Synesthesia

Integration of sensory information is essential to identifying and interacting with our environment. Developing the capacity to understand an object in its visual, olfactory, tactile, auditory, and gustatory is fundamentally important for manipulating their environment[1]. For example, we are capable of identifying the pie on a kitchen table as an edible apple pie (as opposed to a table ornament) by integrating its visual characteristics, taste, smell, and even feel. However, approximately 1/200 individuals report experiencing an automatic, exclusive relationship between two senses, meaning that input from one sensory modality evokes through the experience of another sensory modality consistently thought-out their life and in all environments[2]. Synesthesia is defined as “a neurological phenomenon in which stimulation of one sensory or cognitive pathway leads to automatic, involuntary experiences in a second sensory or cognitive pathway.[3]” There are multiple combinations of exclusive sensory pairs in synesthetes, such as: number-form, sound to color, and lexical-gustatory, which describes taste associated with sound. Both synesthetes and non-synesthetes similarly process and integrate incoming sensory stimuli in the somatosensory cortex in order to identify and manipulate their world; however, recent research attributes synesthetic experience to activity in the temporal lobe and angular gyrus.


Review of Somatosensory Process:
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****[4]****Figure 1
Three types of cutaneous receptors:
Mechanoreceptors- respond to mechanical stimuli such as sound or touch, travels in dorsal columns medial lemniscus pathway
  • Merkle’s Disk-
    • SA type I
    • Edges, bumps, pressure
  • Meisner’s Corpuscles-
    • RA type I
    • Dynamic pressure such as sliding or stroking
  • Ruffini’s Endings-
    • SA type II
    • Stretching
  • Pacinian Corpuscles-
    • RA type II
    • Deeper pressure, fluttering, vibration
  • Thermoreceptors- detects temperature, travels on spinothalamic tract
    • Cold receptors- 10°C-30°C
    • Heat receptors- 32°C -45°C
  • Nociceptors- detect pain, travels on spinothalamic tract
    • Free nerve ending- carry pain sensation from nociceptors to spinal cord [5]

The somatosensory cortex is the main association area for touch and is located in each cerebral hemisphere behind the central sulcus in the post central gyrus of each parietal lobe. It is comprised of somatosensory area I & II. Somatosensory area I is comprised of Broadman’s area 1, 2, 3a, and 3b. Somatosensory area II consists of Broadman’s area 5- primary association area and 7- multimodal association area. The primary sensory area receives elementary information then recognizes and interprets touch giving identity and meaning to the object. Broadman’s area 7 is involved in intermodal processing’s in that it integrates information between body parts in interpersonal and extra personal space. Different areas of the cortex receive different information; for example, sensory input enters in layer 4, layer 3a receives information from muscle spindles, and layers 1&2 receive information on discriminative touch. Cortical communication occurs in layers 2 and 3 and is sent ipsilaterally and contralaterally to somatic sensory cortical areas and the motor cortex. This has the ability to modulate ascending information coming through the thalamus, dorsal column nucleus, and spinal cord. There is a direct corticocortal connection between the primary sensory cortex and the adjacent primary motor cortex.

Broadman’s areas
1. Activated by cutaneous afferents, receives input from 3b
2. Rotation of joints- joint receptors
3a. Transitional area between pre-& post-central gyrus, afferents from muscle receptors ex: GTO & Spindles
3b. Cutaneous Stimulation (SA and RA fibers)

Cross modality processing or intermodal processing- information from various sensory receptors are integrated to give information about movement of a body area.

Role of Somatosensory Integration in Synesthetes
Interestingly enough, some synesthetes experience other sensory modalities along with touch such as visual- tactile and tactile-emotion [6]. In tactile- emotion synesthesia, one experiences a strong emotion in reaction to various textures and tactile inputs[7]. For example, patient AW reported feelings of feelings of confusion when touching corduroy, depression when touching denim, and happiness when feeling silk. When one touches an object, the tactile input is detected and transmitted to somatosensory area I. Somatosensory area II is involved in higher processing of tactile input and discerns what the touch is. The insula in the brain “maps ones internal feelings from their body and uses this information to represent how you feel in relation to the outside world and your immediate environment7.” In extension, the insula is in communication with other emotional centers in ones brain such as theamygdala, autonomic nervous system, and orbitofrontal cortex. Normally, non-synesthetes do exhibit some level of emotion to touch for example, if one’s skin brushes against another, or feeling disgust when handling a slimy substance. However, it is apparent that touch-triggered emotions are heightened in some synesthetes. Researchers attribute the exaggeration of emotions to increased neural cross wirings between the amygdala, S2, insula, and orbitofrontal cortex.

Review of Visual Processing Pathways:
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Figure 2
Screen Shot 2016-11-30 at 2.29.02 PM.png****[8]****
Figure 3

Visual detection of form and color begins with the parvocellular ventral pathway. Initially, the visual input is further processed down the ventral stream through the P cells of the retinal ganglion. From the retinal ganglion, the information of form travels to parvocellular layer while information about color travels through the parvocellular and intralaminar regions of the Lateral Geniculate Nucleus (LGN) of the thalamus via cranial nerve 2, the optic nerve, and then the optic track. There are six layers of the LGN, and information concerning color and form travel through layers 3-6 (ispi, contra, ipsi, contra). Next, it travels through the optic radians to layer 4Cβ of the primary visual cortex. 4Cβ receives information from the parvocellular pathways and relays it to layers II and II of the interblobs in the visual association areas. Information about form travels along the pale stripe, also known as the inter stripe via V2 to the inferior occipitotemporal cortex for higher processing. The interstripe contains complex cells, which are orientation selective to objects anywhere in the receptive field. Information pertaining to color is sent to higher order visual association areas, specifically the inferior occipitotemporal cortex via the thin stripe. The thin stripe contains simple cells, which are only orientation selective.


Neurological Areas Involved:

FMRI technology illuminated the brain locations active during sensory experiences. Integration of sensory information evident during synesthetic experience may be a result of cross activation of sensory information in the brain due to close proximity, which can allow for neural networks to form between the two sensory regions. FMRI studies illuminated the close anatomical proximity of V4 and the fusiform gyrus, affirming researchers cross activation hypothesis7. Visual area 4 contributes to color perception and the fusiform gyrus. For example, Esmeralda sees colors in response to musical notes. In the brain, the temporal lobe is in close proximity to the brain center that receives color signals7. Researchers suggest that cross activation between the temporal lobe and color recognition centers are responsible for her synesthetic experience. In extension, the angular gyrus is thought to play an active role in cross-sensory integration and explain textual synesthesia. For example, a memory of a cat is comprised of its visual appearance, the sound it makes, how it feels, and its scent. The cat’s collective qualities are integrated in the angular gyrus to evoke a memory or recognition. Perhaps synesthetes have an abnormal connection in the angular gyrus that over associates ones sensation pathway with another’s.
Interestingly enough, damage to the angular gyrus inhibits sensory integration; one may recognize individual characteristics of an object or figure but have trouble making sense of the object as a whole and its function. For example, one might recognize the number two and the numberseven but not understand that one number is greater in quantity. Exploring the middle ground between synesthetic processes in the angular gyrus and exploring damage to the angular gyrus collectively shed light onto the important function it holds, which is in interpreting and manipulating our perceived world. I find it fascinating that true development of a deep understanding of brain anatomy and neurological processes is found through studying abnormalities. The relationship between abnormalities can collectively complete ones understanding such as the middle ground between synesthesia and damage to the angular gyrus.


Research:
Skeptics have questioned the validity of synesthetic experience. Many question whether it is a contrived sensation that is merely a figment of their imagination rather than a true experience. Therefore, researchers conducted an array experiments to test the authenticity of synesthetic experiences. Various tests such as GSR, FMRI, and identification of various shapes and symbols shed light into the brain structures involved in sensory integration and provide evidence that synesthetic experiences are real.


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Figure 4

Those with grapheme-color synesthesia report consistently perceiving colors and/or numbers in specific, designated color. For example, when viewing the letter ‘A’ it will constantly appear lilac, and ‘B’ may appear brown to the individual throughout their life. Visual perception of characters involuntarily produces a secondary perception of color. Interestingly enough, the color-to-letter relationship is inconsistent among synesthetes in that some perceive the letter M as green and other perceive the letter M as blue.

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Figure 5
One study tested the low-level sensory effects of synesthetes who saw numbers in color in efforts to find evidence that their experience is consistent with known brain processes [9]. Both synesthetes and non-synesthetes were asked to identify the ratio of 5’s and 2’s jumbled up in a black and white picture7. Then non-synesthetes were asked to identify the ratio of 5’s and 2’s in a jumbled picture where the 5’s and 2’s were shaded in distinctly different colors. Synesthetes consistently answered correctly 80-90 percent of the time because they always saw the numbers in color, even if they were hard to identify figuratively. On the contrary, non-synesthetes could only identify the ratio between 5’s and 2’s 80-90 percent of the time when they were color-coded because the color difference was easier to detect than the figure difference. Although given the same black and white picture (presented to the left of figure 5), synesthetes inherently perceived the right portion of figure 5. This led researchers to believe that their experience is genuine because if their synesthetic experience were purely figurative, they would not be able to quickly identify the ratio faster than non-synesthetes who guessed correct ratios about 50% of the time. However, because synesthetes answered correctly 80-90 percent of the time, we can conclude that they genuinely saw the numbers in two different colors, supporting sensory integration. These findings suggest that synesthetic experiences are authentically sensory (Ramachandran 93).

Future:
Ramachandran suggests that further exploration of synesthesia may open a window into understanding evolutionary origins of creativity and imagination. He suggests that having full-blown disorders may be valuable in boosting creativity, intelligence and social-emotional abilities7. While synesthesia is not a giant metaphor, it does not deter from how there are real biological processes at play; there is a deep connection between the two. The Origin of metaphors and how one comes to understand metaphors has always been a mystery to scientists because they make no literal sense. For example: how does one “feel” blue. Blue is a color not an emotion. In this way, metaphors are comparable to synesthetic experiences as synesthetes feel colors. Ramachandran suggests that further exploration of synesthetic experiences in the brain can shed light on how we come to understand such metaphors. Synesthetic qualities are evident in our understanding of metaphors, which is interesting because we do not have any idea how metaphors work or are represented in the brain. If the evolution metaphors, and by extension, higher thought and abstract thinking, are connected to synesthesia, then it is essential to explore because higher thought is unique to our humanity.7

Glossary:
Synesthesia- a neurological phenomenon in which stimulation of one sensory or cognitive pathway leads to automatic, involuntary experiences in a second sensory or cognitive pathway.[3]
mechanoreceptors- respond to mechanical stimuli such as sound or touch, travels in dorsal columns medial lemniscus pathway
Lexical- gustatory synesthesia- word and taste [10]
Chromesthesia- when a color evokes a sound
GSR- (Galvic skin response) detects perspiration rates
FMRI- (functional magnetic response imaging) measures brain activity by measuring blood flow to various areas of the brain

Questions:
True or False:
1. Studies have shown that synesthesia is a true sensory experience

2. There is only one type of synesthesia

3. understanding synesthesia can provide insight to evolutionary origins of metaphors

4. All synesthetes see the same letters in the same colors.

5. Damage to the angular gyrus inhibits sensory integration

Multiple Choice:

6. Information is integrated in the ...
a. somatosensory cortex
b. angular gyrus
c. occipital lobe
d. medulla


7. What neurological brain imaging technique was used to test the synesthetic experiences?
a. EEG
b. EKG
c. FMRI
d. CT scan

8. Broadman's area is responsible for intermodal processing.
a. 3a
b. 4
c. 2
d. 7

9. cutaneous receptors respond to...
a. touch
b. vibration
c. pressure
d. all the above

10. which of the following is not an example of synesthetic experience?
a. colors and numbers
b. emotion and taste
c. auditory and emotion
d. auditory and colors

Answers:
T, F, T, F, T, a, c, d, d, c.


[1] Types of Synesthesia. (n.d.). Retrieved November 30, 2016, from http://www.synesthesiatest.org/types-of-synesthesia
[2] Cytowic, R. E. (2002, July 01). Cerebrum. Retrieved November 30, 2016, from http://www.dana.org/Cerebrum/Default.aspx?id=39297
[3] Langtree, I. (2016). Synesthesia: Seeing Sounds & Hearing Colors. Retrieved November 30, 2016, from https://www.disabled-world.com/health/neurology/brain/synesthesia.php
[4] Sensory systems I. (n.d.). Retrieved November 30, 2016, from http://www.utdallas.edu/~tres/integ/sen1/display5_03.html
[5] Bryne, J. H. (1997). Pain Principles (Section 2, Chapter 6) Neuroscience Online: An Electronic Textbook for the Neurosciences | Department of Neurobiology and Anatomy - The University of Texas Medical School at Houston. Retrieved November 30, 2016, from http://neuroscience.uth.tmc.edu/s2/chapter06.html
[6] Simner, J., & Ludwig, V. U. (2012). The color of touch: A case of tactile–visual synaesthesia. Neurocase, 18(2), 167-180. doi:10.1080/13554794.2011.568503
[7] Romm, C. (2016, May 22). The People Who Store Their Emotions in Their Fingertips. Retrieved November 30, 2016, from http://nymag.com/scienceofus/2016/05/the-people-who-store-their-emotions-in-their-fingertips.html
[8] Introduction To Visual Prostheses by Eduardo Fernandez and Richard Normann. (2016). Retrieved November 30, 2016, from http://webvision.med.utah.edu/book/part-xv-prosthetics/introduction-to-visual-prostheses-by-eduardo-fernandez-and-richard-normann/
[9] Ramachandran, V. S. (2011). The tell-tale brain: A neuroscientist's quest for what makes us human. New York: W.W. Norton.
[10] H. (2016). Synesthesia: Tasting Words, Seeing Sounds, Hearing Colours And More. Retrieved December 04, 2016, from http://www.spring.org.uk/2014/05/6-intriguing-types-of-synesthesia-tasting-words-seeing-sounds-hearing-colours-and-more.php