Self-consciousness is the basis of higher cognitive functions in humans. It is an essential concept in philosophy, psychology, and cognitive neuroscience. Self-consciousness serves as a framework to understand how individuals interact with the world and contemplate both their own behavior and the behavior of others. Within this framework, the self-system processes self-relevant information and adjusts the impact of self-consciousness on cognitive processes, considering factors such as goal sensitivity (Cunningham and Turk, 2017). Previous studies have suggested that the perpetual goal of the self-system is to ensure that self-relevant information can be preferentially processed (Cunningham and Turk, 2017). This preference for self-relevant information manifests as systematic biases in perception, memory, and attention, where individuals tend to prioritize information that pertains to the self over information related to others. This phenomenon is called self-bias (Sui and Humphreys, 2015, 2017; Cunningham and Turk, 2017). Therefore, self-bias serves as a manifestation of self-consciousness (Sui and Humphreys, 2017). Self-bias can also lead to more accurate processing of familiar people, enabling better inference of their emotions and thoughts in a given situation (Nijhof et al., 2020). To date, evidence of self-bias has been provided in a variety of areas, including perception (Sui et al., 2012; Sui and Humphreys, 2015, 2017), memory (Leshikar et al., 2015; Andrews et al., 2020), and attention (Yang et al., 2013; Humphreys and Sui, 2016).

Although self-bias has been extensively studied and confirmed in self-related stimuli, such as self-face (Sugiura et al., 2008; Malaspina et al., 2018, 2019), self-name (Sugiura et al., 2008; Yang et al., 2013), self-associated geometric shapes (Sui et al., 2012), and self-appraisal (Yue et al., 2021), whether the self-body, a fundamental part of self-consciousness, can induce recognition advantage remains controversial. Several studies have reported that self-hand stimuli are processed faster and more accurately than the hand images of other individuals in implicit tasks (Frassinetti et al., 2011; Richetin et al., 2012; De Bellis et al., 2017; Malaspina et al., 2019). However, this kind of self-advantage has not been found (Frassinetti et al., 2011; Richetin et al., 2012) and even reversed to a self-disadvantage (Ferri et al., 2011; Conson et al., 2015; Candini et al., 2016; Fukui et al., 2020) when participants are required to explicitly make “own hand” or “another person’s hand” judgments. Thus, it has been proposed that there is a possible dissociation between implicit and explicit bodily self-processing. To date, few studies have directly investigated such a possible dissociation, and the findings remain inconclusive. Experiments in patients with brain injury seem to provide anatomical evidence supporting such dissociation, with different areas of the right hemisphere underpinning implicit and explicit self-body knowledge (Candini et al., 2016). However, one neuroimaging study found that explicit and implicit self-processing recruit similar neural networks (Rameson et al., 2010). Recently, Conson et al. (2017) observed self-advantage in both implicit and explicit tasks by adding hand stimuli in palm view. They proposed that palm views, which have a high sensorimotor load, may enhance motor imagery engagement and sensorimotor processing to generate self-advantage. However, it should be noted that they also found self-advantage in back views as well as palm views of the hands. Thus, these findings cannot fully explain the controversial results in implicit and explicit self-body recognition. More recently, based on meta-analysis and experimental evidence, Holmes and colleagues even suggested that hand recognition may have no self-advantage (Holmes et al., 2022). Thus, it remains unclear whether and how self-advantage exists in body recognition.

Notably, in contrast to the observed inconsistency in self-body bias, self-advantage has been found in both implicit and explicit self-face recognition in healthy participants (Devue et al., 2007; Keyes and Brady, 2010; Ma and Han, 2010) and in patients with congenital prosopagnosia (Malaspina et al., 2018, 2019), even in the same explicit self-discrimination task (e.g., Malaspina et al., 2018) as for self-body recognition (e.g., Ferri et al., 2011). These findings indicate that the inconsistency between implicit and explicit tasks does not persist uniformly across different self-related domains. Since self-face and self-body are both fundamental components of self-consciousness, it is crucial to thoroughly examine the inconsistencies in self-body bias to enhance our understanding of the underlying mechanisms of self-bias.

It has been demonstrated that body recognition is a critical self-recognition element (Gillihan and Farah, 2005). Self-body recognition may prove particularly beneficial when primary sources of one’s identity (e.g., the face) are inaccessible (Frassinetti et al., 2010). Especially, the self-body recognition paradigm can be applied to animals, which may have never seen their own faces but possess a relatively detailed visual representation of their anterior extremities (Heschl and Burkart, 2006). Therefore, self-body recognition allows for the exploration of the evolutionary origin of self-awareness. Additionally, in comparison to faces, bodies exhibit less variability; as such, self-body recognition could offer a more generalized method for assessing self-bias and may even help quantify the ability to distinguish oneself from others (Jenkins et al., 2011). Therefore, it is important to resolve the debate associated with the self-body to elucidate the extent to which self-bias is conserved across different self-related information and across implicit and explicit measures.

After carefully examining previous explicit and implicit experiments related to self-body processing, we proposed that participants’ strategies in explicit tasks may contribute to the previously confused findings at the explicit level. In classic implicit tasks, participants are usually required to choose the stimuli that match the target. That is, participants need to observe the target first, compare the choices to the target, and then make a decision. If the target is the self-hand, it may facilitate observation and comparison, and thereby impact subsequent decisions. Therefore, it is not surprising that self-advantage is reported in such implicit tasks (Frassinetti et al., 2011; Richetin et al., 2012; Malaspina et al., 2019). On the other hand, in classic explicit tasks (i.e., self-other discrimination task), participants are required to judge whether the hand image displayed on the screen is their own or another’s. If participants perform the task by determining whether the hand on screen belongs to them (referred to as “Self” judgment) or someone else (referred to as “Other” judgment), self-advantage, if it exists, would facilitate faster identification of their own hands compared to identifying others’, vice versa. However, it should be noted that participants are unlikely to make an “Other” judgment based on recognizing an unfamiliar hand. In other words, when participants make an “Other” judgment, they may also rely on whether they can identify the hand on the screen as their own. That is, both positive (resulting in a “Self” judgment) and negative responses (resulting in an “Other” judgment) may be made based on identifying the “Self.” As such, the designed investigation in classic explicit tasks, whether there is a difference between identifying the own and other’s hand at the explicit level, actually becomes whether there is a difference between positive and negative judgments of identifying the “Self.” Generally, it takes longer to make a “yes” decision than a “no” decision (Mynatt et al., 1977). Additionally, in hand recognition tasks, previous studies have suggested that participants may pay more attention to local details compared to processing faces holistically (Conson et al., 2017; Kuroki and Fukui, 2020). Consequently, once participants identify a local feature that does not belong to themselves, they can quickly choose the negative (“Other”) response. However, they may need multiple clues (features) to confirm that the images represent their own hands and then make the positive (“Self”) decision since others’ hands may also display one or more similarities to their own hands. Indeed, research has shown that participants are more likely to make “not their own” judgments, especially when the task is difficult, e.g., with shorter presentation times (Holmes et al., 2022). In this context, the advantage of making a “no” decision (recognizing the hand on the screen as not one’s own) leads to the advantage for “other” judgments, which may be interpreted as self-disadvantage in the explicit task.

In the present study, we hypothesized that the inconsistency between self-body bias in explicit and implicit tasks may be related to behavioral strategies in explicit tasks. To overcome this strategy issue, we aimed to design a novel explicit task in which participants are required to judge the identity of both “Self” and “Other” rather than only determining whether the hand image on the screen is their own or not. For comparison, we also conducted classic explicit and implicit tasks. We further performed two additional modified explicit tasks based on classic tasks to eliminate the potential influence of stimuli richness. Based on our novel explicit task, we expected that self-hand recognition advantage existed in the explicit task as well as in the implicit task.

We designed a novel explicit task (Experiment 3) and performed the same explicit (Experiment 2 series) and implicit (Experiment 1) tasks as in previous studies to investigate whether the self-hand can induce recognition advantage and, if so, whether such advantage could be found in both implicit and explicit self-hand recognition. Results showed self-advantage in the implicit task and self-disadvantage in the explicit task in self-hand recognition, consistent with previous findings (Ferri et al., 2011; Conson et al., 2015; Candini et al., 2016; Fukui et al., 2020). Importantly, with the newly designed explicit task, we found clear and robust evidence of self-hand recognition advantage, thus providing new insights into the long-standing inconsistencies in previous studies. Below, we discuss the importance of these findings for understanding self-advantage.

For self-related information (e.g., faces), recognition advantage exists at both the explicit and implicit levels (Devue et al., 2007; Keyes and Brady, 2010; Malaspina et al., 2019). However, findings related to self-body recognition are inconsistent at these two levels. That is, self-advantage for self-face and self-body is strongly correlated in implicit tasks but not explicit tasks (Malaspina et al., 2019). How can this self-bias puzzle be explained?

In previous studies, participants were required to make “self” or “non-self” judgments in the explicit tasks but match “self” and “non-self” stimuli to sample stimuli in the implicit tasks. In this case, the method of exclusion (it is not mine) may be applied in the explicit tasks, whereas the method of direct selection (it is the sample) may be applied in the implicit tasks. Obviously, it is easier to judge that it is not A (e.g., self) than it is A (Mynatt et al., 1977). Thus, the self-disadvantage found in explicit experiments may arise from the advantage of judging “it is not mine” versus “it is mine.” To test this possibility, we asked participants to perform the same actions on both self- and non-self hands: i.e., to which individual does the stimulus belong — self, friend, visually familiar individual, or stranger? With this newly designed explicit task, we did find self-advantage same as that found in the implicit task, thereby confirming our hypothesis. Of note, self-advantage was consistently found across all three measures (i.e., ACC, RTs, and ES). These findings suggest the existence of self-advantage in explicit hand recognition.

In previous implicit and explicit studies (Aranda et al., 2010; Ferri et al., 2011; Frassinetti et al., 2011; Candini et al., 2016; Conson et al., 2017; Fukui et al., 2020; Kuroki and Fukui, 2020), to familiarize participants with the task, it has been a common procedure to include a practice phase, which utilized the same stimuli as the main experiments and exposed participants to both self-relevant and other-relevant images prior to the main experiments. As a result, compared with previous studies, even if the brief preview phase in the present study had any impact on subsequent experiments, it would likely be minimal and primarily limited to the implicit task. Note that our results showed self-advantage in the implicit task, suggesting that the brief preview phase in the present study did not cause significant differences between our findings and previous implicit studies (Frassinetti et al., 2011; Richetin et al., 2012; De Bellis et al., 2017; Malaspina et al., 2018). Furthermore, it is noteworthy that our main objective was not to re-examine the well-documented self-advantage observed in implicit tasks. Instead, the main purpose of the present study was to develop a novel explicit task and demonstrate whether the inconsistency between self-body bias in explicit and implicit tasks is related to behavioral strategies in explicit tasks. The brief preview phase in the present study was similar to the practice phase in previous studies (Ferri et al., 2011; Frassinetti et al., 2011; Candini et al., 2016; Conson et al., 2017; Fukui et al., 2020; Kuroki and Fukui, 2020). Indeed, our results in the classic explicit task showed self-disadvantage, aligning with previous findings, further confirming that any potential impact of the brief preview phase in our study on the explicit task, if present, would be comparable to previous studies. More importantly, the impact of stimulus exposure was comparable between our newly designed explicit task and the classic explicit tasks. Therefore, it is unlikely that the observed dissociation in self-bias during the newly designed and classic explicit tasks could be solely attributed to the mere presentation of self-hands before the experimental tasks.

Furthermore, our results showed significant correlations between the self-bias indices in the classic implicit task and newly designed explicit task, indicating potentially shared strategies or even neural pathways involved in implicit and explicit processing. Note that our findings could not preclude the possibility that implicit and explicit self-body recognition pathways may be distinct, despite the presence of self-advantage at both implicit and explicit levels. Further studies employing neuroimaging approaches may elucidate the underlying neural mechanisms.

The same stimulus set used in the implicit task was presented in the newly designed explicit task. To exclude the possibility that the inconsistency between the classic explicit and implicit tasks may be caused by differences in the stimulus set (e.g., the number of identities enrolled in tasks), we modified the classic explicit task and conducted two experiments (Experiments 2b and 2c) using the same paradigm as Experiment 2a, except for the stimulus set. In addition to self-hand images, there were eight images from each of the two strangers in Experiment 2a (equal number of stranger and self images), 16 images from each of the two strangers in Experiment 2b (equal number of images from each individual), and 64 images from friend, stranger, and visually familiar stranger (i.e., same stimulus set as Experiments 1 and 3) in Experiment 2c. We found similar results in all three self-other discrimination experiments as well as significant correlations among them, suggesting that stimulus richness was not the cause for self-disadvantage in self-hand recognition. These results are consistent with previous findings showing that if self-other discrimination tasks are conducted to assess self-body recognition, no self-advantage is found (Frassinetti et al., 2011; Richetin et al., 2012), or even self-disadvantage can be detected (Ferri et al., 2011; Conson et al., 2015; Candini et al., 2016; Kuroki and Fukui, 2020), regardless of the type of stimuli used in the explicit task.

We did not find significant correlations between scores in self-related questionnaires (i.e., private self-consciousness and self-esteem) and self-advantage found in the implicit and newly designed explicit tasks. While several studies have found relationships between scores acquired from self-related questionnaires and self-bias measured by the implicit and explicit tasks (Richetin et al., 2012), not all studies have identified such relationships (for exceptions, see Williams et al., 2018; Jiang et al., 2019; Nijhof et al., 2020; Amodeo et al., 2021). The self-concept has been considered ambiguous and abstract, posing challenges for accurate measurement through questionnaires (Grace and Cramer, 2003). The reliability of questionnaire measures has been questioned due to inconsistent results (Levine et al., 2006). As a result, it may not be surprising that no significant correlations were found between the questionnaire measures and self-advantage in the implicit and newly designed explicit tasks. Note that private self-consciousness scores were correlated with the self-bias indices measured in the classic explicit task (Experiment 2a). To the best of our knowledge, previous studies have mainly found relationships between self-esteem but not self-consciousness measured with questionnaires and self-bias in behavioral tasks (Richetin et al., 2012). Therefore, the reliability of this correlation may need to be re-tested in future studies, which may also be designed to capture the underlying basis of this relationship.

Taken together, we designed a novel explicit paradigm to detect self-advantage in hand recognition. By using this task and a series of classic explicit and implicit tasks, we revealed the reason for inconsistencies between implicit and explicit results in previous studies: participants may utilize different strategies in classic explicit and implicit tasks. Importantly, we obtained compelling evidence that the self-hand recognition advantage exists at both the explicit and implicit levels, and that the self-advantage processing by these two pathways may be linked. Our findings reveal the extent to which self-bias is conserved across self-related information. We anticipate that these results will deepen our understanding of self-bias and open a new avenue for studying self-bias using self-body stimuli, which are less affected by social information carried by faces (e.g., emotion and personality) (Lee et al., 2008; Quevedo et al., 2018).

The raw data supporting the conclusions of this article will be made available by the authors, without undue reservation.

The studies involving human participants were reviewed and approved by Institutional Review Board of Institute of Biophysics. The patients/participants provided their written informed consent to participate in this study.

SX and MZ: conceptualization, methodology, validation, data collection, investigation, formal analysis, writing – original draft, writing – review, and editing. LY: data collection. NL: conceptualization, methodology, writing – original draft, writing – review and editing, supervision, and project administration. All authors contributed to the article and approved the submitted version.

This work was supported by STI2030-Major Projects (Grant nos. 2021ZD0200200 and 2021ZD0204200), the National Natural Science Foundation of China (Grant no. 32071094), Strategic Priority Research Program of Chinese Academy of Science (XDB32020207), Chinese Academy of Sciences (QYZDB-SSW-SMC033).

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

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