John P. Wann

Does limb proprioception drift

SummaryThe hypothesis that proprioceptively perceived limb position drifts during visual occlusion was re-examined by combining some of the protocols used in previous experiments. Sixteen adult subjects made judgements of static limb position during visual occlusion lasting up to 2 min. In addition, the effect of brief 250 ms “glimpses” of the limb, occasional proprioceptive stimulation and directed attention were examined. Despite, conflicting evidence from earlier experiments, there was clear evidence of a drift in perceived limb position, towards the body, during visual occlusion. This drift was halted if brief glimpses of the limb were provided, or minor re-positioning (without vision) was allowed. In neither case, however, did the supplementary cues reset limb position to its originally perceived position. Drift was amplified when subjects attempted to attend to limb position rather than perform a secondary tracking task. The results are not easily accounted for if drift is considered purely as an effect of peripheral sensor adaptation. A notion of central-drift between visual and proprioceptive maps is suggested as an alternative hypothesis.

Weighted combination of size and disparity: A computational model for timing a ball catch.

How do we time hand closure to catch a ball? Binocular disparity and optical looming provide two sources of information about an objects motion in depth, but the relative effectiveness of the two cues depends on ball size. Based on results from a virtual reality ball–catching task, we derive a simple model that uses both cues. The model is sensitive to the relative effectiveness of size and disparity and implicitly switches its response to the cue that specifies the earliest arrival and away from a cue that is lost or below threshold. We demonstrate the models robustness by predicting the response of participants to some very unusual ball trajectories in a virtual reality task.

What does virtual reality need?: human factors issues in the design of three-dimensional computer environments

Virtual reality (VR) has invaded the publics awareness through a series of media articles that have promoted it as a new and exciting form of computer interaction. We discuss the extent to which VR may be a useful tool in visualization and attempt to disambiguate the use of VR as a general descriptor for any three-dimensional computer presentation. The argument is presented that, to warrant the use of the term virtual environment (VE), the display should satisfy criteria that arise from the nature of human spatial perception. It directly follows, therefore, that perceptual criteria are the foundations of an effective VE display. We address the task of making a VE system easy to navigate, traverse and engage, by examining the ways in which three-dimensional perception and perception of motion may be supported, and consider the potential conflict that may arise between depth cues. We propose that the design of VE systems must centre on the perceptual-motor capabilities of the user, in the context of the task to be undertaken, and establish what is essential, desirable and optimal in order to maximize the task gains, while minimizing the learning required to operate within three-dimensional interactive displays.

Relation between velocity and curvature in movement: equivalence and divergence between a power law and a minimum-jerk model

Unconstrained hand movements typically display a decrease in hand speed around highly curved sections of a trajectory. It has been suggested that this relation between tangential velocity and radius of curvature conforms to a one-third power law. We demonstrate that a one-third power law can be explained by models taking account of trajectory costs such as a minimum-jerk model. Data were analyzed from 6 subjects performing elliptical drawing movements of varying eccentricities. Conformity to the one-third power law in the average was obtained but is shown to be artifactual. It is demonstrated that asymmetric velocity profiles may result in consistent departures from a one-third power law but that such differences may be masked by inappropriate analysis procedures. We introduce a modification to the original minimum-jerk model by replacing the assumption of a Newtonian point-mass with a visco-elastic body. Simulations with the modified model identify a basis for asymmetry of velocity profiles and thereby predict departures from a one-third law commensurate with the empirical findings.

Anticipating arrival: is the tau margin a specious theory?

A critical review is presented of previous studies that have examined the use of the tau margin (τ) in the temporal judgment of arrival in natural contexts. This body of evidence is frequently cited as providing strong support for the use of τ for interceptive timing. Critical flaws are demonstrated in either the analysis or interpretation of many of these studies. It is suggested that temporal control in a number of these experiments could have been effected using a relative distance estimate (ζ ratio). Results are also presented and discussed that conflict with the τ control hypothesis. It is concluded that although the τ hypothesis provides an appealing account of interceptive timing, its broad acceptance is unwarranted on the evidence available.

Why you should look where you are going.

It is controversial whether head and eye movement information are required to discern locomotor heading from visual motion information. We present a new theory of steering based on active gaze and retinal flow, which demonstrates that future paths could be judged using known properties of visual cortex neurons, without recovering current heading or integrating extra-retinal signals. This theory is consistent with the gaze-sampling behavior promoted in advanced driving instruction.

Improving vision: neural compensation for optical defocus

Anecdotal reports abound of vision improving in myopia after a period of time without refractive correction. We explored whether this effect is due to an increased tolerance of blur, or whether it reflects a genuine improvement in vision. Our results clearly demonstrated a marked improvement in the ability to detect and recognize letters following prolonged exposure to optical defocus. We ensured that ophthalmic change did not occur, and thus the phenomenon must be due to a neural compensation for thedefocus condition. A second set of experiments measured contrast sensitivity and found a decrease in sensitivity to mid–range (5–25) cycles deg−1 spatial frequencies following exposure to optical defocus. The results of the two experiments may be explained by the unmasking of low contrast, high spatial frequency information via a two–stage process: (1) the pattern of relative channel outputs is maintained during optical defocus by the depression of mid–range spatial frequency channels; (2) channel outputs are pooled prior to the production of the final percept. The second set of experiments also provided some evidence of inter–ocular transfer, indicating that the adaptation process is occurring at binocular sites in the cortex.

Controlling steering and judging heading: Retinal flow, visual direction, and extraretinal information.

The contribution of retinal flow (RF), extraretinal (ER), and egocentric visual direction (VD) information in locomotor control was explored. First, the recovery of heading from RF was examined when ER information was manipulated; results confirmed that ER signals affect heading judgments. Then the task was translated to steering curved paths, and the availability and veracity of VD were manipulated with either degraded or systematically biased RF. Large steering errors resulted from selective manipulation of RF and VD, providing strong evidence for the combination of RF, ER, and VD. The relative weighting applied to RF and VD was estimated. A point-attractor model is proposed that combines redundant sources of information for robust locomotor control with flexible trajectory planning through active gaze.

Steering with or without the flow: is the retrieval of heading necessary?

A separation column and method for separating anion and cation exchange resins in an upwardly flowing stream of water employing ultrasonic level sensing means. The sensing means include first and second transducer means for respectively sensing and receiving an ultrasonic signal in the frequency range of 1 to 5 megacycles per second. In one embodiment of the invention, the level of resin within the column is sensed when the resin interrupts transmission of the ultrasonic signal at a probe. In a second embodiment, the ultrasonic signal is directed toward the surface of the resin, and the time delay for receipt of an echo is measured. This time delay, in turn, gives an indication of the level of ion exchange resin within the column.

Eye-movements aid the control of locomotion

Eye-movements have long been considered a problem when trying to understand the visual control of locomotion. They transform the retinal image from a simple expanding pattern of moving texture elements (pure optic flow), into a complex combination of translation and rotation components (retinal flow). In this article we investigate whether there are measurable advantages to having an active free gaze, over a static gaze or tracking gaze, when steering along a winding path. We also examine patterns of free gaze behavior to determine preferred gaze strategies during active locomotion. Participants were asked to steer along a computer-simulated textured roadway with free gaze, fixed gaze, or gaze tracking the center of the roadway. Deviation of position from the center of the road was recorded along with their point of gaze. It was found that visually tracking the middle of the road produced smaller steering errors than for fixed gaze. Participants performed best at the steering task when allowed to sample naturally from the road ahead with free gaze. There was some variation in the gaze strategies used, but sampling was predominantly of areas proximal to the center of the road. These results diverge from traditional models of flow analysis.