Biomedical subjects

Guy Wallis

Publications and source records attributed to Guy Wallis.

6 recordsLinked to original sources

PubMed2006

The temporal and spatial limits of compensation for fixational eye movements.

High-fidelity eye tracking is combined with a perceptual grouping task to provide insight into the likely mechanisms underlying the compensation of retinal image motion caused by movement of the eyes. The experiments describe the covert detection of minute temporal and spatial offsets incorporated into a test stimulus. Analysis of eye motion on individual trials indicates that the temporal offset sensitivity is actually due to motion of the eye inducing artificial spatial offsets in the briefly presented stimuli. The results have strong implications for two popular models of compensation for fixational eye movements, namely efference copy and image-based models. If an efference copy model is assumed, the results place constraints on the spatial accuracy and source of compensation. If an image-based model is assumed then limitations are placed on the integration time window over which motion estimates are calculated.

Eye Movements↗

PubMed2006

Reflexive optokinetic nystagmus in younger and older observers under photopic and mesopic viewing conditions.

PURPOSE: To investigate the effect of age on optokinetic nystagmus (OKN) in response to stimuli designed to preferentially stimulate the M-pathway. METHOD: OKN was recorded in 10 younger (32.3 +/- 5.98 years) and 10 older (65.6 +/- 6.53) subjects with normal vision. Vertical gratings of 0.43 or 1.08 cpd drifting at 5 degrees /s or 20 degrees /s and presented at either 8% or 80% contrast were displayed on a large screen as full-field stimulation, central stimulation within a central Gaussian-blurred window of 15 degrees diameter, or peripheral stimulation outside this window. All conditions apart from the high-contrast condition were presented in a random order at two light levels, mesopic (1.8 cdm(-2)) and photopic (71.5 cdm(-2)). RESULTS: Partial-field data indicated that central stimulation, mesopic light levels, and lower temporal frequency each significantly increased slow-phase velocity (SPV). Although there was no overall difference between groups for partial-field stimulation, full-field stimulation, or low-contrast stimulation, a change in illumination revealed a significant interaction with age: there was a larger decrease in SPV going from photopic to mesopic conditions for the older group than the younger group, especially for higher temporal frequency stimulation. CONCLUSIONS: OKN becomes reflexive in conditions conducive to M-pathway stimulation, and this rOKN response is significantly diminished in older healthy adults than in younger healthy adults, indicative of decreased M-pathway sensitivity.

Adult↗

PubMed2005

Stability criteria for unsupervised temporal association networks.

A biologically realizable, unsupervised learning rule is described for the online extraction of object features, suitable for solving a range of object recognition tasks. Alterations to the basic learning rule are proposed which allow the rule to better suit the parameters of a given input space. One negative consequence of such modifications is the potential for learning instability. The criteria for such instability are modeled using digital filtering techniques and predicted regions of stability and instability tested. The result is a family of learning rules which can be tailored to the specific environment, improving both convergence times and accuracy over the standard learning rule, while simultaneously insuring learning stability.

Learning↗

PubMed2004

Contradictory influence of context on predominance during binocular rivalry.

BACKGROUND: Binocular rivalry is a complex process characterised by alternations in perceptual suppression and dominance that result when two different images are presented simultaneously to the left and right eyes. It has been reported recently that the addition of contextual cues will promote the predominance of the context consistent rivalry target. In contrast to Levelt's second proposition (1965), this effect has been found to result exclusively from an increase in the dominance phase duration, while the suppression phase duration remains unaffected. METHODS: Human subjects were simultaneously presented with a small (2 degrees ) disc consisting of gratings (four cycles per degree) of different orientations to the two eyes. Four experiments were conducted to ascertain the effects of background gratings and contextual colour information on target predominance and phase duration. For each of the four experimental conditions, the orientation and colour of the target gratings and surrounding contextual background were systematically manipulated. RESULTS: In this study, we report an effect opposite to that of Levelt. Contradictory contextual information increases target predominance and phase duration during binocular rivalry. Our results demonstrate that it is possible to promote the dominance of the context contradictory percept with co-linearity, co-chromaticity and orientation cues. In line with previous studies involving context, we find that this effect on predominance is due to an increase in the duration of the dominance rather than the suppression phase. DISCUSSION: We discuss our findings in respect to those from previous studies and consider high- and low-level processes that may be responsible for these apparently 'contradictory' roles of context on binocular rivalry. In addition, we discuss how the apparent 'anti-Levelt' effect of context can be reinterpreted in a manner that brings it back in line with Levelt's second proposition and raises the question of whether 'suppressability' plays a disproportionately large role in determining the duration of perceptual phases in binocular rivalry.

Dominance, Ocular↗

PubMed2002

An unexpected role for visual feedback in vehicle steering control.

Some motor tasks can be completed, quite literally, with our eyes shut. Most people can touch their nose without looking or reach for an object after only a brief glance at its location. This distinction leads to one of the defining questions of movement control: is information gleaned prior to starting the movement sufficient to complete the task (open loop), or is feedback about the progress of the movement required (closed loop)? One task that has commanded considerable interest in the literature over the years is that of steering a vehicle, in particular lane-correction and lane-changing tasks. Recent work has suggested that this type of task can proceed in a fundamentally open loop manner, with feedback mainly serving to correct minor, accumulating errors. This paper reevaluates the conclusions of these studies by conducting a new set of experiments in a driving simulator. We demonstrate that, in fact, drivers rely on regular visual feedback, even during the well-practiced steering task of lane changing. Without feedback, drivers fail to initiate the return phase of the maneuver, resulting in systematic errors in final heading. The results provide new insight into the control of vehicle heading, suggesting that drivers employ a simple policy of "turn and see," with only limited understanding of the relationship between steering angle and vehicle heading.

Automobile Driving↗

PubMed1996

Using Spatio-temporal Correlations to Learn Invariant Object Recognition.

A competitive network is described which learns to classify objects on the basis of temporal as well as spatial correlations. This is achieved by using a Hebb-like learning rule which is dependent upon prior as well as current neural activity. The rule is shown to be capable of outperforming a supervised rule on the cross validation test of an invariant character recognition task, given a relatively small training set. It is also shown to outperform the supervised version of Fukushima's Neocognitron ([Fukushima, 1980]), on a larger training set. Copyright 1996 Elsevier Science Ltd.

Journal Article↗