The purpose of this thesis was to improve our understanding of the progression of fatigue and recovery during repetitive work and to examine selected methods for predicting fatigue. In Chapter 2, a psychophysical methodology was used to validate the Maximum Acceptable Effort (MAE) equation of Potvin (2012) at duty cycles of greater than 0.5. The results from that study were used to evaluate the MAE equation in the higher duty cycle range. In Chapter 3, the fatigue process during complex MVC-relative force profiles was examined in a repetitive handgrip task. In Chapter 4, I examined the effect of manipulating the order of presentation of various MVC-relative force levels for a repetitive thumb flexion task. Additionally, the influence of post-activation potentiation was examined by stimulating the flexor pollicis longus (FPL) at specific time points during the complex profile. In Chapter 5, Xia and Frey Law’s (2008) three-compartment model (3CMXFL) of muscle fatigue was modified to more accurately reflect physiological processes. The model, with physiological modifications (3CMGMU), as well as the original 3CM optimized for complex tasks (3CMOPT), was optimized to predict the fatigue levels from the experiments described in Chapters 3 and 4, as well as 4 other similar experimental protocols. The predicted fatigue from the 3CMXFL was also compared to the experimental data. The 3CMOPT and 3CMGMU were compared against known endurance times. The 3CMGMU is proposed as an ergonomic tool for evaluating fatigue in repetitive tasks, and the future directions for fatigue modelling and using the MAE equation for complex force-time histories are addressed. This thesis provides the first studies of fatigue accumulation during complex MVC-relative time histories. The findings from this thesis can be applied to the workplace to reduce the risk of injury as a result of muscle fatigue. / Thesis / Doctor of Philosophy (PhD)

No description available.

The human visual system gathers information about three-dimensional object shape from a wide range of sources. How effectively we can use these sources, and how they are combined to form a consistent and accurate percept of the 3D world is the focus of much research. In complex scenes inter-reflections of light between surfaces (mutual illumination) can occur, creating chromatic illumination gradients. These gradients provide a source of information about 3D object shape, but little research has been conducted into the capabilities of the visual system to use such information. The experiments described here were conducted with the aim of understanding the influence of chromatic gradients from mutual illumination on 3D shape perception. Psychophysical experiments are described that were designed to investigate: If the human visual system takes account of mutual illumination when estimating 3D object shape, and how this might occur; How colour shading cues are integrated with other shape cues; The relative influence on 3D shape perception of achromatic (luminance) shading and chromatic shading from mutual illumination. In addition, one chapter explores a selection of mathematical models of cue integration and their applicability in this case. The results of the experiments suggest that the human visual system is able to quickly assess and take account of colour mutual illuminations when estimating 3D object shape, and use chromatic gradients as an independent and effective cue. Finally, mathematical modelling reveals that the chromatic gradient cue is likely integrated with other shape cues in a way that is close to statistically optimal.

No description available.

<p>In recent years, many studies have reported that the tactile spatial acuity of blind participants is enhanced relative to that of sighted participants, but it is unclear what factors drive this enhancement. In the series of three psychophysics studies (of tactile spatial acuity) presented in this thesis, we attempted to tease apart two hypotheses explaining tactile spatial acuity enhancement in the blind: visual deprivation and tactile experience. To measure tactile spatial acuity in these studies, we used a grating orientation task. In the first study (Chapter 2), we found that blind participants outperformed sighted participants, but only on body parts where tactile experience is presumably greater in blind than in sighted participants (i.e., fingertips, not lips); we found additionally that blind participants’ tactile acuity correlated with their Braille reading behaviour (e.g., style, frequency of reading). In the second study (Chapter 3), we found that visual deprivation of sighted participants for periods up to 110 minutes did not enhance their sense of touch. In the third study (Chapter 4), we found that extensive training on a tactile task can substantially improve sighted participants’ sense of touch. The findings from our three studies thus provide consistent support for the hypothesis that tactile experience, but not visual deprivation, drives tactile spatial acuity enhancement in the blind.</p> / Doctor of Philosophy (PhD)

The human visual system has the remarkable capacity to transform spatio-temporal patterns of light into structured units of perception. Much research has focused on how the visual system integrates information around the perimeter of closed contours to form the perception of shape. This dissertation extends previous work by investigating how the perception of curvature along closed-contour shapes is affected by the presence of additional shapes that appear close to the target shape in space and/or time. Chapter 2 examined the ability of shape mechanisms at representing low frequency curvature in the presence of a higher frequency component along contours in multi-shape displays. We found that additions of high amplitude, high frequency curvature along a contour path can modulate the strength of interaction observed between shapes, and thus attenuates the contribution of low frequency components in interactions between neighbouring contours. Chapter 3 examined what curvature features are of importance in modulating phase dependent interactions between shapes. Results revealed that phase-dependent masking does not depend on curvature frequency, but is related to sensitivity for phase shifts in isolated contours, and is affected by both positive and negative curvature extrema. Computational simulations aimed at modelling the population responses evoked in intermediate shape processing areas (i.e., V4) suggest sensitivity to shifts in phase of shapes is not well captured by such a population code, and therefore alternative explanations are required. Chapter 4 examined how sensitivity to curvature deformations along the contour of a closed shape changes as a function of polar angle, angular frequency, and spatial uncertainty. Results show that human observers are, at first approximation, uniformly sensitivity to curvature deformations across all polar angles tested, and this result holds despite changes in angular frequency and spatial uncertainty. Chapter 5 examined whether the strength of spatial masking between shapes is affected by the presentation of a temporal mask. Our results demonstrate that a temporal mask affected spatial masking only when it preceded the target-mask stimulus by 130-180ms. Furthermore, the effects of a temporal mask on spatial masking are approximately additive, suggesting that separate components contribute to spatial and temporal interactions between shapes. / Thesis / Doctor of Philosophy (PhD)

Advancements in computer technology are making video conferencing a viable communication medium for desktop computers. These same advancements are changing the structure and means by which information workers conduct business. From a human factors perspective, however, the study of new communication technologies and their relationships with end users presents a challenging research domain. This study employed two diverse research approaches to the problem of reduced video frame rate in desktop video conferencing. In the first study, a psychophysical method was used to evaluate video image quality as a function of frame rate for a series of different scenes. Scenes varied in terms of level of detail, velocity of panning, and content. Results indicate that for most scenes, differences in frame rate become less detectable above approximately 10 frames per second (fps), suggesting a curvilinear relationship between image quality and frame rate. For a traditional conferencing scene, however, a linear increase in frame rate produced a linear improvement in perceived image quality. High detail scenes were perceived to be of lower quality than the low detail scenes, while panning velocity had no effect. In the second study, a collection of research methods known as ethnography was used to examine long-term use of desktop video by collaborators in a real work situation. Participants from a graduate course met each week for seven weeks and worked on a class project under one of four communication conditions: face-to-face, 1 fps, 10 fps, and 25 fps. Dependent measures included interviews, questionnaires, interaction analysis measures, and ethnomethodology. Recommendations are made regarding the utility and expense of each method with respect to uncovering human factors issues in video-mediated collaboration. It is believed that this research has filled a significant gap in the human factors literature of advanced telecommunications and research methodology. / Ph. D.

Yes / This study aimed to develop a clinical test of face perception which is applicable to a wide range of patients and can capture normal variability. The Caledonian face test utilises synthetic faces which combine simplicity with sufficient realism to permit individual identification. Face discrimination thresholds (i.e. minimum difference between faces required for accurate discrimination) were determined in an "odd-one-out" task. The difference between faces was controlled by an adaptive QUEST procedure. A broad range of face discrimination sensitivity was determined from a group (N=52) of young adults (mean 5.75%; SD 1.18; range 3.33-8.84%). The test is fast (3-4min), repeatable (test-re-test r2=0.795) and demonstrates a significant inversion effect. The potential to identify impairments of face discrimination was evaluated by testing LM who reported a lifelong difficulty with face perception. While LM's impairment for two established face tests was close to the criterion for significance (Z-scores of -2.20 and -2.27) for the Caledonian face test, her Z-score was -7.26, implying a more than threefold higher sensitivity. The new face test provides a quantifiable and repeatable assessment of face discrimination ability. The enhanced sensitivity suggests that the Caledonian face test may be capable of detecting more subtle impairments of face perception than available tests. / None

Yes / Human neuropsychological and neuroimaging studies have raised the possibility that different attributes of optic flow stimuli, namely radial direction and the position of the focus of expansion (FOE), are processed within separate cortical areas. In the human brain, visual areas V5/MT+ and V3A have been proposed as integral to the analysis of these different attributes of optic flow stimuli. In order to establish direct causal relationships between neural activity in V5/MT+ and V3A and the perception of radial motion direction and FOE position, we used Transcranial Magnetic Stimulation (TMS) to disrupt cortical activity in these areas whilst participants performed behavioural tasks dependent on these different aspects of optic flow stimuli. The cortical regions of interest were identified in seven human participants using standard fMRI retinotopic mapping techniques and functional localisers. TMS to area V3A was found to disrupt FOE positional judgements, but not radial direction discrimination, whilst the application of TMS to an anterior sub-division of hV5/MT+, MST/TO-2, produced the reverse effects, disrupting radial direction discrimination but eliciting no effect on the FOE positional judgement task. This double dissociation demonstrates that FOE position and radial direction of optic flow stimuli are signalled independently by neural activity in areas hV5/MT+ and V3A. / This work was funded by the BBSRC (grant B/N003012/1).

Yes / Faces are highly complex stimuli that contain a host of information. Such complexity poses the following questions: (a) do observers exhibit preferences for specific information? (b) how does sensitivity to individual face parts compare? These questions were addressed by quantifying sensitivity to different face features. Discrimination thresholds were determined for synthetic faces under the following conditions: (i) ‘full face’: all face features visible; (ii) ‘isolated feature’: single feature presented in isolation; (iii) ‘embedded feature’: all features visible, but only one feature modified. Mean threshold elevations for isolated features, relative to full-faces, were 0.84x, 1.08, 2.12, 3.34, 4.07 and 4.47 for head-shape, hairline, nose, mouth, eyes and eyebrows respectively. Hence, when two full faces can be discriminated at threshold, the difference between the eyes is about four times less than what is required when discriminating between isolated eyes. In all cases, sensitivity was higher when features were presented in isolation than when they were embedded within a face context (threshold elevations of 0.94x, 1.74, 2.67, 2.90, 5.94 and 9.94). This reveals a specific pattern of sensitivity to face information. Observers are between two and four times more sensitive to external than internal features. The pattern for internal features (higher sensitivity for the nose, compared to mouth, eyes and eyebrows) is consistent with lower sensitivity for those parts affected by facial dynamics (e.g. facial expressions). That isolated features are easier to discriminate than embedded features supports a holistic face processing mechanism which impedes extraction of information about individual features from full faces.