype, neurotypical adults show aftereffects: their prototype shifts in the direction of the adapting face. We measured aftereffects following adaptation to one distorted gender. There were no significant group differences in the size or direction of the aftereffects; both groups showed sex-selective aftereffects after adapting to expanded female faces but showed aftereffects for both sexes after adapting to contracted face of either sex, demonstrating that adults with and without ASD show evidence of partially dissociable male and female face prototypes. This is the first study to examine sex-selective prototypes using figural aftereffects in adults with ASD and replicates the findings of previous studies examining aftereffects in adults with ASD. The results contrast with studies reporting diminished adaptation in children with ASD.

Face identity aftereffects increase monotonically with adaptor extremity over, but not beyond, the range of natural faces

Face identity aftereffects have been used to test theories of the neural coding underlying expert face recognition. Previous studies reported larger aftereffects for adaptors that are morphed further from the average face than for adaptors closer to the average, which appeared to support opponent coding along face-identity dimensions. However, only two levels were tested and it is not clear where they were located relative to the range of naturally occurring faces. This range is of interest given the functional need of the visual system both to produce good discrimination of real everyday faces and to process novel kinds of faces that we may encounter. Here, Experiment 1 establishes the boundary of faces judged as being able to occur in everyday life. Experiment 2 then shows that aftereffects increase with adaptor extremity up to this natural-range boundary, drop significantly immediately outside the boundary, and then remain stable with no drop towards zero even for highly distorted adaptors far beyond the boundary. Computational modelling shows that this unexpected pattern cannot be explained either by a simple opponent or by a classic multichannel model. However, its qualitative features can be captured either by a combination of opponent and multichannel coding (raising the possibility that not all identity-related face dimensions are opponent coded), or by a 3-pool model containing two S-shaped-response channels and a central bell-shaped channel around the average face (raising the possibility of unexpected similarities with coding of eye and head direction).