Synesthesia strengthens sound-symbolic cross-modal correspondences

PHONETIC UNDERPINNINGS OF SOUND SYMBOLISM ACROSS MULTIPLE DOMAINS OF MEANING

Sound symbolism occurs when the sound of a word alone can convey its meaning, e.g. 'balloon' and 'spike' sound rounded and pointed, respectively. Sound-symbolic correspondences are widespread in natural languages, but it is unclear how they are instantiated across different domains of meaning. Here, participants rated auditory pseudowords on opposing scales of seven different sound-symbolic domains: shape (rounded-pointed), texture (hard-soft), weight (light-heavy), size (small-big), brightness (bright-dark), arousal (calming-exciting), and valence (good-bad). Ratings showed cross-domain relationships, some mirroring those between corresponding physical domains, e.g. size and weight ratings were associated, reflecting a physical size-weight relationship, while others involved metaphorical mappings, e.g., bright/dark mapped onto good/bad, respectively. The phonetic features of the pseudowords formed unique sets with characteristic feature weightings for each domain and tended to follow the cross-domain ratings relationships. These results suggest that sound-symbolic correspondences rely on domain-specific patterns of phonetic features, with cross-domain correspondences reflecting physical or metaphorical relationships.

Perceptual Dissimilarity Analysis Distinguishes Grapheme-Color Synesthetes from Nonsynesthetes

Synesthetes can be distinguished from nonsynesthetes on a variety of experimental tasks because their concurrent synesthetic experiences can affect task performance if these experiences match or conflict with some aspect of the stimulus. Here, we tested grapheme-color synesthetes and nonsynesthetic control participants using a novel perceptual similarity task to assess whether synesthetes' concurrent color experiences influence perceived grapheme similarity. Participants iteratively arranged graphemes and, separately, their associated synesthetic colors in a display, such that similar items were placed close together and dissimilar items further apart. The resulting relative inter-item distances were used to calculate the pair-wise (dis)similarity between items in the set, and thence to create separate perceptual representational dissimilarity matrices (RDMs) for graphemes and colors, on an individual basis. On the assumption that synesthetes' similarity judgments for graphemes would be influenced by their concurrent color experiences, we predicted that grapheme and color RDMs would be more strongly correlated for synesthetes than nonsynesthetes. We found that the mean grapheme-color RDM correlation was indeed significantly higher in synesthetes than nonsynesthetes; in addition, synesthetes' grapheme-color RDM correlations were more likely to be individually statistically significant, even after correction for multiple tests, than those of nonsynesthetes. Importantly, synesthetes' grapheme-color RDM correlations were scaled with the consistency of their grapheme-color associations as measured by their Synesthesia Battery (SB) scores. By contrast, the relationship between SB scores and grapheme-color RDM correlations for nonsynesthetes was not significant. Thus, dissimilarity analysis quantitatively distinguished synesthetes from nonsynesthetes, in a way that meaningfully reflects a key aspect of synesthetic experience.

Exploring Group Differences in the Crossmodal Correspondences

There has been a rapid growth of interest amongst researchers in the cross-modal correspondences in recent years. In part, this has resulted from the emerging realization of the important role that the correspondences can sometimes play in multisensory integration. In turn, this has led to an interest in the nature of any differences between individuals, or rather, between groups of individuals, in the strength and/or consensuality of cross-modal correspondences that may be observed in both neurotypically normal groups cross-culturally, developmentally, and across various special populations (including those who have lost a sense, as well as those with autistic tendencies). The hope is that our emerging understanding of such group differences may one day provide grounds for supporting the reality of the various different types of correspondence that have so far been proposed, namely structural, statistical, semantic, and hedonic (or emotionally mediated).

Neural Basis of the Sound-Symbolic Crossmodal Correspondence Between Auditory Pseudowords and Visual Shapes

Sound symbolism refers to the association between the sounds of words and their meanings, often studied using the crossmodal correspondence between auditory pseudowords, e.g., 'takete' or 'maluma', and pointed or rounded visual shapes, respectively. In a functional magnetic resonance imaging study, participants were presented with pseudoword-shape pairs that were sound-symbolically congruent or incongruent. We found no significant congruency effects in the blood oxygenation level-dependent (BOLD) signal when participants were attending to visual shapes. During attention to auditory pseudowords, however, we observed greater BOLD activity for incongruent compared to congruent audiovisual pairs bilaterally in the intraparietal sulcus and supramarginal gyrus, and in the left middle frontal gyrus. We compared this activity to independent functional contrasts designed to test competing explanations of sound symbolism, but found no evidence for mediation via language, and only limited evidence for accounts based on multisensory integration and a general magnitude system. Instead, we suggest that the observed incongruency effects are likely to reflect phonological processing and/or multisensory attention. These findings advance our understanding of sound-to-meaning mapping in the brain.

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.

Visual and Proprioceptive Perceptions Evoke Motion-Sound Symbolism: Different Acceleration Profiles Are Associated With Different Types of Consonants

A growing body of literature has shown that one perceptual modality can be systematically associated with sensation in another. However, the cross-modal relationship between linguistic sounds and motions (i.e., motion-sound symbolism) is an extremely understudied area of research. Against this background, this paper examines the cross-modal correspondences between categories of consonants on one hand and different acceleration profiles of motion stimuli on the other. In the two experiments that we conducted, we mechanically manipulated the acceleration profiles of the stimuli while holding the trajectory paths constant, thus distinguishing the effect of acceleration profiles from that of motion path shapes. The results show that different acceleration profiles can be associated with different types of consonants; in particular, movements with acceleration and deceleration tend to be associated with a class of sounds called obstruents, whereas movements without much acceleration tend to be associated with a class of sounds called sonorants. Moreover, the current experiments show that this sort of cross-modal correspondence arises even when the stimuli are not presented visually, namely, when the participants' hands were moved passively by a manipulandum. In conclusion, the present study adds an additional piece of evidence demonstrating that bodily action-based information, i.e., proprioception as a very feasible candidate, could lead to sound symbolic patterns.

Stimulus Parameters Underlying Sound-Symbolic Mapping of Auditory Pseudowords to Visual Shapes

Sound symbolism refers to non-arbitrary mappings between the sounds of words and their meanings and is often studied by pairing auditory pseudowords such as "maluma" and "takete" with rounded and pointed visual shapes, respectively. However, it is unclear what auditory properties of pseudowords contribute to their perception as rounded or pointed. Here, we compared perceptual ratings of the roundedness/pointedness of large sets of pseudowords and shapes to their acoustic and visual properties using a novel application of representational similarity analysis (RSA). Representational dissimilarity matrices (RDMs) of the auditory and visual ratings of roundedness/pointedness were significantly correlated crossmodally. The auditory perceptual RDM correlated significantly with RDMs of spectral tilt, the temporal fast Fourier transform (FFT), and the speech envelope. Conventional correlational analyses showed that ratings of pseudowords transitioned from rounded to pointed as vocal roughness (as measured by the harmonics-to-noise ratio, pulse number, fraction of unvoiced frames, mean autocorrelation, shimmer, and jitter) increased. The visual perceptual RDM correlated significantly with RDMs of global indices of visual shape (the simple matching coefficient, image silhouette, image outlines, and Jaccard distance). Crossmodally, the RDMs of the auditory spectral parameters correlated weakly but significantly with those of the global indices of visual shape. Our work establishes the utility of RSA for analysis of large stimulus sets and offers novel insights into the stimulus parameters underlying sound symbolism, showing that sound-to-shape mapping is driven by acoustic properties of pseudowords and suggesting audiovisual cross-modal correspondence as a basis for language users' sensitivity to this type of sound symbolism.

Crossmodal Visuospatial Effects on Auditory Perception of Musical Contour

The crossmodal correspondence between auditory pitch and visuospatial elevation (in which high- and low-pitched tones are associated with high and low spatial elevation respectively) has been proposed as the basis for Western musical notation. One implication of this is that music perception engages visuospatial processes and may not be exclusively auditory. Here, we investigated how music perception is influenced by concurrent visual stimuli. Participants listened to unfamiliar five-note musical phrases with four kinds of pitch contour (rising, falling, rising-falling, or falling-rising), accompanied by incidental visual contours that were either congruent (e.g., auditory rising/visual rising) or incongruent (e.g., auditory rising/visual falling) and judged whether the final note of the musical phrase was higher or lower in pitch than the first. Response times for the auditory judgment were significantly slower for incongruent compared to congruent trials, i.e., there was a congruency effect, even though the visual contours were incidental to the auditory task. These results suggest that music perception, although generally regarded as an auditory experience, may actually be multisensory in nature.

Crossmodal Association Between Linguistic Sounds and Motion Imagery: Voicing in Obstruents Connects With Different Strengths of Motor Execution

The present study investigates whether obstruent voicing may or may not affect the imagery of different strengths of motor execution. In a modified version of the implicit association test, participants responded to discrimination tasks that include viewing static pictures of athletes in motion and hearing mono-syllabic linguistic sounds. The results suggest that voiced obstruents are compatible with the motion imagery that implies stronger motor executions, whereas voiceless obstruents are compatible with the imagery that implies weaker motor executions. These results provide experimental support for crossmodal associations between the auditory perception of linguistic sounds, namely, the voicing of obstruents, and the visually induced imagery of different levels of strength in motor actions.

Implicit associations between individual properties of color and sound

We report a series of 22 experiments in which the implicit associations test (IAT) was used to investigate cross-modal correspondences between visual (luminance, hue [R-G, B-Y], saturation) and acoustic (loudness, pitch, formants [F1, F2], spectral centroid, trill) dimensions. Colors were sampled from the perceptually accurate CIE-Lab space, and the complex, vowel-like sounds were created with a formant synthesizer capable of separately manipulating individual acoustic properties. In line with previous reports, the loudness and pitch of acoustic stimuli were associated with both luminance and saturation of the presented colors. However, pitch was associated specifically with color lightness, whereas loudness mapped onto greater visual saliency. Manipulating the spectrum of sounds without modifying their pitch showed that an upward shift of spectral energy was associated with the same visual features (higher luminance and saturation) as higher pitch. In contrast, changing formant frequencies of synthetic vowels while minimizing the accompanying shifts in spectral centroid failed to reveal cross-modal correspondences with color. This may indicate that the commonly reported associations between vowels and colors are mediated by differences in the overall balance of low- and high-frequency energy in the spectrum rather than by vowel identity as such. Surprisingly, the hue of colors with the same luminance and saturation was not associated with any of the tested acoustic features, except for a weak preference to match higher pitch with blue (vs. yellow). We discuss these findings in the context of previous research and consider their implications for sound symbolism in world languages.

The cerebral bases of the bouba-kiki effect

The crossmodal correspondence between some speech sounds and some geometrical shapes, known as the bouba-kiki (BK) effect, constitutes a remarkable exception to the general arbitrariness of the links between word meaning and word sounds. We have analyzed the association of shapes and sounds in order to determine whether it occurs at a perceptual or at a decisional level, and whether it takes place in sensory cortices or in supramodal regions. First, using an Implicit Association Test (IAT), we have shown that the BK effect may occur without participants making any explicit decision relative to sound-shape associations. Second, looking for the brain correlates of implicit BK matching, we have found that intermodal matching influences activations in both auditory and visual sensory cortices. Moreover, we found stronger prefrontal activation to mismatching than to matching stimuli, presumably reflecting a modulation of executive processes by crossmodal correspondence. Thus, through its roots in the physiology of object categorization and crossmodal matching, the BK effect provides a unique insight into some non-linguistic components of word formation.

On the Relative Nature of (Pitch-Based) Crossmodal Correspondences

This review deals with the question of the relative vs absolute nature of crossmodal correspondences, with a specific focus on those correspondences involving the auditory dimension of pitch. Crossmodal correspondences have been defined as the often-surprising crossmodal associations that people experience between features, attributes, or dimensions of experience in different sensory modalities, when either physically present, or else merely imagined. In the literature, crossmodal correspondences have often been contrasted with synaesthesia in that the former are frequently said to be relative phenomena (e.g., it is the higher-pitched of two sounds that is matched with the smaller of two visual stimuli, say, rather than there being a specific one-to-one crossmodal mapping between a particular pitch of sound and size of object). By contrast, in the case of synaesthesia, the idiosyncratic mapping between inducer and concurrent tends to be absolute (e.g., it is a particular sonic inducer that elicits a specific colour concurrent). However, a closer analysis of the literature soon reveals that the distinction between relative and absolute in the case of crossmodal correspondences may not be as clear-cut as some commentators would have us believe. Furthermore, it is important to note that the relative vs absolute question may receive different answers depending on the particular (class of) correspondence under empirical investigation.

Neural basis of the crossmodal correspondence between auditory pitch and visuospatial elevation

Crossmodal correspondences refer to associations between otherwise unrelated stimulus features in different sensory modalities. For example, high and low auditory pitches are associated with high and low visuospatial elevation, respectively. The neural mechanisms underlying crossmodal correspondences are currently unknown. Here, we used functional magnetic resonance imaging (fMRI) to investigate the neural basis of the pitch-elevation correspondence. Pitch-elevation congruency effects were observed bilaterally in the inferior frontal and insular cortex, the right frontal eye field and right inferior parietal cortex. Independent functional localizers failed to provide strong evidence for any of three proposed mechanisms for crossmodal correspondences: semantic mediation, magnitude estimation, and multisensory integration. Instead, pitch-elevation congruency effects overlapped with areas selective for visually presented non-word strings relative to sentences, and with regions sensitive to audiovisual asynchrony. Taken together with the prior literature, the observed congruency effects are most consistent with mediation by multisensory attention.

Interactions Between Auditory Elevation, Auditory Pitch and Visual Elevation During Multisensory Perception

Cross-modal correspondences refer to associations between apparently unrelated stimulus features in different senses. For example, high and low auditory pitches are associated with high and low visual elevations, respectively. Here we examined how this crossmodal correspondence between visual elevation and auditory pitch relates to auditory elevation. We used audiovisual combinations of high- or low-frequency bursts of white noise and a visual stimulus comprising a white circle. Auditory and visual stimuli could each occur at high or low elevations. These multisensory stimuli could be congruent or incongruent for three correspondence types: cross-modal featural (auditory pitch/visual elevation), within-modal featural (auditory pitch/auditory elevation) and cross-modal spatial (auditory and visual elevation). Participants performed a 2AFC speeded classification (high or low) task while attending to auditory pitch, auditory elevation, or visual elevation. We tested for modulatory interactions between the three correspondence types. Modulatory interactions were absent when discriminating visual elevation. However, the within-modal featural correspondence affected the cross-modal featural correspondence during discrimination of auditory elevation and pitch, while the reverse modulation was observed only during discrimination of auditory pitch. The cross-modal spatial correspondence modulated the other two correspondences only when auditory elevation was being attended, was modulated by the cross-modal featural correspondence only during attention to auditory pitch, and was modulated by the within-modal featural correspondence while performing discrimination of either auditory elevation or pitch. We conclude that the cross-modal correspondence between auditory pitch and visual elevation interacts strongly with auditory elevation.

Introduction to the Special Issue on Individual Differences in Multisensory Perception: an Overview

The world is full of objects that can be perceived through multiple different senses to create an integrated understanding of our environment. Since each of us has different biological and psychological characteristics, different people may perceive the world in quite different ways. However, the questions of how and why our multisensory perceptions differ have not been explored in any great depth. This special issue, arising from a series of British Psychological Society-funded seminars, presents new research and opinions on the impacts of a variety of individual differences on multisensory perception. We hope that readers will enjoy this collection of eight papers on individual differences in multisensory perception arising from developmental changes, autism, Down syndrome, migraine, sensory loss and substitution, and personality.