Multimodal cue integration in balance and spatial orientation
Creators: name: Quadir, Shamim
Publisher: name: Imperial College London
Year: 2013

description: The global objective of this thesis was to make a significant contribution to our understanding of how the human brain integrates multisensory, multimodal information to inform our motion through space. The primary objectives were to discern whether visual system differentially encodes visual motion coherence and how both allocentric visual cues interact with vestibular system to tell us where and when we are in physical space. A secondary objective was to develop current techniques for the recording and analysis of visuo-vestibular sensory information for the purpose of multisensory, multimodal integration. I studied the response of cortical visual motion area V5/MT+ to random dot kinematograms (RDK) of varying motion coherence, from complete coherence to random. I used the probability of observing TMS (transcranial magnetic stimulation) evoked phosphenes before and after the RDK as a measure of cortical excitability change. I could not show what I had hypothesised: that coherent and random motion elicited a similar net effect upon V5/MT+ excitability, with intermediary coherences of motion having comparatively less effect. However, I argue that a large factor was insufficient sample size to find the effects given the analyses used. The results do show trends consistent with coherent and random net effects being achieved by different modes of cortical activation, and the study will inform future investigation with the paradigm used. I also measured cortical excitability change at a range of relative TMS intensities. This elicited a significant differential effect consistent with the theory that TMS facilitates neurons as a function of the amount of signal they carry. In a separate TMS evoked phosphene study, I show an interaction between whole body rotation in yaw and the ability to observe phosphenes in V5/MT+; as a function of the TMS intensity used and the velocity of whole body rotation used, relative to perceptual thresholds. As I found no main effects, I could not show whether the findings were consistent with a model of reciprocal visual and vestibular cortical inhibition. My work can be considered a feasibility study to inform further investigation. I also used a visual-vestibular mismatch paradigm to probe how erroneous visual landmark cues update veridical vestibular estimates of angular position and motion duration. I used visual masking to reduce the reliability of the visual landmark cues, prevent visual capture and to also elicit subliminal encodement. I found that reversion to vestibular estimates of angular position was made as a function of the noise inherent in the masked visual landmark cues. I found that it was possible to subliminally encode visual landmarks to update vestibularly derived estimates of motion duration. Lastly, I investigated the combination of a two-interval forced choice technique to record estimates of vestibularly derived angular position and a Bayesian Inference technique to parameterize the characteristics of the angular position estimates. I show this combination provides accurate estimates at the subject level and is suitable for incorporation in a Bayesian inference model of multimodal integration. The hypothesis I aim to test in the future is that if visual landmark and vestibular cues of angular position operate within different spatial reference frames, they cannot be optimally integrated in the brain analogous to a Bayesian Inference model of the multimodal integration.