training-synesthetic-letter-color-associations-by-reading-in-color

Training Synesthetic Letter-color Associations by Reading in Color

Synesthesia is a rare condition in which a stimulus from one modality automatically and consistently triggers unusual sensations in the same and/or other ...

University of Amsterdam

Diffusion tensor imaging allowed us to validate for the first time the hypothesis that hyperconnectivity causes the added sensations in synesthesia. Grapheme-color synesthetes (n = 18), who experience specific colors with particular letters or numbers (for example, 'R is sky blue'), showed greater anisotropic diffusion compared with matched controls. Greater anisotropic diffusion indicates more coherent white matter. Anisotropy furthermore differentiated subtypes of grapheme-color synesthesia. Greater connectivity in the inferior temporal cortex was particularly strong for synesthetes who see synesthetic color in the outside world ('projectors') as compared with synesthetes who see the color in their 'mind's eye' only ('associators'). In contrast, greater connectivity (as compared with non-synesthetes) in the superior parietal or frontal cortex did not differentiate between subtypes of synesthesia. In conclusion, we found evidence that increased structural connectivity is associated with the presence of grapheme-color synesthesia, and has a role in the subjective nature of synesthetic color experience.

Increased structural connectivity in grapheme-color synesthesia.

Little is known about how the properties of our private mental world relate to the physical and functional properties of our brain. Studying synesthesia, where a particular experience evokes a separate additional sensory experience, offers the unique opportunity to study phenomenological experiences as a stable trait in healthy subjects. A common form of synesthesia is grapheme-color synesthesia, where a particular letter or number evokes a particular color experience. We studied the neural basis of qualitative different properties of the synesthetic experience by using individual differences in grapheme-color synesthesia. Specifically, the synesthetic color can be experienced "in the mind" (associator synesthetes) or "in the outside world" (projector synesthetes). Gray matter structure and functioning (imaged using voxel-based morphometry and functional magnetic resonance imaging, respectively) were examined in grapheme-color synesthetes (N = 42, 16 projectors and 26 associators) and nonsynesthetes. Results indicated partly shared mechanisms for all grapheme-color synesthetes, particularly in posterior superior parietal lobe, which is involved in the integration of sensory information. In addition, the nature of synesthetic experience was found to be mediated by distinct neural mechanisms. The outside-world experience is related to brain areas involved in perceiving and acting in the outside world (visual cortex, auditory cortex, motor cortex) as well as frontal brain areas. In contrast, the in-the-mind experience is related to the hippocampus and parahippocampal gyrus, known for their role in memory. Thus, the different subjective experiences are related to distinct neural mechanisms. Moreover, the properties of subjective experiences are in accordance with functional properties of the mediating brain mechanisms.

Neural basis of individual differences in synesthetic experiences.

Grapheme-color synesthesia is a neurological condition in which viewing numbers or letters (graphemes) results in the concurrent sensation of color. While the anatomical substrates underlying this experience are well understood, little research to date has investigated factors influencing the particular colors associated with particular graphemes or how synesthesia occurs developmentally. A recent suggestion of such an interaction has been proposed in the cascaded cross-tuning (CCT) model of synesthesia, which posits that in synesthetes connections between grapheme regions and color area V4 participate in a competitive activation process, with synesthetic colors arising during the component-stage of grapheme processing. This model more directly suggests that graphemes sharing similar component features (lines, curves, etc.) should accordingly activate more similar synesthetic colors. To test this proposal, we created and regressed synesthetic color-similarity matrices for each of 52 synesthetes against a letter-confusability matrix, an unbiased measure of visual similarity among graphemes. Results of synesthetes' grapheme-color correspondences indeed revealed that more similarly shaped graphemes corresponded with more similar synesthetic colors, with stronger effects observed in individuals with more intense synesthetic experiences (projector synesthetes). These results support the CCT model of synesthesia, implicate early perceptual mechanisms as driving factors in the elicitation of synesthetic hues, and further highlight the relationship between conceptual and perceptual factors in this phenomenon.

Similarly shaped letters evoke similar colors in grapheme-color synesthesia.

'Special cases': neural mechanisms and individual differences in synaesthesia.

Despite a recent upsurge of research, much remains unknown about the neurobiological mechanisms underlying synaesthesia. By integrating results obtained so far in Magnetic Resonance Imaging (MRI) studies, this contribution sheds light on the role of particular brain regions in synaesthetic experiences. First, in accordance with its sensory nature, it seems that the sensory brain areas corresponding to the type of synaesthetic experience are activated. Synaesthetic colour experiences can activate colour regions in occipito-temporal cortex, but this is not necessarily restricted to V4. Furthermore, sensory and motor brain regions have been obtained that extend beyond the particular type of synaesthesia studied. Second, differences in experimental setup, number and type of synaesthetes tested, and method to delineate regions of interest may help explain inconsistent results obtained in the BOLD-MRI (Blood Oxygen Level Dependent functional MRI) studies. Third, an overview of obtained results shows that a network of brain areas rather than a single brain region underlies synaesthesia. Six brain regions of overlapping results emerge, these regions are in sensory and motor regions as well as 'higher level' regions in parietal and frontal lobe. We propose that these regions are related to three different cognitive processes inherently part of synaesthesia; the sensory processes, the (attentional) 'binding' processes, and cognitive control processes. Finally, we discuss how these functional and structural brain properties might relate to the development of synaesthesia. In particular, we believe this relationship is better understood by separating the question what underlies the presence of synaesthesia ('trait') from what determines particular synaesthetic associations ('type').

Brain areas involved in synaesthesia: a review.

Synesthesia is a phenomenon where a stimulus produces consistent extraordinary subjective experiences. A relatively common type of synesthesia involves perception of color when viewing letters (e.g. the letter 'a' always appears as light blue). In this study, we examine whether traits typically regarded as markers of synesthesia can be acquired by simply reading in color.

Pseudo-synesthesia through reading books with colored letters.

Over 100,000 verbal and visuospatial immediate and delayed memory tests were presented via the Internet to over 28,000 participants in the age range of 11 to 80. Structural equation modeling pointed to the verbal versus visuospatial dimension as an important factor in individual differences, but not the immediate versus delayed dimension. We found a linear decrease of 1% to 3% per year in overall memory performance past the age of 25. For visuospatial tests, this decrease started at age 18 and was twice as fast as the decrease of verbal memory. There were strong effects of education, with the highest educated group sometimes scoring one full standard deviation above the lowest educated group. Gender effects were small but as expected: women outperformed men on the verbal memory tasks; men outperformed women on the visuospatial tasks. We also found evidence of increasing proneness to false memory with age. Memory for recent news events did not show a decrease with age.

The rise and fall of immediate and delayed memory for verbal and visuospatial information from late childhood to late adulthood.

In grapheme-color synesthesia, a number or letter can evoke two different and possibly conflicting (real and synesthetic) color sensations at the same time. In this study, we investigate the relationship between synesthesia and executive control functions. First, no general skill differences were obtained between synesthetes and non-synesthetes in classic executive control paradigms. Furthermore, classic executive control effects did not interact with synesthetic behavioral effects. Third, we found support for our hypothesis that inhibition of a synesthetic color takes effort and time. Finally, individual differences analyses showed no relationship between the two skills; performance on a 'normal' Stroop task does not predict performance on a synesthetic Stroop task. Across four studies, the current results consistently show no clear relationship between executive control functions and synesthetic behavioral effects. This raises the question of which mechanisms are at play in synesthetic 'management' during the presence of two conflicting (real and synesthetic) sensations.

Executive functions in synesthesia.

Misophonia is a relatively unexplored chronic condition in which a person experiences autonomic arousal (analogous to an involuntary "fight-or-flight" response) to certain innocuous or repetitive sounds such as chewing, pen clicking, and lip smacking. Misophonics report anxiety, panic, and rage when exposed to trigger sounds, compromising their ability to complete everyday tasks and engage in healthy and normal social interactions. Across two experiments, we measured behavioral and physiological characteristics of the condition. Interviews (Experiment 1) with misophonics showed that the most problematic sounds are generally related to other people's behavior (pen clicking, chewing sounds). Misophonics are however not bothered when they produce these "trigger" sounds themselves, and some report mimicry as a coping strategy. Next, (Experiment 2) we tested the hypothesis that misophonics' subjective experiences evoke an anomalous physiological response to certain auditory stimuli. Misophonic individuals showed heightened ratings and skin conductance responses (SCRs) to auditory, but not visual stimuli, relative to a group of typically developed controls, supporting this general viewpoint and indicating that misophonia is a disorder that produces distinct autonomic effects not seen in typically developed individuals.

Misophonia: physiological investigations and case descriptions.

Gustatory forms of synesthesia involve the automatic and consistent experience of tastes that are triggered by non-taste related inducers. We present a case of lexical-gustatory and sound-gustatory synesthesia within one individual, SC. Most words and a subset of non-linguistic sounds induce the experience of taste, smell and physical sensations for SC. SC's lexical-gustatory associations were significantly more consistent than those of a group of controls. We tested for effects of presentation modality (visual vs. auditory), taste-related congruency, and synesthetic inducer-concurrent direction using a priming task. SC's performance did not differ significantly from a trained control group. We used functional magnetic resonance imaging to investigate the neural correlates of SC's synesthetic experiences by comparing her brain activation to the literature on brain networks related to language, music, and sound processing, in addition to synesthesia. Words that induced a strong taste were contrasted to words that induced weak-to-no tastes ("tasty" vs. "tasteless" words). Brain activation was also measured during passive listening to music and environmental sounds. Brain activation patterns showed evidence that two regions are implicated in SC's synesthetic experience of taste and smell: the left anterior insula and left superior parietal lobe. Anterior insula activation may reflect the synesthetic taste experience. The superior parietal lobe is proposed to be involved in binding sensory information across sub-types of synesthetes. We conclude that SC's synesthesia is genuine and reflected in her brain activation. The type of inducer (visual-lexical, auditory-lexical, and non-lexical auditory stimuli) could be differentiated based on patterns of brain activity.

Article	Year
Increased structural connectivity in grapheme-color synesthesia. Nature neuroscience\| PubMed ID: 17515901	2007
Neural basis of individual differences in synesthetic experiences. The Journal of neuroscience : the official journal of the Society for Neuroscience\| PubMed ID: 20445046	2010
Similarly shaped letters evoke similar colors in grapheme-color synesthesia. Neuropsychologia\| PubMed ID: 21219918	2011
'Special cases': neural mechanisms and individual differences in synaesthesia. Journal of neuropsychology\| PubMed ID: 21923783	2011
Brain areas involved in synaesthesia: a review. Journal of neuropsychology\| PubMed ID: 21923787	2011
Pseudo-synesthesia through reading books with colored letters. PloS one\| PubMed ID: 22761905	2012
The rise and fall of immediate and delayed memory for verbal and visuospatial information from late childhood to late adulthood. Acta psychologica\| PubMed ID: 23261419	2013
Executive functions in synesthesia. Consciousness and cognition\| PubMed ID: 23313937	2013
Misophonia: physiological investigations and case descriptions. Frontiers in human neuroscience\| PubMed ID: 23805089	2013
A taste for words and sounds: a case of lexical-gustatory and sound-gustatory synesthesia. Frontiers in psychology\| PubMed ID: 24167497	2013

Romke Rouw

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