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The studies presented in this thesis address current issues in visual attention and awareness research. The first three experimental chapters investigate saccadic remapping of location and orientation information, with a particular focus on saccadic remapping in hemianopia. The results suggest that residual visual abilities in the blind field are necessary in order for a stimulus to be remapped from the blind to the sighted visual field. The results also suggest that remapping underpins our ability to maintain attention at specific spatiotopic locations across a series of saccades. Further evidence from both hemianopic and neurologically intact participants suggest that some orientation information is remapped across saccades. The second three experimental chapters investigate binocular rivalry in previously unstudied paradigms. The results show that the established face dominance and emotion dominance effects in binocular rivalry persist when the stimuli are viewed in peripheral vision. The results also suggest that a stable image presented in the opposite hemifield from the rival pair does not affect the perceived dominance of the separate images within the rival pair, but that the percepts in the rival pair tend to synchronise with those of a second rival pair presented in the opposite hemifield. Using Diaz-Caneja stimuli (half of each image presented to each eye) the results of the final experiments suggest a combination of eye- and object-dominance mechanisms in binocular rivalry in both the intact and the split-brain.

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Many event-related potential (ERP) studies have tried to find out which brain processes are responsible for the subjective experience of seeing. The contribution of these studies has been crucial in order to identify the temporal and spatial dynamics of visual awareness. The negative difference wave named visual awareness negativity (VAN), observed around 200 ms after the stimulus onset, has been claimed by many as a plausible candidate for reflecting the processes correlating with conscious visual perception. Other studies argue instead that only the P3 wave, a positive wave observed around 300-400 ms, correlates with visual awareness. The aim of the present study was to shed light on the issue of the presence of VAN even when using an experimental procedure that allows to dissociate the ERP correlate of subjective awareness from those of unconscious perception, allowing a separate analysis. Data from 24 participants was collected in the present study. The experimental framework included a forced-choice localization task of a low-contrast stimulus, followed by the subjective rating of awareness. The results of the study support the idea that the VAN is the earliest electrophysiological correlate of subjective visual awareness and that the phenomenon of visual awareness emerges early in the visual area.

published_or_final_version / Physics / Master / Master of Philosophy

A tridimensional optical display is described which is capable of producing real images with no optics between the observer and the image. This display, which is called the Visually Integrated Volumetric Image (VIVID), was developed as a laboratory system designed to be used in vision research. Three dimensional Lissajous patterns were produced, whose rotational position and axial depth could be precisely controlled by a general purpose digital minicomputer. By reversing the direction of rotation of the pattern in a known manner, and recording a subject's ability to perceive the true rotational direction for various amounts of axial image depth, the transition between the kinetic depth effect and true depth perception could be investigated. This transition forms the basis for defining a depth perception threshold for this type of a dynamic three dimensional image. Two experiments were performed using this approach. In the first experiment, ten undergraduate and graduate students (both male and female) observed the pattern during 24 one-minute trials. Eight different axial depth values were used, with three trials at each depth. The patterns were made to reverse rotation direction in a pseudo-random manner which was different for each trial presented to a given subject. The subject indicated the perceived direction of rotation by controlling a two position rotary switch, which was monitored by the computer. The total time during which the subject correctly tracked the rotation was integrated by the computer. The ratio of this correct tracking time to the total trial time was taken as a measure of the subject's ability to perceive the axial depth of the image. Plots of this ratio as a function of image depth are presented and discussed. The results averaged over all subjects produce a remarkably smooth curve, yielding a depth perception threshold of 2 mm for the subject population under the given experimental conditions. The characteristics of the stimulus were: (a) 2 cm high by 2 cm wide pattern; (b) Lissajous pattern frequency ratio of 6:1; (c) Axial distance from observer of 55 cm; (d) Wavelength of 533 nm (green); (e) Horizontal rotation axis; (f) Rotation period of 7 seconds; (g) Neutral background field illumination over a 40° come angle; (h) Image line width of 0.5 mm. The second experiment was a pilot experiment designed to investigate the effect of wavelength variations on this form of depth perception. A single subject was presented with a total of 192 one-minute trials at four wavelengths spanning the visible region (i.e., 6 one-minute trials for each of eight depth values at each wavelength). No significant wavelength dependence was observed in this pilot experiment.

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