Color Compensatory Mechanism of Chromatic Adaptation at the Cortical Level.

Reportedly, some chromatic adaptations have extremely short temporal properties, while others have rather long ones. We aimed to dynamically measure the transition of a neutral point as an aftereffect during chromatic adaptation to understand the temporal characteristics of chromatic adaptation. The peripheral retina was exposed to a yellow light to progress color adaptation, while the transition of a neutral point was measured at the fovea. In Experiment 1, the aftereffect had initially progressed but subsequently recovered despite ongoing chromatic adaptation and regardless of the retinal exposure size, suggesting that the adaptation mechanism at the cortical level continues to readjust the color appearance based on daylight conditions. Experiment 2 was similar to Experiment 1, except that it included participants of varying ages. Older eyes behaved in a homologous manner with younger eyes in Experiment 2, albeit quantitative differences. Regardless of age, similar recalibration of neutral points shifted by color adaptation suggests the color compensation function in older eyes may not change due to long-term chromatic adaptation by optical yellowing. In conclusion, the chromatic adaptation mechanism at the cortical level readjusts color perception, even in younger eyes, according to the daylight neutral point. This daylight information may be stored in the neural mechanism of color vision.

Color criteria of facial skin tone judgment.

Facial skin tone recognition provides significant social and ecological information to humans. This study utilized a five-alternative forced-choice test design wherein participants were asked to judge which color (red, yellow, green, blue, or white) they perceived as too strong in stimulus skin tone images. The results showed that the participants' reference point of facial skin tone judgement was closer to the centroid facial skin tone observed on a daily basis than the chromaticity of measured or remembered facial skin tones of the observer. This result was similar for observers from Japan and the United Kingdom. The distance between the reference point of facial skin tone judgment and the average skin chromaticity of each local group was smaller when the stimulus image was recognized as a face compared to when a uniform facial skin tone patch was used. Therefore, humans unconsciously memorize the facial skin tones they encounter in daily life and judge facial skin tones based on the centroid. Furthermore, it is critical to recognize an image as a face for the evaluation of facial skin tone.

Perception of human skin conditions and image statistics.

The color and texture of human skin provide useful information in evaluating a person's health, emotion, attractiveness, and so on. Whereas the appearance of skin is a product of optical properties originating from its own biophysical substrates, recent psychophysical studies of surface material perception indicate a possibility that humans can perceive skin conditions from relatively simple image features such as texture statistics. The present study investigated the structure of multidimensional perceptual rating data for 289 female skin images and analyzed diagnostic low-level image statistics. We found that various skin appearances can be summarized into two dimensions, pleasantness and glossiness, and either dimension is well correlated with a small set of subband luminance and color statistics. We confirmed that manipulations of these critical image statistics significantly alter the appearance of skin in the expected direction. The results support a notion that humans perceive skin properties essentially based on two cardinal dimensions as predicted by simple image statistics.

Desaturation-Induced Brightness in Face Color Perception.

The distinctiveness of perception of face from nonface objects has been noted previously. However, face brightness is often confounded with whiteness in the beauty industry; few studies have examined these perceptual differences. To investigate the interactions among face color attributes, we measured the effect of saturation on brightness and whiteness in both uniform color patches and face images to elucidate the relationship between these two perceptions. We found that, at constant luminance, a uniform color patch looked brighter with an increase in saturation (i.e., the Helmholtz-Kohlrausch effect occurred), while in contrast, brightness of a facial skin image looked less bright with increased saturation (i.e., contrary to the Helmholtz-Kohlrausch effect), which suggested this interaction of color attributes was influenced by top-down information. We conclude that this inverse effect of saturation on brightness for face images is not due to face recognition, color range of the skin tone, the luminance distribution, or recognition of human skin but due to the composite interactions of these facial skin factors in higher order recognition mechanisms.

Evidence for a central component in adaptation to chromatic light.

Adaptation to environmental light allows our visual system to compensate for dynamic changes in the visual environment for avoiding everyday hazards (e.g., misreading traffic lights) and for accurate reaching. We investigated the hypothesis that adaptation to coloured light is achieved not only via photoreceptors in the retina and monocular contrast adaptation, but also by a binocular process that may occur at the level of the cerebral cortex. In the present study, to determine the role of higher-order cortical binocular processes in adaptation to coloured light, participants were adapted to chromatic light such that the duration of adaptation during monocular processing differed from that during binocular processing. A dichoptic device was used to adapt each eye independently. The extent of after-effects, measured as the distance between the neutral points before and after adaptation to coloured light, depended on the duration of adaptation not only at the monocular level but also at a higher cortical level downstream from binocular fusion. Thus, contrast adaptation to coloured light occurs on at least two levels; it is a result of monocular processes at one level and binocular processes at the other, and each type of process exhibits different temporal characteristics. The results of this study suggest a significant cortical role in adaptation to changes in lighting conditions or the optical environment, including the effects of age on the eye, and the necessity of further investigation to clarify the functional connection between chromatic adaptation by photoreceptors and chromatic adaptation by cortical systems.

The light-makeup advantage in facial processing: Evidence from event-related potentials.

The effects of makeup on attractiveness have been evaluated using mainly subjective measures. In this study, event-related brain potentials (ERPs) were recorded from a total of 45 Japanese women (n = 23 and n = 22 for Experiment 1 and 2, respectively) to examine the neural processing of faces with no makeup, light makeup, and heavy makeup. To have the participants look at each face carefully, an identity judgement task was used: they were asked to judge whether the two faces presented in succession were of the same person or not. The ERP waveforms in response to the first faces were analyzed. In two experiments with different stimulus probabilities, the amplitudes of N170 and vertex positive potential (VPP) were smaller for faces with light makeup than for faces with heavy makeup or no makeup. The P1 amplitude did not differ between facial types. In a subsequent rating phase, faces with light makeup were rated as more attractive than faces with heavy makeup and no makeup. The results suggest that the processing fluency of faces with light makeup is one of the reasons why light makeup is preferred to heavy makeup and no makeup in daily life.