I was initially attracted to the area of this thesis by the fundamental aspect of the organisation of the visual cortex in mammals into multiple representations of the visual field. There is little consensus regarding this organisation in terms of number, boundaries or visuotopy, except possibly for areas V1 (primary visual area) and V2 (second visual area), and the middle temporal area (MT). Traditionally, techniques such as electrophysiology and anatomy were used to describe the functional and anatomical organisation of the visual cortex. Recently, however, new non-invasive techniques have become available, such as functional magnetic resonance imaging (fMRI), positron-emission tomography (PET) and transcranial magnetic stimulation (TMS). These techniques have allowed studies of the functional and anatomical organisation of visual cortex in humans while they perform tasks which target functions such as attention, perception and memory.
This thesis comprises a series of studies which investigated the functional contribution of various cortical areas to the visual system using a diversity of techniques. Initially, the thesis set out to examine response properties of neurons in V2 following lesion of V1 in the flying fox. I decided not to pursue animal studies of this kind, but the findings raised a new set of question that I thought might be answerable with non-invasive techniques in humans. In particular, the possibility was raised that one could investigate higher level visual function, such as motion perception by experiments involving visual cortical areas beyond VI in humans. The possibility then arose of carrying out these studies using the new non-invasive technique of TMS. TMS causes temporary disruption similar to the immediate effects of lesion, without any permanent damage. In this case, area MT and the posterior parietal cortex were chosen as the focus of investigation, due to their proximity to the surface of the cortex and therefore accessibility via TMS, and their known functional properties. The use of a human model also allowed the development of tasks which involved the perception of complex illusory stimuli such as motion-induced blindness (MIB), and the use of behavioural measures such as level of confidence. These types of measures are not available in animal studies of visual perception, and allow for the possibility of new functional revelations in the human visual system.
Chapter 2 investigates a fundamental issue concerning how the mammalian visual system functions i.e. hierarchically versus parallel. One way to address this question is to inactivate or lesion area V1 (the primary visual cortex) and examine the functional consequences by recording neuronal activity in area V2 (the secondary visual cortex). In primates, lesion of V1 renders V2 neurons unresponsive to visual stimulation, while in non-primates V2 activity is reduced but not completely abolished. We examined the effects of V1 lesion in the flying fox (Pteropus poliocephalus) as this model had been in development in the laboratory for several years. We knew of the normal visuotopy of areas V1 and V2, but had not yet examined the functional relationship between these 2 areas. In choosing the flying fox it also allowed us to address the controversial evolutionary relationship of Pteropus, namely whether flying foxes and other megachiropterans might form a sister-group of the Order Primates. We found that V2 cells remain visually responsive following destruction of V1, supporting the notion that V1 and V2 can process information in parallel. Our results do not completely exclude the possibility that flying foxes may form a sister-group of primates.
Chapters 3 and 4 involve lesion paradigms, which address the functional contributions of area MT and the posterior parietal cortex in the primate visual system using a human model. The lesion was temporarily induced using a relatively new technique known as TMS, which is both safe and painless. The use of TMS in the visual system was a technique which had not yet been developed in Australia, but had made serious progress in the United States and Europe. Consequently, in addition to the scientific contribution, it was also important that the technqiue be made available in this country. Chapter 3 investigated the functional contribution of area MT and the posterior parietal area to motion perception. Subjects viewed a coherently moving checkerboard stimulus and had to indicate the direction of motion, and rate how confident they were about their choice, i.e. their perceptual ability. The optimal time to deliver TMS was found to be 60ms before the onset of stimulus motion. Delivery of TMS to area MT resulted in a greater reduction in confidence compared with posterior parietal TMS; however error frequency was not different. A hemispheric asymmetry was found whereby a greater deficit in motion perception was observed following left posterior parietal magnetic stimulation. The final major finding was a visual hemifield/motion direction interaction, whereby motion stimuli presented to the left visual hemifield and moving centrifugally were perceived more weakly and induced more errors.
The optimal latency reported in this study might reflect disruption to feedback projections from area MT to area V1. The difference between confidence and error frequency may also reflect functional differences between feedforward and feedback projections. The hemispheric asymmetry observed following posterior parietal TMS might involve a combination of lower and higher order processes, whereby a left hemisphere bias for stimuli with straight edges interacts with attentional switching mechanisms. The visual hemifield/motion direction finding is thought to reflect timing differences in the activation of right and left ipsilateral visual fields.
Chapter 4 investigated the effect of posterior parietal TMS on motion-induced blindness, a phenomenon whereby yellow discs presented on a coherently moving blue background disappear and reappear cyclically. We found that the disappearance phase is more effectively disrupted by left posterior parietal TMS and the reappearance phase by right hemisphere stimulation. The results are explained in the context of hemispheric differences observed in anosognosia (a clinical derivative of unilateral spatial neglect) and motion perception. These are then related to the interhemispheric switch model, which has been proposed to underlie other ambiguous stimuli (or visual disappearance phenomena) such as binocular rivalry.
Chapter 5 was a purely psychophysical study which examined the line-motion illusion (LMI), whereby subjects perceive a line to grow out of a visual cue, even though the entire line is presented instantaneously. The LMI was compared with apparent motion stimuli, and both were presented in each visual quadrant and at three different orientations (horizontal, oblique and vertical). Subjects had to rate how strongly they perceived motion within each stimulus presentation. The results reveal a lower visual field and oblique orientation bias for the LMI, but not for apparent motion stimuli. The biases observed for the LMI may reflect the influence of spatial distortions inherent to the human visual system.
I benefited greatly from using such a diversity of techniques throughout my thesis. It has broadened my view of the processes which may be involved in visual perception, and made me appreciate that there are many ways to interpret how the brain functions. The studies described in this thesis revealed interesting functional consequences of inducing either permanent or temporary lesions in mammalian visual cortex. The TMS study of area MT and the posterior parietal cortex was the first of its type in Australia. The studies also showed new ways of applying TMS to experimental paradigms, especially in reference to the perturbation of the alternating phases associated with MTR While these are early days, the use of TMS as a lesion technique in the visual system holds much promise, especially when it is used in combination with complex and interesting visual stimuli.