Chiang, T-C.; (2006) Segregated and integrated hemispheric processes in vision and movement. Doctoral thesis , University of London.

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Abstract

Anatomical facts dictate that a foveally fixated word is represented by nodes of a neural network distributed across the two hemispheres. However, the mechanisms and implications of this lateralization are poorly understood. This thesis aimed to explore segregated and integrated hemispheric processes in the visual and motor cortices by applying unilateral transcranial magnetic stimulation (TMS) over cortical areas involved in visual and movement perception. In a visual letter recognition task, unilateral TMS over the visual cortex impaired processing in the contra-, but not ipsilateral visual field. Crucially, such selective contralateral effects were found for the left and right sides of foveal targets. The results imply that each primary visual cortex initially processes the contralateral part of foveal stimuli. To extend these results from letters to whole word recognition processes, a left or right precue was followed by a word/pseudoword. Visual word recognition was not affected by attentional cues unless the characters were scattered into letter arrays such that word shape was completely distorted. For distorted words, the lateralized cues had significant ipsilateral effects, in line with the single letter recognition findings. To establish the neural correlates of early hemispheric integration of visual stimuli, the crossed-uncrossed difference (CUD) paradigm was applied. The CUD was increased by inhibiting the crossed latencies, but only when TMS was applied to the hemisphere receiving visual information. Interhemispheric transmission was further studied by recording blood flow change at the unstimulated motor cortex by near infrared spectroscopy. The results revealed that the significantly increased oxy-haemoglobin and slightly decreased deoxy-haemoglobin outlasted the 1 Hz stimulation by up to 40 minutes.

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