Mirror-touch synesthesia is a rare neurological phenomenon that causes individuals to experience the same physical sensations that they observe occurring to another individual. Studies have shown that there are at least two spatial frames under which mirror-touch synesthesia can be experienced: mirrored, where an individual that observes someone being touched on the right side would experience the same sensation on their left, or anatomical, where an individual that observes someone being touched on the right side would experience the same sensation on their right (Banissy & Ward, 2007). Current statistics estimate that between 1.6% and 2.5% of the general population meet the criteria to be diagnosed with mirror-touch synesthesia, and mirror-touch synesthesia is near equally prevalent in both men and women (Banissy, 2013). The first reported case of mirror-touch synesthesia occurred in 2005 (Blakemore, Bristow, Bird, Frith, & Ward, 2005), and in recent years, psychologists have begun to conduct more extensive research into this unique form of synesthesia to discover its cause and the effect it has on the individuals that experience it.
There is not a sole test to verify and diagnosis mirror-touch synesthesia. Over the years, scientists and researchers have developed several different testing methods to diagnosis mirror-touch synesthesia, and, in most cases, two or more methods are used in conjunction to diagnosis an individual.
Sensory Interference Task
Mirror-touch synesthesia was first tested through a sensory interference task, and this method of testing is still used to this day. During the sensory interference task, participants would be touched on either the right cheek, the left cheek, or would not be touched at all; simultaneously, they would observe an assistant being touched in either the same or different location. Participants were then asked to report what location, if any, they felt a sensation. Mirror-touch synesthetes showed higher rates of error than non-synesthetes during incongruent trials, when they observed an assistant being touched in a location that was different from theirs (Holle, Banissy, Wright, Bowling, & Ward, 2011). A high rate of errors on the incongruent trials of the sensory interference task shows a positive correlation with mirror-touch synesthesia diagnosis.
Scientists have conducted extensive research into the empathy quotient results in both mirror-touch synesthetes and non-synesthetes. Mirror-touch synesthetes have shown to score significantly higher on the emotional reactivity subscale than non-synesthetes, and tend to trend higher on the subscales of cognitive empathy and social skills, but not enough to consider it a remarkable difference (Bannisey et al., 2007). An above average score on the emotional reactivity scale is an indicator of mirror-touch synesthesia.
A newer form of testing for mirror-touch synesthesia is fMRI, functional magnetic resonance imaging. This testing is split into three sections: the first is a ‘touch’ session, followed by two ‘video’ sections. During the ‘touch’ session, the participant lies on an examination table while the researcher touches either the right or left side of the participant’s neck or cheek, or does not touch the participant at all. The participant is touched in varying locations in twenty-second increments five separate times. Following the ‘touch’ session, the participant watches two sections of videos, the first containing real people being touched on their neck and face and the second containing inanimate objects being touched on their equivalent neck and face regions. While the participants engage in the test, they are simultaneously undergoing fMRI scanning. Scans from the fMRI show that the somatosensory activation caused by the observation of touch to humans in the first video session was significantly greater in mirror-touch synesthetes than non-synesthetes (Blakemore et al., 2005).
Theories of Cause
There are no proven causes of mirror-touch synesthesia, but many theories have surfaced since its discovery. The two most widely-known and plausible theories are the threshold theory and the self-other theory. The cross-activation theory, while newer and lesser-known, is also discussed, as well as genetic factors that may cause mirror-touch synesthesia.
The Threshold Theory
According to the threshold theory, mirror-touch synesthesia is caused by a hyperactive mirror neuron system, and this level of hyperactivity exceeds a threshold of awareness in mirror-touch synesthetes that results in conscious perceptions of touch from observation. Non-synesthetes, who have a normal level of activity in their mirror neuron system, remain below this threshold, allowing them to observe touch without perceiving it on their own person (Blakemore et al., 2005).
The Self-Other Theory
The self-other theory states that mirror-touch synesthesia is caused by an individual’s inability to distinguish one’s self from others. Mirror-touch synesthetes over-extend and project their bodily self onto others, and consequently blur self-other boundaries (Blakemore et al., 2005). This theory is supported by a study that revealed that after mirror-touch synesthetes experienced both observed and felt touch, when observing images that they originally perceived as equally ‘self’ and ‘other’, they began to recognize the images as the ‘self’ and no longer ‘other’ (Banissy ; Ward, 2013).
The Cross-Activation Theory
The cross-activation theory, also known as the crosswire theory, states that neural circuits that are contained in the sight sensory system and the touch sensory system in mirror-touch synesthetes are overlapped, unlike non-synesthetes, causing individuals with mirror-touch synesthesia to feel what they see (Hubbard, Brang, ; Ramachandran, 2011).
Suggestions of genetic links in mirror-touch synesthesia have also been made, as it is not unlikely that a mirror-touch synesthete will also have a close relative with mirror-touch synesthesia; the first patient ever diagnosed with mirror-touch synesthesia was later found to have a first cousin who also had the same condition (Blakemore et al., 2005).
Mirror-touch synesthesia has been proven to affect more than just an individual’s perception of touch. Multiple studies, as shown below, reveal that mirror-touch synesthesia affects an individual’s perception of others’ emotions and expressions as well as their perception of the self.
Research shows that mirror-touch synesthetes possess superior abilities when recognizing and understanding others’ emotional expressions. Study analyses of accuracy performances reveal that mirror-touch synesthetes outperform non-synesthete participants on expression recognition, scoring an average of 92% correct compared to non-synesthetes’ 82%. It is believed that this increase in ability is due to heightened sensorimotor simulation mechanisms in mirror-touch synesthetes (Banissy, Garrido, Kusnir, Duchaine, Walsh, & Ward, 2011).
Along with superior emotional expression recognition, research has confirmed that mirror-touch synesthetes have significantly heightened empathy compared to non-synesthetes. Following the theory that mirror-touch synesthesia is caused by hyperactive mirror neurons, it is logical to believe that this hyperactivity allows for mirror-touch synesthetes to effortlessly experience observable emotions on another individual, thus making them highly empathetic (Banissy, 2013).
Research has also suggested that mirror-touch synesthesia can create an altered perception of one’s self and others, theorizing that observing touch not only triggers a conscious perception of touch in mirror-touch synesthetes, but also changes the mental representation of the self. The results of a study conducted in 2013 revealed that observing touch on another’s face caused a notable change in mirror-touch synesthetes’ self-face recognition, and that mirror-touch synesthetes would alter their existing idea of their facial representation to incorporate another person’s features (Maister, Banissy, & Tsakiris, 2013).