New Investigations into the Visual Representation of Objects and Scenes

This thesis deals with human visual perception of scenes, where ‘scene’ is a term used to refer to a complex visual stimulus consisting of a large number of different elements/objects and subtending most, if not all, of the visual field. This thesis deals less with initial, low-level perceptual processing of such stimuli and more with the high-level processing that follows. Specifically, the thesis deals with processes in vision that act on information about the real-world objects in the visual scene. In visual terms, an object can be defined as an entity that is coherent in space and time, is restricted to a localised portion of the scene and also has some meaning to the observer viewing it (i.e., they can use and/or identify it). In keeping with this object-oriented approach, areas such as the attentional selection of objects in a visual scene, the recognition of those objects and the maintenance of information about objects in memory will all be dealt with by the thesis; whereas areas such as the perception of basic visual qualities (e.g., orientation, colour, luminance and contrast) will not. The thesis begins by reviewing literature on scene perception, an area which deals with how humans explore a complicated visual environment (the scene) and select objects from it to assist in their current behavioural goals. Generally, it is thought that observers use covert visual attention and overt eye-movements to orient the high-resolution parts of the retinas (the foveas) towards salient parts of their visual environment. The conventional view is that processing related to recognition of objects then ‘takes over’ in a serial manner after this exploration. Despite the apparent separation of these attentional and recognition processes, they are still critically dependent on one another, especially in the context of the perception of a complete scene (this may not be apparent in the perception of isolated visual stimuli). The experiments of this thesis aim to elucidate this interaction by using paradigms from the scene perception and visual attention literature. Although this is a large body of literature, there are still several key questions that it has not answered. The first of these relates to the perception of dynamic (motion-based) scenes. The literature on scene perception has largely concentrated on the perception and representation of static (motionless) scenes, generally presented in photographic format, either in isolation or in a series with other scenes. Because the visual world of humans is filled with motion, studies of scene perception must head towards incorporating dynamic, motion-based elements into their experimental programs to retain and expand their ecological validity. This is something the current thesis attempts to do. The first few experiments investigate the perception of dynamic scenes by using a basic visual search paradigm. The purpose of these initial experiments is to precisely quantify the relative contribution different static and dynamic stimulus attributes have in guiding and focusing visual attention. The second unanswered question in the scene perception literature relates to the interaction between the immediate selection and recognition of objects and their maintenance in memory for performance of a later task. There is much physiological evidence, coming from studies using single cell recordings, that looks at connections between temporal and frontal cortical areas thought to sub-serve the maintenance of visual information. There is also brain imaging evidence that looks at specific loci in the brain where working visual representations may be stored and/or supported. However, there is little work using EEG technology, which can give a picture of gross physiological function at small time scales. Because of the immense speed of temporo-frontal interactions, and the wide spatial extent of the areas involved, EEG is an excellent candidate for examining such activity. Therefore, this thesis attempts to examine the neural underpinnings of working visual representations in humans using EEG. The first EEG experiment of this thesis uses the same, simple visual stimuli as the behavioural experiments in order attain maximum control over the difficulty of the attentional and recognition tasks under consideration. The remaining EEG experiments use more complex stimuli, in order to further generalise the findings of the initial EEG experiment and the previous behavioural experiments. The data from the behavioural and EEG experiments will most likely complement each other, as the behavioural experiments aim to quantify the extent to which various visual features contribute to the formation of working visual representations while the EEG experiments seek to elucidate the neurophysiology behind the formation of such representations. Overall then, the behavioural experiments are looking primarily at how such representations are formed whereas the EEG experiments are looking primarily at where they are formed.