Cortico-cortical interactions in visual awareness

• Main Author: Silvanto, J. T.
• Published: University College London (University of London) 2007
• Subjects: 612.8
• Online Access: https://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.719064

This thesis investigated the role of cortico-cortical interactions and the role of striate cortex (VI) in human visual awareness in both normal subjects and the blindsight subject GY. In Chapter 3, the critical time windows of VI and V5/MT activity in awareness of moving visual stimuli were compared using transcranial magnetic stimulation (TMS). The results demonstrate the importance of backprojections from V5/MT to VI in awareness of real motion stimuli. In Chapter 4, the role of VI in conscious perception of moving phopshenes induced by stimulation of V5/MT was studied. By varying the activation level of VI, it is shown that the amount of activity in VI determines whether activity in V5/MT reaches awarereness. Furthermore, the activity in V5/MT influences the information content in VI, but it is VI that determines whether that information reaches awareness. In chapter 5, it is reported that the blindsight subject GY can experience visual sensations, elicited by TMS, in his blind field. Importantly, such blind field percepts (phosphenes) could only be induced when GY's contralesional extrastriate area V5/MT was stimulated in close temporal proximity with the ipsilesional V5/MT. Stimulation of his ipsilesional V5/MT also altered to appearance of phosphene induced from the intact VI, but did not induce a blind field percept. The necessity of the contralesional stimulation in eliciting phosphenes from extrastriate cortex points to the importance of VI in visual awareness. In chapter 6, the role of FEF in exerting top-down influences on the extrastriate visual cortex prior to eye movement preparation was studied. It was established, using TMS, that activity in the human frontal eye fields has a direct effect on the sensitivity of extrastriate visual area V5/MT, and that the spatial organisation of this top-down effect is lateralised in the human brain.