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Colored-speech synaesthesia is triggered by multisensory, not unisensory, perception.

Psychol Sci. 2009 May; 20(5): 529-33
Bargary G, Barnett KJ, Mitchell KJ, Newell FN

Although it is estimated that as many as 4% of people experience some form of enhanced cross talk between (or within) the senses, known as synaesthesia, very little is understood about the level of information processing required to induce a synaesthetic experience. In work presented here, we used a well-known multisensory illusion called the McGurk effect to show that synaesthesia is driven by late, perceptual processing, rather than early, unisensory processing. Specifically, we tested 9 linguistic-color synaesthetes and found that the colors induced by spoken words are related to what is perceived (i.e., the illusory combination of audio and visual inputs) and not to the auditory component alone. Our findings indicate that color-speech synaesthesia is triggered only when a significant amount of information processing has occurred and that early sensory activation is not directly linked to the synaesthetic experience.

 

'When birds of a feather flock together': synesthetic correspondences modulate audiovisual integration in non-synesthetes.

PLoS ONE. 2009; 4(5): e5664
Parise CV, Spence C


BACKGROUND: Synesthesia is a condition in which the stimulation of one sense elicits an additional experience, often in a different (i.e., unstimulated) sense. Although only a small proportion of the population is synesthetic, there is growing evidence to suggest that neurocognitively-normal individuals also experience some form of synesthetic association between the stimuli presented to different sensory modalities (i.e., between auditory pitch and visual size, where lower frequency tones are associated with large objects and higher frequency tones with small objects). While previous research has highlighted crossmodal interactions between synesthetically corresponding dimensions, the possible role of synesthetic associations in multisensory integration has not been considered previously.
METHODOLOGY: Here we investigate the effects of synesthetic associations by presenting pairs of asynchronous or spatially discrepant visual and auditory stimuli that were either synesthetically matched or mismatched. In a series of three psychophysical experiments, participants reported the relative temporal order of presentation or the relative spatial locations of the two stimuli.
PRINCIPAL FINDINGS: The reliability of non-synesthetic participants' estimates of both audiovisual temporal asynchrony and spatial discrepancy were lower for pairs of synesthetically matched as compared to synesthetically mismatched audiovisual stimuli.
CONCLUSIONS: Recent studies of multisensory integration have shown that the reduced reliability of perceptual estimates regarding intersensory conflicts constitutes the marker of a stronger coupling between the unisensory signals. Our results therefore indicate a stronger coupling of synesthetically matched vs. mismatched stimuli and provide the first psychophysical evidence that synesthetic congruency can promote multisensory integration. Synesthetic crossmodal correspondences therefore appear to play a crucial (if unacknowledged) role in the multisensory integration of auditory and visual information.

 

Multiple dimensions in bi-directional synesthesia.

Eur J Neurosci. 2009 Apr; 29(8): 1703-10
Gebuis T, Nijboer TC, Van der Smagt MJ

Grapheme-color synesthetes report seeing a specific color when a number is perceived. The reverse, the synesthetic experience of a specific grapheme after the percept of a color is extremely rare. However, recent studies have revealed these interactions at both behavioral and neurophysiological levels. We investigated whether similar neuronal processes (i.e. perceptual and/or attentional) may underlie this bi-directional interaction by measuring event-related potentials (ERPs) during both a number-color and color-number priming task. In addition, we investigated the unitarity of synesthesia by comparing two distinct subtypes of synesthetes, projectors and associators, and assessed whether consistencies between measures (i.e. behavioral and electrophysiological) were present across synesthetes. Our results show longer reaction times for incongruent compared with congruent trials in both tasks. This priming effect is also present in the P3b latency (parietal electrode site) and P3a amplitude (frontal electrode site) of the ERP data. Interestingly, projector and associator synesthetes did not reveal distinct behavioral or electrophysiological patterns. Instead, a dissociation was found when synesthetes were divided in two groups on the basis of their behavioral data. Synesthetes with a large behavioral priming effect revealed ERP modulation at the frontal and parietal electrode sites, whereas synesthetes with a small priming effect revealed a frontal effect only. Together, these results show, for the first time, that similar neural mechanisms underlie bi-directional synesthesia in synesthetes that do not report a synesthetic experience of a grapheme when a color is perceived. In addition, they add support for the notion of the existence of both 'lower' and 'higher' synesthetes.

 

The neurophysiology of bi-directional synesthesia (Commentary on Gebuis et al.).

Eur J Neurosci. 2009 Apr; 29(8): 1701-2
Shalgi S, Foxe JJ

 

Prevalence, characteristics and a neurocognitive model of mirror-touch synaesthesia.

Exp Brain Res. 2009 May 3;
Banissy MJ, Kadosh RC, Maus GW, Walsh V, Ward J

In so-called 'mirror-touch synaesthesia', observing touch to another person induces a subjective tactile sensation on the synaesthete's own body. It has been suggested that this type of synaesthesia depends on increased activity in neural systems activated when observing touch to others. Here we report the first study on the prevalence of this variant of synaesthesia. Our findings indicate that this type of synaesthesia is just as common, if not more common than some of the more frequently studied varieties of synaesthesia such as grapheme-colour synaesthesia. Additionally, we examine behavioural correlates associated with the condition. In a second experiment, we show that synaesthetic experiences are not related to somatotopic cueing-a flash of light on an observed body part does not elicit the behavioural or subjective characteristics of synaesthesia. Finally, we propose a neurocognitive model to account for these characteristics and discuss the implications of our findings for general theories of synaesthesia.

 

Touching sounds: thalamo-cortical plasticity and the neural basis of multisensory integration.

J Neurophysiol. 2009 Apr 29;
Naumer MJ, van den Bosch JJ

To date, non-invasive neuroimaging research on multisensory perception has focused on cortical activations. In a series of elegant fMRI experiments, Beauchamp and Ro (2008) recently investigated altered cortical activations associated with acquired sound-touch synaesthesia resulting from a thalamic lesion. Their findings highlight the important role of intact thalamo-cortical projections for preventing illusory crossmodal perception and for underlying reliable multisensory integration.

 

Synaesthesia: learned or lost?

Dev Sci. 2009 Apr; 12(3): 484-91
Cohen Kadosh R, Henik A, Walsh V

The question why synaesthesia, an atypical binding within or between modalities, occurs is both enduring and important. Two explanations have been provided: (1) a congenital explanation: we are all born as synaesthetes but most of us subsequently lose the experience due to brain development; (2) a learning explanation: synaesthesia is related to some learning process during childhood. Three recent studies provide conflicting support for these explanations. Two studies supported the idea that synaesthesia is learned by showing that the frequency of everyday language implicitly modulates the synaesthetic experience. Another study argued that synaesthesia reflects basic, innate magnitude representations. In this paper we reassess these points of view, and show that it is possible for both to be valid. These findings are integrated into an interactive specialization account of development in order to explain the neuronal mechanism underlying synaesthesia.

 

Do synesthetes have a general advantage in visual search and episodic memory? A case for group studies.

PLoS ONE. 2009; 4(4): e5037
Rothen N, Meier B


BACKGROUND: Some studies, most of them case-reports, suggest that synesthetes have an advantage in visual search and episodic memory tasks. The goal of this study was to examine this hypothesis in a group study.
METHODOLOGY/PRINCIPAL FINDINGS: In the present study, we tested thirteen grapheme-color synesthetes and we compared their performance on a visual search task and a memory test to an age-, handedness-, education-, and gender-matched control group. The results showed no significant group differences (all relevant ps>.50). For the visual search task effect sizes indicated a small advantage for synesthetes (Cohen's d between .19 and .32). No such advantage was found for episodic memory (Cohen's d<.05).
CONCLUSIONS/SIGNIFICANCE: The results indicate that synesthesia per se does not seem to lead to a strong performance advantage. Rather, the superior performance of synesthetes observed in some case-report studies may be due to individual differences, to a selection bias or to a strategic use of synesthesia as a mnemonic. In order to establish universal effects of synesthesia on cognition single-case studies must be complemented by group studies.

 

Training grapheme-colour associations produces a synaesthetic Stroop effect, but not a conditioned synaesthetic response.

Neuropsychologia. 2009 Mar; 47(4): 1208-11
Meier B, Rothen N

The goal of this study was to investigate whether behavioural and physiological consequences of synaesthesia can be elicited by training specific letter-colour associations. Towards this goal 20 non-synaesthetic individuals were trained for 10 min on 7 consecutive days to associate four different letters with four specific colours. After training, we administered the synaesthetic Stroop test and the synaesthetic conditioning test. The results showed that a 1-week letter-colour association-training was sufficient to elicit a synaesthetic Stroop effect. In contrast, there was no evidence for a conditioned synaesthetic response as measured by skin conductance response in the synaesthetic conditioning test. These results indicate that the presence of a synaesthetic Stroop effect is not unique to true synaesthetes. We discuss methodological, experiential, and neuronal reasons for the dissociation between the synaesthetic Stroop test and the synaesthetic conditioning test.

 

Attention, automaticity, and awareness in synesthesia.

Ann N Y Acad Sci. 2009 Mar; 1156: 141-67
Mattingley JB

The phenomenon of synesthesia has occupied the thoughts of philosophers and artists for decades. With the advent modern behavioral and brain imaging techniques, scientific research on synesthesia has also moved into the mainstream of thought. Here I provide a cognitive neuroscience perspective on the condition, with a particular emphasis on grapheme-color synesthesia, the most common variant, in which individuals report vivid and consistent experiences of color in association with numerals, letters, and words. Behavioral studies have revealed several fundamental properties of induced synesthetic colors. First, although they seem to arise automatically, without the need for voluntary control, they are strongly modulated by selective attention. Second, they attain salience relatively early in visual processing, and so can influence perceptual judgments and guide focal attention in cluttered, achromatic displays. Third, brain activity during synesthetic color experiences arises from within the ventral temporal lobe, including color-selective area V4. It has been speculated that grapheme-color synesthesia arises from disinhibited feedback or abnormal cross-wiring between brain regions involved in extracting visual form and color.

 

The neuroanatomy of grapheme-color synesthesia.

Eur J Neurosci. 2009 Mar; 29(6): 1287-93
Jäncke L, Beeli G, Eulig C, Hänggi J

Grapheme-color synesthetes perceive particular colors when seeing a letter, word or number (grapheme). Functional neuroimaging studies have provided some evidence in favor of a neural basis for this type of synesthesia. Most of these studies have reported extra activations in the fusiform gyrus, which is known to be involved in color, letter and word processing. The present study examined different neuroanatomical features (i.e. cortical thickness, cortical volume and cortical surface area) in a sample of 48 subjects (24 grapheme-color synesthetes and 24 control subjects), and revealed increased cortical thickness, volume and surface area in the right and left fusiform gyrus and in adjacent regions, such as the lingual gyrus and the calcarine cortex, in grapheme-color synesthetes. In addition, we set out to analyze structural connectivity based on fractional anisotropy (FA) measurements in a subsample of 28 subjects (14 synesthetes and 14 control subjects). In contrast to the findings of a recent neuroanatomical study using modern diffusion tensor imaging measurement techniques, we did not detect any statistically significant difference in FA between synesthetes and non-synesthetes in the fusiform gyri. Our study thus supports the hypothesis of local anatomical differences in cortical characteristics in the vicinity of the V4 complex. The observed altered brain anatomy in grapheme-color synesthetes might be the anatomical basis for this particular form of synesthesia but it is also possible that the detected effects are a consequence (rather than the primary cause) of the life-long experience of grapheme-color synesthesia.

 

When 9 is not on the right: implications from number-form synesthesia.

Conscious Cogn. 2009 Jun; 18(2): 366-74
Gertner L, Henik A, Kadosh RC

Number-form synesthetes consciously experience numbers in spatially-defined locations. For non-synesthete individuals, a similar association of numbers and space appears in the form of an implicit mental number line as signified by the distance effect-reaction time decreases as the numerical distance between compared numbers increases. In the current experiment, three number-form synesthetes and two different non-synesthete control groups (Hebrew speaking and English speaking) performed a number comparison task. Synesthete participants exhibited a sizeable distance effect only when presented numbers were congruent with their number-form. In contrast, the controls exhibited a distance effect regardless of congruency or presentation type. The findings suggest that: (a) number-form synesthesia impairs the ability to represent numbers in a flexible manner according to task demands; (b) number-form synesthesia is a genuine tangible experience, triggered involuntarily; and (c) the classic mental number line can be more pliable than previously thought and appears to be independent of cultural-lingo direction.

 

Of colored numbers and numbered colors.

Exp Psychol. 2009; 56(3): 180-7
Gebuis T, Nijboer TC, van der Smagt MJ

Grapheme-color synesthetes experience a specific color when they see a grapheme but they do not report to perceive a grapheme when a color is presented. In this study, we investigate whether color can still evoke number-processes even when a vivid number experience is absent. We used color-number and number-color priming, both revealing faster responses in congruent compared to incongruent conditions. Interestingly, the congruency effect was of similar magnitude for both conditions, and a numerical distance effect was present only in the color-number priming task. In addition, a priming task in which synesthetes had to judge the parity of a colored number revealed faster responses in parity congruent than in parity incongruent trials. These combined results demonstrate that synesthesia is indeed bi-directional and of similar strength in both directions. Furthermore, they illustrate the precise nature of these interactions and show that the direction of these interactions is determined by task demands, not by the more vividly experienced aspect of the stimulus.

 

Implicit versus explicit interference effects in a number-color synesthete.

Cortex. 2009 Feb 13;
Berteletti I, Hubbard EM, Zorzi M

A fundamental question in the study of consciousness is the connection between subjective report and objective measures. We explored this question by testing NM, a grapheme-color synesthete, who experiences colors when viewing digits but not dot patterns. Synesthesia research has traditionally used variants of the Stroop paradigm as an objective correlate of these subjective synesthetic reports. We used both a classical synesthetic Digit Stroop task and a novel Numerosity Stroop task, in which random dot patterns were colored either congruently or incongruently with the colors NM reported for digits. We observed longer response times in the incongruent condition for both tasks, despite the fact that NM denied experiencing colors for random dot patterns, constituting a clear dissociation between subjective and objective measures of synesthetic experience. We argue that distinguishing synesthesia from learned synesthesia-like associations (pseudosynesthesia) should depend primarily on the presence of subjective reports, validated by objective measures. More generally, we suggest that consciously and unconsciously mediated interference may arise from qualitatively different mechanisms.

 

Synesthesia: a new approach to understanding the development of perception.

Dev Psychol. 2009 Jan; 45(1): 175-89
Spector F, Maurer D

In this article, the authors introduce a new theoretical framework for understanding intersensory development. Their approach is based upon insights gained from adults who experience synesthesia, in whom sensory stimuli induce extra cross-modal or intramodal percepts. Synesthesia appears to represent one way that typical developmental mechanisms can play out by magnifying connections present in early life that are pruned and/or inhibited during development but persist in muted form in all adults. As such, the study of synesthesia provides valuable insights into the nature of intersensory development. The authors review evidence on the perceptual reality and neural basis of synesthesia, then summarize developmental models and evidence that its underlying mechanisms are universal in adults. They illustrate how evidence for consistent sensory associations in adults leads to predictions about toddlers' perception and present 3 bodies of work that have confirmed those hypotheses. They end by describing novel hypotheses about intersensory development that arise from this framework. Such intersensory associations appear to reflect intrinsic sensory cortical organization that influences the development of perception and of language and that may constrain the learning of environmentally based associations.

 

A whole-genome scan and fine-mapping linkage study of auditory-visual synesthesia reveals evidence of linkage to chromosomes 2q24, 5q33, 6p12, and 12p12.

Am J Hum Genet. 2009 Feb; 84(2): 279-85
Asher JE, Lamb JA, Brocklebank D, Cazier JB, Maestrini E, Addis L, Sen M, Baron-Cohen S, Monaco AP

Synesthesia, a neurological condition affecting between 0.05%-1% of the population, is characterized by anomalous sensory perception and associated alterations in cognitive function due to interference from synesthetic percepts. A stimulus in one sensory modality triggers an automatic, consistent response in either another modality or a different aspect of the same modality. Familiality studies show evidence of a strong genetic predisposition; whereas initial pedigree analyses supported a single-gene X-linked dominant mode of inheritance with a skewed F:M ratio and a notable absence of male-to-male transmission, subsequent analyses in larger samples indicated that the mode of inheritance was likely to be more complex. Here, we report the results of a whole-genome linkage scan for auditory-visual synesthesia with 410 microsatellite markers at 9.05 cM density in 43 multiplex families (n = 196) with potential candidate regions fine-mapped at 5 cM density. Using NPL and HLOD analysis, we identified four candidate regions. Significant linkage at the genome-wide level was detected to chromosome 2q24 (HLOD = 3.025, empirical genome-wide p = 0.047). Suggestive linkage was found to chromosomes 5q33, 6p12, and 12p12. No support was found for linkage to the X chromosome; furthermore, we have identified two confirmed cases of male-to-male transmission of synesthesia. Our results demonstrate that auditory-visual synesthesia is likely to be an oligogenic disorder subject to multiple modes of inheritance and locus heterogeneity. This study comprises a significant step toward identifying the genetic substrates underlying synesthesia, with important implications for our understanding of the role of genes in human cognition and perception.

 

Colored-hearing synesthesia.

Jpn Hosp. 2008 Jul; 51-6
Takayanagi K

According to the experimental study of synesthetic tendencies in a total of 104 subjects, the angry voice of the queen of the night from Mozart is pictured as the colors red and yellow. The voice of lament was pictured as yellow. The colors of 'angelic tones' were perceived as cool colors. Though differences between male and female were not clear, synesthesia toward musical phrases were slightly different between seventeen-year-old boys and adults. It is important to choose proper colors to mediate synesthetic tendencies in hospital settings. Angry voices tended to engender warm colors. 'Angelic tones' were cold colors. Patients in ICU may have two attitudes. One is a panic stage which needs calm surroundings, and the other is a fighting stage trying to escape from their serious condition in ICU settings. Common synesthetic tendencies can be used for choosing the color of ICU settings. Hospices have a different attitude to patient care. Colors should be carefully chosen accordingly.

 

Induced cross-modal synaesthetic experience without abnormal neuronal connections.

Psychol Sci. 2009 Feb; 20(2): 258-65
Cohen Kadosh R, Henik A, Catena A, Walsh V, Fuentes LJ

Are the kinds of abnormal cross-modal interactions seen in synaesthesia or following brain damage due to hyperconnectivity between or within brain areas, or are they a result of lack of inhibition? This question is highly contested. Here we show that posthypnotic suggestion induces abnormal cross-modal experience similar to that observed in congenital grapheme-color synaesthesia. Given the short time frame of the experiment, it is unlikely that new cortical connections were established, so we conclude that synaesthesia can result from disinhibition between brain areas.

 

Is undiagnosed synaesthesia a confounding factor in the interpretation of MRI images?

Conf Proc IEEE Eng Med Biol Soc. 2008; 2008: 5778-81
Zayed N, Goodyear B, Smith M

Synaesthesia is a condition in which stimulation of a sensory modality evokes another sensation in the same or a different sensory modality. Currently, synaesthesia is considered a neurological condition that involves crosstalk between brain regions. Given the numerous anatomical and functional connections within the brain, it is possible that undiagnosed synaesthesia may influence the results of functional magnetic resonance imaging (fMRI) studies or even structural MRI. In this paper, we investigated the currently available literature to determine if and how the sensations invoked by synaesthesia could impact fMRI and structural MRI. Our investigation found that synaesthesia can have a profound impact on fMRI studies of sensory and cognitive functions, and there is evidence to suggest structural connections in the brain are also altered. Given the low prevalence of synaesthesia, the likelihood of synaesthesia being a confounding factor in fMRI studies of patient groups is small; however, determining the presence of synaesthesia is important for investigating individual patients.

 

An exploratory study of linguistic-colour associations across languages in multilingual synaesthetes.

Q J Exp Psychol (Colchester). 2009 Jul; 62(7): 1343-55
Barnett KJ, Feeney J, Gormley M, Newell FN

In one of the most common forms of synaesthesia, linguistic-colour synaesthesia, colour is induced by stimuli such as numbers, letters, days of the week, and months of the year. It is not clear, however, whether linguistic-colour synaesthesia is determined more by higher level semantic information-that is, word meaning-or by lower level grapheme or phoneme structure. To explore this issue, we tested whether colour is consistently induced by grapheme or phoneme form or word meaning in bilingual and trilingual linguistic-colour synaesthetes. We reasoned that if the induced colour was related to word meaning, rather than to the acoustic or visual properties of the words, then the induced colours would remain consistent across languages. We found that colours were not consistently related to word meaning across languages. Instead, induced colours were more related to form properties of the word across languages, particularly visual structure. However, the type of inducing stimulus influenced specific colour associations. For example, colours to months of the year were more consistent across languages than were colours to numbers or days of the week. Furthermore, the effect of inducing stimuli was also associated with the age of acquisition of additional languages. Our findings are discussed with reference to a critical period in language acquisition on synaesthesia.

 
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