Synesthesia LOs Flashcards
What is synesthesia
A stimulus in one sensory modality evokes an additional experience in an unrelated sensory modality (intermodal synesthesia) or in a different aspect of the same modality (intramodel synesthesia)
Etymology: ‘joined sensation’ from greek syn = together + aisthesis = perception
Links two normally independent qualia
Stimulus triggers synesthesia is inducer and modality in which resulting synesthesia is experienced is the concurrent
Types identified by using inducer -> concurrent
Over 100 different types
Cytowic Diagnositc criteria:
automatic/involuntary
Spatially extended
Consistent, elementary and specific
Highly memorable
Loaded with affect
How common is synesthesia and what is the etiology
1 in 23 people have it
Equal in males and females
1 in 90 have grapheme -> colour synesthesia
Most common types: grapheme -> colour and days of the week -> colour
8x more common in artists, poets and novelists
Congenital: present from birth, heritable up to 40% of synesthetes first / second degree relatives also have it, can skip generations,
Adventitious: acquired after birth, may result from stroke, closed head trauma, temporal lobe epileptic seizures or blindness. Can be permanent, may be temporary (hallucinogenic drugs, meditation, sensory deprivation)
What is evidence synesthesia is bottom up sensory phenomena
Expt 1: constructed displays of randomly placed graphemes (5s) but other graphemes (2s) were placed in the display forming a certain shape (eg. a triangle) difficult for non-synesthetes to detect the shape (~60% correct) for grapheme -> colour synesthetes the shape popped out (~90%). Pop out is due to early visual processing
Expt. 2: graphemes were placed at point of fixation on a screen and gradually moved outwards to the periphery. Size of grapheme was scaled up to ensure it was still identifiable. Beyond 11° of eccentricity the grapheme was clearly visible but lots its color
Expt 3: individual graphemes presented in the periphery ( < 11°) are easily identified, however when other letters flanked the target it was difficult to identify. These crowded graphemes still evoked the appropriate colour, this process must take place at an early sensory level rather than a higher cognitive level. Conclusion: Synesthesia is a genuine sensory phenomenon?
Perception vs Imagination: spoken word -> colour synesthetes placed in PET scanner, spoken words caused activation in V4 but not other visual areas, non-synesthes scanned while they imagined colour; no activity in V4. Synesthesia is likely perceiving not imagining
What is evidence synesthesia is top down conceptual phenomena
Attention: created large graphemes (5s) out of different smaller graphemes (2s) called Navon figures. When grapheme -> colour synesthete attended to figure’s component graphemes (ie. the 2s) it was seen by the colour evoked by the 2, when figure attended as a global form (a 5) it was seen in the colour evoked by a 5. Attention is important for binding colours to forms
Context: synesthete shown ambiguous character in two different contexts: black digits or black letters. The digit 5 and letter S were exactly same stimulus. Digit 5 seen as pink, letter S seen as green. Top-down processing (context and meaning) affects synesthetic experience
Concepts: tested grapheme -> colour synesthete. She saw 7 as yellow; identified 7 slower when it was printed in blue ink. Was shown a series of slides each containing one symbol eg. 5 then + then 2 then =. Final slide showed colour patch (yellow or blue). Task: name colour as quickly as possible. Result: over 250 ms slower in mismatch condition (blue) than in match (yellow) Conclusion: synesthetic experience can be elicited by concepts not just by stimuli that are physically present
How can this evidence be reconciled
Two subtypes can exist for grapheme -> colour synesthesia, based on either perceptual or conceptual stimulus attributes
Lower synesthetes: processing is primarily bottom-up. Colours elicited by actual visual appearance of grapheme (eg. 7 -> yellow)
Higher synesthetes: experience is affected by top-down processing. Colours elicited by concept conveyed by the stimuli (eg. arabic numerals, roman numerals and days of the week which ll represent ordinality)
Dixon et al. (2004) proposed two subtypes based on experience in external or internal space:
Projectors: strongly experience colours as overlays “projected” and bound to a grapheme in the outside world = lower synesthetes
Associators: experience weaker synesthetic colours in the minds eye only = higher synesthetes
Cross-activation theory
gene in synesthesia fails to prune the excess synapses that are present in all infants. Result is cross activation from a certain stimulus due to cross-wiring between different brain areas. Extra connections exist.
Visual word form area (VWFA) in the fusiform gyrus responds to visual letters words and number, this is adjacent to V4. Hypothesis: in lower synesthetes there is cross activation between VWFA and V4 (would explain colour -> grapheme synesthesia)
Areas around angular gyrus (TPO junction) implicated in representing more abstract numerical ideas - concepts like ordinality
Anterior inferior temporal (AIT) cortex also found to encode conceptual representations of words, letter and numbers. Hypothesis: in higher synesthetes there is cross-activation between areas around the angular gyrus and V4 or another higher colour-processing area in the superior temporal gyrus (explain month -> colour synesthesia: each month has an ordinal value)
Difference between lower and higher synesthesia results from where the genes for synesthesia are expressed
Evidence: visual representation of graphemes (eg. font) affects lower not higher synesthetes. Accounts for the fact that upper/lowercase makes no difference because neither VWFA nor AIT respond differently to capitalization
Hubbard et al (2006): visually identifiable graphemes presented at lower contrast (eg. light grey on white background) failed to produce synesthetic colours, VFWA less activated by low-contrast letters more by high contrast
Rouw / Scholte (2007): used diffusion tensor imaging (DTI) to map white matter (tracts of myelinated axons), found increasing cross-wiring in the right inferior temporal cortex (near VWFA and V4), additional anatomical connectivity in left posterior IPS (intraparietal sulcus, involved in binding) and frontal lobes.
Problems: what about adventitious synesthesia (no new connections created by hallucinogens). What is synesthesia not apparent until mid-childhood (should be present from birth, because the brain of a neonate is more highly interconnected)
Disinhibited neural feedback theory
degree of neural connectivity is identical in synesthetes and non synesthetes. Activity is normally balanced by equal excitation and inhibition Synesthesia is due to (genetically determined) reduced inhibition in normally existing feedback pathways. Feedback may come from a multisensory nexus (like TPO junction)
accounts for top-down effects because feedback comes from multisensory areas (more connections also accounts for this)
Acquired synesthesia may be due to brain damage, meditation, hallucinogenic drug use (cannot result from more connections)
LSD blocks receptors for serotonin (primarily inhibitory) the resulting disinhibition allows neural targets to be more easily activated, hence synesthesia
Visual cortex can take on a representation of tactile space (eg. in newly blind or even in blindfolded sighted individuals after a few days); evidence for activation of existing somatosensory-visual connections
Problems: adventitious synesthesia and LSD-induced hallucinations are often complex, including visualizations of animals and complex scenes in contrast to the generic experiences of congenital synesthesia which generally are simply of colour. Does not account for synesthesia emerging in mid-childhood
Reduced Neural Plasticity
if a certain letter is seen in a certain colour this will strengthen the connection between them, because we see letters in arious colours this tends to average out. If plasticity is reduced an initial letter colour pairing may not change, accounts for emergence of synesthesia in mid-childhood.
Ward/Simmer (2003): JIW’s spoken word -> taste synesthesia is likely to hae originated during vocab acquisition. Phonemes that trigger certain taste also tend to appear in the name of the corresponding foodstuff (eg. college -> sausage, sydney -> kidney). Often a semantic association between triggering word and taste (eg. blue -> inky, bar -> milk chocolate). Synesthetic tastes are generally childhood foods rather than foods first eaten in adulthood. Implies synesthesia is not due to innate connections but can be influenced by conceptual knowledge.
Simner et al. (2005): tested grapheme -> colour synesthetes, classified responses into 11 common colour terms. Found non-random patterns of colour associations with letters (43% linked A -> red, 58% linked B-> blue or brown, 29% linked C-> yellow, 49% linked O -> white, 43% linked Y -> Yellow). Also people who lack synesthesia showed some evidence of same trends (43% red for A, 47% yellow for Y, 49% blue for B). High frequency letters are general associated with high frequency colours. Specific grapheme-colour links in synesthesia are not arbitrary perhaps due to orderly topographic mapping of many cortical areas.
Witthoft / Winawer (2013): 11 grapheme -> colour synesthetes evaluated. Colour-letter associations closely matched those in Fihser-Price magnetic letter sets which were owned by 10 of the synesthetes. Colours matched closely to at least 14 of 26 letters in the set, a probability of less than 1 in 1 billion. In addition to genetic and perceptual factors, synesthesia must also involve experience and memory
Problems: unclear how this theory accounts for other forms of synesthesia (eg. day of the week -> gender). Does not account for adventitious synesthesia
What are the implications of synesthesia for multisensory perception
Synesthesia may not be unique, interaction across senses is actually the rule rather than the exception.
Multisensory perception occurs in everyone (unitary experience resulting from combined stimulation of more than one sensory system).
In food for example sensory attributes tend to reliability co-occur (flavour comprised of taste, smell, touch, temperature, sight, sound and pain/irritation).
Watching as movie binds vision + audition
What are implications of synesthesia for binding problem
Bhow are separate aspects of a single stimulus (eg. colour, form, motion) reintegrated into a coherent holistic percept? Synesthetic experiences are not integrated but are synchronized and separate. Binding of qualia may be accomplished by coordinating subsystems using feedback connections which requires interconnections between differently specialied areas, resulting in holistic percept.
Hyperbinding model suggests synesthesia arises through an overactivation of parietal binding mechanisms
Applying TMS to parietal binding regions disrupts synesthesia
What are implications of synesthesia for metaphor/creativity/language
Synesthesia may reveal neurological basis for metaphor, creativity and language
Metaphor: seeing similar in the dissimilar. Important potential component of creativity. Perhaps makes synesthesia gene adaptive (more common in artists). Damage to left angular gyrus (implicated in higher synesthesia) leaves patients with impaired understanding of dual nature of metaphors (eg. all that glitters is not gold would only be treated literally)
Creativity: inward synesthesia allows novel connections to be made, emphasizes subjective nature of individual experience
Language: in both English / Tamil speakers, 98% named a rounded shape “bouba” and a jagged one “kiki”, also shown in children as young as 2.5. Effect is not obtained in patients with damage to left angular gyrus, implies objects are not named arbitrarily but are due to synesthesia like mapping (sound symbolism), type of synesthesia that we all share may be part of the basis for language
