Edward M. Hubbard

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 brains 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.

Travel broadens the mind.

The onset of locomotion heralds one of the major life transitions in early development and involves a pervasive set of changes in perception, spatial cognition, and social and emotional development. Through a synthesis of published and hitherto unpublished findings, gathered from a number of converging research designs and methods, this article provides a comprehensive review and reanalysis of the consequences of self-produced locomotor experience. Specifically, we focus on the role of locomotor experience in changes in social and emotional development, referential gestural communication, wariness of heights, the perception of self-motion, distance perception, spatial search, and spatial coding strategies. Our analysis reveals new insights into the specific processes by which locomotor experience brings about psychological changes. We elaborate these processes and provide new predictions about previously unsuspected links between locomotor experience and psychological function. The research we describe is relevant to our broad understanding of the developmental process, particularly as it pertains to developmental transitions. Although acknowledging the role of genetically mediated developmental changes, our viewpoint is a transactional one in which a single acquisition, in this case the onset of locomotion, sets in motion a family of experiences and processes that in turn mobilize both broad-based and context-specific psychological reorganizations. We conclude that, in infancy, the onset of locomotor experience brings about widespread consequences, and after infancy, can be responsible for an enduring role in development by maintaining and updating existing skills.

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 subjects 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.

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.

Recruitment of an Area Involved in Eye Movements During Mental Arithmetic

Addition to the Right, Subtraction to the Left High-level cognitive achievements, such as writing and mathematics, have appeared relatively recently in the evolutionary record in comparison to low-level skills, such as the perception of bright-dark boundaries. The latter have been demonstrated to arise from the coding properties of neurons in visual cortical centers, but what do the former map onto? Knops et al. (p. 1583, published online 7 May) provide evidence that addition and subtraction are encoded within the same cortical region that is responsible for eye movements to the right and left, such that the neural activity associated with addition could be distinguished from that associated with subtraction by a computational classifier trained to discriminate between rightward and leftward eye movements. Addition and subtraction are encoded in the same part of the brain that is responsible for eye movements and spatial attention. Throughout the history of mathematics, concepts of number and space have been tightly intertwined. We tested the hypothesis that cortical circuits for spatial attention contribute to mental arithmetic in humans. We trained a multivariate classifier algorithm to infer the direction of an eye movement, left or right, from the brain activation measured in the posterior parietal cortex. Without further training, the classifier then generalized to an arithmetic task. Its left versus right classification could be used to sort out subtraction versus addition trials, whether performed with symbols or with sets of dots. These findings are consistent with the suggestion that mental arithmetic co-opts parietal circuitry associated with spatial coding.

Inverse retinotopy: Inferring the visual content of images from brain activation patterns

Traditional inference in neuroimaging consists in describing brain activations elicited and modulated by different kinds of stimuli. Recently, however, paradigms have been studied in which the converse operation is performed, thus inferring behavioral or mental states associated with activation images. Here, we use the well-known retinotopy of the visual cortex to infer the visual content of real or imaginary scenes from the brain activation patterns that they elicit. We present two decoding algorithms: an explicit technique, based on the current knowledge of the retinotopic structure of the visual areas, and an implicit technique, based on supervised classifiers. Both algorithms predicted the stimulus identity with significant accuracy. Furthermore, we extend this principle to mental imagery data: in five data sets, our algorithms could reconstruct and predict with significant accuracy a pattern imagined by the subjects.

The cross-activation theory at 10

In 2001, Ramachandran and Hubbard introduced the cross-activation model of grapheme-colour synaesthesia. On the occasion of its 10-year anniversary, we review the evidence from experiments that have been conducted to test the model to assess how it has fared. We examine data from behavioural, functional magnetic resonance imaging (fMRI), anatomical studies (diffusion tensor imaging and voxel-based morphometry), and electroencephalography (EEG) and magnetoencephalography (MEG) studies of grapheme-colour synaesthesia. Although much of this evidence has supported the basic cross-activation hypothesis, our growing knowledge of the neural basis of synaesthesia, grapheme, and colour processing has necessitated two specific updates and modifications to the basic model: (1) our original model assumed that binding and parietal cortex functions were normal in synaesthesia; we now recognize that parietal cortex plays a key role in synaesthetic binding, as part of a two-stage model. (2) Based on MEG data we have recently collected demonstrating that synaesthetic responses begin within 140 ms of stimulus presentation, and an updated understanding of the neural mechanisms of reading as hierarchical feature extraction, we present a revised and updated version of the cross-activation model, the cascaded cross-tuning model. We then summarize data demonstrating that the cross-activation model may be extended to account for other forms of synaesthesia and discuss open questions about how learning, development, and cortical plasticity interact with genetic factors to lead to the full range of synaesthetic experiences. Finally, we outline a number of future directions needed to further test the cross-activation theory and to compare it with alternative theories.

Magnetoencephalography reveals early activation of V4 in grapheme-color synesthesia

Grapheme-color synesthesia is a neurological phenomenon in which letters and numbers (graphemes) consistently evoke particular colors (e.g. A may be experienced as red). The cross-activation theory proposes that synesthesia arises as a result of cross-activation between posterior temporal grapheme areas (PTGA) and color processing area V4, while the disinhibited feedback theory proposes that synesthesia arises from disinhibition of pre-existing feedback connections. Here we used magnetoencephalography (MEG) to test whether V4 and PTGA activate nearly simultaneously, as predicted by the cross-activation theory, or whether V4 activation occurs only after the initial stages of grapheme processing, as predicted by the disinhibited feedback theory. Using our high-resolution MEG source imaging technique (VESTAL), PTGA and V4 regions of interest (ROIs) were separately defined, and activity in response to the presentation of achromatic graphemes was measured. Activation levels in PTGA did not significantly differ between synesthetes and controls (suggesting similar grapheme processing mechanisms), whereas activation in V4 was significantly greater in synesthetes. In synesthetes, PTGA activation exceeded baseline levels beginning 105-109ms, and V4 activation did so 5ms later, suggesting nearly simultaneous activation of these areas. Results are discussed in the context of an updated version of the cross-activation model, the cascaded cross-tuning model of grapheme-color synesthesia.

Neural mechanisms of attentional shifts due to irrelevant spatial and numerical cues.

Studies of endogenous (cue-directed) attention have traditionally assumed that such shifts must be volitional. However, recent behavioural experiments have shown that participants make automatic endogenous shifts of attention when presented with symbolic cues that are systematically associated with particular spatial directions, such as arrows and numerals, even when such cues were not behaviourally relevant. Here we used event-related potentials (ERPs) to test whether these automatic shifts of attention use the same mechanisms as volitional shifts of attention. We presented participants with non-predictive (50% valid) task-irrelevant arrow and numeral cues while measuring cue- and target-locked ERPs. Although the cues were task-irrelevant, they elicited attention-related ERP components previously found in studies that used informative and/or task-relevant cues. These findings further substantiate the dissociation between endogenous and volitional attentional control, and suggest that the same fronto-parietal networks involved in volitional shifts of attention are also involved in reflexive endogenous shifts of attention.

What information is critical to elicit interference in number-form synaesthesia?

Numerous behavioural paradigms have demonstrated a close connection between numbers and space, suggesting that numbers may be represented on an internal mental number line. For example, in the Spatial Numerical Association of Response Codes (SNARC) effect, reaction times are faster for left-sided responses to smaller numbers and for right-sided responses to larger numbers. One valuable tool for exploring such numerical-spatial interactions is the study of number-form synaesthesia, in which participants report vivid, automatic associations of numerical and other ordinal sequences with precise, idiosyncratic, spatial layouts. Recent studies have demonstrated the influence of synaesthetic spatial experiences on behavioural number tasks. The aim of the present study is to further explore these internal spatial representations by presenting a case-study of an unusual synaesthete, DG, who reports highly detailed representations not only of numerical sequences (including representations of negative and Roman numbers), but also different representations for other ordinal sequences, such as time sequences (months, days and hours), the alphabet, financial sequences and different units of measure (e.g., kilograms, kilometres and degrees). Here, we describe DGs synaesthetic experiences and a series of behavioural experiments on numerical tasks concerning the automaticity of this phenomenon. DGs performance on number comparison and cued-detection tasks was modulated by his synaesthetic mental representation for the numerical sequence, such that his reaction times were slower when the spatial layout was incompatible with the orientation of his mental number line. We found that the spatial presentation of stimuli, rather than the implicit or explicit access to numerosity required by tasks, was essential to eliciting DGs number-forms. These results are consistent with previous studies and suggest that numerical-spatial interactions may be most strongly present in synaesthetes when both numerical and spatial information are explicitly task-relevant, consistent with a growing body of literature regarding the SNARC and other related effects.