Thinking left and right : neurocognitive studies on spatial relation processing

Abstract

In many of our interactions with our environment, we make use of spatial relations. These spatial relations can be either categorical or coordinate. Categorical spatial relations concern abstract relations like “left of” and “above”, whereas coordinate spatial relations are metric in nature and concern exact distance like “2 meters apart”. In many studies it has been found that there is a left hemisphere advantage for processing categorical relations and a right hemisphere advantage for processing coordinate relations. This lateralisation pattern suggests that there are two separate processing mechanisms underlying the two types of spatial relations. However, there are also some alternative theories suggesting that there might be a singular underlying mechanism, or that other factors, like task design, determine the lateralisation pattern. Therefore, evaluation of these opposing theories is appropriate. The first part of this thesis concentrates on the issue of time; there appear to be some temporal characteristics crucial to the lateralisation pattern. A new working memory task design, the “cross dot task” is introduced in this thesis; it entails the comparison of two sequentially presented stimuli. The retention interval between these two stimuli was altered to study the temporal characteristics of spatial relation processing. The lateralisation pattern was found convincingly for shorter intervals and additional neuroimaging studies indicated that the lateralisation was mostly located in the superior parietal cortex and during the encoding of the first stimulus of the two. In the second part of this thesis the issue of stimulus format and strategy use is addressed. Some have argued that instead of the distinction between categorical and coordinate processing, it would be more appropriate to use a distinction between verbal and visuospatial processing of spatial relations, with regard to the left and right hemisphere respectively. This opposes the original definition of spatial relation processing, that states they are completely spatial in nature. However, our results indicate that verbal and visuospatial task format can only influence the extent of lateralisation, not the direction of lateralisation. Furthermore, additional studies on naturally occurring strategies indicate that both categorical and coordinate relations are processed in a truly spatial way. The last part of this thesis focuses on the generalizability of the findings on spatial relation processing, which have mainly resulted from experiments with very simple and abstract stimuli. Therefore, we have conducted studies making use of virtual depictions of natural scenes, like living rooms and kitchens. Participants have shown that they use both categorical and coordinate information to perceive and memorize object locations in such complex and lifelike stimuli. Moreover, visual field manipulations and a study on patients with unilateral brain damage suggest that categorical left hemisphere and coordinate right hemisphere advantages are present within such natural settings, congruent with what has been found for the abstract stimuli.