Math anxiety differentially impairs symbolic, but not nonsymbolic, fraction skills across development

Although important for the acquisition of later math skills, fractions are notoriously difficult. Previous studies have shown that higher math anxiety (MA) is associated with lower performance in symbolic fraction tasks in adults and have suggested that MA may negatively impact the acquisition of fractions in children. However, the effects of MA on fraction skills in school-aged children remain underexplored. We, therefore, investigated the impact of MA on the performance of younger (second and third graders) and older (fifth and sixth graders) children in math fluency (MF), written calculation, fraction knowledge (FK), and symbolic fraction and nonsymbolic ratio processing. On the basis of our prior work suggesting a perceptual foundation for fraction processing, we predicted that symbolic, but not nonsymbolic, math skills (especially fractions) would be impaired by MA. As predicted, higher MA was associated with lower performance in general mathematics achievement and symbolic fraction tasks, but nonsymbolic ratio processing was not affected by MA in either age group. Furthermore, working memory capacity partially mediated the effects of MA on general mathematics achievement, FK, and symbolic fraction processing. These results suggest that understanding the bidirectional interactions between MA and fractions may be important for helping children acquire these critical skills.

Taking the Relational Structure of Fractions Seriously: Relational Reasoning Predicts Fraction Knowledge in Elementary School Children

Understanding and using symbolic fractions in mathematics is critical for access to advanced STEM concepts. However, children and adults consistently struggle with fractions. Here, we take a novel perspective on symbolic fractions, considering them within the framework of relational structures in cognitive psychology, such as those studied in analogy research. We tested the hypothesis that relational reasoning ability is important for reasoning about fractions by examining the relation between scores on a domain-general test of relational reasoning (TORR Jr.) and a test of fraction knowledge consisting of various types of fraction problems in 194 second grade and 145 fifth grade students. We found that relational reasoning was a significant predictor of fractions knowledge, even when controlling for non-verbal IQ and fractions magnitude processing for both grades. The effects of relational reasoning also remained significant when controlling for overall mathematics knowledge and skill for second graders but was attenuated for fifth graders. These findings suggest that this important subdomain of mathematical cognition is integrally tied to relational reasoning and opens the possibility that instruction targeting relational reasoning may prove to be a viable avenue for improving children's fractions skills.

Registered Replication Report on Fischer, Castel, Dodd, and Pratt (2003)

The attentional spatial-numerical association of response codes (Att-SNARC) effect (Fischer, Castel, Dodd, & Pratt, 2003)—the finding that participants are quicker to detect left-side targets when the targets are preceded by small numbers and quicker to detect right-side targets when they are preceded by large numbers—has been used as evidence for embodied number representations and to support strong claims about the link between number and space (e.g., a mental number line). We attempted to replicate Experiment 2 of Fischer et al. by collecting data from 1,105 participants at 17 labs. Across all 1,105 participants and four interstimulus-interval conditions, the proportion of times the effect we observed was positive (i.e., directionally consistent with the original effect) was .50. Further, the effects we observed both within and across labs were minuscule and incompatible with those observed by Fischer et al. Given this, we conclude that we failed to replicate the effect reported by Fischer et al. In addition, our analysis of several participant-level moderators (finger-counting habits, reading and writing direction, handedness, and mathematics fluency and mathematics anxiety) revealed no substantial moderating effects. Our results indicate that the Att-SNARC effect cannot be used as evidence to support strong claims about the link between number and space.

No calculation necessary: Accessing magnitude through decimals and fractions

Research on how humans understand the relative magnitude of symbolic fractions presents a unique case of the symbol-grounding problem with numbers. Specifically, how do people access a holistic sense of rational number magnitude from decimal fractions (e.g. 0.125) and common fractions (e.g. 1/8)? Researchers have previously suggested that people cannot directly access magnitude information from common fraction notation, but instead must use a form of calculation to access this meaning. Questions remain regarding the nature of calculation and whether a division-like conversion to decimals is a necessary process that permits access to fraction magnitudes. To test whether calculation is necessary to access fractions magnitudes, we carried out a series of six parallel experiments in which we examined how adults access the magnitude of rational numbers (decimals and common fractions) under varying task demands. We asked adult participants to indicate which of two fractions was larger in three different conditions: decimal-decimal, fraction-fraction, and mixed decimal-fraction pairs. Across experiments, we manipulated two aspects of the task demands. 1) Response windows were limited to 1, 2 or 5 s, and 2) participants either did or did not have to identify when the two stimuli were the same magnitude (catch trials). Participants were able to successfully complete the task even at a response window of 1 s and showed evidence of holistic magnitude processing. These results indicate that calculation strategies with fractions are not necessary for accessing a sense of a fractions meaning but are strategic routes to magnitude that participants may use when granted sufficient time. We suggest that rapid magnitude processing with fractions and decimals may occur by mapping symbolic components onto common amodal mental representations of rational numbers.

Implicit and explicit spatial-numerical representations diverge in number-form synesthetes

In number-form (NF) synesthesia-a condition in which people report vivid, automatic and consistent mental layouts for numerical sequences-numbers and space are closely linked. These explicit associations are similar to the implicit associations demonstrated by the Spatial-Numerical Association of Response Codes (SNARC) effect. Thus, NF synesthesia offers a unique opportunity to investigate spatial-numerical associations. We tested implicit and explicit representations in NF synesthetes using a multiple case-study design. Over two sessions, synesthetes participated in a semi-structured interview focusing on the nature of their associations, as well as SNARC and number line estimation tasks. Contrary to our hypotheses, only one synesthete demonstrated SNARC effects congruent with her reported form, whereas two others exhibited SNARC effects that were the opposite of their explicit NFs. While this inconsistency between implicit and explicit representations may indicate separate underlying cognitive mechanisms, factors such as task-specific constraints and strategic variability must also be considered.

Individual Differences in Implicit and Explicit Spatial Processing of Fractions

Recent studies have explored the foundations of mathematical skills by linking basic numerical processes to formal tests of mathematics achievement. Of particular interest is the relationship between spatial-numerical associations-specifically, the Spatial Numerical Association of Response Codes (SNARC) effect-and various measures of math ability. Thus far, studies investigating this relationship have yielded inconsistent results. Here, we investigate how individual implicit and explicit spatial representations of fractions relate to fraction knowledge and other formal measures of math achievement. Adult participants (n = 105) compared the magnitude of single digit, irreducible fractions to ½, a task that has previously produced a reliable SNARC effect. We observed a significant group-level SNARC effect based on overall fraction magnitude, with notable individual variability. While individual SNARC effects were correlated with performance on a fraction number-line estimation (NLE) task, only NLE significantly predicted scores on a fractions test and basic standardized math test, even after controlling for IQ, mean accuracy, and mean reaction time. This suggests that-for fractions-working with an explicit number line is a stronger predictor of math ability than implicit number line processing. Neither individual SNARC effects nor NLE performance were significant predictors of algebra scores; thus, the mental number line may not be as readily recruited during higher-order mathematical concepts, but rather may be a foundation for thinking about simpler problems involving rational magnitudes. These results not only characterize the variability in adults' mental representations of fractions, but also detail the relative contributions of implicit (SNARC) and explicit (NLE) spatial representations of fractions to formal math skills.

A colorful advantage in iconic memory

Synesthesia is a benign neurodevelopmental condition in which stimulation of one sensory modality evokes experiences in a second, unstimulated modality (Simner and Hubbard, 2013). In grapheme-color synesthesia (GCS), which is experienced by 1-2% of adults, synesthetes reliably and involuntarily experience specific colors when viewing blackand-white graphemes. Previous case-studies have identified synesthetes with spectacular memory (Luria, 1968; Smilek, Dixon, Cudahy, & Merikle, 2001) and group studies have found advantages for synesthetes compared to nonsynesthetes in long-term memory (Rothen, Meier, & Ward, 2012). Here, we tested whether similar advantages are also present in earlier stages of memory. We tested visual iconic memory-the sensory store for vision-which has a very large capacity, but decays approximately 1000 ms after stimulus offset (Chow, 1985; Sergent et al., 2013; Sperling, 1960). We tested 20 synesthetes and 20 nonsynesthetes in a direct replication of the Sperling (1960) Partial Report Paradigm using letters (Experiment 1) and non-alphanumeric symbols (Experiment 2) as stimuli. Overall, synesthetes had a greater iconic memory capacity than nonsynesthetes when presented with synesthesia-inducing letter stimuli. This advantage was reduced when they were presented with non-synesthesia inducing symbol stimuli. Critically this advantage was most prominent when memory was stressed by asking participants to remember large arrays. Our results demonstrate that synesthetic memory advantages extend to the earliest stages of memory, and suggest that advantages in later stages of memory may arise from these earlier advantages.

On the genesis of spatial-numerical associations: Evolutionary and cultural factors co-construct the mental number line

Mapping numbers onto space is a common cognitive representation that has been explored in both behavioral and neuroimaging contexts. Empirical work probing the diverse nature of these spatial-numerical associations (SNAs) has led researchers to question 1) how the human brain links numbers with space, and 2) whether this link is biologically vs. culturally determined. We review the existing literature on the development of SNAs and situate that empirical work within cognitive and neuroscientific theoretical frameworks. We propose that an evolutionarily-ancient frontal-parietal circuit broadly tuned to multiple magnitude dimensions provides the phylogenetic substrate for SNAs, while enculturation and sensorimotor experience shape their specific profiles. We then use this perspective to discuss educational implications and highlight promising avenues for future research.

The fractions SNARC revisited: Processing fractions on a consistent mental number line

The ability to understand fractions is key to establishing a solid foundation in mathematics, yet children and adults struggle to comprehend them. Previous studies have suggested that these struggles emerge because people fail to process fraction magnitude holistically on the mental number line (MNL), focusing instead on fraction components. Subsequent studies have produced evidence for default holistic processing but examined only magnitude processing, not spatial representations. We explored the spatial representations of fractions on the MNL in a series of three experiments. Experiment 1 replicated Bonato et al.; 30 naïve undergraduates compared unit fractions (1/1-1/9) to 1/5, resulting in a reverse SNARC (Spatial-Numerical Association of Response Codes) effect. Experiment 2 countered potential strategic biases induced by the limited set of fractions used by Bonato et al. by expanding the stimulus set to include all irreducible, single-digit proper fractions and asked participants to compare them against 1/2. We observed a classic SNARC effect, completely reversing the pattern from Experiment 1. Together, Experiments 1 and 2 demonstrate that stimulus properties dramatically impact spatial representations of fractions. In Experiment 3, we demonstrated within-subjects reliability of the SNARC effect across both a fractions and whole number comparison task. Our results suggest that adults can indeed process fraction magnitudes holistically, and that their spatial representations occur on a consistent MNL for both whole numbers and fractions.

Making Space for Spatial Proportions

The three target articles presented in this special issue converged on an emerging theme: the importance of spatial proportional reasoning. They suggest that the ability to map between symbolic fractions (like 1/5) and nonsymbolic, spatial representations of their sizes or magnitudes may be especially important for building robust fractions knowledge. In this commentary, we first reflect upon where these findings stand in a larger theoretical context, largely borrowed from mathematics education research. Next, we emphasize parallels between this work and emerging work suggesting that nonsymbolic proportional reasoning may provide an intuitive foundation for understanding fraction magnitudes. Finally, we end by exploring some open questions that suggest specific future directions in this burgeoning area.

Individual Differences in Nonsymbolic Ratio Processing Predict Symbolic Math Performance

What basic capacities lay the foundation for advanced numerical cognition? Are there basic nonsymbolic abilities that support the understanding of advanced numerical concepts, such as fractions? To date, most theories have posited that previously identified core numerical systems, such as the approximate number system (ANS), are ill-suited for learning fraction concepts. However, recent research in developmental psychology and neuroscience has revealed a ratio-processing system (RPS) that is sensitive to magnitudes of nonsymbolic ratios and may be ideally suited for supporting fraction concepts. We provide evidence for this hypothesis by showing that individual differences in RPS acuity predict performance on four measures of mathematical competence, including a university entrance exam in algebra. We suggest that the nonsymbolic RPS may support symbolic fraction understanding much as the ANS supports whole-number concepts. Thus, even abstract mathematical concepts, such as fractions, may be grounded not only in higher-order logic and language, but also in basic nonsymbolic processing abilities.

Rates of white matter hyperintensities compatible with the radiological profile of multiple sclerosis within self-referred synesthete populations

Synesthesia is an inherited condition causing unusual secondary sensations (e.g, sounds might be experienced as both auditory and visual percepts). The condition has been linked with cognitive and perceptual benefits and is considered a benign alternative form of perception. Here, we investigate self-referred synesthete populations and their rates of radiologically determined white matter hyperintensities (WMH) of a type compatible with the McDonald imaging criteria for the diagnosis of multiple sclerosis (MS). MS is a chronic condition resulting in damage to myelination surrounding nerve fibers of the central nervous system (CNS). Magnetic resonance imaging (MRI) features highly suggestive of MS without overt clinical symptoms are termed radiologically isolated syndrome (RIS). We present data showing that the shared MRI profile of MS and RIS has been significantly overrepresented in synesthetes who have participated in neuroimaging research. We present validation of the clinical and MRI status of these synesthetes and an analysis showing the significant probability their unusual numbers may not have arisen by chance. We discuss how to interpret significant data based on small case numbers and consider the implications of our findings for synesthesia's clinical status.

The Organization of Spatial Reference Frames Involved in the Snarc Effect

The SNARC effect refers to faster reaction times for larger numbers with right-sided responses and for smaller numbers with left-sided responses, even when numerical magnitude is irrelevant. Although the SNARC is generally thought to reflect a mapping between numbers and space, the question of which spatial reference frame(s) are critical for the effect has not been systematically explored. We propose a dynamic hierarchical organization of the reference frames (from a global left–right frame to body- and object-related frames), where the influence of each frame can be modulated by experimental context. We conducted two experiments based on predictions derived from this organizational system. Experiment 1 compared instructions that differed only in focusing participants’ attention on either the response buttons or the hands. Instructions focusing on a hand-based reference frame eliminated the SNARC. Experiment 2 provided the opportunity for an object-centred reference frame to manifest itself in the SNARC. Although we did not observe an effect of an object-centred reference frame, we observed the influence of other reference frames in a context where an object-centred reference frame was emphasized. Altogether, these results support the proposed organization of the reference frames.

The cognitive mechanisms of the SNARC effect: an individual differences approach

Access to mental representations of smaller vs. larger number symbols is associated with leftward vs. rightward spatial locations, as represented on a number line. The well-replicated SNARC effect (Spatial-Numerical Association of Response Codes) reveals that simple decisions about small numbers are facilitated when stimuli are presented on the left, and large numbers facilitated when on the right. We present novel evidence that the size of the SNARC effect is relatively stable within individuals over time. This enables us to take an individual differences approach to investigate how the SNARC effect is modulated by spatial and numerical cognition. Are number-space associations linked to spatial operations, such that those who have greater facility in spatial computations show the stronger SNARC effects, or are they linked to number semantics, such that those showing stronger influence of magnitude associations on number symbol decisions show stronger SNARC effects? Our results indicate a significant correlation between the SNARC effect and a 2D mental rotation task, suggesting that spatial operations are at play in the expression of this effect. We also uncover a significant correlation between the SNARC effect and the distance effect, suggesting that the SNARC is also related to access to number semantics. A multiple regression analysis reveals that the relative contributions of spatial cognition and distance effects represent significant, yet distinct, contributions in explaining variation in the size of the SNARC effect from one individual to the next. Overall, these results shed new light on how the spatial-numerical associations of response codes are influenced by both number semantics and spatial operations.

Developmental changes in mu suppression to observed and executed actions in autism spectrum disorders

There has been debate over whether disruptions in the mirror neuron system (MNS) play a key role in the core social deficits observed in autism spectrum disorders (ASD). EEG mu suppression during the observation of biological actions is believed to reflect MNS functioning, but understanding of the developmental progression of the MNS and EEG mu rhythm in both typical and atypical development is lacking. To provide a more thorough and direct exploration of the development of mu suppression in individuals with ASD, a sample of 66 individuals with ASD and 51 typically developing individuals of 6-17 years old were pooled from four previously published studies employing similar EEG methodology. We found a significant correlation between age and mu suppression in response to the observation of actions, both for individuals with ASD and typical individuals. This relationship was not seen during the execution of actions. Additionally, the strength of the correlation during the observation of actions did not significantly differ between groups. The results provide evidence against the argument that mirror neuron dysfunction improves with age in individuals with ASD and suggest, instead, that a diagnosis-independent developmental change may be at the root of the correlation of age and mu suppression.

Number line compression and the illusory perception of random numbers

Developmental studies indicate that children initially possess a compressed intuition of numerical distances, in which larger numbers are less discriminable than small ones. Education then "linearizes" this responding until by about age eight, children become able to map symbolic numerals onto a linear spatial scale. However, this illusion of compression of symbolic numerals may still exist in a dormant form in human adults and may be observed in appropriate experimental contexts. To investigate this issue, we asked adult participants to rate whether a random sequence of numbers contained too many small numbers or too many large ones. Participants exhibited a large bias, judging as random a geometric series that actually oversampled small numbers, consistent with a compression of large numbers. This illusion resisted training on a number-space mapping task, even though performance was linear on this task. While the illusion was moderately reduced by explicit exposure to linear sequences, responding was still significantly compressed. Thus, the illusion of compression is robust in this task, but linear and compressed responding can be exhibited in the same participants depending on the experimental context.

Superior size-weight illusion performance in patients with schizophrenia: evidence for deficits in forward models

When non-psychiatric individuals compare the weights of two similar objects of identical mass, but of different sizes, the smaller object is often perceived as substantially heavier. This size-weight illusion (SWI) is thought to be generated by a violation of the common expectation that the large object will be heavier, possibly via a mismatch between an efference copy of the movement and the actual sensory feedback received. As previous research suggests that patients with schizophrenia have deficits in forward model/efference copy mechanisms, we hypothesized that schizophrenic patients would show a reduced SWI. The current study compared the strength of the SWI in schizophrenic patients to matched non-psychiatric participants; weight discrimination for same-sized objects was also assessed. We found a reduced SWI for schizophrenic patients, which resulted in better (more veridical) weight discrimination performance on illusion trials compared to non-psychiatric individuals. This difference in the strength of the SWI persisted when groups were matched for weight discrimination performance. The current findings are consistent with a dysfunctional forward model mechanism in this population. Future studies to elucidate the locus of this impairment using variations on the current study are also proposed.

Neurophysiology of synesthesia

Synesthesia is an experience in which stimulation in one sensory or cognitive stream leads to associated experiences in a second, unstimulated stream. Although synesthesia is often referred to as a "neurological condition," it is not listed in the DSM IV or the ICD classifications, as it generally does not interfere with normal daily functioning. However, its high prevalence rate (one in 23) means that synesthesia may be reported by patients who present with other psychiatric symptoms. In this review, I focus on recent research examining the neural basis of the two most intensively studied forms of synesthesia, grapheme --> color synesthesia and tone --> color synesthesia. These data suggest that these forms of synesthesia are elicited through anomalous activation of color-selective areas, perhaps in concert with hyperbinding mediated by the parietal cortex. I then turn to questions for future research and the implications of these models for other forms of synesthesia.

Variants of synesthesia interact in cognitive tasks: Evidence for implicit associations and late connectivity in cross-talk theories

This study examines the interaction between two types of synesthesia: ordinal linguistic personification (OLP; the involuntary association of animate qualities such as gender/personality to linguistic units such as letters/numbers/days) and grapheme-color synesthesia (the involuntary association of colors to letters and/or numbers). By examining both variants in the same individual we aim to: (a) show that features of different synesthetic variants interact in cognitive tasks, (b) provide a cognitive model of this interaction, and (c) constrain models of the underlying neurological roots of this connectivity. Studies have shown inhibition in Stroop-type tasks for naming font colors that clash with synesthetic colors (e.g. slower naming of green font for synesthetically red letters). We show that Stroop-type slow-down occurs only when incongruent colors come from other letters with matching (but not mis-matching) gender (experiment 2). We also measure the speed of OLP gender judgments (e.g. a=female; experiment 1) and show that response times are slowed by incongruent colors from other letters with mis-matching (but not matching) genders. Our studies suggest that synesthetic variants interact and that their concurrents can become implicitly connected without mediation from inducing stimuli. We interpret these findings in light of recent developmental data showing protracted heterochronous neuronal development in humans, which continues through adolescence in parietal, frontal and perisylvian areas.

Neurocognitive mechanisms of synesthesia

Synesthesia is a condition in which stimulation of one sensory modality causes unusual experiences in a second, unstimulated modality. Although long treated as a curiosity, recent research with a combination of phenomenological, behavioral, and neuroimaging methods has begun to identify the cognitive and neural basis of synesthesia. Here, we review this literature with an emphasis on grapheme-color synesthesia, in which viewing letters and numbers induces the perception of colors. We discuss both the substantial progress that has been made in the past fifteen years and some open questions. In particular, we focus on debates in the field relating to the neural basis of synesthesia, including the relationship between synesthesia and attention and the role of meaning in synesthetic colors. We propose that some, but probably not all, of these differences can be accounted for by differences in the synesthetes studied and discuss some methodological implications of these individual differences.

Contrast Affects the Strength of Synesthetic Colors

Grapheme-color synesthesia is an automatic, involuntary experience of seeing colors when viewing numbers, letters or words on a printed page. Previous research has demonstrated that synesthesia is a genuine perceptual phenomenon, but crucially, all of these experiments have used high-contrast letters and numbers. Our synesthete, JC, anecdotally reported that the strength of his synesthetic colors varied depending on whether the graphemes were presented in high or low contrast. To test this, we asked JC to rate the strength of his experiences to letters of different contrasts on three different dates. JC's ratings of the strength of his synesthetic colors consistently declined monotonically with contrast, suggesting that his synesthetic colors were reduced or absent at low contrasts. To more precisely quantify the impact of this, we then tested JC on modified versions of our embedded figures task (Ramachandran and Hubbard, 2001a) and crowding task (Ramachandran and Hubbard, 2001b) by presenting displays with varying contrast between the graphemes and the background. Behavioral data in the contrast variant of our embedded figures task showed that JC performed significantly better than controls at high contrast, replicating our previous findings. However, at low contrast this advantage was eliminated, consistent with his reports of weaker or absent colors. A similar, but weaker pattern of results was found in the modified version of our crowding task. These results suggest that JC's synesthetic colors may be elicited at contrast dependent stages of visual processing. We propose that regions of the fusiform gyrus specialized for letter and number grapheme recognition that have been shown to respond in a contrast dependent manner mediate JC's synesthetic colors. However, whether this is true for all grapheme-color synesthetes or is only true of the group we refer to as lower synesthetes, remains to be seen.

EEG evidence for mirror neuron dysfunction in autism spectrum disorders

Autism spectrum disorders (ASD) are largely characterized by deficits in imitation, pragmatic language, theory of mind, and empathy. Previous research has suggested that a dysfunctional mirror neuron system may explain the pathology observed in ASD. Because EEG oscillations in the mu frequency (8-13 Hz) over sensorimotor cortex are thought to reflect mirror neuron activity, one method for testing the integrity of this system is to measure mu responsiveness to actual and observed movement. It has been established that mu power is reduced (mu suppression) in typically developing individuals both when they perform actions and when they observe others performing actions, reflecting an observation/execution system which may play a critical role in the ability to understand and imitate others' behaviors. This study investigated whether individuals with ASD show a dysfunction in this system, given their behavioral impairments in understanding and responding appropriately to others' behaviors. Mu wave suppression was measured in ten high-functioning individuals with ASD and ten age- and gender-matched control subjects while watching videos of (1) a moving hand, (2) a bouncing ball, and (3) visual noise, or (4) moving their own hand. Control subjects showed significant mu suppression to both self and observed hand movement. The ASD group showed significant mu suppression to self-performed hand movements but not to observed hand movements. These results support the hypothesis of a dysfunctional mirror neuron system in high-functioning individuals with ASD.

Interactions between number and space in parietal cortex

Since the time of Pythagoras, numerical and spatial representations have been inextricably linked. We suggest that the relationship between the two is deeply rooted in the brain's organization for these capacities. Many behavioural and patient studies have shown that numerical-spatial interactions run far deeper than simply cultural constructions, and, instead, influence behaviour at several levels. By combining two previously independent lines of research, neuroimaging studies of numerical cognition in humans, and physiological studies of spatial cognition in monkeys, we propose that these numerical-spatial interactions arise from common parietal circuits for attention to external space and internal representations of numbers.

Individual differences among grapheme-color synesthetes: brain-behavior correlations

Grapheme-color synesthetes experience specific colors associated with specific number or letter characters. To determine the neural locus of this condition, we compared behavioral and fMRI responses in six grapheme-color synesthetes to control subjects. In our behavioral experiments, we found that a subject's synesthetic experience can aid in texture segregation (experiment 1) and reduce the effects of crowding (experiment 2). For synesthetes, graphemes produced larger fMRI responses in color-selective area human V4 than for control subjects (experiment 3). Importantly, we found a correlation within subjects between the behavioral and fMRI results; subjects with better performance on the behavioral experiments showed larger fMRI responses in early retinotopic visual areas (V1, V2, V3, and hV4). These results suggest that grapheme-color synesthesia is the result of cross-activation between grapheme-selective and color-selective brain areas. The correlation between the behavioral and fMRI results suggests that grapheme-color synesthetes may constitute a heterogeneous group.

Psychophysical investigations into the neural basis of synaesthesia

We studied two otherwise normal, synaesthetic subjects who 'saw' a specific colour every time they saw a specific number or letter. We conducted four experiments in order to show that this was a genuine perceptual experience rather than merely a memory association. (i) The synaesthetically induced colours could lead to perceptual grouping, even though the inducing numerals or letters did not. (ii) Synaesthetically induced colours were not experienced if the graphemes were presented peripherally. (iii) Roman numerals were ineffective: the actual number grapheme was required. (iv) If two graphemes were alternated the induced colours were also seen in alternation. However, colours were no longer experienced if the graphemes were alternated at more than 4 Hz. We propose that grapheme colour synaesthesia arises from 'cross-wiring' between the 'colour centre' (area V4 or V8) and the 'number area', both of which lie in the fusiform gyrus. We also suggest a similar explanation for the representation of metaphors in the brain: hence, the higher incidence of synaesthesia among artists and poets.

Psychophysical investigations into the neural basis of synaesthesia

We studied two otherwise normal, synaesthetic subjects who 'saw' a specific colour every time they saw a specific number or letter. We conducted four experiments in order to show that this was a genuine perceptual experience rather than merely a memory association. (i) The synaesthetically induced colours could lead to perceptual grouping, even though the inducing numerals or letters did not. (ii) Synaesthetically induced colours were not experienced if the graphemes were presented peripherally. (iii) Roman numerals were ineffective: the actual number grapheme was required. (iv) If two graphemes were alternated the induced colours were also seen in alternation. However, colours were no longer experienced if the graphemes were alternated at more than 4 Hz. We propose that grapheme colour synaesthesia arises from 'cross-wiring' between the 'colour centre' (area V4 or V8) and the 'number area', both of which lie in the fusiform gyrus. We also suggest a similar explanation for the representation of metaphors in the brain: hence, the higher incidence of synaesthesia among artists and poets.