Neuropsychological, neurophysiological and psychophysical evidence support the notion of two separate and largely independent cortical visual systems: a dorsal system mediating visually guided action and a ventral system mediating object perception and recognition (Goodale & Milner, 1992). This thesis is divided into three parts that explore questions related to the two-visual-systems model, two in humans and one in rats. The first part explores whether dorsal representations are based on the veridical properties of the stimuli or whether they include information produced by filling-in mechanisms of cortical visual areas. All human experiments were carried out with the ELITE and SMART motion tracking systems. Kinematic analysis showed that grasping Kanizsa illusory squares and partly-occluded objects was as accurate as grasping luminance-defined targets and it is concluded that information about interpolated regions is available to the dorsal system for the calibration of the movement parameters. A Vernier acuity task confirmed that the perceptual localization of Kanizsa and luminance-defined contours is not equally accurate in the ventral visual system. The second part explores the effect of target dimensionality on grasping, focusing on the possibility that actions aimed at targets that contain two-dimensional information could be modulated by ventral visual mechanisms. The Diagonal Illusion (DI) was chosen to investigate this possibility because it is entirely the product of three- dimensional objects. The DI exerted an effect on both perception and action, although the latter was smaller, suggesting that the effects of illusions on action previously reported are not attributable to the presence of 2D information and, by implication, that 2D information in the target array does not elicit modulation by the ventral visual system. These conclusions were confirmed by a study that found similar kinematic profiles from grasps aimed at 3D, 2D and 2D-enhanced targets. Control studies ruled out potential confounding effects resulting from curvatures of the stimuli that could have acted as obstacles and from differences in haptic feedback. It is concluded that object-directed action is mediated by dorsal visual mechanisms, irrespective of target dimensionality. The third part seeks to find evidence of ventral visual processing in rats by measuring the perception of visual illusions and object recognition in this species. The aim is to establish whether rats could provide a suitable model to further investigate the dorsal and ventral visual systems. An automated apparatus with a touch-screen and computer generated stimuli was developed to train the animals. The results from the illusion studies are not conclusive as only one out of three groups of rats was able to solve a discrimination with Kanizsa illusory figures. The preliminary results from the object recognition studies are however clearer. Rats were able to use aspect ratio to solve a discrimination with stimuli that varied in size and location suggesting that size- and location-independent object recognition occurs in this species. Probe trials confirmed these results. It is concluded that rats may have visual processes comparable to those occurring in the ventral visual system of humans and primates.

Repetition of target properties across consecutive visual search trials affects response time: a phenomenon known as "repetition priming" in visual search. Ten visual search experiments are reported which were designed to better characterise the mechanism(s) producing this phenomenon. After replication of the effects reported in the literature with a new paradigm (Experiment 1), two further experiments (Experiments 2 and 3) were designed to identify the optimum paradigm with which to study these effects further, and revealed that repetition priming occurs only under particular circumstances. This conclusion was extended by Experiments 4-6, which showed that repetition priming effects depend on the exact stimuli and task used. These findings challenge the suggestion in the literature that repetition priming in visual search is driven by an automatic bottom-up process, and instead suggest that it is rather more flexible, and depends on the exact stimulus characteristics and task requirements.

Both the gaze and actions of another are powerful cues for guiding visual attention in the environment. Nonetheless, empirical investigations of action as a cue for orienting are curiously rare when compared to those for gaze. This is more remarkable in the light of diverse evidence signifying that uniquely for observed action; specialised mechanisms may directly map attentional processes used during the performance of the same actions.

When viewing complex images, discomfort was best predicted from how contrast energy was distributed across spatial frequencies. Images rated as uncomfortable to view ded to have greater amplitude close to 3 cycles/degree; within their radially averaged Fourier amplitude spectra, relative to elsewhere in the spectrum. Luminance gratings with spatial frequencies of 3 cycles/degree are those most likely to elicit epileptiform activity, migraine, discomfort, and visuo-perceptual distortions (Wilkins et al., 1984). We conclude that uncomfortable images contain greater relative contrast energy at those spatial frequencies to which the visual system is generally most sensitive.

The present study explores the importance of visual impairments, in the form of visual acuity, stereopsis, dorsal and ventral processing problems and Cerebral Visual Impairment (CVI) as determinants of motor co-ordination difficulties. The Movement ABC was used to identify 49 nine to ten year old primary school pupils with hand-eye co-ordination problems (HECP) and 49 control pupils (without HECP). A pilot study comparing the performance of 30 case and 30 control pupils in one primary school on visual acuity, stereopsis and six different visual processing tasks was completed and the most discriminating visual tasks were then administered to the remaining 19 case and 19 control pupils in a second primary school.

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Human control of locomotion relies heavily upon vision. Through our eyes we sample information about the illuminated environment that aids us in moving skilfully through it. The exact visual information that supports control of high-speed locomotion around bends has been a matter of debate. Two models of steering (the Tangent Point and Active Gaze model) propose different visual inputs to enable this process. In two studies (Chapter 2) these models were tested against each other, and it was found that humans look at points on their future path (consistent with the Active Gaze model). Not only do participants look where they want to go (Chapters 2 & 3), but they go where they look (Chapter 4), even when there is an additional cognitive load at the point of fixation (Chapter 7). However, gaze direction is not the sole source of information when steering: visual direction and optic flow may also play a role. Three experiments (Chapters 3, 4, & 5) highlight the importance of the visual direction information provided by road-edges. When information from road-edges was strong (Chapters 4 & 5) gaze direction ceased to bias steering. Optic flow also caused systematic steering biases when the temporal and spatial flow characteristics from regions either side of the bending roadway were manipulated (Chapter 6). Two distinct behaviours were elicited by humans steering in the presence of optic flow asymmetries: i) steering was biased towards one region in order to equalise asymmetries, and ii) oversteer increased at higher perceived speeds averaged across global flow asymmetries. The observed interaction between flow, road- edges and gaze direction is indicative of the visuomotor-control system's propensity to use multiple sources of information in a flexible manner according to information quality and availability.

No description available.

No description available.

Transient motion blindness is a visual phenomenon where temporary blindness for first-order visual motion is induced in observers by placing the target motion episode within a temporal stream of irrelevant distractors (Sahraie, Milders & Niedeggen, 2001).  Observers are asked to ignore all irrelevant motion occurring prior to a clear signal (the appearance of a red fixation), whereupon they are asked to pay attention to any subsequent motion and to report its direction.  Under these circumstances, observers are found to be completely unaware of any motion episodes that occur within 300ms of the red cue, even though first-order motion is traditionally thought to ‘pop-out’ of visual displays without the need for attention (Dick, Ullman & Sagi, 1987).  Sahraie et al (2001) proposed that motion blindness (MB) may result from the carry-over of motion distractor inhibition.  The present thesis evaluated the accuracy of this hypothesis over a series of 10 experiments, finding that: 1) MB is a robust and easily replicable phenomenon; 2) MB cannot be explained purely in terms of the task switching and spatial shifting costs incurred during the task; 3) MB is contingent upon the presence of irrelevant distractor motion; 4) MB only arises when this distractor motion is selectively inhibited; 5) the severity of MB can be predicted from knowledge of an observer’s inhibitory strength; 6) perceptual motion sensitivity is measurably reduced during periods of MB; and 7) the magnitude of the MB effect varies predictably with the amount of inhibition thought to have been generated by the task.  Overall, it is argued that motion blindness does arise as the result of distractor inhibition in line with the inhibitory hypothesis of Sahraie et al (2001), and that the impairment reflects general operating principles of the selective attention which controls sensory information processing.