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Content archived on 2024-05-29

The neural basis of number space: probing connectivity and interactions between number and space processing with Transcranial Magnetic Stimulation (TMS)

Final Activity Report Summary - NBNS (The Neural Basis of Number Space: probing connectivity and interactions between number and space processing with Transcranial Magnetic Stimulation (TMS))

This 3-year project was aimed to map the brain circuitry responsible for the interaction between numbers and space in healthy adults. Some people possess a mental image of the number sequence that is spatially distributed and can be used to carry out calculations. People without an explicit number form nevertheless represent numbers spatially, in Western cultures with small numbers toward the left of mental space and large numbers toward the right. It is still unclear to what extent space and number processing are intermingled in the brain and whether number magnitude representations can be dissociated from their mental spatial representation. Also it is unclear whether mapping numbers onto a mental space is an instance of a domain-general cognitive mechanism rather than a domain-specific feature.

During the first year, several behavioural studies have been performed while setting up and piloting TMS and fMRI experiments which will be carried out in a near future. The first remarkable finding concerned the dependency of the behavioural effects of the spatial representation of numbers on the hand that is used to respond to numbers. Unimanual right two-choice key-presses are indeed to spatially primed from number magnitude, left-side responses (performed with the right index) being faster to small than large numbers and right-side responses, right-side responses (performed with the right middle) being faster irrespective of the task (either a magnitude comparison or a parity judgment). Unimanual left two-choice key-presses, instead, performed with the index (right-side response) and middle (left-side responses) fingers, did not show such preferential mapping effect. This immediately suggested that an additional variable should be taken in consideration when exploring the neural correlates and necessary substrates of mental number space - that is the connection between finger identity and number magnitude representations. To this aim a parallel line of research was started, investigating the brain mechanisms involved in a high level (i.e. beyond primary somatosensory) representations of fingers.

A variant of the classical in-between task (i.e. say how many fingers lie in-between two fingers touched on the same hand), the intermanual in-between task (i.e. say whether the finger-distance between the two touches on one hand is the same as the finger-distance between the two touches on the other hand) has been recently developed. Behavioural studies with tactile stimulation performed manually by an experimenter with an fMRI-compatible piezo-electric apparatus have brought evidence for the existence of an intermediate body representation that is accessed by somatosensory stimuli and shows properties that naturally map onto abstract number concepts (e.g. ordinality), in addition to having a spatial layout. An fMRI study is now being set up to explore the brain areas and circuits whose activity correlates with the use of such intermediate representation. In turn this will help identifying the areas that may be specific to number space and those in which converging processes are applied to fingers and numbers.

The mapping of the neural basis of number space with TMS (as soon as the necessary lab equipment will become available) will therefore take advantage of two types of functional-anatomical information: one related to the circuits involved in spatial attention and the other specifically related to cognitive finger representations. With regard to the domain-specificity issue, a dual-task experiment has been performed to verify whether the preferential mapping of number magnitude and pitch height onto response positions, as elicited by hearing sung number names (i.e. stimuli conveying both magnitude and pitch information), is equally or differentially disrupted by two types of concomitant tasks (a visual working memory task and a spatial working memory task). Results are not yet available, since are in the process of being analysed.