Abstract: Patients with brain function disorders due to stroke or dementia may show inability to recognize themselves in the mirror. Although the cognitive ability to recognize mirror images has been investigated in many animal species, the animal species that can be used for experimentation and the mechanisms involved in recognition remain unclear. We investigated whether mice have the ability to recognize their mirror images. Demonstrating evidence of this in mice would be useful for researching the psychological and biological mechanisms underlying this ability. We examined whether mice preferred mirrors, whether plastic tapes on their heads increased their interest, and whether mice accustomed to mirrors learnt its physical phenomenon. Mice were significantly more interested in live stranger mice than mirrors. Mice with tape on their heads spent more time before mirrors. Becoming accustomed to mirrors did not change their behaviour. Mice accustomed to mirrors had significantly increased interest in photos of themselves over those of strangers and cage-mates. These results indicated that mice visually recognized plastic tape adherent to reflected individuals. Mice accustomed to mirrors were able to discriminate between their images, cage-mates, and stranger mice. However, it is still unknown whether mice recognize that the reflected images are of themselves.
4. Discussion
In this study, we applied a plastic tape to the heads of mice and investigated their change in interest towards the mirror. The interest in the mirror when the plastic tape was applied to the heads of mice significantly increased from before they were accustomed to the mirror to after they were accustomed to the mirror. Furthermore, we found that mice frequently contacted the mirror, suggesting that they could distinguish the image on the mirror from the faces of the cage-mate and stranger mice.
Animals that are thought to be able to perceive their reflections in the mirror as their own figures, in many cases, follow four steps when faced with a mirror: (1) make social reactions, (2) explore the physical sense (such as checking the back of the mirror), (3) perform repetitive actions to test the mirror, and (4) understand that the image reflected is their own [9]. In the tests used in this study, mice did not show social reactions or exploratory behaviours of reacting positively to mirrors as did chimpanzees, dogs, and fish in previous studies. The interest of the mice to the opaque board was comparable to that to the mirror. Previous reports show that the mirror slightly disgusted the mice, and that unlike with other animal species, mirrors are not environmentally enriching material for mice [22, 23]. For this reason, chambers composed of mirrors are used to test the effects of anxiolytic drugs in mice [24, 25]. The difference in our results may be due to the differences in the familiarity of the mice to the mirror, the reflective state of the plastic breeding home cages, single breeding versus mass breeding, and the illuminance of the experimental environment. Specular reflection provides only visual information, whereas live animals provide multiple sensory information. Therefore, live animals have richer stimuli than mirrors. Since mice are animals that prioritize olfaction rather than vision and hearing, it is considered reasonable that their interest in the mirror without smell quickly diminishes.
In this study, all the mice showed a stronger interest in the live stranger mouse than in the mirror. Previous studies have reported that mice are more interested in mirrors than stranger mice [26]. The difference in these results may have been because of accustoming the mice to the mirror, which may have affected the result. However, it is reasonable that the mice would show interest in live stranger mice that provide multiple sensory information rather than in specular reflections that provide only visual information. Moreover, even if mice do not understand the reflection in the mirror as the reflected image of itself, it is a natural reaction to ignore the mirror image as a harmless stimulus for themselves rather than to recognize it as a homogeneous individual to react with socially [27]. In fact, the mental process and cognitive ability of the mouse in response to the mirror are unknown, and further research is needed for elucidation. This study shows that mice are not interested in mirrors.
We applied small plastic tapes to the heads of mice for the tape on the head test. Mice with the tapes on their heads showed an increased interest in the mirror. Even after becoming accustomed to the mirror, the interest of the taped mouse to the mirrors remained high. However, the taped mice did not show behaviour suggestive of trying to eliminate the tape. During the mark test, the mark may not be perceived as abnormal by the animals, and they may not feel the impulse to touch it. Pigs have been reported to be able to recognize their movements in the mirror in a very short time, and to be self-conscious [28]. However, it is thought that since pigs are accustomed to the state of mud being attached to their bodies, even if they are marked during experiments, they do not mind this. This does not mean that they are not self-perceiving. Even if they can perceive themselves, if the motivation to remove dirt from their faces is small, they would not show action suggestive of trying to touch the mark. It may be possible that mice, like pigs, do not feel the necessity to remove foreign objects attached to their bodies. The mark test is a compound task in which the ability of the subject to use tools, recognize itself, and detect visual information is questioned. The mark test has been used as a method for confirming the presence or absence of self-awareness, but opinion is divided on its validity [16]. The mark test represents one aspect of self-recognition that has, in recent years, been considered to be different from the overall self-recognition that human beings experience. Moreover, it may not be so meaningful to target an animal that mainly uses a sense other than vision during the mirror test [29]. The results of the present study did not necessarily indicate the existence of self-recognition capability in mice. However, it showed that mice visually perceived unusual states through the mirror. Further research is needed to clarify factors that increase the interest of mice towards mirrors.
Other than the head, the throat is another location that can be used to attach the tape. A similar study analysed the behaviour of a magpie with a tape attached to the throat [10]. However, the throat is a motile part of the animal body, and a tape attached to the throat would provide a tactile stimulus. However, the skin on the head is immobile, and a level of similar tactile stimulus would be implausible.
This study showed that, by spending time with a mirror in the home cage, mice changed their interest in photos of themselves over photos of cage-mates and stranger mice. This mouse behaviour indicated that by learning through the mirror, the mice recognized that the image in the mirror was different from the figure of their cage-mate mice. It should be noted that the results of this study are not evidence that mice recognize the images in the mirror as their own. However, some animals have shown that their self-perception of the mirror image can be enabled through experience [30, 31]. It has been reported that self-perception of the mirror image occurs naturally in rhesus macaques after training for accurate visual-specific receptor association to mirror images [32, 33]. It has also been reported that pigeons pass the mark test after thorough training by voluntary and mirror-based pecking [30, 34]. In recent years, it has been reported that the cleaner wrasse (Labroides dimidiatus) is able to self-recognize by learning [12]. In addition, it is indicated that the age of acquiring mirror image self-recognition in humans is related to the frequency of postnatally experiencing the mirror and to cultural differences [35]. Among infants in Africa, where opportunities to see mirrors are less frequent than in developed countries, it is reported that the age at which mirror images of the self can be clearly perceived is somewhat higher. The results of this study are consistent with previous reports, indicating the possibility that more animals show that there is a sense of “self” than we think. Since mirror image self-recognition increased as the mirror was experienced more frequently, it is considered that changing the cognitive evaluation of the mirror at the stage when the reflecting property of the mirror and the reflecting object are learnt becomes the turning point.
The mental processes of mice and other animal species, such as apes are unknown, and it is difficult to decipher the cognitive abilities of such animals. Our results show that the mouse is an animal that alters recognition to the mirror by learning. Further research is needed to clarify the mirror recognition of the self by mice. Having a mouse as an effective model for behavioural research, such as mirror self-recognition, opens doors to study aspects of this behaviour that would otherwise be impossible to study.
Patients who suffer from failure of brain function due to a stroke or dementia may show symptoms of being able to recognize the images of their family members and others in the mirror, while not being able to recognize their own images. This phenomenon is called mirror self-misidentification [36]. Mirror self-misidentification is also a symptom of dissociative disorder [37]. However, the mechanism by which this occurs is not clear. Furthermore, when the function of the upper part of the medial prefrontal cortex is temporarily stopped by the transcranial magnetic stimulation method, the person manifests symptoms of being unable to recognize themselves when looking at the mirror [38]. These reports suggest that specific neural circuits are involved in the perception of mirror images of oneself in humans. It is therefore useful to develop a method to clarify these neural mechanisms, to treat cranial nerve disease, and to further clarify the evolutionary basis of the cognitive ability of recognizing mirror images of oneself. New knowledge obtained by conducting experiments on animals focus on whether or not mirror self-recognition is possible for those specific species. Furthermore, many other questions on the neural infrastructure remain. This study shows the potential of using mice for elucidating neural circuits.
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