Biology subjects

Nakauchi, S.

Publications and source records attributed to Nakauchi, S..

16 recordsLinked to original sources

bioRxiv2026

What makes an angry face uncomfortable? Distinct contributions of facial expression, interpersonal distance, facial stimulus type, and gaze in virtual reality

Why does an angry face feel uncomfortable? The answer is that it signals a threat. However, a face is only part of an encounter, and distance, facial stimulus type, and gaze may shape discomfort regardless of perceived anger. To separate these cues, we conducted three within-subjects virtual reality experiments. In each experiment, 24 adults viewed avatars at intimate, personal, and social distances (30, 100, and 300 cm, respectively) and rated the face's perceived anger and their own discomfort; head movement was recorded in Experiments 2 and 3. In Experiment 1, the expression (angry, neutral) and facial color (natural, red) were crossed with distance; in Experiment 2, a featureless mannequin served as a nonface comparison; and in Experiment 3, the gaze direction (direct, averted) was manipulated. Expression primarily determined perceived anger, whereas distance predominantly determined discomfort: A nearby neutral face was uncomfortable despite low perceived anger (Experiment 1). A neutral human face was more uncomfortable than a mannequin, although both received similarly low perceived-anger ratings (Experiment 2). Direct gaze increased the discomfort without changing perceived anger (Experiment 3). Backward head movement exhibited a similar pattern, with participants leaning back more from human faces than from the mannequin. These results indicate that the discomfort associated with an angry face is not merely explained by perceived anger. Instead, social discomfort was differentially associated with interpersonal distance and gaze direction and differed between the human-face and mannequin conditions.

neuroscience↗

bioRxiv2026

The congruency between anger intensity and reddish facial color modulates the early posterior negativity (EPN)

Facial color is associated with the perceptual evaluation of emotions, and compared with faces with original facial or greenish color, reddish angry faces are often judged as having higher emotion intensity. Although perceptual modulation by the relationship between anger and red has also been reported from the perspective of electroencephalography (EEG), how variations in perceived emotion intensity are reflected in brain activity remains unclear. This study investigated whether EEG activity associated with face and facial expression processing is modulated as a function of the interaction between facial color and perceived emotion intensity. In the experiment, we recorded EEGs while participants evaluated emotion intensity using facial stimuli created by combining morph continua from neutral to angry expressions with three types of facial color conditions (original, red, and green). The results revealed that, in the red facial color condition, the early posterior negativity (EPN) amplitude significantly increased as a function of emotion intensity compared with those in the original and green facial color conditions. These findings suggest that the early, automatic affective processing of facial expressions, reflected in the EPN, is modulated by the combination of facial color and emotion intensity. Our findings provide new evidence that early, automatic affective processing of facial expressions, as indexed by the EPN, is modulated by the congruency between high anger intensity and a reddish facial color. HighlightO_LIReddish angry faces increase the ERP component associated with emotion evaluation. C_LIO_LIThe relationship between anger and red is evident in the left hemisphere. C_LIO_LIThe interaction between facial expression and color occurs at a later cognitive processing stage than facial expression or facial color processing alone. C_LI

neuroscience↗

bioRxiv2026

Direction-dependent bias in the perceived approach speed and its implications for robot motion design

Humans frequently interact with robots that approach from different directions; however, how the approach direction systematically biases the perceived speed remains unclear. Here, we demonstrate a robust direction-dependent bias in the perceived approach speed, whereby objects approaching from behind are consistently perceived as moving faster than those approaching from the front are. Across five experiments using both physical robots and immersive virtual reality, this bias manifests as a systematic shift in the point of subjective equality without corresponding changes in discrimination sensitivity, indicating a perceptual bias rather than altered precision. We further show that this effect can be explained by body-induced differences in interocular velocity that arise when observers orient themselves toward stimuli approaching from behind. This bias persists in virtual environments and for nonrobotic objects, suggesting that it is driven primarily by visual factors rather than multisensory cues or robot-specific appearance. Together, these findings provide a perceptual foundation for incorporating approach direction into the design and evaluation of the motion behavior of robots in human-centered environments.

neuroscience↗

bioRxiv2025

Human trust and emotion in the context of collision avoidance with an autonomous mobile robot: An investigation of predictability and smoothness in virtual reality

The integration of mobile robots into human environments requires that they behave in trustworthy ways. Trust develops through interaction, and erratic movements can easily break it. Previous research examined motion predictability and smoothness separately, but their combined effect remains unclear. We investigated how predictability (consistent versus random) and smoothness (gradual versus abrupt) influence human trust and emotion during collision avoidance in virtual reality. Twenty-six participants encountered a robot across repeated trials. We measured subjective valence, arousal, and trust, along with skin conductance responses. Predictability dominated the results. Consistent robot behavior led to increased trust and positive feelings over time. Conversely, unpredictable behavior kept trust low. Smoothness acted mainly as a moderator for arousal; smooth paths reduced the stress caused by unpredictable moves. We also found that prolonged proximity in gradual movements raised physiological arousal, even if users reported feeling calm. These findings suggest predictability drives social acceptance more than smoothness. In order to promote trust, robot designers should prioritize consistent, learnable behaviors. HighlightsO_LIWe examined AMR motion predictability and smoothness in VR collision avoidance. C_LIO_LIPredictable motion significantly improved human trust and valence over time. C_LIO_LIUnpredictable and abrupt behavior increased physiological arousal. C_LIO_LISmoothness modulated arousal primarily when behavior was unpredictable. C_LIO_LIPredictability is more critical than smoothness for social acceptance of AMRs. C_LI

neuroscience↗

bioRxiv2025

Material perception relies on context-dependent active sensing strategies

Material perception is typically studied under passive and highly constrained viewing conditions, thus leaving it unclear whether humans rely on active sampling strategies to resolve perceptual ambiguities. In everyday vision, however, observers naturally move their heads and manipulate objects with their hands to obtain informative cues, thus raising the question of how such exploratory actions contribute to material recognition. We used immersive virtual reality to examine whether viewpoint changes and object manipulations support the discrimination of visually challenging materials, specifically metal versus glass, whose appearances depend strongly on the associated illumination and viewing geometry. On the basis of three experiments, we revealed that observers systematically increased their exploration tendencies when the identity of a material was ambiguous and that this increased movement was associated with higher discrimination accuracy. By independently manipulating head- and hand-based motions, we identified the context-dependent contributions of each modality, revealing that viewpoint changes dominated when multiple objects could be compared simultaneously, whereas object manipulation was more effective when only a single target was available. Moreover, the participants differed in terms of the efficiency of their sampling strategies, and those who employed more informative exploration patterns exhibited both higher accuracy and greater performance gains across different trials. These results demonstrate that material recognition depends on flexible, context-dependent active sensing rather than passive evaluations of static images. Our findings introduce a new framework, according to which material perception emerges from an adaptive perception-action loop, thus highlighting the functional role of exploratory behaviors in complex visual environments. Significance StatementThe task of material perception in natural environments depends not only on the optical properties of surfaces but also on the actions that observers take to reveal informative visual cues. However, most laboratory studies rely on passive viewing, and how humans actively regulate their own movements to reduce perceptual uncertainty remains unknown. On the basis of three virtual-reality experiments, we showed that observers flexibly perform head- and hand-based explorations to enhance the discriminability of visually ambiguous materials. These exploratory strategies were strongly context-dependent and exhibited systematic individual differences that predicted learning-related improvements. Our findings demonstrate that material perception emerges from an adaptive perception-action loop rather than static image analyses, providing a framework for understanding how humans actively acquire diagnostic sensory information in complex environments.

neuroscience↗

bioRxiv2025

Task-dependent pupillary responses to glossiness and attractiveness judgments

Human pupillary responses are influenced not only by low-level visual properties but also by cognitive and affective factors related to task demands. However, the temporal dynamics of how different evaluative tasks modulate pupil size remain poorly understood. In this study, we investigated how pupillary responses vary when observers evaluate the same set of object images for either glossiness or attractiveness. These two perceptual attributes were selected to exemplify distinct cognitive demands: one rooted in surface-level visual analysis and the other involving emotional valuation. The stimuli consisted of 60 grayscale object photographs selected from the THINGS database, representing common real-world items without social or facial content. Low-level image statistics were controlled using histogram matching to equalize luminance and contrast across all stimuli. Participants viewed each image for 3000 ms while maintaining central fixation and rated either its glossiness or attractiveness on a 7-point scale in separate task blocks. Ground-average waveforms revealed task-dependent modulations of pupil size across rating levels, consistent with prior reports that pupillary responses vary with evaluative context even for identical stimuli. Using temporal principal component analysis and generalized additive modeling, we found that higher glossiness ratings were associated with greater pupil constriction at early, light-reflex-like latencies, whereas higher attractiveness ratings elicited greater pupil dilation at later time points. These findings suggest that distinct temporal profiles of pupil size reflect task-specific processing demands, potentially aligning with the notion that visual and affective evaluations may unfold in temporally distinct stages, as suggested in prior theoretical models. Our results underscore the value of pupillometry as a non-invasive tool for dissociating task-dependent perceptual processes, with potential applications in cognitive neuroscience and affective computing. Impact statementThis study reveals that pupil size reflects not only what we see, but also how we evaluate it. By showing distinct temporal signatures of pupillary responses for glossiness versus attractiveness judgments of the same images, our findings suggest temporally dissociable stages of sensory and affective processing. These results highlight the value of pupillometry as a non-invasive index of task-dependent cognitive evaluation.

neuroscience↗

bioRxiv2025

Illuminating the Unconscious: Physical and Perceived Brightness in Cognitive Processing

This study investigated how physical luminance and perceived brightness affect breakthrough time (BT) under continuous flash suppression (CFS). Experiment 1 examined whether the glare illusion--which increases subjective brightness without altering actual luminance--would shorten BT compared to physically identical controls. The results revealed no difference, suggesting that subjective brightness alone does not expedite emergence into awareness. Experiment 2 assessed whether the partial suppression of the illusions inducers influenced detection speed and revealed that subjective brightness stimuli gained a BT advantage. Experiment 3 tested participants ability to discriminate real versus illusory brightness while stimuli remained suppressed; performance above chance for both conditions indicated that physical and perceived brightness cues were processed unconsciously. Together, these findings suggest that contextual brightness illusions are not simply lost below awareness--they can be discriminated in unconscious vision. Statement of RelevanceUnderstanding how the visual system processes brightness illusions--even prior to awareness--is crucial for uncovering the brains deeper mechanisms of perception. This research clarifies the limits of unconscious visual processing by showing that illusions can be registered without consciousness but do not necessarily expedite emergence into awareness. These insights advance theoretical models of hierarchical vision and demonstrate that while early cortical areas prioritize actual luminance in determining whether a stimulus reaches awareness, higher areas can still encode illusory brightness beneath the threshold of consciousness. Thus, the studys findings have broad implications for both basic neuroscience--refining how we think about the boundary between unconscious and conscious perception--and for applied fields seeking to harness or mitigate perceptual illusions. Research Transparency StatementO_ST_ABSPreregistrationC_ST_ABSAll experiments were not preregistered. Conflicts of interestThe authors declare that they have no competing interests. Data availabilityThe code and data underlying the results presented in the study are available from the Open Science Framework repository (https://osf.io/u5der/). EthicsThe Committee for Human Research of the Toyohashi University of Technology approved the experiments (2023-20). Written informed consent was obtained from all participants after the procedural details were explained to them. FundingThis work was supported by JSPS KAKENHI (Grant Numbers JP22K17987 to H.T., JP24H01551 to T.M., JP23KK0183 to T.M., JP20H05956 to S.N.), JSPS Grant-in-Aid for JSPS Fellows (Grant Number JP24KJ1313), and Young Principal Investigator fund JPMJFS2121. Artificial intelligenceDuring the preparation of this work, the authors used ChatGPT o1 and Grammarly to improve the language, and the manuscript was proofread by native English speakers through an English editing service. After using the tool and service, the authors reviewed and edited the content as needed, and they take full responsibility for the content of the publication.

biochemistry↗

bioRxiv2025

Dynamic vs. Static Facial Color Changes: Evidence for Terminal Color Dominance in Expression Recognition

Facial color is closely linked to the perception of emotion, with reddish tones often associated with anger. While previous studies have shown that static reddish facial tones enhance this perception, it remains unclear whether dynamic changes in facial color further amplify this effect. This study investigated how variations in facial color influence the perception of expression using a judgment task that involved morphed facial stimuli (fearful to angry). The participants evaluated facial expressions under two conditions: faces with dynamic color changes and faces with static colors. Experiment 1 compared red (CIELAB a*+) faces to original-colored faces, and Experiment 2 compared green (a*-) faces to original-colored faces. Neither experiment revealed significant differences between dynamic and static facial colors. However, faces with a final reddish color (higher a* value) were more likely to be perceived as angry. These findings suggest that the final facial color influences the perception of anger independent of whether the color change is dynamic or static. Our findings support the idea that the recognition of anger is modulated by the relationship between an angry expression and the color red and provide a new perspective on facial color changes in the interaction between facial expression and facial color.

neuroscience↗

bioRxiv2024

Interaction between facial expression and color in modulating EPR P3

The relationships between facial expression and color affect human cognition functions such as perception and memory. However, whether these relationships influence attention remains unclear. Additionally, whether facial expressions affect selective attention is unknown; for example, reddish angry faces increase negative social evaluation or emotion intensity, but it is unclear whether selective attention is similarly enhanced. To investigate these questions, we examined whether event-related potentials for faces vary depending on facial expression and color by recording electroencephalography (EEG) data. We conducted an oddball task using stimuli that combined facial expressions (angry, neutral) and facial colors (original, red, green). The participants counted the number of times a rarely appearing target face stimulus appeared among the standard face stimuli. The results indicated that the difference in P3 amplitudes for the target and standard faces depended on the combinations of facial expressions and facial colors; the P3 amplitudes for red angry faces were greater than those for red neutral faces. Additionally, there was no significant main effect or interaction effect of facial expression or facial color on P1 amplitudes for the target, and there were significant main effects of facial expression only on the N170 amplitude. These findings suggest that the intensity of a humans selective attention to facial expressions varies according to the higher-order semantic processing of the relationship between emotion and color rather than simple facial expression or facial color effects individually. Our results support the idea that red color increases the human response to anger from an EEG perspective. Significance StatementIt remains unclear whether selective attention to faces is modulated by the relationships between facial expression and color. Using an oddball task and recording EEGs, we showed that the event-related potentials reflecting selective attention are modulated by the interaction between facial expression and facial color, although the interaction was not found at earlier ERP stages. These findings suggest that the intensity of selective attention to facial expressions is influenced more by the interaction between facial expression and facial color than by facial expression or facial color alone and that the interaction occurs as a higher-order processing step than facial expression or color recognition. Our results provide EEG evidence supporting the idea that red color increases the human response to anger.

neuroscience↗

bioRxiv2024

Visual cues for moisture perception of facial skin: Enhancing high-frequency components of skin lightness increases perceived dryness

Facial skin texture provides crucial visual cues that reflect an individuals impressions and health conditions. In this study, we focused on the texture attribute of "moisture" and investigated which visual cues influenced skin moisture perception. The stimuli consisted of images from three facial areas (the whole face, cheek, and eyebrow areas) with and without makeup under two lighting directions. The participants rated the presented stimuli on the three texture attributes (moisture, glossiness, and attractiveness) using a five-point scale. The results from Experiment 1 revealed correlations between the ratings and histogram statistics of each channel in the CIELAB color space, with variations depending on the conditions and facial regions. A negative correlation was observed between the cheek moisture perception and the variance in the L* channel. We subsequently obtained similar ratings by enhancing the high-frequency components of skin lightness for artificially dried stimuli (Experiment 2) and for images depicting different skin conditions due to various types of makeup (Experiment 3). Both experiments confirmed a decrease in moisture and attractiveness and an increase in glossiness; these were correlated with the degree of artificial drying. These findings indicated that the high-frequency components of skin lightness could be visual cues for determining the perceived dryness.

neuroscience↗

bioRxiv2024

Facial expression recognition is modulated by approach-avoidance behavior

Facial expression recognition influences approach-avoidance behaviors, but do approach-avoidance behaviors affect facial expression recognition? We conducted psychophysical experiments to this end, indicating a reverse causal relationship. In a virtual reality space, 3D face stimulus facial expressions varied on seven levels--from happy to angry in Experiments 1 and 3 and from happy to fearful in Experiment 2. Participants were asked to perform according to one of the following conditions in response to the stimulus. Participants 1) approached (one-meter forward), 2) avoided (one-meter backward), 3) were approached by, or 4) were avoided by the 3D model. Then, participants selected facial expressions. Experiment 1 revealed that participants recognized the face as angrier when they avoided it rather than when it avoided them. Experiment 2 showed that participants recognized the face as happy when approaching and fearful when avoiding, irrespective of who acted. Experiment 3 revealed that participants recognized the face as angrier when the face approached them rather than when they approached if both parties were physically close. Accordingly, approach-avoidance behavior changes facial expression recognition, indicating a reverse causal relationship. We posit that unconscious learning rooted in biological instincts creates this connection.

neuroscience↗

bioRxiv2024

Spatial modulation of facial expression: enhanced recognition of faces behind the observer

Our ability to recognize facial expressions is crucial for understanding others emotions1 and facilitating smooth communication2. Numerous studies have explored how we perceive these cues, considering factors such as health3, social signals4 and personality traits5. However, most of this research involves observers facing a monitor and assessing facial stimuli presented directly in front of them. Real-life scenarios offer more diverse spatial dynamics, such as conversing with someone at a table or glancing back at a passerby. Thus, faces behind the observer might trigger heightened recognition, akin to reacting swiftly to a perceived threat. Herein, we demonstrate that facial expression recognition is influenced by spatial relationships, i.e., faces in front of versus behind the observer. Participants judged the expressions of faces appearing in front of or behind them in virtual space. The findings of three experiments reveal an enhanced level of recognition for faces behind the participant. Interestingly, this effect varies with emotional valence; anger is amplified merely by the presence of a face behind the observer, while happiness requires actively turning to the rear for enhancement to occur. These findings suggest a biological instinct for perceiving threats behind us, potentially influencing subsequent actions. Hence, spatial relationships may modulate facial expression recognition.

neuroscience↗

bioRxiv2023

Human behavior and comfort during load carrying to autonomous mobile robot

Interactions between humans and autonomous mobile robots (AMRs) are expected to grow in smart cities to improve logistics operations, such as depositing packages on AMRs for pickup on the street. However, the way that humans walk and pass objects to an AMR when approaching each other remains largely unknown. We conducted two psychophysical experiments to clarify the behavior and comfort of humans when carrying a package and placing it on an AMR for load carrying. Participants were asked to approach a programmed AMR and pass the package in two experiments: 1) changing the stop distance and AMR speed and 2) changing the stop distance and package weight. Motion trackers quantified the participants walking speed and frequency of hesitation to walk. In addition, the subjective heaviness and comfort were recorded through a questionnaire during each trial. The results indicated that the participants speed decreased and hesitation probability increased when the stop distance of the AMR decreased. Nevertheless, the participants felt more comfortable with the close approach, whereas the package weight did not affect their behavior. By contrast, they felt uncomfortable when AMR remained still. These findings suggest that humans regard the AMR approach as load-carrying assistance and not as invading their personal space. To achieve a comfortable interaction in load carrying from humans to AMRs, we suggest that the AMR can closely approach a person without eliciting personal space invasion.

neuroscience↗

bioRxiv2022

Human collision avoidance behavior against autonomous mobile robot

Currently, the probability of pedestrians on the streets of smart cities encountering autonomous mobile robots (AMRs) is increasing. Previous studies have discussed collision avoidance between humans as they cross paths. Their avoidance behavior toward AMRs, however, remains unclear. To address this, we experimentally investigated the avoidance direction a human would choose against an AMR approaching head-on. This experiment included recording human locomotion under such a scenario. Further, an AMR was programmed to approach from various starting points, including directly through the participants (head-on collision). The participants were directed to evade it by moving rightward or leftward, and their paths were tracked. We found that the participants did not strongly prefer either direction, suggesting that the avoidance direction is not solely determined by the participants attributes, such as their adherence to the traffic rules of their region. The probability of rightward evasion when the AMR approached head-on indicated that humans use different avoidance strategies when encountering other humans and obstacles. Moreover, the participants motion analysis revealed that they involuntarily twisted their waists in the avoidance direction before they evaded the AMR. These results suggest that this twist is the most important predictor of the avoidance direction. These findings could be encoded into the programs of AMRs to adapt these vehicles to our locomotory responses more organically.

neuroscience↗

bioRxiv2022

Glossiness perception and its pupillary response

Recent studies have discovered that pupillary response changes depend on cognitive factors, such as subjective brightness caused by optical illusions and luminance. However, it remains unclear how the cognitive factor derived from the glossiness perception of object surfaces affects pupillary response. We investigated the relationship between glossiness perception and pupillary response through a gloss rating experiment that includes recording pupil diameter. For the stimuli, we prepared general object images (original) and randomized images (shuffled) that comprised of the same images with randomized small square regions. The image features were then controlled by matching the luminance histogram. The observers were asked to rate the perceived glossiness of the stimuli presented for 3,000 ms and changes in their pupil diameter were recorded. Consequently, if glossiness of the original images were rated as high, those of the shuffled were rated as low, and vice versa. High-gloss images constricted the pupil size more than the low-gloss ones near the pupillary light reflex. By contrast, the shuffled images dilated the pupil size more than the original image at a relatively later stage. These results suggest that local features comprising specular highlights involve the cognitive factor for pupil constriction, and this process is faster than pupil dilation derived from the inhibition of object recognition.

neuroscience↗

bioRxiv2020

Team flow is a unique brain state associated with enhanced information integration and neural synchrony

Team flow occurs when a group of people reaches high task engagement while sharing a common goal as in sports teams and music bands. While team flow is a superior enjoyable experience to individuals experiencing flow or regular socialization, the neural basis for such superiority is still unclear. Here, we addressed this question utilizing a music rhythm task and electroencephalogram hyper-scanning. Experimental manipulations held the motor task constant while disrupted the hedonic musical correspondence to blocking flow or occluded the partners body and task feedback to block social interaction. The manipulations effectiveness was confirmed using psychometric ratings and an objective measure for the depth of flow experience through the inhibition of the auditory-evoked potential to a task-irrelevant stimulus. Spectral power analysis revealed higher beta/gamma power specific to team flow at the left temporal cortex. Causal interaction analysis revealed that the left temporal cortex receives information from areas encoding individual flow or socialization. The left temporal cortex was also significantly involved in integrated information at both the intra- and inter-brains levels. Moreover, team flow resulted in enhanced global inter-brain integrated information and neural synchrony. Thus, our report presents neural evidence that team flow results in a distinct brain state and suggests a neurocognitive mechanism by which the brain creates this unique experience. Data AvailabilityAll data and analysis codes used in the preparation of this article are available at https://osf.io/3b4hp.

neuroscience↗