Publikationsserver der RWTH Aachen University
On the structure and neural correlates of the numerical magnitude representation and its influence in the assessment of verbal working memory
Abstract
dc:descriptionThis thesis deals with two issues: First, it elaborates on the nature and neural correlates of the mental representation of two-digit numbers. Second, it investigates whether numbers are well suited to serve as stimulus material in the assessment of verbal working memory (vWM). Dehaene, Dupoux and Mehler (1990) claimed that two-digit numbers are mentally represented along a single, continuous mental number line. Nuerk, Weger and Willmes (2001), however, found evidence for separate representations of decade and unit digits. They presented unit-decade-compatible (henceforth compatible) and incompatible number pairs. In compatible number pairs unit and decade digit magnitude comparison lead to identical response tendencies (e.g. 89_74, 8 > 7 and 9 > 4). For incompatible number pairs this is not the case (e.g. 63_48, 6 > 4 but 3 < 8). Faster reaction times (RT) for compatible number pairs (compatibility effect) suggest separate unit and decade representations. Study 1 contains three experiments which use the manipulation of stimulus onset asynchronies (SOA) for units and decades. In Experiment 1, the SOA range of Dehaene et al. was extended to gain a better understanding of unit-specific interference effects. In the remaining two experiments, the salience of the unit digits was increased. Most importantly, Study 1 shows that attentional and strategic factors exert some influence of the magnitude representation of two-digit numbers, which underlines the assumption of separate representations of units and decades. Functional magnetic resonance imaging (fMRI) was used in Study 2 to test whether numbers – just like visual information is organized retinotopically in the Cortex – are represented in spatially ordered parts of the brain (numerotopy). A parametric approach of data analysis was chosen to show that (a) the idea of numerotopy does not seem to hold on a neural basis and (b) semantic interference activates neural assemblies in the areas located in the anterior medial aspects of cortex along the intraparietal sulcus (IPS), while numerical magnitude information activates posterior lateral parts of the IPS. In Study 3 transcranial magnetic stimulation (TMS) was used to induce a virtual lesion in the left IPS. If the lesioned area is functionally relevant for the processing of numerical magnitude information, a modulation of related cognitive effects (distance and compatibility effect) should be observed. Indeed, significant portions of the tested population showed a reliable modulation of these effects. This establishes a direct link between the stimulated brain areas and the cognitive functions indexed by the effects investigated. Based on the results of Study 1 to 3, the model of two-digit number magnitude comparison of Nuerk & Willmes (2005) is extended. A neural implementation of the model is proposed. In Study 4, fMRI was used to test whether numbers are a good operationalisation for the assessment of vWM, which is postulated to rely on a purely phonological, asemantic code (Baddeley, 2003). Numbers automatically convey semantic meaning, however. The necessity to recruit such semantic meaning was manipulated in two versions of the n-back paradigm, using numbers and letters as stimuli. Stimulus-specific modulation of neural activity in the IPS was observed, implying semantic processing in the context of a vWM task. This questions the employment of numbers for the assessment of vWM and suggests the use of asemantic stimuli (e.g. letters).
Degree
thesis:*- Grantor dc:publisher
- Publikationsserver der RWTH Aachen University
- Year dc:date
- 2006
Author and committee
dc:creator, dc:contributor.*- Author dc:creator
-
- Knops, André
- Contributors dc:contributor
-
- Willmes-von Hinckeldey, Klaus
Subjects
dc:subject × 2Rights
dc:rights- Statement dc:rights
-
- info:eu-repo/semantics/openAccess
- Language dc:language
- eng