Tone interval - taste synaesthetes were observed and demonstrated by Beeli et al. (2005). In his research, Beeli described a rare case of 27 year old female musician who had a taste as a concurrent perception. She reported the automaticity experience of different taste upon hearing specific intervals 

Over a year of study, it was confirmed that E.S.'s synaesthetic experience is authentic. It is considered an abnormal form of synaesthesia in which the inducing stimuli are complicated and required extra-musical training. This compelling fact leads to the thought that synaesthesia could be used to solve cognitive problems.

The topic of synaesthesia is relatively common in the musical world. Numerous musicians including world-renowned composers and performers; for instance, Alexander Scriabin, Jean Sibelius, Franz Liszt, György Ligeti, Olivier Messiaen, Itzhak Perlman, Pharrell Williams, and Billy Joel, have reported or showed a signal of synaesthesia. Most of the musician’s synaesthesia cases experience a colour as a concurrent perception.

According to the research by Brang and Ramacgandran (2011), it suggests that synaesthesia occurs from increased communication between sensory regions and sensory deprivation or brain damage, possibly in response to drugs. In other words, inducing stimuli can evoke the corresponding sensory modality and, at the same time, activate another sensory region in the brain, which causes poly-sense experience.

Thought-provoking information on synesthesia is that the condition of synesthesia is highly transmittable from parents to offspring. This observation leads to the hypothesis that synaesthesia is caused by a unique gene or set of genes.

In the following section, two rare synaesthesia cases that are entailing and relevant to the tone interval (research topic), taste synaesthesia and sound/lexical – synaesthesia, will be discussed.

The topic of synaesthesia has been monitoring and studying by scientists since the 1800s. There are at least 60 variants of synaesthesia which can be in any combination of sensory modality. Synaesthesia usually occurs unidirectional except for a rare case that has the bidirectional synaesthesia. According to statistic reports, around 4 percent of the world population has synaesthesia.

Since synaesthesia and crossmodal correspondences are a correlating topic in many aspects, it enables the possibility to use the information from synaesthesia experiment to substitute missing element in crossmodal correspondence experiment to create a final product for research and as an element for future experimental performances.

In summary, the case of participant S.C. is appealing for this research, as well as for future projects. As an idea for musicians, composers, and artists, the case might be a model for extended musical boundaries. For example, one might think of experimental lied song, large scale opera, or any types of a musical genre, which include linguistic property, the idea of combining taste with language attributes correlated with musical elements, not only for performers and composers (in general, musical creators), but also for audiences and artistic observers. This finding, from a personal point of view, is considered beneficial for both the aesthetical and the economic aspects of performing art.

The definition of synaesthesia has been given by many scientists, researchers, and neurologists. These are some beautiful renditions of the definition of the word "synaesthesia":

• "Synesthesia refers to the experience of cross-modal sensory (and conceptual) mappings, in which the corresponding external stimulation of the additional perceived sense is absent." (Colizoli et al., 2013);
• "Synesthesia is a rare experience where one property of a stimulus evokes a second experience not associated with the first." (Banissy et al., 2014);
• "Synaesthesia is the involuntary physical experience of a cross-modal linkage, for example, hearing a tone (inducing stimulus) evokes an additional sensation of seeing a colour (concurrent perception)." (Beeli et al., 2005).

From these definitions given by professional experimenters, it reveals that "Synaesthesia is an involuntary and autonomous experience induced by a singular sensory stimulus that generates additional non-related sensory perception without any additional stimulus."

In 2013, Colizoli et al. observed a rare case of lexical-gustatory and sound-gustatory synaesthesia in participant S.C. Lexical-gustatory synaesthesia refers to the consistent and non-voluntary experience of complex taste induced by spoken or written languages. Less than 0.2 percent of the world population is estimated being lexical-related synaesthetic.  It was first reported in 1907 by A.H. Pierce. Individual cases of lexical-gustatory synaesthesia are different, but the common mechanism is found to be modulating factors. Due to a growing body of research, it has been shown that phonological, conceptual, semantic, and learned experiences have played an essential role in lexical-gustatory synaesthesia.

Participant S.C. was a 29 years old female musician, a performing artist, and a teacher. Her native language is Dutch. S.C. reported that she consistently experienced tastes, smells, and texture in her mouth upon hearing, speaking, reading, and thinking about certain words/letters, musical sounds, and environmental sounds. Her mother also had colour-related synaesthesia; consistent with the observation that synaesthesia is highly transmitted from parents. However, the fact that S.C. had a lexical synaesthesia induced by languages that she was familiar with and able to understand, it correlates to the fact that synaesthetic experience is modulated by semantical attribute. S.C. synaesthetic experience manner was unidirectional, and her most potent inducers were auditory linguistic stimuli. Most of the words and letters of the alphabet can cause a synaesthetic experience. For non-linguistic sounds (musical and environmental), they have to be discrete, i.e., musical piece does not elicit the experience. She did not report that certain types of chords could produce the taste or had the ability to induce her experience. In controversial from Pierce's report, S.C. synaesthetic taste was less intense than the veridical taste.