Spatial information processing in humans and monkeys

Abstract

In this thesis a series of experiments are described on human volunteers and rhesus monkeys (Macaca mulatta) in the context of spatial information processing. In the first single-unit recording experiments in monkeys a spatial summation algorithm was investigated. The responses of single neurons to the presentation of two stimuli in their receptive field were compared to the activity elicited by the individual presentation of these stimuli. Parietal area 7a neurons yielded responses to multiple stimuli that resembled a winner-take-all algorithm where the most optimal stimulus determines the neuron’s firing rate. The following neurophysiological study of working memory delay-period activity revealed that persistent activity commonly regarded the core of working memory processes, does not necessarily contribute to solving a delayed match-to-sample task. The alternative we propose is that a comparison of the memorized and the current stimuli can be solved by implementation of a matched filter mechanism. Both monkey studies ensured important insights into the mechanisms underlying cognitive performance and have to be taken into account in refining the existing network models of spatial attention and working memory. In human subjects two hypotheses were examined concerning the possible interactions between perception and oculomotor action and the distinction between the processes underlying spatial categorisation and distance assessment. Although, both experiments involved a behavioural methodology, in both cases the experimental design was guided by knowledge of physiological properties of visual neurons. In both cases we demonstrated that the most parsimonious explanation of the results is based on the basic neurophysiological properties of cortical neurons. It is very likely that a number of similar studies neglected these simple interpretations of the experimental outcome. The final chapter consists of a critical literature review showing that nonhuman primates have a functionally symmetric brain in the context of spatial cognition. This is in contrast to the right hemisphere specialisation in the spatial cognition in humans. One the one hand, this indicates that the left hemisphere dominance in language processing in humans evolved either concurrently or earlier than the right hemisphere expertness in spatial cognition. On the other hand, these conclusions offer an explanation why in monkeys the hemineglect syndrome is much less severe than in human patients. That is, in humans neglect occurs mainly after a right hemisphere damage, whereas in monkeys the remaining healthy hemisphere can to a great extent compensate the neglect syndrome of the other hemisphere due to their functional symmetry. Taken together the studies reported in this thesis offer new insights and up to now less explored perspectives on the issues such as spatial information integration, spatial working memory, manipulation and retrieval of spatial information and finally the lateralization of spatial cognition within the context of brain evolution.