Annaliese M. Plooy

The influence of computer monitor height on head and neck posture

A storage element responsive to static and dynamic input signals which generates complementary static and dynamic output signals and incorporates scan test logic. The invention includes a first circuit for receiving dynamic and static input signals and providing static output signals in response thereto and a second circuit connected to the first circuit for providing dynamic output signals. In the illustrative embodiment, the first circuit includes a static flip-flop constructed with a multiplexer, a static input (master) latch and a static output (slave) latch. The static input latch provides first and second intermediate complementary outputs on first and second intermediate output terminals respectively. In the illustrative embodiment, the second circuit is an arrangement which includes a first switching element with a first terminal connected to a first node, a control terminal responsive to the first intermediate complementary output signal and a third terminal for providing a third intermediate complementary output signal. The second circuit includes a second switching element having a first terminal connected to the first node, a control terminal responsive to a second complementary input signal and a third terminal for providing a fourth intermediate complementary output signal. The second circuit further includes a third switching element having a first terminal connected to the first node, a second terminal connected to a source of supply and a control terminal connected to a source of a clock signal.

The attentional demands of preferred and non-preferred gait patterns

The purpose of this study was to determine the attentional demands of natural and imposed gait, as well as the attentional costs of transitions between the walking and running co-ordination patterns. Seven healthy young men and four healthy young women undertook an auditory probe reaction time task concurrently with self-selected gait (Experiment 1) and imposed walking and running (Experiment 2) at different speeds on a motor-driven treadmill. In Experiment 1, where participants were free to choose their own movement pattern to match the speed of travel of the treadmill, normal gait control was shown to have a significant attentional cost, and hence not be automatic in the classical sense. However, this attentional cost did not differ between the two gait modes or at the transition point. In Experiment 2, where participants were required to maintain specific gait modes regardless of the treadmill speed, the maintenance of walking at speeds normally associated with running was found to have an attentional cost whereas this was not the case for running at normal walking speeds. Collectively the findings support a model of gait control in which the normal switching between gait modes is determined with minimal attention demand and in which it is possible to sustain non-preferred gait modes although, in the case of walking, only at a significant attentional/cognitive cost.

Wrist posture during computer pointing device use

OBJECTIVE This research examines individual differences in the wrist postures adopted during the use of two pointing devices (mouse and trackball). DESIGN A multiple case study of twelve participants was employed. BACKGROUND The use of pointing devices may lead to musculoskeletal discomfort and injury as a consequence of prolonged exposure to postures involving wrist extension and ulnar deviation. METHODS Wrist flexion/extension and radial/ulnar deviation was measured while twelve participants completed two standardised tasks involving horizontal and vertical cursor movements respectively. RESULTS Exposure to extreme ulnar deviation and wrist extension was observed in the use of computer mouse and trackball. The trackball involved decreased ulnar deviation and increased wrist extension, however considerable individual differences were observed. CONCLUSIONS Some users may be placed at risk of injury by prolonged exposure to the use of such devices, while others may not. A trackball may reduce the exposure to extreme ulnar deviation, but in some cases, a trackball may increase exposure to extreme wrist extension.

Effects of acoustic startle stimuli on interceptive action.

In reaction time (RT) tasks, presentation of a startling acoustic stimulus (SAS) together with a visual imperative stimulus can dramatically reduce RT while leaving response execution unchanged. It has been suggested that a prepared motor response program is triggered early by the SAS but is not otherwise affected. Movements aimed at intercepting moving targets are usually considered to be similarly governed by a prepared program. This program is triggered when visual stimulus information about the time to arrival of the moving target reaches a specific criterion. We investigated whether a SAS could also trigger such a movement. Human experimental participants were trained to hit moving targets with movements of a specific duration. This permitted an estimate of when movement would begin (expected onset time). Startling and sub-startle threshold acoustic probe stimuli were delivered unexpectedly among control trials: 65, 85, 115 and 135 ms prior to expected onset (10:1 ratio of control to probe trials). Results showed that startling probe stimuli at 85 and 115 ms produced early response onsets but not those at 65 or 135 ms. Sub-threshold stimuli at 115 and 135 ms also produced early onsets. Startle probes led to an increased vigor in the response, but sub-threshold probes had no detectable effects. These data can be explained by a simple model in which preparatory, response-related activation builds up in the circuits responsible for generating motor commands in anticipation of the GO command. If early triggering by the acoustic probes is the mechanism underlying the findings, then the data support the hypothesis that rapid interceptions are governed by a motor program.

The Utilisation of Visual Information in the Control of Rapid Interceptive Actions

When intercepting a moving target, accurate timing depends, in part, upon starting to move at the right moment. It is generally believed that this is achieved by triggering motor command generation when a visually perceived quantity such as the targets time-to-arrival reaches a specific criterion value. An experimental method that could be used to determine the moment when this visual event happens was introduced by Whiting and coworkers in the 1970s, and it involves occluding the vision of the target at different times prior to the time of movement onset (MO). This method is limited because the experimenter has no control over MO time. We suggest a method which provides the needed control by having people make interceptive movements of a specific duration. We tested the efficacy of this method in two experiments in which the accuracy of interception was examined under different occlusion conditions. In the first experiment, we examined the effect of changing the timing of an occlusion period (OP) of fixed duration (200 ms). In the second experiment, we varied the duration of the OP (180-430 ms) as well as its timing. The results demonstrated the utility of the proposed method and showed that performance deteriorated only when the participants had their vision occluded from 200 ms prior to MO. The results of Experiment 2 were able to narrow down the critical interval to trigger the interceptive action to within the period from 200 to 150 ms prior to MO, probably closer to 150 ms. In addition, the results showed that the execution of brief interceptive movements (180 ms) was not affected by the range of OPs used in the experiments. This indicates that the whole movement was prepared in advance and triggered by a visual stimulus event that occurred at about 150 ms before onset.

Manual interception of moving targets in two dimensions: performance and space-time accuracy.

We report results from four experiments that examined performance of an interceptive task that restricted movement of the hand and moving target to a horizontal plane. The task required accurate control over both where and when interception takes place. Three experiments studied the effects of four independent variables: target speed, target size, manipulandum size and movement amplitude. For small amplitude movements, small, fast targets were hit harder than larger slower ones and targets were hit harder with smaller manipulanda; movement time (MT) was unaffected by target size, but was shorter when the manipulandum was smaller. For larger amplitude movements, smaller, faster targets were also hit harder, but MTs tended to be greater when targets were smaller. The results support the idea that MT and peak movement speed can be independently controlled to some degree in order to meet the accuracy demands of the task. Analysis of the task showed that spatial and temporal accuracy demands are interdependent, indicating that the spatial and temporal variable errors should covary such that increases in one are accompanied by decreases in the other. This can be tested if there is no variation in interception location; which was not the case in the first three experiments. In a final experiment variation in interception location was restricted by requiring that the target be struck through an aperture. Both spatial and temporal variable errors could be estimated. As predicted, it was found that when spatial errors were small, temporal errors were large.

Reducing Contrast Makes Speeds in a Video-Based Driving Simulator Harder to Discriminate as Well as Making Them Appear Slower

We investigated the effect of reducing image contrast on speed perception using a video-based driving simulator in which participants viewed pairs of scenes and were asked to judge whether the second scene was faster or slower than the first scene. We predicted two outcomes: (i) that vehicle speeds would become harder to discriminate, and (ii) that vehicle speeds would appear slower. There is previous evidence confirming the latter prediction in a less realistic computer-based driving simulation, but none demonstrating the former. Our results supported both predictions, each of which may have traffic-safety implications when reduced-contrast conditions are experienced in real life, such as with fog or when the driver has cataracts.

The contribution of vision and proprioception to judgements of finger proximity

Abstract We sought to determine whether an increase in judged egocentric distance created by increasing vergence-specified distance would be negated when participants pointed at their own finger. It was found that ocular position dominates limb proprioception in the judgement of finger distance in the sagittal plane when vision is available. In contrast, an increase in perceived egocentric distance was largely attenuated by the presence of limb proprioception in reduced visual cue conditions. We conclude that the relative contribution of vergence to perceived distance depends upon the strength of the vergence effort signal when there are other cues present. Furthermore, if the distance percept includes a major contribution from retinal cues, then the visual component will dominate the limb proprioception component. If the visual component is largely determined by vergence information, limb proprioception will make a significant contribution and actually dominate when the vergence effort signal is weak. The results extend previous studies that have found a similar relationship between ocular position and limb proprioception in the perception of a finger′s location in the coronal plane.

Auditory feedback influences perceived driving speeds.

Reducing the level of internal noise is seen as a goal when designing modern cars. One danger of such a philosophy is that one is systematically attempting to alter one of the cues that can be used by drivers to estimate speed and this could bias speed judgments and driving behaviour. Seven participants were presented with pairs of video-based driving scenes and asked to judge whether the second scene appeared faster or slower than the first (2-alternative forced-choice task using the method of constant stimuli). They either heard in-car noise at the level it occurred in the real world or reduced in volume by 5 dB. The reduction in noise led to participants judging speeds to be significantly slower and this effect was evident for all participants. This finding indicates that, when in-car noise is attenuated, drivers are likely to underestimate their speed, potentially encouraging them to drive faster and placing them at greater risk of crashing.

A helmet mounted display system with active gaze control for visual telepresence

The objective of a Visual Telepresence System is to provide the operator with a high fidelity image from a remote stereo camera pair linked to a pan/tilt device such that the operator may reorient the camera position by use of head movement. Systems such as these which utilise virtual reality style helmet mounted displays have a number of limitations. The geometry of the camera positions and of the displays is generally fixed and is most suitable only for viewing elements of a scene at a particular distance. To address such limitations, a prototype system has been developed where the geometry of the displays and cameras is dynamically controlled by the eye movement of the operator. This paper explores why it is necessary to actively adjust the display system as well as the cameras and justifies the use of mechanical adjustment of the displays as an alternative to adjustment by electronic or image processing methods. The electronic and mechanical design is described including optical arrangements and control algorithms. The performance and accuracy of the system is assessed with respect to eye movement.