Cognitive Science: An Introduction/Synesthesia

Synesthesia

IntroductionEdit

Some people can see or hear a word, yet they taste food; detect sounds yet see shapes; or, recognize sounds after smelling a specific scent. These are just three of the 61 different variants of the condition called synesthesia. The precise definition of synesthesia is hard to define as it needs to be broad enough to capture the different variants of it [1]. However, the most commonly used definition of synesthesia is the phenomenon where a stimulus evokes a consistent sensation that is not derivable from the stimulus [2]. The word synesthesia has Greek roots from which it translates to perceive together, and the people who have this ability are called the synesthetes [3]. Throughout this research, synesthesia will be explored such as its history, development, types, neural basis and its heredity.

History of SynesthesiaEdit

Synesthesia is a rare neurological trait from which it interests scientists for nearly 200 years [4]. Within the period of 1876 to 1895, there was considerable debate over the nature of synesthesia, what it should be called, and its causes, that is why this period is known as the golden age of synesthesia [5]. During this period, the conception of synesthesia was broadened away from purely color-based experiences into a wide diversity of diverse phenomena. In 1876, Gustav Theodor Fechner, a renowned figure in neuroscience and psychology, conducted an experiment from which he confirmed that music can be matched with distinct associations that can replace words, and afterward described color associations to vowels given to him by four acquaintances [6].This was the first attempt by any researcher to find correlations between vowels and colors. In 1881, two 20 years old men named Bleuler, and Lehmann reported the first case of synesthesia in history in which smell, touch, pain, and taste act as inducers [5]. Blueler and Lehmann listed colors for each letter of the alphabet from about a dozen people, and beyond letter-color associations. As a result of their inquiries, they produce double sensations such as sound photisms associated with light, color, and form sensations elicited through hearing.

Development of SynesthesiaEdit

Synesthesia is a variation of human experience that involves the automatic activation of unusual concurrent experiences in response to ordinary inducing stimuli [7]. Synesthesia development is not yet understood well, but it may have a genetic basis resulting in enhanced cortical connectivity during development [7]. Some hypotheses explore the development of synesthesia: immune theory, serotonergic hyperactivity hypothesis, and neonatal hypothesis. The immune hypothesis states that genes responsible for normal cortical development are also involved in the development of synesthesia [8]. The serotonergic hyperactivity hypothesis proposed that psychedelic drugs—psilocybin, LSD, and mescaline—can induce synesthesia [9]. A neurotransmitter suspected to be central to perceptual changes is serotonin. Excessive serotonin in the brain can cause symptoms of psychedelic intoxication and may also play a role in synesthesia after brain injury. The neonatal hypothesis argues that the newborn does not keep sensations separate from one another, and they mix sights, sounds, feelings, and smells [10]. As a result, newborns' perception has sounds, and their feelings have a taste.

Types of SynesthesiaEdit

There may be as many as 61 types of synesthesia, depending on which senses paired together. The three most studied and prevalent synesthesia are grapheme-color, sound-to-color, and lexical-gustatory synesthesia [11]. People with grapheme-color synesthesia associate letters and digits with colors [12]. In a given synesthete, the associations between graphemes and colors are specific, in that each grapheme corresponds to a particular hue that is mostly consistent across the lifespan [13]. An example of people who have this kind of synesthesia is that Z may be green, and the number 10 can be black. The grapheme-color synesthetes are typically identified based on the consistency of their grapheme-color matchings [12]. Sound-to-color synesthesia is the type of synesthesia where, for example, the E chord on a piano might be blue, but the note three octaves lower might be red. This synesthesia is the focus of the present investigation [14]. The sound-color synesthesia utilizes pathways applied to integrate visual and auditory information as part of the ordinary mechanisms of cross-modal perception. The cross-modal audiovisual areas of the brain appear to respond more to the combined presence of vision and sound than to either vision or sound alone [14]. Amanda and her team experimented with this type of synesthesia. According to them, three families who experience color when listening to sounds are connected by rare genetic variants affecting genes that contribute to axonogenesis [15]. Furthermore, some people do also experience lexical-gustatory synesthesia. Lexical-gustatory synesthesia is the phenomenon where words often trigger vivid but subjective experiences of food that have texture and temperature as well as complex tastes [16]. In lexical-gustatory synesthesia, reports have explored the episodic links between inducers and their concurrence. Inducer words are often familiar names that may have semantic or episodic links to the concurrent taste [17]. An example of this is, hearing the brand Quaker from which the synesthete might perceive oatmeal taste.

Neural Basis of SynesthesiaEdit

Considerable attention has been devoted to the neural mechanisms underlying synesthesia, a healthy condition involving atypical brain activation and the concurrent experience of color photisms in response to letters, numbers, and words [18]. The neuroimaging research of synesthesia has focused on brain areas activated during the experience of synesthesia and associated structural brain differences [18]. Synesthesia offers a unique opportunity to study the neural basis of subjective experiences in healthy brains. Synesthesia does exist and is not related to any psychological, neurological, or psychiatric disease [19]. Comparing synesthetes to non-synesthetes shows structural differences in their grey and white matter properties. The structural differences are obtained in the modality-specific regions as well as in other brain areas. Brain areas involved in further processing, reactivating, and combining information can mediate synesthesia [19].

Heredity in SynesthesiaEdit

About 2% - 4% of the population has the condition of synesthesia [4]. Children born with the neurological trait are surprised to discover that other people do not experience the world the way they do [20]. The children who have the phenomenon often keep their perception to themselves as others disbelieve what they are experiencing. Kylie Barnett and her team conducted a methodological experiment from which they recruited a first-degree relative with synesthesia. The study proved that different types of synesthesia occur within the same family. The findings strongly indicate that various types of synesthesia are fundamentally related at the genetic level, but the individual differences between synesthetes are influence by other factors [21].

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