How Magnitudes are processed in both hemispheres: An Imaging and behavioural Approach

Numerical cognition is a recent research area that has received more structure through the triplecode model of Dehaene and colleagues (Dehaene, Piazza, Pinel, & Cohen, 2003). This model assumes three distinct numerical representation systems that can be anatomically localised and dissociated. Firstly, there is a system supporting spatial orientation on the mental ‘number line’ and is located in the bilateral posterior superior parietal areas. Secondly, a verbal representation of numbers is located in the language areas of the left hemisphere and contains orthographic, phonological and syntactic codes. Finally, numerical processes can also rely on a core quantity system. This quantity system is the semantic representation of numbers that is used for magnitude comparison, mental arithmetic and approximate calculation. It is believed that this system is analogue to an internal ‘number line’, on which magnitudes are represented independently of their modality. The core quantity system is bilaterally represented in the horizontal part of the intraparietal sulci and forms the focus of my Phd. The first part of my Phd is twofold. In the first section, I investigate in more detail how magnitudes are encoded in the intraparietal sulci of the brain. This study is an event-related fMRI adaptation study that aims at gaining insight into the neural mechanism behind the quantity priming effect, an important marker of magnitude coding. In the second section, I will investigate the influence of task demands on the localisation of the neural origin the quantity priming effect. Behavioural studies, using tasks such as naming or parity, have found similar quantity priming effects, suggesting a common underlying neural mechanism. However, it is possible that both tasks lead to the same behavioural responses but originate in different parts of the brain. In the second part, I will focus on hemispheric differences in magnitude representation. Firstly, I want to investigate whether both hemispheres have a qualitatively different magnitude representation. It is argued by some authors that the left hemisphere is more exact in its magnitude representation, whereas the right hemisphere would have a more coarsely tuned magnitude representation. To this purpose, the VHF technique is applied and compares the numerical distance effect and the quantity priming effect in both hemispheres. Secondly, I will investigate the effect of number size on hemispheric differences in number processing. Numbers are - certainly during early stages of acquirement - presented on a number line, spatially organised from left to right, with smaller numbers presented on the left and larger numbers on the right side of space. This spatial organisation of numbers possibly resulted in a hemispheric specialization of the RH (small numbers) and the LH (larger numbers). This hypothesis will be tested with an event-related adaptation fMRI study. The final part of my Phd contains two studies that will validate the VHF method as a technique to investigate hemispheric differences in their magnitude representation. It is likely that information presented to one hemisphere is rapidly transferred through the corpus callosum to the other hemisphere, making this method unsuitable for revealing (subtle) hemispheric differences in processing magnitude information. This will firstly be investigated with an event-related fMRI study. If the corpus callosum rapidly transfers magnitude information from one to the other hemisphere after lateralised presentation of a target number, parietal activation should be observed in both hemispheres. Finally, I will further investigate whether the inhibitory/excitatory influence of the corpus callosum on interhemispheric communication can be stimulated by manipulating the percentage of trials where a prime number contains (in)valid information for the reaction to the target number in a VHF study. When most of the trials lead to response conflict, we can expect the corpus callosum to exert an inhibitory rather than an excitatory influence on hemispheric communication of magnitude information. The results of this study will have implications for the use of the VHF method to reveal hemispheric differences in magnitude processing.