Intrinsic network connectivity reflects consistency of synesthetic experiences

Brain circuits in autonomous sensory meridian response and related phenomena

Autonomous sensory meridian response (ASMR) is characterized by a tingling sensation with a feeling of relaxation and a state of flow. We explore the neural underpinnings and comorbidities of ASMR and related phenomena with altered sensory processing. These phenomena include sensory processing sensitivity (SPS), synaesthesia, Alice in Wonderland syndrome and misophonia. The objective of this article is to uncover the shared neural substrates and distinctive features of ASMR and its counterparts. ASMR, SPS and misophonia exhibit common activations in the brain regions associated with social cognition, emotion regulation and empathy. Nevertheless, ASMR responders display reduced connectivity in the salience network (SN), while individuals with SPS exhibit increased connectivity in the SN. Furthermore, ASMR induces relaxation and temporarily reduces symptoms of depression, in contrast to SPS and misophonia, which are linked to depression. These observations lead us to propose that ASMR is a distinct phenomenon owing to its attention dispatch mechanism and its connection with emotion regulation. We suggest that increased activations in the insula, along with reduction in connectivity within the salience and default mode networks in ASMR responders, may account for their experiences of relaxation and flow states. This article is part of the theme issue 'Sensing and feeling: an integrative approach to sensory processing and emotional experience'.

The human brain networks mediating the vestibular sensation of self-motion

Vestibular Agnosia - where peripheral vestibular activation triggers the usual reflex nystagmus response but with attenuated or no self-motion perception - is found in brain disease with disrupted cortical network functioning, e.g. traumatic brain injury (TBI) or neurodegeneration (Parkinson's Disease). Patients with acute focal hemispheric lesions (e.g. stroke) do not manifest vestibular agnosia. Thus, brain network mapping techniques, e.g. resting state functional MRI (rsfMRI), are needed to interrogate functional brain networks mediating vestibular agnosia. Hence, we prospectively recruited 39 acute TBI patients with preserved peripheral vestibular function and obtained self-motion perceptual thresholds during passive yaw rotations in the dark and additionally acquired whole-brain rsfMRI in the acute phase. Following quality-control checks, 26 patient scans were analyzed. Using self-motion perceptual thresholds from a matched healthy control group, 11 acute TBI patients were classified as having vestibular agnosia versus 15 with normal self-motion perception thresholds. Using independent component analysis on the rsfMRI data, we found altered functional connectivity in bilateral lingual gyrus and temporo-occipital fusiform cortex in the vestibular agnosia patients. Moreover, regions of interest analyses showed both inter-hemispheric and intra-hemispheric network disruption in vestibular agnosia. In conclusion, our results show that vestibular agnosia is mediated by bilateral anterior and posterior network dysfunction and reveal the distributed brain mechanisms mediating vestibular self-motion perception.

Similar but different: High prevalence of synesthesia in autonomous sensory meridian response (ASMR)

Autonomous sensory meridian response (ASMR) is a complex sensory-emotional experience characterized by pleasant tingling sensations initiating at the scalp. ASMR is triggered in some people (called ASMR-responders) by stimuli including whispering, personal attention, and crisp sounds (termed ASMR triggers). Since its inception, ASMR has been likened to synesthesia, but convincing empirical data directly linking ASMR with synesthesia is lacking. In this study, we examined whether the prevalence of synesthesia is indeed significantly higher in ASMR-responders than non-responders. A sample of working adults and students (N = 648) were surveyed about their experience with ASMR and common types of synesthesia. The proportion of synesthetes who were classified as ASMR-responders was 52%, whereas 22% of ASMR-responders were also synesthetes. These results suggest that: (1) over half of those identifying as synesthetes also experience ASMR, and (2) that synesthesia is up to four times as common among ASMR-responders as among non-responders (22% vs. 5%). Findings also suggest a prevalence rate for ASMR of approximately 20%. Overall, the co-occurrence of ASMR and synesthesia lends empirical support to the idea that ASMR may be driven by synesthetic mechanisms, but future research would benefit from examining how ASMR and synesthesia are different, as well as similar.

"No pain, no gain": The impact of autonomous sensory meridian response on pain perception

Autonomous sensory meridian response (ASMR) is a phenomenon characterised by a static-like tingling sensation spreading from the scalp and neck to the periphery in response to a variety of audio, visual, and tactile triggers resulting in a highly relaxed state and boosted positive affect. The limited literature on this phenomenon points to a potential of ASMR to alleviate pain. Emerging evidence also suggests that ASMR may be linked to increased sensory sensitivity more broadly. This study aimed to objectively address these claims by administering an algometer (measure of pain tolerance), and a visual analog scale (VAS) (measure of subjective pain sensitivity) to ASMR experiencers and controls at baseline, following an ASMR video, and a control video. Findings indicate that ASMR experiencers have a higher pain sensitivity than controls; however, there was no difference between the two groups in terms of pain tolerance. In addition, any potential analgesic properties associated with experiencing ASMR may reflect protective properties of ASMR buffering against the increased pain sensitivity among ASMR experiencers relative to controls.

Synaesthesia as a model system for understanding variation in the human mind and brain

The aim of this article is to reposition synaesthesia as model system for understanding variation in the construction of the human mind and brain. People with synaesthesia inhabit a remarkable mental world in which numbers can be coloured, words can have tastes, and music is a visual spectacle. Key questions remain unanswered about why it exists, and how the study of synaesthesia might inform theories of the human mind. This article argues we need to rethink synaesthesia as not just representing exceptional experiences, but as a product of an unusual neurodevelopmental cascade from genes to brain to cognition of which synaesthesia is only one outcome. Specifically, differences in the brains of synaesthetes support a distinctive way of thinking (enhanced memory, imagery etc.) and may also predispose towards particular clinical vulnerabilities. In effect, synaesthesia can act as a paradigmatic example of a neuropsychological approach to individual differences.

Consistency and strength of grapheme-color associations are separable aspects of synesthetic experience

Consistency of synesthetic associations over time is a widely used test of synesthesia. Since many studies suggest that consistency is not a completely reliable feature, we compared the consistency and strength of synesthetes' grapheme-color associations. Consistency was measured by scores on the Synesthesia Battery and by the Euclidean distance in color space for the specific graphemes tested for each participant. Strength was measured by congruency magnitudes on the Implicit Association Test. The strength of associations was substantially greater for synesthetes than non-synesthetes, suggesting that this is a novel, objective marker of synesthesia. Although, intuitively, strong associations should also be consistent, consistency and strength were uncorrelated, indicating that they are likely independent, at least for grapheme-color synesthesia. These findings have implications for our understanding of synesthesia and for estimates of its prevalence since synesthetes who experience strong, but inconsistent, associations may not be identified by tests that focus solely on consistency.

Synesthesia does not help to recover perceptual dominance following flash suppression

Grapheme-colour synesthesia occurs when letters or numbers elicit an abnormal colour sensation (e.g., printed black letters are perceived as coloured). This phenomenon is typically reported following explicit presentation of graphemes. Very few studies have investigated colour sensations in synesthesia in the absence of visual awareness. We took advantage of the dichoptic flash suppression paradigm to temporarily render a stimulus presented to one eye invisible. Synesthetic alphanumeric and non-synesthetic stimuli were presented to 21 participants (11 synesthetes) in achromatic and chromatic experimental conditions. The test stimulus was first displayed to one eye and then masked by a sudden presentation of visual noise in the other eye (flash suppression). The time for an image to be re-perceived following the onset of the suppressive noise was calculated. Trials where there was no flash suppression performed but instead mimicked the perceptual suppression of the flash were also tested. Results showed that target detection by synesthetes was significantly better than by controls in the absence of flash suppression. No difference was found between the groups in the flash suppression condition. Our findings suggest that synesthesia is associated with enhanced perception for overt recognition, but does not provide an advantage in recovering from a perceptual suppression. Further studies are needed to investigate synesthesia in relation to visual awareness.

A distinct electrophysiological signature for synaesthesia that is independent of individual differences in sensory sensitivity

People with synaesthesia have been reported to show atypical electrophysiological responses to certain simple sensory stimuli, even if these stimuli are not inducers of synaesthesia. However, it is unclear whether this constitutes a neural marker that is relatively specific to synaesthesia or whether it reflects some other trait that co-occurs with synaesthesia, but is not specific to it. One candidate is atypical sensory sensitivity (e.g., strong aversion to certain lights and sounds, 'sensory overload') which is a feature of both synaesthesia and autism and that varies greatly in the neurotypical population. Using visual evoked-potentials (to stimuli varying in spatial frequency) and auditory-evoked potentials (to stimuli varying in auditory frequency), we found that synaesthetes had a modulated visual evoked-potential around P1/N1 (emanating from fusiform cortex), a greater auditory N1, as well as differences in the time-frequency domain (increased alpha and beta induced power for visual stimuli). This was distinct from that found in non-synaesthetes. By contrast, no significant electrophysiological differences were found that were linked to neurotypical variation in sensory sensitivity.

"Mickey Mousing" in the Brain: Motion-Sound Synesthesia and the Subcortical Substrate of Audio-Visual Integration

Motion-sound synesthesia is characterized by illusory auditory sensations linked to the pattern and rhythms of motion (dubbed "Mickey Mousing" as in cinema) of visually experienced but soundless object, like an optical flow array, a ball bouncing or a horse galloping. In an MRI study with a group of three synesthetes and a group of eighteen control participants, we found structural changes in the brains of synesthetes in the subcortical multisensory areas of the superior and inferior colliculi. In addition, functional magnetic resonance imaging data showed activity in motion-sensitive regions, as well as temporal and occipital areas, and the cerebellum. However, the synesthetes had a higher activation within the left and right cuneus, with stronger activations when viewing optical flow stimuli. There was also a general difference in connectivity of the colliculi with the above mentioned regions between the two groups. These findings implicate low-level mechanisms within the human neuroaxis as a substrate for local connectivity and cross activity between perceptual processes that are "distant" in terms of cortical topography. The present findings underline the importance of considering the role of subcortical systems and their connectivity to multimodal regions of the cortex and they strengthen a parsimonious account of synesthesia, at the least of the visual-auditory type.

Synaesthesia and autism: Different developmental outcomes from overlapping mechanisms?

Synaesthesia, a mixing of the senses, is more common in individuals with autism. Here, we review the evidence for the association between synaesthesia and autism with regard to their genetic background, brain connectivity, perception, cognitive mechanisms and their contribution to exceptional talents. Currently, the overlap between synaesthesia and autism is established most convincingly at the level of alterations in sensory sensitivity and perception, with synaesthetes showing autism-like profiles of sensory sensitivity and a bias towards details in perception. Shared features may include a predominance of local over global connectivity in the brain. When autism and synaesthesia co-occur in the same individual, the chance of developing heightened cognitive and memory abilities is increased. We discuss how the same theoretical models could potentially explain both conditions. Given the evidence, we believe the phenotypical overlap between autism and synaesthesia has been established clearly enough to invite future research to confirm overlapping mechanisms.

A case of co-occuring synesthesia, autism, prodigious talent and strong structural brain connectivity

BACKGROUND

Synesthesia is a sensory phenomenon where certain domain-specific stimuli trigger additional sensations of e.g. color or texture. The condition occurs in about 4% of the general population, but is overrepresented in individuals with Autism Spectrum Disorder (ASD), where it might also be associated with the presence of prodigious talents.

CASE PRESENTATION

Here we describe the case of a young transsexual man with Asperger Syndrome, synesthesia and a prodigious talent for foreign language acquisition. In our case, not only letters, numbers, spoken words, music, noises, weekdays and months lead to highly consistent, vivid color sensations but also his own and others' emotions, geometric shapes, any mathematical symbol, and letters from an unfamiliar alphabet (Hebrew). These color associations seem to aid categorization, differentiation and storage of information and might thereby contribute to the young man's language acquisition ability. We investigated the young man's structural brain connectivity in comparison to adults with or without ASD, applying global fiber tracking to diffusion-weighted Magnetic Resonance Imaging (MRI) data. The case presented with increased connectivity, especially between regions involved in visual and emotion processing, memory, and higher order associative binding regions. An electroencephalography experiment investigating synesthetic color and shape sensations while listening to music showed a negligible occipital alpha suppression, indicating that these internally generated synesthetic sensations derive from a different brain mechanism than when processing external visual information.

CONCLUSIONS

Taken together, this case study endorses the notion of a link between synesthesia, prodigious talent and autism, adding to the currently still sparse literature in this field. It provides new insights into the possible manifestations of synesthesia in individuals with ASD and its potential contribution to prodigious talents in people with an otherwise unexceptional cognitive profile. Additionally, this case impressively illustrates how synesthesia can be a key element not only of sensory perception but also social and emotional processing and contributes to existing evidence of increased brain connectivity in association with synesthesia.

Reduced perceptual narrowing in synesthesia

Synesthesia is a neurologic trait in which specific inducers, such as sounds, automatically elicit additional idiosyncratic percepts, such as color (thus "colored hearing"). One explanation for this trait-and the one tested here-is that synesthesia results from unusually weak pruning of cortical synaptic hyperconnectivity during early perceptual development. We tested the prediction from this hypothesis that synesthetes would be superior at making discriminations from nonnative categories that are normally weakened by experience-dependent pruning during a critical period early in development-namely, discrimination among nonnative phonemes (Hindi retroflex /d̪a/ and dental /ɖa/), among chimpanzee faces, and among inverted human faces. Like the superiority of 6-mo-old infants over older infants, the synesthetic groups were significantly better than control groups at making all the nonnative discriminations across five samples and three testing sites. The consistent superiority of the synesthetic groups in making discriminations that are normally eliminated during infancy suggests that residual cortical connectivity in synesthesia supports changes in perception that extend beyond the specific synesthetic percepts, consistent with the incomplete pruning hypothesis.

Stochastic resonance model of synaesthesia

In synaesthesia, stimulation of one sensory modality evokes additional experiences in another modality (e.g. sounds evoking colours). Along with these cross-sensory experiences, there are several cognitive and perceptual differences between synaesthetes and non-synaesthetes. For example, synaesthetes demonstrate enhanced imagery, increased cortical excitability and greater perceptual sensitivity in the concurrent modality. Previous models suggest that synaesthesia results from increased connectivity between corresponding sensory regions or disinhibited feedback from higher cortical areas. While these models explain how one sense can evoke qualitative experiences in another, they fail to predict the broader phenotype of differences observed in synaesthetes. Here, we propose a novel model of synaesthesia based on the principles of stochastic resonance. Specifically, we hypothesize that synaesthetes have greater neural noise in sensory regions, which allows pre-existing multisensory pathways to elicit supra-threshold activation (i.e. synaesthetic experiences). The strengths of this model are (a) it predicts the broader cognitive and perceptual differences in synaesthetes, (b) it provides a unified framework linking developmental and induced synaesthesias, and (c) it explains why synaesthetic associations are inconsistent at onset but stabilize over time. We review research consistent with this model and propose future studies to test its limits. This article is part of a discussion meeting issue 'Bridging senses: novel insights from synaesthesia'.

Cannabis-induced altered states of consciousness are associated with specific dynamic brain connectivity states

BACKGROUND

Cannabis, and specifically one of its active compounds delta-9-tetrahydrocannabinol in recreational doses, has a variety of effects on cognitive processes. Most studies employ resting state functional magnetic resonance imaging techniques to assess the stationary effects of cannabis and to-date one report addressed the impact of delta-9-tetrahydrocannabinol on the dynamics of whole-brain functional connectivity.

METHODS

Using a repeated-measures, within-subjects design, 19 healthy occasional cannabis users (smoking cannabis ⩽2 per week) underwent resting state functional magnetic resonance imaging scans. Each subject underwent two scans: in the intoxicated condition, shortly after smoking a cannabis cigarette, and in the non-intoxicated condition, with the subject being free from cannabinoids for at least one week before. All sessions were randomized and performed in a four-week interval. Data were analysed employing a standard independent component analysis approach with subsequent tracking of the functional connectivity dynamics, which allowed six connectivity clusters (states) to be individuated.

RESULTS

Using standard independent component analysis in resting state functional connectivity, a group effect was found in the precuneus connectivity. With a dynamic independent component analysis approach, we identified one transient connectivity state, characterized by high connectivity within and between auditory and somato-motor cortices and anti-correlation with subcortical structures and the cerebellum that was only found during the intoxicated condition. Behavioural measures of the subjective experiences of changed perceptions and tetrahydrocannabinol plasma levels during intoxication were associated with this state.

CONCLUSIONS

With the help of the dynamic connectivity approach we could elucidate neural correlates of the transitory perceptual changes induced by delta-9-tetrahydrocannabinol in cannabis users, and possibly identify a biomarker of cannabis intoxication.

A functional magnetic resonance imaging investigation of the autonomous sensory meridian response

BACKGROUND

Autonomous Sensory Meridian Response (ASMR) is a sensory-emotional experience in which specific stimuli (ASMR "triggers") elicit tingling sensations on the scalp, neck, and shoulders; these sensations are accompanied by a positive affective state. In the current research, functional magnetic resonance imaging (fMRI) was used in order to delineate the neural substrates of these responses.

METHODS

A total of 17 individuals with ASMR and 17 age- and sex-matched control participants underwent fMRI scanning while watching six 4-minute videos. Three of the videos were designed to elicit ASMR tingling and three videos were not.

RESULTS

The results demonstrated that ASMR videos have a distinct effect on the neural activity of individuals with ASMR. The contrast of ASMR participants' responses to ASMR videos showed greater activity in the cingulate gyrus as well as in cortical regions related to audition, movement, and vision. This activity was not observed in control participants. The contrast of ASMR and control participants' responses to ASMR-eliciting videos detected greater activity in right cingulate gyrus, right paracentral lobule, and bilateral thalamus in ASMR participants; control participants showed greater activity in the lingula and culmen of the cerebellum.

CONCLUSIONS

Together, these results highlight the fact that ASMR videos elicit activity in brain areas related to sensation, emotion, and attention in individuals with ASMR, but not in matched control participants.

Atypical Functional Connectivity Associated with Autonomous Sensory Meridian Response: An Examination of Five Resting-State Networks

Autonomous sensory meridian response (ASMR) is a perceptual phenomenon in which specific auditory and/or visual stimuli consistently elicit tingling sensations on the neck, scalp, and shoulders, as well as a positive and relaxed emotional state. The “ASMR triggers” that initiate these responses generally consist of soft sounds (e.g., whispering), repetitive noises (e.g., tapping sounds), or videos of people performing socially intimate acts (e.g., watching someone brush her hair). Despite being a relatively common phenomenon, little is known about the neural substrates of ASMR. In the current research, resting-state functional magnetic resonance imaging (fMRI) was used to examine whether ASMR was associated with atypical patterns of functional connectivity. Seventeen individuals with ASMR and 17 matched control participants underwent an anatomical MRI scan and a resting-state fMRI scan. An independent components analysis was used to identify the default mode, salience, central executive, sensorimotor, and visual networks. An analysis of variance with group (ASMR vs. control) as a between-subjects variable was performed to contrast the functional connectivity of each of these networks. The results demonstrated that ASMR was associated with reduced functional connectivity in the salience and visual networks, and with atypical patterns of connectivity in the default mode, central executive, and sensorimotor networks.

Assessing Lateral Interaction in the Synesthetic Visual Brain

In grapheme-color synesthesia, letters and numbers evoke abnormal colored perceptions. Although the underlying mechanisms are not known, it is largely thought that the synesthetic brain is characterized by atypical connectivity throughout various brain regions, including the visual areas. To study the putative impact of synesthesia on the visual brain, we assessed lateral interactions (i.e., local functional connectivity between neighboring neurons in the visual cortex) by recording steady-state visual evoked potentials (ssVEPs) over the occipital region in color-grapheme synesthetes (n = 6) and controls (n = 21) using the windmill/dartboard paradigm. Discrete Fourier Transform analysis was conducted to extract the fundamental frequency and the second harmonics of ssVEP responses from contrast-reversing stimuli presented at 4.27 Hz. Lateral interactions were assessed using two amplitude-based indices: Short-range and long-range lateral interactions. Results indicated that synesthetes had a statistically weaker signal coherence of the fundamental frequency component compared to the controls, but no group differences were observed on lateral interaction indices. However, a significant correlation was found between long-range lateral interactions and the type of synesthesia experience (projector versus associator). We conclude that the occipital activity related to lateral interactions in synesthetes does not substantially differ from that observed in controls. Further investigation is needed to understand the impact of synesthesia on visual processing, specifically in relation to subjective experiences of synesthete individuals.

Windows into the Visual Brain: New Discoveries about the Visual System, Its Functions, and Implications for Practitioners

Introduction

In recent years, major progress has been made in understanding the human visual system because of new investigative techniques. These developments often contradict older concepts about visual function.

Methods

A detailed literature search and interprofessional discussions.

Results

Recent innovative neurological tests are described that are able to show much more accurately the visual pathways, the process of vision, and the close relationships among sensory modalities. These tests also reveal the remarkable neuroplasticity of the human brain and disorders of connectivity that frequently involve visual function.

Discussion

How these recent neurological advances may benefit service providers is discussed.

Implications for practitioners

It is important that from time to time new neurological and ophthalmic developments are summarized for professionals who are involved in the clinical management of individuals with visual disorders and how the newly acquired knowledge affects the diagnosis and intervention strategies. Visual rehabilitation must be based on up-to-date science, which continually changes and grows with research.

Increased misophonia in self-reported Autonomous Sensory Meridian Response

BACKGROUND

Autonomous Sensory Meridian Response (ASMR) is a sensory experience elicited by auditory and visual triggers, which so far received little attention from the scientific community. This self-reported phenomenon is described as a relaxing tingling sensation, which typically originates on scalp and spreads through a person's body. Recently it has been suggested that ASMR shares common characteristics with another underreported condition known as misophonia, where sounds trigger negative physiological, emotional and behavioural responses. The purpose of this study was to elucidate whether ASMR is associated with heightened levels of misophonia.

METHODS

The Misophonia Questionnaire (MQ) was administered to individuals reporting to experience ASMR and to age and gender matched controls.

RESULTS

Compared to controls ASMR group scored higher on all subscales of MQ including the Misophonia Symptom Scale, the Misophonia Emotions and Behaviors Scale and the Misophonia Severity Scale.

DISCUSSION

Individuals reporting ASMR experience have elevated levels of misophonia.

Exploring the functional nature of synaesthetic colour: Dissociations from colour perception and imagery

Individuals with grapheme-colour synaesthesia experience anomalous colours when reading achromatic text. These unusual experiences have been said to resemble 'normal' colour perception or colour imagery, but studying the nature of synaesthesia remains difficult. In the present study, we report novel evidence that synaesthetic colour impacts conscious vision in a way that is different from both colour perception and imagery. Presenting 'normal' colour prior to binocular rivalry induces a location-dependent suppressive bias reflecting local habituation. By contrast, a grapheme that evokes synaesthetic colour induces a facilitatory bias reflecting priming that is not constrained to the inducing grapheme's location. This priming does not occur in non-synaesthetes and does not result from response bias. It is sensitive to diversion of visual attention away from the grapheme, but resistant to sensory perturbation, reflecting a reliance on cognitive rather than sensory mechanisms. Whereas colour imagery in non-synaesthetes causes local priming that relies on the locus of imagined colour, imagery in synaesthetes caused global priming not dependent on the locus of imagery. These data suggest a unique psychophysical profile of high-level colour processing in synaesthetes. Our novel findings and method will be critical to testing theories of synaesthesia and visual awareness.

Top-down signal transmission and global hyperconnectivity in auditory-visual synesthesia: Evidence from a functional EEG resting-state study

Auditory-visual (AV) synesthesia is a rare phenomenon in which an auditory stimulus induces a "concurrent" color sensation. Current neurophysiological models of synesthesia mainly hypothesize "hyperconnected" and "hyperactivated" brains, but differ in the directionality of signal transmission. The two-stage model proposes bottom-up signal transmission from inducer- to concurrent- to higher-order brain areas, whereas the disinhibited feedback model postulates top-down signal transmission from inducer- to higher-order- to concurrent brain areas. To test the different models of synesthesia, we estimated local current density, directed and undirected connectivity patterns in the intracranial space during 2 min of resting-state (RS) EEG in 11 AV synesthetes and 11 nonsynesthetes. AV synesthetes demonstrated increased parietal theta, alpha, and lower beta current density compared to nonsynesthetes. Furthermore, AV synesthetes were characterized by increased top-down signal transmission from the superior parietal lobe to the left color processing area V4 in the upper beta frequency band. Analyses of undirected connectivity revealed a global, synesthesia-specific hyperconnectivity in the alpha frequency band. The involvement of the superior parietal lobe even during rest is a strong indicator for its key role in AV synesthesia. By demonstrating top-down signal transmission in AV synesthetes, we provide direct support for the disinhibited feedback model of synesthesia. Finally, we suggest that synesthesia is a consequence of global hyperconnectivity. Hum Brain Mapp 39:522-531, 2018. © 2017 Wiley Periodicals, Inc.

Graphemes Sharing Phonetic Features Tend to Induce Similar Synesthetic Colors

Individuals with grapheme-color synesthesia experience idiosyncratic colors when viewing achromatic letters or digits. Despite large individual differences in grapheme-color association, synesthetes tend to associate graphemes sharing a perceptual feature with similar synesthetic colors. Sound has been suggested as one such feature. In the present study, we investigated whether graphemes of which representative phonemes have similar phonetic features tend to be associated with analogous synesthetic colors. We tested five Korean multilingual synesthetes on a color-matching task using graphemes from Korean, English, and Japanese orthography. We then compared the similarity of synesthetic colors induced by those characters sharing a phonetic feature. Results showed that graphemes associated with the same phonetic feature tend to induce synesthetic color in both within- and cross-script analyses. Moreover, this tendency was consistent for graphemes that are not transliterable into each other as well as graphemes that are. These results suggest that it is the perceptual-i.e., phonetic-properties associated with graphemes, not just conceptual associations such as transliteration, that determine synesthetic color.

An Examination of Personality Traits Associated with Autonomous Sensory Meridian Response (ASMR)

Autonomous Sensory Meridian Response (ASMR) is a perceptual condition in which the presentation of particular audio-visual stimuli triggers intense, pleasurable tingling sensations in the head and neck regions, which may spread to the periphery of the body. These triggering stimuli are often socially intimate in nature, and usually involve repetition of movements and/or sounds (e.g., hearing whispering, watching someone brush her hair). Reports of ASMR experiences first appeared in online communities in 2010; since this time, these communities have expanded, with some groups consisting of over 100,000 members. However, despite the apparent prevalence of ASMR, there is currently no research on the personality characteristics that co-occur with this condition. In the current study, 290 individuals with ASMR and 290 matched controls completed the Big Five Personality Inventory (BFI; John et al., 1991); participants with ASMR also completed a questionnaire related to their ASMR phenomenology. Individuals with ASMR demonstrated significantly higher scores on Openness-to-Experience and Neuroticism, and significantly lower levels of Conscientiousness, Extraversion, and Agreeableness compared to matched controls. Further, ratings of subjective ASMR intensity in response to 14 common ASMR stimuli were positively correlated with the Openness-to-Experience and Neuroticism dimensions of the BFI. These results provide preliminary evidence that ASMR is associated with specific personality traits and suggest avenues for further investigation.

Why Saturday could be both green and red in synesthesia

It has long been observed that certain words induce multiple synesthetic colors, a phenomenon that has remained largely unexplored. We report here on the distinct synesthetic colors two synesthetes experienced with closed sets of concepts (digits, weekdays, months). For example, Saturday was associated with green, like other word starting with s; however, Saturday also had its specific color (red). Auditory priming and Visual Color Stroop tasks were used to understand the cognitive mechanisms supporting the distinct synesthetic colors. Results revealed that processing of word segments and whole words was specifically involved in each type of synesthetic colors. However, these mechanisms differed between participants, as they could relate either to orthography (and written words) or phonology (and spoken words). Further differences concerned the word representations, which varied as to whether or not they encoded serial positions. In addition to clarifying the cognitive mechanisms underlying the distinct synesthetic colors, our results offer some clues for understanding the neurocognitive underpinnings of a rather common form of synesthesia.

Synesthesia strengthens sound-symbolic cross-modal correspondences

Synesthesia is a phenomenon in which an experience in one domain is accompanied by an involuntary secondary experience in another, unrelated domain; in classical synesthesia, these associations are arbitrary and idiosyncratic. Cross-modal correspondences refer to universal associations between seemingly unrelated sensory features, e.g., auditory pitch and visual size. Some argue that these phenomena form a continuum, with classical synesthesia being an exaggeration of universal cross-modal correspondences, whereas others contend that the two are quite different, since cross-modal correspondences are non-arbitrary, non-idiosyncratic, and do not involve secondary experiences. Here, we used the implicit association test to compare synesthetes' and non-synesthetes' sensitivity to cross-modal correspondences. We tested the associations between auditory pitch and visual elevation, auditory pitch and visual size, and sound-symbolic correspondences between auditory pseudowords and visual shapes. Synesthetes were more sensitive than non-synesthetes to cross-modal correspondences involving sound-symbolic, but not low-level sensory, associations. We conclude that synesthesia heightens universally experienced cross-modal correspondences, but only when these involve sound symbolism. This is only partly consistent with the idea of a continuum between synesthesia and cross-modal correspondences, but accords with the idea that synesthesia is a high-level, post-perceptual phenomenon, with spillover of the abilities of synesthetes into domains outside their synesthesias. To our knowledge, this is the first demonstration that synesthetes, relative to non-synesthetes, experience stronger cross-modal correspondences outside their synesthetic domains.

Metabolic connectivity mapping reveals effective connectivity in the resting human brain

Directionality of signaling among brain regions provides essential information about human cognition and disease states. Assessing such effective connectivity (EC) across brain states using functional magnetic resonance imaging (fMRI) alone has proven difficult, however. We propose a novel measure of EC, termed metabolic connectivity mapping (MCM), that integrates undirected functional connectivity (FC) with local energy metabolism from fMRI and positron emission tomography (PET) data acquired simultaneously. This method is based on the concept that most energy required for neuronal communication is consumed postsynaptically, i.e., at the target neurons. We investigated MCM and possible changes in EC within the physiological range using "eyes open" versus "eyes closed" conditions in healthy subjects. Independent of condition, MCM reliably detected stable and bidirectional communication between early and higher visual regions. Moreover, we found stable top-down signaling from a frontoparietal network including frontal eye fields. In contrast, we found additional top-down signaling from all major clusters of the salience network to early visual cortex only in the eyes open condition. MCM revealed consistent bidirectional and unidirectional signaling across the entire cortex, along with prominent changes in network interactions across two simple brain states. We propose MCM as a novel approach for inferring EC from neuronal energy metabolism that is ideally suited to study signaling hierarchies in the brain and their defects in brain disorders.

The Merit of Synesthesia for Consciousness Research

Synesthesia is a phenomenon in which additional perceptual experiences are elicited by sensory stimuli or cognitive concepts. Synesthetes possess a unique type of phenomenal experiences not directly triggered by sensory stimulation. Therefore, for better understanding of consciousness it is relevant to identify the mental and physiological processes that subserve synesthetic experience. In the present work we suggest several reasons why synesthesia has merit for research on consciousness. We first review the research on the dynamic and rapidly growing field of the studies of synesthesia. We particularly draw attention to the role of semantics in synesthesia, which is important for establishing synesthetic associations in the brain. We then propose that the interplay between semantics and sensory input in synesthesia can be helpful for the study of the neural correlates of consciousness, especially when making use of ambiguous stimuli for inducing synesthesia. Finally, synesthesia-related alterations of brain networks and functional connectivity can be of merit for the study of consciousness.

A systematic review of non-motor rTMS induced motor cortex plasticity

Motor cortex excitability can be measured by single- and paired-pulse transcranial magnetic stimulation (TMS). Repetitive transcranial magnetic stimulation (rTMS) can induce neuroplastic effects in stimulated and in functionally connected cortical regions. Due to its ability to non-invasively modulate cortical activity, rTMS has been investigated for the treatment of various neurological and psychiatric disorders. However, such studies revealed a high variability of both clinical and neuronal effects induced by rTMS. In order to better elucidate this meta-plasticity, rTMS-induced changes in motor cortex excitability have been monitored in various studies in a pre-post stimulation design. Here, we give a literature review of studies investigating motor cortex excitability changes as a neuronal marker for rTMS effects over non-motor cortical areas. A systematic literature review in April 2014 resulted in 29 articles in which motor cortex excitability was assessed before and after rTMS over non-motor areas. The majority of the studies focused on the stimulation of one of three separate cortical areas: the prefrontal area (17 studies), the cerebellum (8 studies), or the temporal cortex (3 studies). One study assessed the effects of multi-site rTMS. Most studies investigated healthy controls but some also stimulated patients with neuropsychiatric conditions (e.g., affective disorders, tinnitus). Methods and findings of the identified studies were highly variable showing no clear systematic pattern of interaction of non-motor rTMS with measures of motor cortex excitability. Based on the available literature, the measurement of motor cortex excitability changes before and after non-motor rTMS has only limited value in the investigation of rTMS related meta-plasticity as a neuronal state or as a trait marker for neuropsychiatric diseases. Our results do not suggest that there are systematic alterations of cortical excitability changes during rTMS treatment, which calls into question the practice of re-adjusting the stimulation intensity according to the motor threshold over the course of the treatment.

A critical review of the neuroimaging literature on synesthesia

Synesthesia refers to additional sensations experienced by some people for specific stimulations, such as the systematic arbitrary association of colors to letters for the most studied type. Here, we review all the studies (based mostly on functional and structural magnetic resonance imaging) that have searched for the neural correlates of this subjective experience, as well as structural differences related to synesthesia. Most differences claimed for synesthetes are unsupported, due mainly to low statistical power, statistical errors, and methodological limitations. Our critical review therefore casts some doubts on whether any neural correlate of the synesthetic experience has been established yet. Rather than being a neurological condition (i.e., a structural or functional brain anomaly), synesthesia could be reconsidered as a special kind of childhood memory, whose signature in the brain may be out of reach with present brain imaging techniques.

What is the link between synaesthesia and sound symbolism?

Sound symbolism is a property of certain words which have a direct link between their phonological form and their semantic meaning. In certain instances, sound symbolism can allow non-native speakers to understand the meanings of etymologically unfamiliar foreign words, although the mechanisms driving this are not well understood. We examined whether sound symbolism might be mediated by the same types of cross-modal processes that typify synaesthetic experiences. Synaesthesia is an inherited condition in which sensory or cognitive stimuli (e.g., sounds, words) cause additional, unusual cross-modal percepts (e.g., sounds trigger colours, words trigger tastes). Synaesthesia may be an exaggeration of normal cross-modal processing, and if so, there may be a link between synaesthesia and the type of cross-modality inherent in sound symbolism. To test this we predicted that synaesthetes would have superior understanding of unfamiliar (sound symbolic) foreign words. In our study, 19 grapheme-colour synaesthetes and 57 non-synaesthete controls were presented with 400 adjectives from 10 unfamiliar languages and were asked to guess the meaning of each word in a two-alternative forced-choice task. Both groups showed superior understanding compared to chance levels, but synaesthetes significantly outperformed controls. This heightened ability suggests that sound symbolism may rely on the types of cross-modal integration that drive synaesthetes' unusual experiences. It also suggests that synaesthesia endows or co-occurs with heightened multi-modal skills, and that this can arise in domains unrelated to the specific form of synaesthesia.

Implicit interactions between number and space in digit-color synesthesia

In digit-color synesthesia, a variant of grapheme-color synesthesia, digits trigger an additional color percept. Recent work on number processing in synesthesia suggests that colors can implicitly elicit numerical representations in digit-color synesthetes implying that synesthesia is bidirectional. Furthermore, morphometric investigations revealed structural differences in the parietal cortex of grapheme-color synesthetes, i.e., in the brain region where interactions between number and space occur in non-synesthetic subjects. Based upon these previous findings, we here examined whether implicitly evoked numerical representations interact with spatial representations in synesthesia in such a way that even a non-numerical, visuo-spatial task (here: line bisection) is modulated, i.e., whether synesthetes exhibit a systematic bisection bias for colored lines. Thirteen digit-color synesthetes were asked to bisect two sets of lines which were colored in their individual synesthetic colors associated with a small or a large digit, respectively. For all colored line stimuli combined, digit-color synesthetes showed--like control subjects (n = 13, matched for age, gender, IQ and handedness)--a pseudo-neglect when bisecting colored lines. Measuring the color-induced change of the bisection bias (i.e., comparing the biases when bisecting lines colored according to a small number vs those lines corresponding to a large number) revealed that only digit-color synesthetes were significantly influenced by line color. The results provide further evidence for the bidirectional nature of synesthesia and support the concept of a mental number line. In addition, they extend previous reports on bidirectionality in synesthesia by showing that even non-numerical, visuo-spatial performance can be modulated by implicit bidirectional processes.

Associative memory advantage in grapheme-color synesthetes compared to older, but not young adults

People with grapheme-color synesthesia perceive enriched experiences of colors in response to graphemes (letters, digits). In this study, we examined whether these synesthetes show a generic associative memory advantage for stimuli that do not elicit a synesthetic color. We used a novel between group design (14 young synesthetes, 14 young, and 14 older adults) with a self-paced visual associative learning paradigm and subsequent retrieval (immediate and delayed). Non-synesthesia inducing, achromatic fractal pair-associates were manipulated in visual similarity (high and low) and corresponded to high and low memory load conditions. The main finding was a learning and retrieval advantage of synesthetes relative to older, but not to younger, adults. Furthermore, the significance testing was supported with effect size measures and power calculations. Differences between synesthetes and older adults were found during dissimilar pair (high memory load) learning and retrieval at immediate and delayed stages. Moreover, we found a medium size difference between synesthetes and young adults for similar pair (low memory load) learning. Differences between young and older adults were also observed during associative learning and retrieval, but were of medium effect size coupled with low power. The results show a subtle associative memory advantage in synesthetes for non-synesthesia inducing stimuli, which can be detected against older adults. They also indicate that perceptual mechanisms (enhanced in synesthesia, declining as part of the aging process) can translate into a generic associative memory advantage, and may contribute to associative deficits accompanying healthy aging.

The genetic basis of music ability

Music is an integral part of the cultural heritage of all known human societies, with the capacity for music perception and production present in most people. Researchers generally agree that both genetic and environmental factors contribute to the broader realization of music ability, with the degree of music aptitude varying, not only from individual to individual, but across various components of music ability within the same individual. While environmental factors influencing music development and expertise have been well investigated in the psychological and music literature, the interrogation of possible genetic influences has not progressed at the same rate. Recent advances in genetic research offer fertile ground for exploring the genetic basis of music ability. This paper begins with a brief overview of behavioral and molecular genetic approaches commonly used in human genetic analyses, and then critically reviews the key findings of genetic investigations of the components of music ability. Some promising and converging findings have emerged, with several loci on chromosome 4 implicated in singing and music perception, and certain loci on chromosome 8q implicated in absolute pitch and music perception. The gene AVPR1A on chromosome 12q has also been implicated in music perception, music memory, and music listening, whereas SLC6A4 on chromosome 17q has been associated with music memory and choir participation. Replication of these results in alternate populations and with larger samples is warranted to confirm the findings. Through increased research efforts, a clearer picture of the genetic mechanisms underpinning music ability will hopefully emerge.

Changes in motor cortex excitability associated with temporal repetitive transcranial magnetic stimulation in tinnitus: hints for cross-modal plasticity?

BACKGROUND

Motor cortex excitability was found to be changed after repetitive transcranial magnetic stimulation (rTMS) of the temporal cortex highlighting the occurrence of cross-modal plasticity in non-invasive brain stimulation. Here, we investigated the effects of temporal low-frequency rTMS on motor cortex plasticity in a large sample of tinnitus patients. In 116 patients with chronic tinnitus different parameters of cortical excitability were assessed before and after ten rTMS treatment sessions. Patients received one of three different protocols all including 1 Hz rTMS over the left temporal cortex. Treatment response was defined as improvement by at least five points in the tinnitus questionnaire (TQ). Variables of interest were resting motor threshold (RMT), short-interval intra-cortical inhibition (SICI), intracortical facilitation (ICF), and cortical silent period (CSP).

RESULTS

After rTMS treatment RMT was decreased by about 1% of stimulator output near-significantly in the whole group of patients. SICI was associated with significant changes with respect to treatment response. The group of treatment responders showed a decrease of SICI over the course of treatment, the group of non-responders the reverse pattern.

CONCLUSIONS

Minor RMT changes during rTMS treatment do not necessarily suggest the need for systematic re-examination of the RMT for safety and efficacy issues. Treatment response to rTMS was shown to be related to changes in SICI that might reflect modulation of GABAergic mechanisms directly or indirectly related to rTMS treatment effects.

The effects of psilocybin and MDMA on between-network resting state functional connectivity in healthy volunteers

Perturbing a system and observing the consequences is a classic scientific strategy for understanding a phenomenon. Psychedelic drugs perturb consciousness in a marked and novel way and thus are powerful tools for studying its mechanisms. In the present analysis, we measured changes in resting-state functional connectivity (RSFC) between a standard template of different independent components analysis (ICA)-derived resting state networks (RSNs) under the influence of two different psychoactive drugs, the stimulant/psychedelic hybrid, MDMA, and the classic psychedelic, psilocybin. Both were given in placebo-controlled designs and produced marked subjective effects, although reports of more profound changes in consciousness were given after psilocybin. Between-network RSFC was generally increased under psilocybin, implying that networks become less differentiated from each other in the psychedelic state. Decreased RSFC between visual and sensorimotor RSNs was also observed. MDMA had a notably less marked effect on between-network RSFC, implying that the extensive changes observed under psilocybin may be exclusive to classic psychedelic drugs and related to their especially profound effects on consciousness. The novel analytical approach applied here may be applied to other altered states of consciousness to improve our characterization of different conscious states and ultimately advance our understanding of the brain mechanisms underlying them.

Achromatic synesthesias - a functional magnetic resonance imaging study

Grapheme-color synesthetes experience consistent, automatic and idiosyncratic colors associated with specific letters and numbers. Frequently, these specific associations exhibit achromatic synesthetic qualities (e.g. white, black or gray). In this study, we have investigated for the first time the neural basis of achromatic synesthesias, their relationship to chromatic synesthesias and the achromatic congruency effect in order to understand not only synesthetic color but also other components of the synesthetic experience. To achieve this aim, functional magnetic resonance imaging experiments were performed in a group of associator grapheme-color synesthetes and matched controls who were stimulated with real chromatic and achromatic stimuli (Mondrians), and with letters and numbers that elicited different types of grapheme-color synesthesias (i.e. chromatic and achromatic inducers which elicited chromatic but also achromatic synesthesias, as well as congruent and incongruent ones). The information derived from the analysis of Mondrians and chromatic/achromatic synesthesias suggests that real and synesthetic colors/achromaticity do not fully share neural mechanisms. The whole-brain analysis of BOLD signals in response to the complete set of synesthetic inducers revealed that the functional peculiarities of the synesthetic brain are distributed, and reflect different components of the synesthetic experience: a perceptual component, an (attentional) feature binding component, and an emotional component. Additionally, the inclusion of achromatic experiences has provided new evidence in favor of the emotional binding theory, a line of interpretation which constitutes a bridge between grapheme-color synesthesia and other developmental modalities of the phenomenon.

State- and trait-related alterations of motor cortex excitability in tinnitus patients

Chronic tinnitus is a brain network disorder with involvement of auditory and non-auditory areas. Repetitive transcranial magnetic stimulation (rTMS) over the temporal cortex has been investigated for the treatment of tinnitus. Several small studies suggest that motor cortex excitability is altered in people with tinnitus. We retrospectively analysed data from 231 patients with chronic tinnitus and 120 healthy controls by pooling data from different studies. Variables of interest were resting motor threshold (RMT), short-interval intra-cortical inhibition (SICI), intra-cortical facilitation (ICF), and cortical silent period (CSP). 118 patients were tested twice - before and after ten rTMS treatment sessions over the left temporal cortex. In tinnitus patients SICI and ICF were increased and CSP was shortened as compared to healthy controls. There was no group difference in RMT. Treatment related amelioration of tinnitus symptoms were correlated with normalisations in SICI. These findings confirm earlier studies of abnormal motor cortex excitability in tinnitus patients. Moreover our longitudinal data suggest that altered SICI may reflect a state parameter, whereas CSP and ICF may rather mirror a trait-like predisposing factor of tinnitus. These findings are new and innovative as they enlarge the knowledge about basic physiologic and neuroplastic processes in tinnitus.

Phenotypic variability in resting-state functional connectivity: current status

We reviewed the extant literature with the goal of assessing the extent to which resting-state functional connectivity is associated with phenotypic variability in healthy and disordered populations. A large corpus of work has accumulated to date (125 studies), supporting the association between intrinsic functional connectivity and individual differences in a wide range of domains-not only in cognitive, perceptual, motoric, and linguistic performance, but also in behavioral traits (e.g., impulsiveness, risky decision making, personality, and empathy) and states (e.g., anxiety and psychiatric symptoms) that are distinguished by cognitive and affective functioning, and in neurological conditions with cognitive and motor sequelae. Further, intrinsic functional connectivity is sensitive to remote (e.g., early-life stress) and enduring (e.g., duration of symptoms) life experience, and it exhibits plasticity in response to recent experience (e.g., learning and adaptation) and pharmacological treatment. The most pervasive associations were observed with the default network; associations were also widespread between the cingulo-opercular network and both cognitive and affective behaviors, while the frontoparietal network was associated primarily with cognitive functions. Associations of somatomotor, frontotemporal, auditory, and amygdala networks were relatively restricted to the behaviors linked to their respective putative functions. Surprisingly, visual network associations went beyond visual function to include a variety of behavioral traits distinguished by affective function. Together, the reviewed evidence sets the stage for testing causal hypothesis about the functional role of intrinsic connectivity and augments its potential as a biomarker for healthy and disordered brain function.

Combined structural and functional imaging reveals cortical deactivations in grapheme-color synaesthesia

Synaesthesia is a heritable condition in which particular stimuli generate specific and consistent sensory percepts or associations in another modality or processing stream. Functional neuroimaging studies have identified potential correlates of these experiences, including, in some but not all cases, the hyperactivation of visuotemporal areas and of parietal areas thought to be involved in perceptual binding. Structural studies have identified a similarly variable spectrum of differences between synaesthetes and controls. However, it remains unclear the extent to which these neural correlates reflect the synaesthetic experience itself or additional phenotypes associated with the condition. Here, we acquired both structural and functional neuroimaging data comparing thirteen grapheme-color synaesthetes with eleven non-synaesthetes. Using voxel-based morphometry and diffusion tensor imaging, we identify a number of clusters of increased volume of gray matter, of white matter or of increased fractional anisotropy in synaesthetes vs. controls. To assess the possible involvement of these areas in the synaesthetic experience, we used nine areas of increased gray matter volume as regions of interest in an fMRI experiment that characterized the contrast in response to stimuli which induced synaesthesia (i.e., letters) vs. those which did not (non-meaningful symbols). Four of these areas showed sensitivity to this contrast in synaesthetes but not controls. Unexpectedly, in two of them, in left lateral occipital cortex and in postcentral gyrus, the letter stimuli produced a strong negative BOLD signal in synaesthetes. An additional whole-brain fMRI analysis identified 14 areas, three of which were driven mainly by a negative BOLD response to letters in synaesthetes. Our findings suggest that cortical deactivations may be involved in the conscious experience of internally generated synaesthetic percepts.

Serotonergic hyperactivity as a potential factor in developmental, acquired and drug-induced synesthesia

Though synesthesia research has seen a huge growth in recent decades, and tremendous progress has been made in terms of understanding the mechanism and cause of synesthesia, we are still left mostly in the dark when it comes to the mechanistic commonalities (if any) among developmental, acquired and drug-induced synesthesia. We know that many forms of synesthesia involve aberrant structural or functional brain connectivity. Proposed mechanisms include direct projection and disinhibited feedback mechanisms, in which information from two otherwise structurally or functionally separate brain regions mix. We also know that synesthesia sometimes runs in families. However, it is unclear what causes its onset. Studies of psychedelic drugs, such as psilocybin, LSD and mescaline, reveal that exposure to these drugs can induce synesthesia. One neurotransmitter suspected to be central to the perceptual changes is serotonin. Excessive serotonin in the brain may cause many of the characteristics of psychedelic intoxication. Excessive serotonin levels may also play a role in synesthesia acquired after brain injury. In brain injury sudden cell death floods local brain regions with serotonin and glutamate. This neurotransmitter flooding could perhaps result in unusual feature binding. Finally, developmental synesthesia that occurs in individuals with autism may be a result of alterations in the serotonergic system, leading to a blockage of regular gating mechanisms. I conclude on these grounds that one commonality among at least some cases of acquired, developmental and drug-induced synesthesia may be the presence of excessive levels of serotonin, which increases the excitability and connectedness of sensory brain regions.

Neural networks of colored sequence synesthesia

Synesthesia is a condition in which normal stimuli can trigger anomalous associations. In this study, we exploit synesthesia to understand how the synesthetic experience can be explained by subtle changes in network properties. Of the many forms of synesthesia, we focus on colored sequence synesthesia, a form in which colors are associated with overlearned sequences, such as numbers and letters (graphemes). Previous studies have characterized synesthesia using resting-state connectivity or stimulus-driven analyses, but it remains unclear how network properties change as synesthetes move from one condition to another. To address this gap, we used functional MRI in humans to identify grapheme-specific brain regions, thereby constructing a functional "synesthetic" network. We then explored functional connectivity of color and grapheme regions during a synesthesia-inducing fMRI paradigm involving rest, auditory grapheme stimulation, and audiovisual grapheme stimulation. Using Markov networks to represent direct relationships between regions, we found that synesthetes had more connections during rest and auditory conditions. We then expanded the network space to include 90 anatomical regions, revealing that synesthetes tightly cluster in visual regions, whereas controls cluster in parietal and frontal regions. Together, these results suggest that synesthetes have increased connectivity between grapheme and color regions, and that synesthetes use visual regions to a greater extent than controls when presented with dynamic grapheme stimulation. These data suggest that synesthesia is better characterized by studying global network dynamics than by individual properties of a single brain region.

Insular Dysfunction Reflects Altered Between-Network Connectivity and Severity of Negative Symptoms in Schizophrenia during Psychotic Remission

Schizophrenia is characterized by aberrant intrinsic functional connectivity (iFC) within and between intrinsic connectivity networks (ICNs), including the Default Mode- (DMN), Salience- (SN), and Central Executive Network (CEN). The anterior insula (AI) of the SN has been demonstrated to modulate DMN/CEN interactions. Recently, we found that the dependence of DMN/CEN interactions on SN's right AI activity is altered in patients with schizophrenia in acute psychosis and related to psychotic symptoms, indicating a link between aberrant AI, DMN, CEN, and psychosis. However, since structural alterations of the insula are also present during psychotic remission and associated with negative symptoms, impaired AI interaction might be relevant even for psychotic remission and corresponding symptoms. Twelve patients with schizophrenia during psychotic remission (SR) and 12 healthy controls were assessed using resting-state fMRI and psychometric examination. High-model-order independent component analysis of fMRI data revealed ICNs including DMN, SN, and CEN. Scores of iFC within (intra-iFC) and between (inter-iFC) distinct subsystems of the DMN, SN, and CEN were calculated, compared between groups and correlated with the severity of symptoms. Intra-iFC was altered in patients' SN, DMN, and CEN, including decreased intra-iFC in the left AI within the SN. Patients' inter-iFC between SN and CEN was increased and correlated with the severity of negative symptoms. Furthermore, decreased intra-iFC of the left AI correlated with both severity of negative symptoms and increased inter-iFC between SN and CEN. Our result provides first evidence for a relationship between AI dysfunction and altered between-network interactions in schizophrenia during psychotic remission, which is related to the severity of negative symptoms. Together with our previous results, data suggest specific SN/DMN/CEN reorganization in schizophrenia with distinct insular pathways for distinct symptom dimensions.

Short- and long-range neural synchrony in grapheme-color synesthesia

Grapheme-color synesthesia is a perceptual phenomenon where single graphemes (e.g., the letter "E") induce simultaneous sensations of colors (e.g., the color green) that were not objectively shown. Current models disagree as to whether the color sensations arise from increased short-range connectivity between anatomically adjacent grapheme- and color-processing brain structures or from decreased effectiveness of inhibitory long-range connections feeding back into visual cortex. We addressed this issue by examining neural synchrony obtained from EEG activity, in a sample of grapheme-color synesthetes that were presented with color-inducing versus non-color-inducing graphemes. For color-inducing graphemes, the results showed a decrease in the number of long-range couplings in the theta frequency band (4-7 Hz, 280-540 msec) and a concurrent increase of short-range phase-locking within lower beta band (13-20 Hz, 380-420 msec at occipital electrodes). Because the effects were both found in long-range synchrony and later within the visual processing stream, the results support the idea that reduced inhibition is an important factor for the emergence of synesthetic colors.

Absolute pitch exhibits phenotypic and genetic overlap with synesthesia

Absolute pitch (AP) and synesthesia are two uncommon cognitive traits that reflect increased neuronal connectivity and have been anecdotally reported to occur together in an individual. Here we systematically evaluate the occurrence of synesthesia in a population of 768 subjects with documented AP. Out of these 768 subjects, 151 (20.1%) reported synesthesia, most commonly with color. These self-reports of synesthesia were validated in a subset of 21 study subjects, using an established methodology. We further carried out combined linkage analysis of 53 multiplex families with AP and 36 multiplex families with synesthesia. We observed a peak NPL LOD = 4.68 on chromosome 6q, as well as evidence of linkage on chromosome 2, using a dominant model. These data establish the close phenotypic and genetic relationship between AP and synesthesia. The chromosome 6 linkage region contains 73 genes; several leading candidate genes involved in neurodevelopment were investigated by exon resequencing. However, further studies will be required to definitively establish the identity of the causative gene(s) in the region.