Do you see what I hear? . . . . This cloudy weather feels so Bauhaus.
Synesthesia is one of the true mysteries of the human brain and human subjective experience. Richard Cytowic, who is cited below by the synesthesia site at MIT, offers a broad overview in the abstract to a 1995 paper (available online) titled "Synesthesia: Phenomenology and Neuropsychology - A Review of Current Knowledge."
Synesthesia (Greek, syn = together + aisthesis = perception) is the involuntary physical experience of a cross-modal association. That is, the stimulation of one sensory modality reliably causes a perception in one or more different senses. Its phenomenology clearly distinguishes it from metaphor, literary tropes, sound symbolism, and deliberate artistic contrivances that sometimes employ the term "synesthesia" to describe their multisensory joinings. An unexpected demographic and cognitive constellation co-occurs with synesthesia: females and non-right-handers predominate, the trait is familial, and memory is superior while math and spatial navigation suffer. Synesthesia appears to be a left-hemisphere function that is not cortical in the conventional sense. The hippocampus is critical for its experience. Five clinical features comprise its diagnosis. Synesthesia is "abnormal" only in being statistically rare. It is, in fact, a normal brain process that is prematurely displayed to consciousness in a minority of individuals.By way of further introduction, here is a brief definition of synesthesia by the same author, quoted by the folks at MIT, from the introduction to his book on the subject:
Synesthesia is an involuntary joining in which the real information of one sense is accompanied by a perception in another sense. In addition to being involuntary, this additional perception is regarded by the synesthete as real, often outside the body, instead of imagined in the mind's eye. It also has some other interesting features that clearly separate it from artistic fancy or purple prose. Its reality and vividness are what make synesthesia so interesting in its violation of conventional perception. Synesthesia is also fascinating because logically it should not be a product of the human brain, where the evolutionary trend has been for increasing separation of function anatomically.
~ R. Cytowic, Synesthesia: A Union of the Senses, Springer-Verlag, NY (p.1)From a site that offers a synesthesia test, here are some of the many various forms of synesthesia:
Okay then, all of this is prelude to introducing a set of article from the Frontiers in Psychology: Cognitive Science series of new review (3) and research (5) articles into the phenomenon of synesthesia.
Types of SynesthesiaSynesthesia is not a phenomenon that manifests itself in one way. In fact, synesthesia can manifest itself in many different forms, as it involves different parts of the human brain. It can range from tasting colors to smelling sounds. Synesthesia can occur between any two senses or perceptual modes. If we take into account only the five basic senses (sight, smell, sound, taste, and touch) and imagine pairings, we already have twenty different types of synesthesia. In reality, there are many more possibilities. Solomon Shereshevsky, a synesthete, reportedly experienced a link between all five of the basic senses. While this is an extreme example, it’s evidence that more than two senses can cross paths.If you’re an auditory learner, check out this video by Jamie Ward and GoCognitive! It’s worth a watch!
While there are plenty of logically possible combinations, there are several synesthesia types that occur most commonly. Let’s take a look!
Grapheme-Color SynesthesiaThis is one of the most common types of synesthesia. A person who experiences this may associate/see individual letters or numbers with a specific color. Usually, two people won’t report the same color for letters and numbers. However, studies have shown that many synesthetes will see some letters the same way (for example, ‘A‘ is likely to be red).
Sound-to-Color SynesthesiaWhen sound triggers the visualization of colored, generic shapes, sound-to-color synesthesia is at play. For certain people, the stimuli are limited, and only a few types of sounds will trigger a perception. However, there are cases wherein many different sounds trigger color visualizations. Usually, the perceived colors appear in generic shapes – squares, circles, etc.
Number-Form SynesthesiaA number form is essentially a mental map that consists of numbers. When a person with number-form synesthesia thinks about numbers, a number map is involuntarily visualized. It is sometimes suggested that the number forms are a product of “cross-activation” between regions in the parietal lobe – a part of the brain that is involved in numerical and spatial cognition.
PersonificationThis type is known as ordinal-linguistic personification, or OLP. An individual who experiences this will associate ordered sequences with various personalities. Ordered sequences may include numbers, letters, months and so on. For example, a person with OLP may look at the letter ‘A’ and think in his mind that ‘A’ is a rude letter. Personally, I think it’s rather amusingly self-deprecating – but that’s just me. Check out Seattle graphic designer, Jesse Jaren’s unique portrayal of ordinal-linguistic synesthesia (shown in the graphic below) in this entertaining and informative blog post. You’ll agree, it’s pretty awesome.
In addition to thinking that certain ordinal sequences have a personality, a synesthete may also imbue a personality within an object. While occurrences have been reported early on, this is a type of synesthesia that has only gained attention from researchers in recent years.
Lexical-Gustatory SynesthesiaLexical-gustatory synesthesia is one of the more rare synesthesia types. Synesthetes who experience this kind of synesthesia evoke different kinds of tastes when they hear certain words or phonemes. According to research, associations between the words and what a synesthete is able taste are constrained by tastes he or she has experienced early in life. So, if an individual hasn’t had mashed potatoes or bacon, they won’t be tasting those flavors as a result of this variation. What a shame!
~ Again, these are just some of the more commonly reported forms. If you have questions about personal experiences (be they similar or dissimilar to those above), feel free to contact us! We’ll do our best to respond in a timely and informative fashion!
The first three articles here are review articles, examining the data we have to work with at this time.
Myrto I. Mylopoulos  and Tony Ro 
1. Philosophy Program, The Graduate Center, City University of New York, New York, NY, USASynesthesia is a fairly common condition in which individuals experience atypical responses (such as color experiences) in association with certain types of stimuli (such as non-colored letters). Although synesthesia has been described for centuries, only very recently has there been an explosive growth of systematic scientific examinations of this condition. In this article, we review and critically evaluate current methods for both assessing synesthesia and examining its psychological basis, including the “test-retest” procedure, online battery assessments, and behavioral experiments. We highlight the limitations of these methods for understanding the nature of this complex condition and propose potential solutions to address some of these limitations. We also provide a set of markers that aid in distinguishing synesthesia from other closely related psychological phenomena.
2. Department of Psychology, The City College and Graduate Center, Program in Cognitive Neuroscience, City University of New York, New York, NY, USA
Mylopoulos MI, and Ro T, (2013, Oct 22). Synesthesia: a colorful word with a touching sound? Frontiers in Psychology; 4:763. doi: 10.3389/fpsyg.2013.00763
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Affect-related synesthesias: a prospective view on their existence, expression and underlying mechanismsNele Dael, Guillaume Sierro and Christine Mohr
Institute of Psychology, University of Lausanne, Lausanne, SwitzerlandThe literature on developmental synesthesia has seen numerous sensory combinations, with surprisingly few reports on synesthesias involving affect. On the one hand, emotion, or more broadly affect, might be of minor importance to the synesthetic experience (e.g., Sinke et al., 2012). On the other hand, predictions on how affect could be relevant to the synesthetic experience remain to be formulated, in particular those that are driven by emotion theories. In this theoretical paper, we hypothesize that a priori studies on synesthesia involving affect will observe the following. Firstly, the synesthetic experience is not merely about discrete emotion processing or overall valence (positive, negative) but is determined by or even altered through cognitive appraisal processes. Secondly, the synesthetic experience changes temporarily on a quantitative level according to (i) the affective appraisal of the inducing stimulus or (ii) the current affective state of the individual. These hypotheses are inferred from previous theoretical and empirical accounts on synesthesia (including the few examples involving affect), different emotion theories, crossmodal processing accounts in synesthetes, and non-synesthetes, and the presumed stability of the synesthetic experience. We hope that the current review will succeed in launching a new series of studies on “affective synesthesias.” We particularly hope that such studies will apply the same creativity in experimental paradigms as we have seen and still see when assessing and evaluating “traditional” synesthesias.
Dael N, Sierro G, and Mohr C, (2013, Oct 18). Affect-related synesthesias: a prospective view on their existence, expression and underlying mechanisms. Frontiers in Psychology, 4:754. doi: 10.3389/fpsyg.2013.00754
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David P. Luke  and Devin B. Terhune 
1. Department of Counselling & Psychology, University of Greenwich, Eltham, UKDespite the general consensus that synaesthesia emerges at an early developmental stage and is only rarely acquired during adulthood, the transient induction of synaesthesia with chemical agents has been frequently reported in research on different psychoactive substances. Nevertheless, these effects remain poorly understood and have not been systematically incorporated. Here we review the known published studies in which chemical agents were observed to elicit synaesthesia. Across studies there is consistent evidence that serotonin agonists elicit transient experiences of synaesthesia. Despite convergent results across studies, studies investigating the induction of synaesthesia with chemical agents have numerous methodological limitations and little experimental research has been conducted. Cumulatively, these studies implicate the serotonergic system in synaesthesia and have implications for the neurochemical mechanisms underlying this phenomenon but methodological limitations in this research area preclude making firm conclusions regarding whether chemical agents can induce genuine synaesthesia.
2. Department of Experimental Psychology, University of Oxford, Oxford, UK
Luke DP, and Terhune DB. (2013, Oct 17). The induction of synaesthesia with chemical agents: a systematic review. Frontiers in Psychology, 4:753. doi: 10.3389/fpsyg.2013.00753
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These next five articles are all original research, adding to what we knew (or correcting what we thought we knew).
Combined structural and functional imaging reveals cortical deactivations in grapheme-color synaesthesiaErik O'Hanlon , Fiona N. Newell  and Kevin J. Mitchell 
1. School of Psychology and Institute of Neuroscience, Trinity College Dublin, Dublin, IrelandSynaesthesia 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.
2. Smurfit Institute of Genetics and Institute of Neuroscience, Trinity College Dublin, Dublin, Ireland
O'Hanlon E, Newell FN, and Mitchell KJ. (2013, Oct 30). Combined structural and functional imaging reveals cortical deactivations in grapheme-color synaesthesia. Frontiers in Psychology, 4:755. doi: 10.3389/fpsyg.2013.00755
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Enhanced recognition memory in grapheme-color synaesthesia for different categories of visual stimuliJamie Ward, Peter Hovard, Alicia Jones and Nicolas Rothen
Sackler Centre for Consciousness Science and School of Psychology, University of Sussex, Brighton, UKMemory has been shown to be enhanced in grapheme-color synaesthesia, and this enhancement extends to certain visual stimuli (that don't induce synaesthesia) as well as stimuli comprised of graphemes (which do). Previous studies have used a variety of testing procedures to assess memory in synaesthesia (e.g., free recall, recognition, associative learning) making it hard to know the extent to which memory benefits are attributable to the stimulus properties themselves, the testing method, participant strategies, or some combination of these factors. In the first experiment, we use the same testing procedure (recognition memory) for a variety of stimuli (written words, non-words, scenes, and fractals) and also check which memorization strategies were used. We demonstrate that grapheme-color synaesthetes show enhanced memory across all these stimuli, but this is not found for a non-visual type of synaesthesia (lexical-gustatory). In the second experiment, the memory advantage for scenes is explored further by manipulating the properties of the old and new images (changing color, orientation, or object presence). Again, grapheme-color synaesthetes show a memory advantage for scenes across all manipulations. Although recognition memory is generally enhanced in this study, the largest effects were found for abstract visual images (fractals) and scenes for which color can be used to discriminate old/new status.
Ward J, Hovard P, Jones A, and Rothen N. (2013, Oct 24). Enhanced recognition memory in grapheme-color synaesthesia for different categories of visual stimuli. Frontiers in Psychology, 4:762. doi:10.3389/fpsyg.2013.00762
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Olympia Colizoli, Jaap M. J. Murre and Romke Rouw
Department of Psychology, Brain and Cognition, University of Amsterdam, Amsterdam, Netherlands
Gustatory forms of synesthesia involve the automatic and consistent experience of tastes that are triggered by non-taste related inducers. We present a case of lexical-gustatory and sound-gustatory synesthesia within one individual, SC. Most words and a subset of non-linguistic sounds induce the experience of taste, smell and physical sensations for SC. SC's lexical-gustatory associations were significantly more consistent than those of a group of controls. We tested for effects of presentation modality (visual vs. auditory), taste-related congruency, and synesthetic inducer-concurrent direction using a priming task. SC's performance did not differ significantly from a trained control group. We used functional magnetic resonance imaging to investigate the neural correlates of SC's synesthetic experiences by comparing her brain activation to the literature on brain networks related to language, music, and sound processing, in addition to synesthesia. Words that induced a strong taste were contrasted to words that induced weak-to-no tastes (“tasty” vs. “tasteless” words). Brain activation was also measured during passive listening to music and environmental sounds. Brain activation patterns showed evidence that two regions are implicated in SC's synesthetic experience of taste and smell: the left anterior insula and left superior parietal lobe. Anterior insula activation may reflect the synesthetic taste experience. The superior parietal lobe is proposed to be involved in binding sensory information across sub-types of synesthetes. We conclude that SC's synesthesia is genuine and reflected in her brain activation. The type of inducer (visual-lexical, auditory-lexical, and non-lexical auditory stimuli) could be differentiated based on patterns of brain activity.
Colizoli O, Murre JMJ, and Rouw R. (2013, Oct 23). A taste for words and sounds: a case of lexical-gustatory and sound-gustatory synesthesia. Frontiers in Psychology, 4:775. doi: 10.3389/fpsyg.2013.00775
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Michelle Jarick , Mark T. Stewart , Daniel Smilek  and Michael J. Dixon 
1. Neurocognition of Attention and Perception Lab, Department of Psychology, MacEwan University, Edmonton, AB, CanadaTime-space synaesthetes “see” time units organized in a spatial form. While the structure might be invariant for most synaesthetes, the perspective by which some view their calendar is somewhat flexible. One well-studied synaesthete L adopts different viewpoints for months seen vs. heard. Interestingly, L claims to prefer her auditory perspective, even though the month names are represented visually upside down. To verify this, we used a spatial-cueing task that included audiovisual month cues. These cues were either congruent with L's preferred “auditory” viewpoint (auditory-only and auditory + month inverted) or incongruent (upright visual-only and auditory + month upright). Our prediction was that L would show enhanced cueing effects (larger response time difference between valid and invalid targets) following the audiovisual congruent cues since both elicit the “preferred” auditory perspective. Also, when faced with conflicting cues, we predicted L would choose the preferred auditory perspective over the visual perspective. As we expected, L did show enhanced cueing effects following the audiovisual congruent cues that corresponded with her preferred auditory perspective, but that the visual perspective dominated when L was faced with both viewpoints simultaneously. The results are discussed with relation to the reification hypothesis of sequence space synaesthesia (Eagleman, 2009).
2. Department of Psychology, Willamette University, Salem, OR, USA
3. Department of Psychology, University of Waterloo, Waterloo, ON, Canada
Jarick M, Stewart MT, Smilek D, and Dixon MJ. (2013, Oct 16). Do you see what I hear? Vantage point preference and visual dominance in a time-space synaesthete. Frontiers in Psychology, 4:695. doi: 10.3389/fpsyg.2013.00695
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Janina Nielsen , Tillmann H. C. Kruger , Uwe Hartmann , Torsten Passie [1,2], Thorsten Fehr  and Markus Zedler 
1. Department of Clinical Psychiatry, Social Psychiatry, and Psychotherapy, Hannover Medical School, Hannover, GermanyIntroduction: Synaesthesia is a phenomenon in which a certain stimulus induces a concurrent sensory perception; it has an estimated prevalence of 4%. Sexual arousal as an inducer for synaesthetic perceptions is rarely mentioned in the literature but can be found sometimes in case reports about subjective orgasmic experiences.
2. Department of Psychiatry, Harvard Medical School, Harvard University, Boston, MA, USA
3. Department of Neuropsychology, Center for Advanced Imaging Bremen/Magdeburg, Center for Cognitive Sciences, University Bremen, Bremen, Germany
Aims: To examine whether synaesthetic perceptions during sexual intercourse have an impact on the sexual experience and the extent of sexual trance compared to non-synaesthetes.
Methods: In total, 19 synaesthetes with sexual forms of synaesthesia (17 female; 2 male) were included as well as corresponding control data of 36 non-synaesthetic subjects (n = 55). Two questionnaires were used to assess relevant aspects of sexual function and dysfunction (a German adaption of the Brief Index of Sexual Functioning, KFSP) as well as the occurrence and extent of sexual trance (German version of the Altered States of Consciousness Questionnaire, OAVAV). Additionally qualitative interviews were conducted in some subjects to further explore the nature of sexual experiences in synaesthetes.
Main Outcome Measures: Sexual experience and extent of sexual trance during intercourse.
Results: Synaesthetes depicted significantly better overall sexual function on the KFSP with increased scores for the subscale “sexual appetence” but coevally significant lower subscale scores for “sexual satisfaction.” Sexual dysfunction was not detected in this sample. Synaesthetes depicted significantly higher levels of the subscales “oceanic boundlessness” and “visionary restructuralization” than controls using the OAVAV. Qualitative interviews revealed varying synaesthetic perceptions during the different states of arousal. Furthermore, synaesthetes reported an unsatisfactory feeling of isolation caused by the idiosyncratic perceptions.
Conclusions: Synaesthetes with sexual forms of synaesthesia seem to experience a deeper state of sexual trance without, however, enhanced satisfaction during sexual intercourse.
Nielsen J, Kruger THC, Hartmann U, Passie T, Fehr T, and Zedler M. (2013, Oct 16). Synaesthesia and sexuality: the influence of synaesthetic perceptions on sexual experience. Frontiers in Psychology, 4:751. doi: 10.3389/fpsyg.2013.00751