Implications of number-space synesthesia on the automaticity of numerical processing

Synesthetic Correspondence: An Overview

Intramodal and cross-modal perceptual grouping based on the spatial proximity and temporal closeness between multiple sensory stimuli, as an operational principle has built a coherent and meaningful representation of the multisensory event/object. To implement and investigate the cross-modal perceptual grouping, researchers have employed excellent paradigms of spatial/temporal ventriloquism and cross-modal dynamic capture and have revealed the conditional constraints as well as the functional facilitations among various correspondence of sensory properties, with featured behavioral evidence, computational framework as well as brain oscillation patterns. Typically, synesthetic correspondence as a special type of cross-modal correspondence can shape the efficiency and effect-size of cross-modal interaction. For example, factors such as pitch/loudness in the auditory dimension with size/brightness in the visual dimension could modulate the strength of the cross-modal temporal capture. The empirical behavioral findings, as well as psychophysical and neurophysiological evidence to address the cross-modal perceptual grouping and synesthetic correspondence, were summarized in this review. Finally, the potential applications (such as artificial synesthesia device) and how synesthetic correspondence interface with semantics (sensory linguistics), as well as the promising research questions in this field have been discussed.

The Impact of Coding Levels of Magnitude and of Spatial-Direction on the Spatial-Numerical Association of Response Codes Effect of Negative Numbers

Whether negative numbers have a fixed spatial–numerical association of response codes effect (SNARC effect), and (if they have) whether the spatial representation of negative numbers is associated with negative numbers’ absolute or signed values remains controversial. In this study, through three experiments, the coding level of the magnitude and the spatial-direction is manipulated. In the first experiment, participants are required to code the magnitude and spatial-direction explicitly by using a magnitude classification task. In the second experiment, participants are forced to code the magnitude implicitly as well as to code the spatial-direction explicitly by utilizing a cuing task. In the third experiment, participants are obliged to code the magnitude explicitly as well as to code the spatial-direction implicitly by adopting a magnitude and arrow-direction classification tasks with Go/No-Go responses. The results show that (1) the absolute value of negative numbers associates with space when the magnitude of negative numbers is explicitly coded, no matter employing the explicit or implicit spatial-direction; (2) the signed value of negative numbers associates with space under the condition of implicit magnitude as well as explicit spatial-direction. In conclusion, the current study indicates that the SNARC effect of negative numbers is variable in different conditions, and the type of SNARC effect about negative numbers is modulated by the joint coding level of the magnitude and spatial-direction.

Cognitive and brain correlates of acquired number-colour synaesthetic-like associations

Synaesthesia is a condition in which one sensory dimension triggers another sensation. The exact contribution of genetic and environmental factors in synaesthesia is not yet fully understood. Most synaesthesia phenotypes involve associations in which the synaesthetic inducer constitutes some form of linguistic/conceptual information acquired during the course of development (e.g., digits, letters, and names of months). To study the role of learning in synaesthesia, we induced synaesthesia by training and examined the behavioural and brain correlates of number-colour associations. We took advantage of the well-known behavioural and neural signature of numbers and used number symbols as inducers. Short (2 weeks) and long (4 weeks) training protocols were conducted with two different groups. Task-related BOLD response was acquired while participants performed Stroop tasks requiring naming colours while ignoring the stimuli (i.e., number symbols, dots, words). If the arbitrary association involving number-colour is automatic, the irrelevant dimension (i.e., numbers) would interfere with the colour response. In addition, if number-colour associations are transferred to linguistic and non-symbolic representations, the passive viewing of stimuli (i.e., words and dots) would disrupt colour naming. Behavioural findings showed automatic associations as both training protocols elicited reliable congruency effects for all stimulus dimensions. Congruency effects following both training protocols produced reliable brain activations in various cortical sites involved in number and in cognitive control. The behavioural and brain patterns reported here support the role of learning in the brain correlates of developmental synaesthesia and provide the first evidence that automatic associations involving different magnitude dimensions can be acquired.

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.

Space counts! Brain correlates of spatial and numerical representations in synaesthesia

Over-learned semantic representations, such as numbers, are strongly associated with space in normal cognition, and in the phenomenon called number-space synaesthesia. In number-space synaesthesia, numbers are linked to spatial locations in an idiosyncratic way. Synaesthetes report numbers as belonging to a specific location, or feelings that a specific location is the right location for that number. What does really differentiate synaesthetes from non-synaesthetes with respect to their number-space representation? Here we present a number-space synaesthete, MkM, whose number-space representation dramatically differs from that of controls. We examined the impact of spatial distance with respect to MkM's mental number line (MNL), and numerical distance with respect to the conceptualized horizontal representation of numbers. In a behavioural experiment, MkM and controls performed number comparison tasks in which they reported either the larger numerical value (number task) or the larger stimulus (physical task) (Experiment 1). A spatial distance effect was found only for MkM. In a brain imaging experiment, MkM and controls compared a single presented digit with an internal reference (Experiment 2). Consistent with the behavioural results, spatial distance elicited significant brain activations only for MkM in different cortical sites including the left supramarginal gyrus. Numerical distance elicited significant brain activations only for controls in the left somatosensory cortex and in the right operculum. We propose that two types of representation are accessed in synaesthesia: one derived by the semantic coding of numbers across space (described by the MNL), and an explicit spatial representation derived from the position of number within the synaesthetic association. The level of overlap between these two forms of representation depends on the shape of the synaesthetic number-space association.