synaesthesia revision

Define synaesthesia.

A condition in which a stimulus in one sensory or cognitive domain automatically triggers an additional involuntary perceptual experience.

What is an inducer?

The trigger stimulus that evokes a synaesthetic experience.

What is a concurrent?

The additional perceptual experience evoked by the inducer.

What are the key hallmarks of natural synaesthesia?

Consistency, automaticity, involuntary activation, lifelong stability, and independence from mood or mental state.

What is phenomenology?

The subjective experience of perception; in synaesthesia, the genuine experience of the concurrent.

What is the most common type of synaesthesia?

Grapheme-colour synaesthesia (letters/numbers evoke colours).

Distinguish between natural, induced, and adventitious synaesthesia.

• Natural (developmental) synaesthesia: present from early life; stable, consistent, automatic, and involuntary.

• Induced synaesthesia: experimentally produced (training, hypnosis, drugs, sensory deprivation).

• Adventitious/acquired synaesthesia: develops later due to unusual circumstances such as brain injury, drugs, meditation, or sensory deprivation.

• Categories can overlap (e.g., drug-induced experiences are both induced and acquired)

Give three other examples of synaesthetic pairings.

Sound → colour, word → taste (lexical-gustatory), visual motion → sound.

What is associative synaesthesia?

The concurrent is experienced internally in the "mind's eye."

What is projective synaesthesia?

The concurrent is perceived externally as if it exists in physical space.

How does consistency support synaesthesia as a genuine phenomenon?

Synaesthetes produce the same inducer-concurrent pairings over many years, showing unusual stability.

What is perceptual presence and why is it important?

Perceptual presence refers to how much an experience feels like a real external perception. Natural synaesthesia typically has low perceptual presence: the concurrent is experienced in addition to reality.

• Some hypnotic and drug-induced experiences may replace or occlude real perception.

Therefore perceptual presence helps distinguish natural from induced synaesthesia

What is the synaesthetic Stroop effect?

Slower responses when presented colours conflict with a synaesthete's concurrent colour.

What does the Stroop effect demonstrate?

That concurrents are activated automatically.

What was shown in the Gestalt triangle study?

Synaesthetes could identify hidden shapes more easily because induced colours helped group visual information.

Why is the Gestalt triangle study important?

It demonstrates that synaesthesia affects perception, not just memory or imagination.

What did fMRI studies of blindfolded synaesthetes listening to spoken words show?

Activation of both language areas and visual cortex, unlike controls.

What did grapheme-presentation studies show?

Activation of V8 (grapheme processing) and V4 (colour processing).

Why do grapheme-presentation studies support the cross-activation hypothesis?

Because viewing graphemes activates both grapheme and colour-processing regions.

What did ERP studies find?

V4 activation occurs 100–150 ms after a stimulus

Why is the ERP finding important?

It is too fast for conscious language processing, suggesting sensory-level processing.

What neurophysiological findings are seen in grapheme-colour synaesthetes?

Lower phosphene thresholds and enhanced visual evoked potentials (VEPs).

What do lower phosphene thresholds indicate?

Increased visual cortex excitability.

What evidence suggests synaesthesia may be learned?

Childhood experiences, semantic influences, and cross-modal associations.

What is the coloured refrigerator magnet theory?

Childhood exposure to coloured letter magnets may influence grapheme-colour pairings.

Give an example of a semantic influence on synaesthesia.

Fourteen → green.

What is the Bouba-Kiki effect?

Most people match rounded shapes with "Bouba" and spiky shapes with "Kiki."

Why is the Bouba-Kiki effect relevant to synaesthesia?

It suggests humans naturally form cross-modal associations.

How do congenital blindness studies support innate tendencies?

Colour-related synaesthesia can occur despite a lack of visual experience.

What is the cross-activation hypothesis?

synaesthesia arises from abnormal neural hyper-connectivity between adjacent sensory regions. this structural wiring occurs because the brain fails to prune away extra connections during early childhood, causing an ‘inducer’ in one sensory modality to automatically trigger a ‘concurrent’ sensation in another

Name four findings that support the cross-activation hypothesis.

DTI connectivity, spoken-word fMRI studies in blindfolded synaesthetes, V4/V8 activation after activation by stimulus, and early ERP responses of V4

What developmental mechanism may explain cross-activation?

Failure to prune excess neural connections during childhood.

How does DTI support the cross-activation hypothesis?

It shows greater structural connectivity between inducer and concurrent-related regions (within the fusiform gyrus and between the visual and auditory cortex).

What is the disinhibition hypothesis?

sensory areas are connected in everyone, but are usually inhibited → reduced inhibition in synaesthetes allows abnormal coactivation in synaesthesia

what evidence supports the disinhibition hypothesis?

synaesthesia can appear rapidly after hypnosis, drugs and sensory deprivation, increased cortical excitability (lower phosphene threshold and enhanced VEPs) and parietal cortex activation

Why do hypnosis, drugs, and sensory deprivation support disinhibition?

Synaesthesia-like experiences appear too rapidly for new structural connections to form.

How do lower phosphene thresholds support disinhibition?

They indicate increased cortical excitability caused by reduced inhibition.

How does parietal cortex activation support disinhibition?

The parietal cortex may disinhibit concurrent-related sensory regions, allowing multiple cortical areas to become active simultaneously.

What was the method used in training-induced synaesthesia?

Repeated learning of grapheme-colour associations.

What were the results of modest training?

Stroop-like effects but little synaesthetic phenomenology.

What were the results of extensive adaptive training?

Consistency, automaticity, synaesthesia-like experiences, lower phosphene thresholds, and enhanced VEPs.

Which induction method provides the strongest evidence that synaesthesia-like experiences can be learned?

Extensive adaptive training.

What effects can hypnotically induced synaesthesia produce?

Automaticity-like effects, Stroop interference, and projector-like reports.

How does hypnotically induced synaesthesia differ from natural synaesthesia?

The suggested colour may replace the real colour rather than coexist with it.

What characterises drug-induced synaesthesia?

It is often inconsistent, state-dependent, and non-automatic.

What does sensory deprivation-induced synaesthesia demonstrate?

Rapid perceptual plasticity.

What synaesthesia-like experiences can occur after brain injury?

Sound-induced touch experiences.

What limitation exists in brain injury-induced synaesthesia?

Concurrents are usually simple sensations such as tingling or pressure.