Bidirectional lexical-gustatory synesthesia

In developmental lexical–gustatory synesthesia, specific words (inducers) can trigger taste perceptions (concurrents) and these synesthetic associations are generally stable. We describe a case of multilingual lexical–gustatory synesthesia for whom some synesthesias were bidirectional as some tastes also triggered auditory word associations. Evoked concurrents could be gustatory but also tactile sensations. In addition to words and pseudowords, many voices were effective inducers, suggesting increased connections between cortical taste areas and both voice-selective and language-selective areas. Lasting changes in some evoked tastes occurred during childhood suggesting that some plasticity can be present after the initial learning of associations. Inducers were often linked to taste concurrents phonologically or semantically, but also through identifiable childhood episodes (persons or events). Several inducers were phonologically linked to episodic inducers suggesting a process of secondary acquisition for many inducers. Implications of these observations are discussed.     

Genuine and drug-induced synesthesia: A comparison

Despite some principal similarities, there is no systematic comparison between the different types of synesthesia (genuine, acquired and drug-induced). This comprehensive review compares the three principal types of synesthesia and focuses on their phenomenological features and their relation to different etiological models. Implications of this comparison for the validity of the different etiological models are discussed. Comparison of the three forms of synesthesia show many more differences than similarities. This is in contrast to their representation in the literature, where they are discussed in many respects as being virtually similar. Noteworthy is the much broader spectrum and intensity with the typical drug-induced synesthesias compared to genuine and acquired synesthesias. A major implication of the phenomenological comparison in regard to the etiological models is that genuine and acquired synesthesias point to morphological substrates, while drug-induced synesthesia appears to be based on functional changes of brain activity.     

Modality and variability of synesthetic experience

In synesthesia, stimulation of one sensory or cognitive pathway leads to additional, involuntary experiences in a second sensory or cognitive pathway. We here review previous surveys on this neurologically based phenomenon and report the results of 63 synesthetes who completed our Internet and paper questionnaire on synesthesia. In addition to asking for personal data and information on the participant's synesthesia, the questionnaire focused on the components of the inducer that elicit or modulate synesthesia. Synesthesia was most often developmental (92%) and of the grapheme-color type (86%). Sixty-two percent of the participants perceived time-related words in a spatial configuration. Music-color synesthesia was common (41%), and synesthesia for natural and artificial sounds (33%) was higher than in previous estimates. Eighty-one percent of participants experienced more than one form of synesthesia. Multimodal synesthesia, in which inducer and concurrent belong to 2 different sensory modalities, occurred in 92% of the participants. Overall, auditory stimuli were most often reported as inducers, and visual concurrents were most common. Modulations of the synesthetic experiences such as changes of the concurrent color, expansion within the same or to a different sensory modality, or reduction of the number of inducers over time were reported by 17% of participants. This challenges the presumed consistency of synesthesia and the adequacy of the test-retest consistency score still most commonly used to assess the veracity of reported synesthesia. Implications of the high prevalence of cross-modal synesthesia and the variability of synesthesia are discussed.     

The impact of mediodorsal thalamic lesions on olfactory attention and flavor perception

Olfactory attention may be important in generating odor-induced tastes - an arguably universal form of synesthesia - by ensuring that the taste concurrent is captured by the nose and olfaction, not by the mouth and gustation (oral-capture). To examine the role of olfactory attention in generating odor-induced tastes and oral capture we tested a small sample (n = 4) of participants with likely impairments in olfactory attention - individuals with mediodorsal thalamic nucleus (MDNT) lesions. These participants were compared to two sets of controls on tests of olfactory attention, oral capture, odor and flavor perception, and control tasks. MDNT participants demonstrated impaired olfactory attention and enhanced oral capture. Greater oral capture was associated with greater olfactory attentional impairment. These findings imply that olfactory attention may be important in attributing odor-induced tastes to the olfactory modality. However, unlike for visual binding and for the neurodevelopmental synesthesias, where attention may be necessary to demonstrate both phenomena, olfactory attention deficits did not impair flavor binding or the experience of odor-induced tastes.     

Synaesthetic perception of colour and visual space in a blind subject: an fMRI case study

In spatial sequence synaesthesia (SSS) ordinal stimuli are perceived as arranged in peripersonal space. Using fMRI, we examined the neural bases of SSS and colour synaesthesia for spoken words in a late-blind synaesthete, JF. He reported days of the week and months of the year as both coloured and spatially ordered in peripersonal space; parts of the days and festivities of the year were spatially ordered but uncoloured. Words that denote time-units and triggered no concurrents were used in a control condition. Both conditions inducing SSS activated the occipito-parietal, infero-frontal and insular cortex. The colour area hOC4v was engaged when the synaesthetic experience included colour. These results confirm the continued recruitment of visual colour cortex in this late-blind synaesthetes. Synaesthesia also involved activation in inferior frontal cortex, which may be related to spatial memory and detection, and in the insula, which might contribute to audiovisual integration related to the processing of inducers and concurrents.     

强联觉的认知加工模型及其脑机制

联觉是当个体感受到一种感觉刺激的同时体验到其它感觉的现象, 主要分为强联觉和弱联觉两种。本文主要介绍了强联觉的各种类型及其相关脑机制的研究, 分析和比较了前人从结构和认知加工角度建构的多个强联觉理论模型, 总结了这些模型对强联觉现象脑神经机制的解释。同时本文还探讨了强联觉与其它(如注意、记忆、创造力等)认知加工过程的关系, 对弱联觉(即联觉隐喻)的研究进展和不足也进行了分析和总结, 指出未来对联觉的研究趋势应集中在弱联觉的研究上。     

Enhanced mental rotation ability in time-space synesthesia

Time-space synesthesia is a variant of sequence–space synesthesia and involves the involuntary association of months of the year with 2D and 3D spatial forms, such as arcs, circles, and ellipses. Previous studies have revealed conflicting results regarding the association between time-space synesthesia and enhanced spatial processing ability. Here, we tested 15 time-space synesthetes, and 15 non-synesthetic controls matched for age, education, and gender on standard tests of mental rotation ability, spatial working memory, and verbal working memory. Synesthetes performed better than controls on our test of mental rotation, but similarly to controls on tests of spatial and verbal working memory. Results support a dissociation between visuo-spatial imagery and spatial working memory capacity, and suggest time-space synesthesia is associated only with enhanced visuo-spatial imagery. These data are consistent with the time-space connectivity thesis that time-space synesthesia results from enhanced connectivity in the parietal lobe between regions supporting the representation of temporal sequences and those underlying visuo-spatial imagery.     

Bright sneezes and dark coughs, loud sunlight and soft moonlight

Synesthetic metaphors (such as "the dawn comes up like thunder") are expressions in which words or phrases describing experiences proper to one sense modality transfer their meanings to another modality. In a series of four experiments, subjects used scales of loudness, pitch, and brightness to evaluate the meanings of a variety of synesthetic (auditory-visual) metaphors. Loudness and pitch expressed themselves metaphorically as greater brightness; in turn, brightness expressed itself as greater loudness and as higher pitch. Although loudness thus shared with brightness a metaphorical connection, pitch and brightness showed a connection that was closer and that applied more generally to different kinds of visual brightness. The ways that people evaluate synesthetic metaphors emulate the characteristics of synesthetic perception, thereby suggesting that synesthesia in perception and synesthesia in language both may emenate from the same source-from a phenomenological similarity in the makeup of sensory experiences of different modalities.     

Precision of synesthetic color matching resembles that for recollected colors rather than physical colors

Grapheme-color synesthesia is an atypical condition in which individuals experience sensations of color when reading printed graphemes such as letters and digits. For some grapheme-color synesthetes, seeing a printed grapheme triggers a sensation of color, but hearing the name of a grapheme does not. This dissociation allowed us to compare the precision with which synesthetes are able to match their color experiences triggered by visible graphemes, with the precision of their matches for recalled colors based on the same graphemes spoken aloud. In six synesthetes, color matching for printed graphemes was equally variable relative to recalled experiences. In a control experiment, synesthetes and age-matched controls either matched the color of a circular patch while it was visible on a screen, or they judged its color from memory after it had disappeared. Both synesthetes and controls were more variable when matching from memory, and the variance of synesthetes' recalled color judgments matched that associated with their synesthetic judgments for visible graphemes in the first experiment. Results suggest that synesthetic experiences of color triggered by achromatic graphemes are analogous to recollections of color. (PsycINFO Database Record (c) 2012 APA, all rights reserved).     

Superior encoding enhances recall in color-graphemic synesthesia

Synesthesia is a phenomenon in which particular stimuli, such as letters or sound, generate a secondary sensory experience in particular individuals. Reports of enhanced memory in synesthetes raise the question of its cognitive and neurological substrates. Enhanced memory in synesthetes could arise from the explicit or implicit use of a synesthetic cue to aid memory, from changes unique to the synesthete brain, or from both, depending on the task. To assess this question, we tested nine color-graphemic synesthetes using standardized neuropsychological measures that should not trigger color-graphemic synesthesia (visuo-spatial tests) and measures that should trigger color-graphemic synesthesia (verbal tasks). We found a synesthetic advantage on both types of tests, primarily in the initial encoding of information. The pattern of results adds to existing evidence of advantages in synesthetic memory, as well as provides novel evidence that synesthetes may have enhanced encoding rather than superior recall. Synesthetes learn more initially, rather than forgetting less over time.     

Do small white balls squeak? Pitch-object correspondences in young children

Adults with auditory-visual synesthesia agree that higher pitched sounds induce smaller, brighter visual percepts. We have hypothesized that these correspondences are remnants of cross-modal neural connections that are present at birth and that influence the development of perception and language even in adults and children without synesthesia. In this study, we explored these correspondences in preschoolers (30-36 months; n=12 per experiment). The children were asked to indicate which of two bouncing balls was making a centrally located sound. The balls varied in size and/or surface darkness; the sound varied in pitch. The children reliably matched the higher pitched sound to a smaller and lighter (white) ball (Experiment 1), to a lighter (white) ball (Experiment 2), and in one of two groups, to a smaller ball (Experiment 3). Children’s matching of pitch and size cannot be attributed to intensity matching or to learning. These data support the hypothesis that some cross-modal correspondences may be remnants of the neural mechanisms underlying neonatal perception.     

Synesthesia and memory: color congruency, von Restorff, and false memory effects

In the current study, we explored the influence of synesthesia on memory for word lists. We tested 10 grapheme-color synesthetes who reported an experience of color when reading letters or words. We replicated a previous finding that memory is compromised when synesthetic color is incongruent with perceptual color. Beyond this, we found that, although their memory for word lists was superior overall, synesthetes did not exhibit typical color- or semantic-defined von Restorff isolation effects (von Restorff, 1933) compared with control participants. Moreover, our synesthetes exhibited a reduced Deese-Roediger-McDermott false memory effect (Deese, 1959; Roediger & McDermott, 1995). Taken as a whole, these findings are consistent with the idea that color-grapheme synesthesia can lead people to place a greater emphasis on item-specific processing and surface form characteristics of words in a list (e.g., the letters that make them up) relative to relational processing and more meaning-based processes. (PsycINFO Database Record (c) 2010 APA, all rights reserved).     

Inducing synesthesia in non-synesthetes: Short-term visual deprivation facilitates auditory-evoked visual percepts

Sounds can modulate activity in visual cortex, facilitating the detection of visual targets. However, these sound-driven modulations are not thought to evoke conscious visual percepts in the general population. In individuals with synesthesia, however, multisensory interactions do lead to qualitatively different experiences such as sounds evoking flashes of light. Why, if multisensory interactions are present in all individuals, do only synesthetes experience abnormal qualia? Competing models differ in the time required for synesthetic experiences to emerge. The cross-activation model suggests synesthesia arises over months or years from the development of abnormal neural connections. Here we demonstrate that after ∼5 min of visual deprivation, sounds can evoke synesthesia-like percepts (vivid colors and Klüver form-constants) in ∼50% of non-synesthetes. These results challenge aspects of the cross-activation model and suggest that synesthesia exists as a latent feature in all individuals, manifesting when the balance of activity across the senses has been altered.     

Does color synesthesia pose a paradox for early-selection theories of attention?

Abstract— P.M. is a synesthete who experiences colors when viewing alphanumeric symbols. Her search for a target differing from distractors by a synesthetic color feature takes the form of a pop-out search. Thus, it would seem that synesthesia can occur preattentively. However, discrepancies between the regression functions of response times observed in target-present trials and target-absent trials, and the fact that fast response times occur only when the target is within a few degrees of visual angle from fixation, indicate that P.M.'s synesthesia does not occur preattentively, but rather is within the focus of attention. We conclude that synesthesia is a genuine perceptual phenomenon that can have substantial influence on visual processing.     

Grapheme–color synesthesia can enhance immediate memory without disrupting the encoding of relational cues

Previous evidence has suggested that grapheme–color synesthesia can enhance memory for words, but little is known about how these photisms cue retrieval. Often, the encoding of specific features of individual words can disrupt the encoding of ordered relations between words, resulting in an overall decrease in recall accuracy. Here we show that the photisms arising from grapheme–color synesthesia do not function like these item-specific cues. The influences of high and low word frequency on the encoding of ordered relations and the accuracy of immediate free recall were compared across a group of 10 synesthetes and 48 nonsynesthetes. The main findings of Experiment 1 showed that the experience of synesthesia had no adverse effect on the encoding of ordered relations (as measured by input–output correspondence); furthermore, it enhanced recall accuracy in a strictly additive fashion across the two word frequency conditions. Experiment 2 corroborated these findings by showing that the synesthetes only outperformed the nonsynesthetes when the materials involved words and letters, not when they involved digits and spatial locations. Altogether, the present findings suggest that synesthesia can boost immediate memory performance without disrupting the encoding of ordered relations.     

Concept driven color experiences in digit-color synesthesia

In digit-color synesthesia, black digits elicit conscious experiences of highly specific colors (photisms). This paper describes C, an undergraduate student who experiences digit-color synesthesia. We evaluated whether C's photisms could occur even in the absence of an externally presented digit. C was shown simple arithmetic problems (e.g., 5 + 2) followed by a color patch. C was significantly faster at naming the color of the patch when it was congruent with the photism associated with the answer to the problem than when the color patch was incongruent with the answer. These results replicate previous findings (Dixon, Smilek, Cudahy, & Merikle, 2000) and strongly suggest that for C, an externally presented inducing stimulus is not necessary to trigger a photism--activating the concept of a digit is sufficient.     

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.     

The color of two alphabets for a multilingual synesthete

Interviews with a multilingual synesthete (MLS), who experiences colored letters for Roman and Cyrillic alphabets and for digits, revealed stable synesthetic experiences over 2 1/2 - 5 years. Colors of Cyrillic letters were based on Roman letters. Four Stroop tests involving both types of letters showed that MLS was able to name print color faster if the colors matched her synesthetic colors, showing that synesthesia is automatic. Letter-naming times for blocks of color were slower than those of actual letters, supporting unidirectionality of synesthesia. Stroop tests with Roman, but not Cyrillic, letters showed MLS acquired new temporary letter-color pairings and her color-naming times for these were not different from those for her original synesthetic colors.