M9, T1, unpacking Synesthesia

Question 1

Synesthesia

Answer 1

• There are 52 different types of synesthesia, but the most common type is grapheme-colour.
• Synesthesia involves inducer-concurrent pairings where the inducer will elicit a concurrent sensory experience in another sensory modality.
• The synesthetic inducer-concurrent experience is different to experiencing an illusion or a hallucination.
• can be developed or acquired

Question 2

Developmental and acquired synesthesia

Answer 2

Developmental Synesthesia: not considered a disorder – more a different experience that can be beneficial. Present from around age 1, and are lifelong experiences. Considered to be automatic – involuntary evoked

Acquired Synesthesia: caused by neuropathological, neurophysiological or neurochemical damage

Question 3

Nikolinakos et al 2013 colour-taste synesthesia, patient TK

Answer 3

• TK 72 year-old right handed male
• Colour is inducer
• Taste is concurrent
• artist, painter, print maker and art teacher

Question 4

Nikolinakos et al 2013, patient TK, unidirectional synesthesia for 3 or 4 basic tastes

Answer 4

Green -> bitterness
Red -> sweetness
Yellow -> sourness
No colour -> saltiness

Colours not resulting in taste qualities = neutral taste

Question 5

TK history of synesthesia

Answer 5

• TK became vividly aware of colour –tastes when he started art school in his 20s
• Prior to this no memory of synesthesia experiences
• TK painted since early childhood
• Synesthesia experiences occur when painting, looking at art and from colours in environment
• As an art teacher students thought he was being metaphorical – he was being literal

Question 6

TK active experience of synesthesia

Answer 6

• TK uses the experience to inform artwork – is the artwork, like a good meal, balanced?
• TK’s synesthesia experiences vary with the purity, amount and intensity of the perceived hue
• Experiences taste on his tongue but it differs from the taste of food (less strong)
• No experiences when forming mental images of colours or when dreaming

Question 7

Fornazzari et al 2012 multimodal synesthesia and thalamic stroke

Answer 7

• 45 year old male who developed acquired multimodal synesthesia 9 months after stroke
• Admitted to hospital ED with a hypertensive crisis most likely evoked by the use of cocaine
• Resulted in right hemiparesis, right hemisensory deficit, right hemianopsia, slurred speech with normal comprehension, and a transient right alien limb phenomenon. He also lost his sense of smell at the time

Question 8

Fornazzari et al 2012 experiences after stroke before diagnosis

Answer 8

• Referred to neurology 14 months after stroke due to subjective memory complaint
• 6 months after stroke he experienced frequent déjà vu and developed petite Madeleine phenomenon triggered by visual stimuli
  -> automatic involuntary evoking of a memory
  -> “the sight of a ginger cookie opened a full childhood scene in his grandmothers kitchen with visual, auditory and olfactory sensations”
• Sense of smell began to return

Question 9

Fornazzari et al 2012, patient experience of synesthesia

Answer 9

• 9 months after the stroke he started having different synesthetic experiences
• Developed sound -> colour, sounds -> tactile (touch), grapheme -> taste synesthesia, and conceptual type (thinking about something can elicit experience)
  -> Blue print (in advertisements) meant chicken tasted bad
  -> Black print chicken OK
• During 3 years of the study his experiences were consistent

Question 10

Fornazzari et al 2012 patient damaged region and cognitive ability

Answer 10

• He could voluntarily suppress all of his synesthesic experiences
• Normal cognitive abilities (MMSE score = 30/30)
• MRI scan at 24 months after the stroke revealed a lesion left posterior part of thalamus
• The thalamus is the central relay station for the brain
• The specific area damaged in the patient is associated with communication to the posterior parietal cortex (associated with attention) and multimodal integrative areas in the brain

Question 11

Why does synesthesia occur?

Answer 11

Cross activation hypothesis
Developmental synesthesia
Acquired synesthesia

Question 12

Cross activation hypothesis (Ramachandran and Hubbard, 2001)

Answer 12

• There appears to be distinct neural mechanisms underlying synesthesia related to differences in connectivity relative to the neurotypical brain.
• differences in grey and white matter from synesthetes v NT
• DTI shows differences in communication between synesthetes v NT
• It has been proposed that some particular types of synesthesia might be more common than others (grapheme-colour) because neural areas associated with each are located close together in the brain
• The colour processing area of the brain (V4) is in the fusiform.
• The visual-grapheme processing area of the brain is in the fusiform, essentially next to V4

Question 13

BUT - cross activation hypothesis (Ramachandran and Hubbard, 2001)

Answer 13

• The idea is that there could be some cross-connectivity in the brain causing synaesthesia, and because these two areas (V4 and fusiform) are so close to one another, it is more likely for this form (grapheme-colour) to occur
• However, not all types of synaesthesia can be explained by this proximity of neural areas
• Many structural and functional brain differences in synesthetes are in areas not directly implicated in the relevant sensory area of the brain for the synesthesia being studied (e.g. the intraparietal sulcus in grapheme-colour – may be involved in attention)

Question 14

Developmental synesthesia and heritability

Answer 14

• Synesthesia is known to run in families (Jewanski et al., 2011)
• However, family members don’t necessarily show the exact same stimuli-perception pairings
• Synesthetes with lexical-gustatory synesthesia (a rare type) tend to have relatives with the more common color-based synesthesia
• Most studies of DS agree that there is a genetic basis
• While there might be a genetic predisposition, it doesn’t seem as though genetics cause synesthesia

Question 15

Developmental synesthesia, conflicting research

Answer 15

• Baron-Cohen (1996) found that DS is more common in females than males, and it seems as though the particular trait is being passed along the X-chromosome (Bailey & Johnson, 1997).
• However, later research did not support this finding
• Furthermore, research suggests that the same genetic deformities are present for both affected and unaffected members of the same families
• Note: it is hard to localise which of many genes might be related

Question 16

Developmental synesthesia pruning of connections

Answer 16

• At a young age, our brains have far more connections than what they do as an adult and as we age experience prunes our connections.
• Therefore, in some types of synaesthesia, there may be a genetic mutation meaning that some connections are not being pruned away as efficiently with experience (or there are just more connections to start with in the brain)

Question 17

Acquired synesthesia

Answer 17

Compared to developmental synaesthesia, acquired synaesthesia tends to have inducers (stimuli) which are simple sensory stimuli rather than learned and meaningful stimuli (like graphemes or days of the week)

Two possibilities of how it comes about:
- Sensory loss
- Pharmacology

Question 18

Acquired synesthesia: sensory loss

Answer 18

• In these cases, the impaired modality acts as the synesthetic concurrent (e.g. acquired touch-vision after going blind)
• It is likely that the brain area associated with the particular sense lost is still functional (e.g. visual cortex), but no longer receives its primary inputs (from the eye/retina).
• Therefore, the concurrent experience may come about because the undamaged brain area instead relies on other existing multisensory inputs, which then take on the role of the inducer for the particular brain area

Question 19

Acquired synesthesia: sensory loss - brain plasticity

Answer 19

• Acquired cases usually develop from two days to two years after sensory loss
• This suggests that the brain has some flexibility in terms of it’s plasticity
• Fast acting inhibition from existing pathways after sensory loss
• Generation of new connections which may take a longer amount of time

Question 20

Acquired synesthesia: pharmacology

Answer 20

• Taking some types of drugs (e.g. LSD; Mescaline, Psilocybin) can induce temporary synesthesia in some cases
• After ingesting these substances, experiences can develop within hours, potentially due to the substance unmasking pre-existing pathways (Hartman & Hollister 1963)
• For LSD, could be due to decreased inhibition via serotonergic receptors in the brain (Marek & Aghajanian, 1998)

Question 21

Modifying stroop task to measure synesthesia experience

Answer 21

• One common way to modify the stroop task and test colour-grapheme synesthesia is to ask participants to name different graphemes (e.g. A, B C) when they are printed in congruent or incongruent colours to the synesthetes self reported experience.
• For instance, if the person has a sensory experience of seeing the colour blue upon viewing the letter A, then, they should be faster to name that letter when it is printed in blue compared to when it is printed in another colour (e.g. green, which might be associated with the letter B)

Question 22

Does the synesthetic experience depend on attentional mechanisms?

Answer 22

• if several ‘A’s are arranged to form a large ‘T’
  -> when the ‘A’ are attended orange may be perceived
  -> when the ‘T’ is attended green may be perceived
• relies on subjective report

Question 23

Synesthetic congruency tasks

Answer 23

• Synesthetic congruency effect =difference in RT to name colours on congruent (letter matches synesthete’s representation) and incongruent trials (letter does not match synesthete’s representation)
• Synesthetes usually slower for incongruent trials whereas controls show no difference

Question 24

Is it useful? Synesthetic congruency tasks

Answer 24

These tasks mean we can measure synesthesia objectively, however:
-> Are people faster for congruent trials, slower for incongruent trials, or both?
-> Is the effect occurring at the perceptual level or the response level?

-> the task can be adapted to test the role of attention by using priming
-> is awareness necessary for the synesthetic effect?

Question 25

Synesthetic priming task

Answer 25

• Present an irrelevant letter prime (congruent or incongruent) that the synesthete associates with a particular colour then present a colour patch to be named.
• Vary awareness of the prime by changing duration letter is shown for.
  -> Congruency effect only emerged when the letter prime was shown for a relatively long time (500ms)
  -> No congruency effect when prime was shown for 28 or 56ms
  -> explicit awareness required

Question 26

Synesthetic priming task: load manipulation

Answer 26

• focus on fixation point then a letter prime (congruent or incongruent) appears for 400ms, followed by an image containing the target colour which is the named
• the prime letter is surrounded by a shape that has multiple gaps, task is to determine where the smallest gap is
• Low load = easy gap discrimination task
• High load = hard gap discrimination task
• High load requires more “spare” attention resources – so changing level of attention directed to the letter

Question 27

Synesthetic priming task: load manipulation results

Answer 27

• smaller congruency effect for high load versus low load task for synesthetes
• smaller interference effects in low load condition as the gap is easier to find, more interference effects from the prime letter in high load conditions
• high load condition consumed all “spare” attention resources and diverted attention away from prime letter
• at the very least, attention modulates synesthetic experience

Question 28

Visual search: pop out search

Answer 28

Pop Out Search: Occurs when the search target is defined by a single feature
- When searching for a single feature, it “pops-out” and is processed effortlessly, (i.e., pre-attentively).
- It is said the search is conducted in parallel - you search the whole scene at once
- response time remains the same regardless of load change
*look up image

Question 29

Visual search: effortful search

Answer 29

Effortful search: occurs when the search target is defined by two or more features (a conjunction of features)
- When doing a conjunction search, the search for the target is far more difficult, and requires direct attentional engagement (attentively).
- The search is conducted serially – attend to small sections of the scene, one step at a time
- response time increase linearly as load increases
*look up image

Question 30

Synesthetic searching

Answer 30

• what may appear as an effortful search task to a NT may appear as a pop out search task to a synesthete *look up image

Synesthetic embedded figures task
- Participants search for the embedded figure made up of letters
- Presumably, if synesthetic colour experience arose pre-attentively, S participants would be equally as fast to detect shapes with increased distractors, whereas controls would get slower and slower
-> however no difference in RT’s for synesthetes v controls as distractors increased
-> limited evidence that shapes “pop-out” or are identified without attention
-> synesthetes slightly more accurate (perhaps once attended colours helped speed decisions)