Pseudoneglect and visual attention networks

Pseudoneglect represents the tendency for healthy individuals to show a slight but consistent bias in favour of stimuli appearing in the left visual field. The bias is often measured using variants of the line bisection task. An accurate model of the functional architecture of the visuospatial attention system must account for this widely observed phenomenon, as well as for modulation of the direction and magnitude of the bias within individuals by a variety of factors relating to the state of the participant and/or stimulus characteristics. To date, the neural correlates of pseudoneglect remain relatively unmapped. In the current thesis, I employed a combination of psychophysical measurements, electroencephalography (EEG) recording and transcranial direct current stimulation (tDCS) in an attempt to probe the neural generator(s) of pseudoneglect. In particular, I wished to utilise and investigate some of the factors known to modulate the bias (including age, time-on-task and the length of the to-be-bisected line) in order to identify neural processes and activity that are necessary and sufficient for the lateralized bias to arise. Across four experiments utilising a computerized version of a perceptual line bisection task, pseudoneglect was consistently observed at baseline in healthy young participants. However, decreased line length (experiments 1, 2 and 3), time-on-task (experiment 1) and healthy aging (experiment 3) were all found to modulate the bias. Specifically, all three modulations induced a rightward shift in subjective midpoint estimation. Additionally, the line length and time-on-task effects (experiment 1) and the line length and aging effects (experiment 3) were found to have additive relationships. In experiment 2, EEG measurements revealed the line length effect to be reflected in neural activity 100 – 200ms post-stimulus onset over source estimated posterior regions of the right hemisphere (RH: temporo-parietal junction (TPJ)). Long lines induced a hemispheric asymmetry in processing (in favour of the RH) during this period that was absent in short lines. In experiment 4, bi-parietal tDCS (Left Anodal/Right Cathodal) induced a polarity-specific rightward shift in bias, highlighting the crucial role played by parietal cortex in the genesis of pseudoneglect. The opposite polarity (Left Cathodal/Right Anodal) did not induce a change in bias. The combined results from the four experiments of the current thesis provide converging evidence as to the crucial role played by the RH in the genesis of pseudoneglect and in the processing of visual input more generally. The reduction in pseudoneglect with decreased line length, increased time-on-task and healthy aging may be explained by a reduction in RH function, and hence contribution to task processing, induced by each of these modulations. I discuss how behavioural and neuroimaging studies of pseudoneglect (and its various modulators) can provide empirical data upon which accurate formal models of visuospatial attention networks may be based and further tested.

Russian social networks on the web: cohesion and coherence in Vkontakte

In this thesis connections between messages on the public wall of the Russian social network Vkontakte are analysed and classified. A total of 1818 messages from three different Vkontakte groups were collected and analysed according to a new framework based on Halliday and Hasan’s (1976) research into cohesion and Simmons’s (1981) adaptation of their classification for Russian. The two categories of textuality, cohesion and coherence, describe the linguistic connections between messages. The main aim was to find out how far the traditional categories of cohesion are applicable to an online social network including written text as well as multimedia-files. In addition to linguistic cohesion the pragmatic and topic coherence between Vkontakte messages was also analysed. The analysis of pragmatic coherence classifies the messages with acts according to their pragmatic function in relation to surrounding messages. Topic coherence analyses the content of the messages, describes where a topic begins, changes or is abandoned. Linguistic cohesion, topic coherence and pragmatic coherence enable three different types of connections between messages and these together form a coherent communication on the message wall. The cohesion devices identified by Halliday and Hasan and Simmons were found to occur in these texts, but additional devices were also identified: these are multimodal, graphical and grammatical cohesion.