S y n e s t h e s i a Grant Heller

What is Synesthesia? (Cytowic, 1993) The word synesthesia comes from the two Greek words, syn (together) and aisthesis (perception). Therefore, synesthesia literally means "joined perception.” Synesthesia is the merging of 2 or more senses (for example, sound and sight). This is often referred to as a cross-modal association in nonsynesthetes. Any of the 5 senses can be joined, and current research is investigating other sensations, such as that of emotions or the passage of time. Most accounts report colored hearing, but colored letters or numbers are also common forms of synesthesia. Synesthetic relationships are usually unique to the individual, and usually only operate in one direction. For example, sight may induce touch, but touch will not induce visual perception.

The word synesthesia comes from the two Greek words, syn (together) and aisthesis (perception). Therefore, synesthesia literally means "joined perception.”

Synesthesia is the merging of 2 or more senses (for example, sound and sight). This is often referred to as a cross-modal association in nonsynesthetes.

Any of the 5 senses can be joined, and current research is investigating other sensations, such as that of emotions or the passage of time.

Most accounts report colored hearing, but colored letters or numbers are also common forms of synesthesia.

Synesthetic relationships are usually unique to the individual, and usually only operate in one direction. For example, sight may induce touch, but touch will not induce visual perception.

The colored alphabets of 2 synesthetes (Duffy, 2001)

Comparison: letters to Chinese characters (Duffy, 2001)

Colored Time (Duffy, 2001)

History (Cytowic, 2003) Synesthesia has been known in the field of medicine for nearly 300 years. However, only at the turn of the 20th century was it settled that this phenomenon was a product of the brain rather than the imagination. The first known medical reference comes from the English opthamologist, Thomas Woolhouse, who described a blind man who reported sound-induced colored visions in 1710. Interest peaked between 1860 and 1920, and dropped significantly by 1930. This was partly due to the inability to find a causal mechanism for the phenomenon, and also due to the shift in the field of psychology towards the objective interests of behaviorism. As of yet, no one has been able to find a physical translation algorithm between the senses.

Synesthesia has been known in the field of medicine for nearly 300 years. However, only at the turn of the 20th century was it settled that this phenomenon was a product of the brain rather than the imagination.

The first known medical reference comes from the English opthamologist, Thomas Woolhouse, who described a blind man who reported sound-induced colored visions in 1710.

Interest peaked between 1860 and 1920, and dropped significantly by 1930. This was partly due to the inability to find a causal mechanism for the phenomenon, and also due to the shift in the field of psychology towards the objective interests of behaviorism.

As of yet, no one has been able to find a physical translation algorithm between the senses.

Prevelance & Characteristics (Cytowic, 1995) It is estimated that about 1/25,000 have some form. It is likely that many who have synesthesia do not even realize this is a phenomenon unique to them. Most report having these joined sensations as far back as they can remember. Many synesthetes have excellent memories (perform in the superior range on the Weschler Memory Scale), but show minor deficits in the areas of mathematics, a poor sense of direction, and right-left confusion. It is thought to be a dominant trait of the X chromosome, and has been observed 3:1 in US women, and 8:1 in UK women. Synesthesia is not referred to as a disorder or a deficit. Synesthetes usually perform normally on neurological tests.

It is estimated that about 1/25,000 have some form. It is likely that many who have synesthesia do not even realize this is a phenomenon unique to them.

Most report having these joined sensations as far back as they can remember.

Many synesthetes have excellent memories (perform in the superior range on the Weschler Memory Scale), but show minor deficits in the areas of mathematics, a poor sense of direction, and right-left confusion.

It is thought to be a dominant trait of the X chromosome, and has been observed 3:1 in US women, and 8:1 in UK women.

Synesthesia is not referred to as a disorder or a deficit. Synesthetes usually perform normally on neurological tests.

Cytowic’s five diagnostic features (1993, 1995) 1.) “ Synesthesia is involuntary but elicited . It is a passive experience that happens to someone. It is unsuppressable, but elicited by a stimulus that is usually identified without difficulty. It cannot be conjured up or dismissed at will, although circumstances of attention and distraction may make the experience seem more or less vivid”. 2.) “ Synesthesia is projected . It is perceived externally in peri-personal space, the limb-axis space immediately surrounding the body, never at a distance…” 3.) “ Synesthetic perceptions are durable, discrete, and generic, never pictorial or elaborated .” Associations last a lifetime, are unique to the individual, and never advance beyond an elementary, unembroidered level. 4.) “ Synesthesia is memorable …many synesthetes use their association as a mnemonic aid”. 5.) “ Synesthesia is emotional and noetic (knowledge from experience)”.

1.) “ Synesthesia is involuntary but elicited . It is a passive experience that happens to someone. It is unsuppressable, but elicited by a stimulus that is usually identified without difficulty. It cannot be conjured up or dismissed at will, although circumstances of attention and distraction may make the experience seem more or less vivid”.

2.) “ Synesthesia is projected . It is perceived externally in peri-personal space, the limb-axis space immediately surrounding the body, never at a distance…”

3.) “ Synesthetic perceptions are durable, discrete, and generic, never pictorial or elaborated .” Associations last a lifetime, are unique to the individual, and never advance beyond an elementary, unembroidered level.

4.) “ Synesthesia is memorable …many synesthetes use their association as a mnemonic aid”.

5.) “ Synesthesia is emotional and noetic (knowledge from experience)”.

Baron-Cohen & Harrison (1997) Simon Baron-Cohen at the Institute of Psychiatry in London tested colored-letter synesthetes for the consistency and durability of their perception. Synesthete and nonsynesthete participants were asked to report their colored letter pairings (the control group was asked to make up their own paring). Without warning a year later the synesthetes were asked to again report their color-letter pairings. 92% accurately identified the same colors for their letters. In comparison, only 37% of the control group participants were able to correctly report their parings after one week (Baron-Cohen and Harrison, 1997).

Simon Baron-Cohen at the Institute of Psychiatry in London tested colored-letter synesthetes for the consistency and durability of their perception.

Synesthete and nonsynesthete participants were asked to report their colored letter pairings (the control group was asked to make up their own paring).

Without warning a year later the synesthetes were asked to again report their color-letter pairings. 92% accurately identified the same colors for their letters.

In comparison, only 37% of the control group participants were able to correctly report their parings after one week (Baron-Cohen and Harrison, 1997).

Colored-letters…in physics (Duffy, 2001)

Colored Music (Duffy, 2001)

Sound induced by visual perception (Duffy, 2001)

Acupuncture & Synesthesia (Duffy, 2001)

(Duffy, 2001)

Colored Music (Duffy, 2001)

Different Types of Synesthesia (Duffy, 2001) Developmental synesthesia: This is the traditional form described by Cytowic’s 5 diagnostic features. These synethetes report having their joined sensations back to their earliest memories. Metaphorical synesthesia: These are synesthetic references made in art, music, or poetry. We commonly use these metaphors: “it looks cold outside, I’m feeling blue, Swiss cheese tastes sharp”, etc. Acquired synesthesia (usually caused by a neurological dysfunction or other dramatic physical change): Most commonly there are case study reports of blind individuals who suddenly pair colored-sight with words or tactile stimulation. Drug-Induced synesthesia: Psychoactive drugs such as LSD, mescaline, or hashish can illicit joined sensations. About 4% of seizures in the hippocampus of the limbic system induce temporary synesthesia. Seizures localized in the hippocampus produce simple sensory experiences, while those that spread into the cortex of the temporal lobe can induce very specific and elaborate sensations (Cytowic, 1995).

Developmental synesthesia: This is the traditional form described by Cytowic’s 5 diagnostic features. These synethetes report having their joined sensations back to their earliest memories.

Metaphorical synesthesia: These are synesthetic references made in art, music, or poetry. We commonly use these metaphors: “it looks cold outside, I’m feeling blue, Swiss cheese tastes sharp”, etc.

Acquired synesthesia (usually caused by a neurological dysfunction or other dramatic physical change): Most commonly there are case study reports of blind individuals who suddenly pair colored-sight with words or tactile stimulation.

Drug-Induced synesthesia: Psychoactive drugs such as LSD, mescaline, or hashish can illicit joined sensations.

About 4% of seizures in the hippocampus of the limbic system induce temporary synesthesia. Seizures localized in the hippocampus produce simple sensory experiences, while those that spread into the cortex of the temporal lobe can induce very specific and elaborate sensations (Cytowic, 1995).

Current Theories of Synesthesia Learned association theory: Proposes that synesthesia is entirely based on learned associations between stimuli, and is thus solely based on memory experiences. This theory was originally proposed by M. Calkins in 1893. While it is weak in explaining complex and durable associations, this theory cannot be entirely ruled out as a possibility. In this theory there would likely be strong functional connections between sensory brain areas in synesthetes.

Learned association theory:

Proposes that synesthesia is entirely based on learned associations between stimuli, and is thus solely based on memory experiences. This theory was originally proposed by M. Calkins in 1893.

While it is weak in explaining complex and durable associations, this theory cannot be entirely ruled out as a possibility.

In this theory there would likely be strong functional connections between sensory brain areas in synesthetes.

Awareness theories: There is a possibility that synesthetic interactions across modalities occur in everyone, but at some point synesthetes become actively conscious of this connection. Thus, synesthesia would occur in our unconscious minds and would be suppressed by most individuals. Cytowic (1993) postulated a similar theory of cortical suppression. Cytowic claimed that synesthesia resides in the subcortical regions of the limbic system, and only emerges to the conscious when the cortical areas are suppressed. Cytowic has also proposed that the limbic system is central to human emotions, and is not necessarily a less evolved region of the brain. Current Theories of Synesthesia

Awareness theories:

There is a possibility that synesthetic interactions across modalities occur in everyone, but at some point synesthetes become actively conscious of this connection.

Thus, synesthesia would occur in our unconscious minds and would be suppressed by most individuals.

Cytowic (1993) postulated a similar theory of cortical suppression. Cytowic claimed that synesthesia resides in the subcortical regions of the limbic system, and only emerges to the conscious when the cortical areas are suppressed.

Cytowic has also proposed that the limbic system is central to human emotions, and is not necessarily a less evolved region of the brain.

Neural connectivity theories: In the past decade, most synesthetic theories have focussed on issues of neural connectivity. Maurer (1997) provides evidence that strongly indicates neural connections between the auditory and visual areas in infants. Normally these connections are pruned as the infant grows, but this could explain synesthesia if some pathways remained intact. This is referred to as the neonatal synesthesia theory. Ramachandran (2001) has hypothesized that functional connections exist in synesthetes between specific senses either through excessive proliferation or defective pruning of neural connections. This is referred to as the cross-wiring theory. Current Theories of Synesthesia

Neural connectivity theories:

In the past decade, most synesthetic theories have focussed on issues of neural connectivity.

Maurer (1997) provides evidence that strongly indicates neural connections between the auditory and visual areas in infants. Normally these connections are pruned as the infant grows, but this could explain synesthesia if some pathways remained intact. This is referred to as the neonatal synesthesia theory.

Ramachandran (2001) has hypothesized that functional connections exist in synesthetes between specific senses either through excessive proliferation or defective pruning of neural connections. This is referred to as the cross-wiring theory.

Cross-wiring in the fusiform gyrus (Leeuwen, 2004)

Disinhibited feedback theories: It’s possible that there are no abnormal neural networks in synesthetes, but that existing and normal connections are used in an abnormal way (Grossenbacher, 1997). Current Theories of Synesthesia

Disinhibited feedback theories:

It’s possible that there are no abnormal neural networks in synesthetes, but that existing and normal connections are used in an abnormal way (Grossenbacher, 1997).

Crowding study with synesthesia (Hubbard & Ramachandran, 2001)

Attention to local or global stimuli (Rich & Mattingly, 2003)

PET Scan: Colored-word synesthetes (top), & nonsynesthetes (bottom) (Paulesu, et. al., 1995)

fMRI: Brain activations for synesthetes vs. control subjects (Leeuwn, 2004; Nunn et. al., 2002)

References

References