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Biomedical subjects

R A Clement

Publications and source records attributed to R A Clement.

At least 19 recordsLinked to original sources
PubMed1992

Gaze angle explanations of the induced effect.

Mayhew and Longuet-Higgins formulated a computational explanation of the induced effect which successfully predicts the conditions under which the induced effect will occur. Underlying their theories are the assumptions that disparity information is separated into horizontal and vertical components and that the vertical disparities are used to calculate the gaze angles. An implementation of the fusional explanation introduced by Petrov makes similar predictions for the induced effect, but does not depend on these two assumptions.

Female↗
PubMed1991

Natural coordinates for specification of eye movements.

Tweed and Vilis (Journal of Neurophysiology, 58, 832-849, 1987) have argued that quaternion algebra provides the most appropriate description of the rotations of the eye, and have derived a three-dimensional model of gaze control based on quaternion operations. Euler angles give a simpler description of the rotations of the eye, and can also be used to formulate an alternative version of the three-dimensional gaze control model. Comparison of the two versions of the model highlights the distinction between the functional predictions of the model, and the predictions which depend only on the choice of mathematical descriptions.

Humans↗
PubMed1990

An extension of Helmholtz's explanation of Listing's law.

Helmoholtz argued that the eye moves in accordance with Listing's law to ensure that objects have the same apparent direction despite changes in the point of fixation. Listing's law ensures that the same pattern of image displacements is associated with a given eye movement, independent of the initial point of fixation. His explanation has been extended to the problem of ensuring that objects remain at the same apparent depth, with both direct and indirect fixation. This problem is simplified if disparity is analysed in the region of the frontal plane.

Depth Perception↗
PubMed1989

An investigation of trained neural networks from a neurophysiological perspective.

The application of theoretical neural networks to preprocessed images was investigated with the aim of developing a computational recognition system. The neural networks were trained by means of a back-propagation algorithm, to respond selectively to computer-generated bars and edges. The receptive fields of the trained networks were then mapped, in terms of both their synaptic weights and their responses to spot stimuli. There was a direct relationship between the pattern of weights on the inputs to the hidden units (the units in the intermediate layer between the input and the output units), and their receptive field as mapped by spot stimuli. This relationship was not sustained at the level of the output units in that their spot-mapped responses failed to correspond either with the weights of the connections from the hidden units to the output units, or with a qualitative analysis of the networks. Part of this discrepancy may be ascribed to the output function used in the back-propagation algorithm.

Algorithms↗
PubMed1989

Computed extraocular muscle activity during blinks.

The rotations and retractions of the eyeball due to co-contraction of the extraocular muscles are investigated using a model of the mechanics of the orbital plant. The hypothesis that the rotational movements are secondary to globe retraction is not supported.

Blinking↗
PubMed1988

Normal visual pathway routing in dissociated vertical deviation.

Flash visually evoked cortical potentials have been recorded in three groups of age- and sex-matched subjects; one comprised of subjects with dissociated vertical deviation, one comprised of subjects with oculocutaneous albinism and one group of controls. The latency of the major positive (P2) component did not show statistically significant contralateral lateralization on monocular stimulation in either the dissociated vertical deviation group or the control group. Contralateral lateralization was found in the albino group at a statistically significant level (P less than 0.01). It is concluded that subjects with dissociated vertical deviation do not possess the typical albino optic pathway misrouting that has been reported.

Adolescent↗
PubMed1987

Line correspondence in binocular vision.

The mathematical analysis of binocular vision introduced by Helmholtz is applied to the problem of the use of disparity information to position a stimulus in depth. It is shown that matching the images from the left and right eyes along radial directions is an alternative to matching images along the horizontal direction only.

Attention↗
PubMed1987

A method for raytracing through schematic eyes with off-axis components.

A linear algebraic method for tracing skew rays through aspheric surfaces is described. The advantage of this method is that the refracting surfaces can be translated and rotated with respect to each other. The method is used to investigate the optical factors affecting the location of Purkinje images.

Cornea↗
PubMed1987

A schematic eye model for the effects of translation and rotation of ocular components on peripheral astigmatism.

The relative contributions of translation and rotation of the cornea and lens to peripheral astigmatic asymmetry have been investigated using a linear algebraic ray tracing method. It is believed that lenticular rotation is responsible for angle alpha, so bringing about peripheral astigmatic asymmetry, as normally occurs in human eyes over the temporal and nasal retina. Rotation of the cornea may be responsible for the small numbers of eyes which exhibit large amounts of peripheral astigmatic asymmetry. The effects of corneal rotation and translation on the dimensions of the entrance pupil are illustrated.

Astigmatism↗
PubMed1987

A model for retinal shape changes in ametropia.

Modeling ametropic retinal shape changes with an ellipsoid of constant equatorial radius has been found to produce types of peripheral astigmatism which are in good agreement with experimental findings.

Astigmatism↗
PubMed1987

Excursion tests of ocular motility.

Three alternative excursion tests of ocular motility have been compared. A computer model of the mechanics of the extraocular muscles has been used to estimate the tensions developed by the muscles in these tests. On the basis of these calculations we recommend the transverse test in which the positions of the eyes are observed as they track a target moving from the right to the left with a fixed elevation and a fixed depression.

Computer Simulation↗
PubMed1986

Visual evoked cortical and subcortical potentials in human albinos.

Albinism is a congenital condition in which hypopigmentation occurs. In addition to this abnormality there is a misrouting of the optic nerve fibers, with some fibers from the temporal retina following a crossed route at the chiasma and terminating in the contralateral cortical hemisphere. This contralateral preponderance of fibers from each eye should be recognizable from recording the visual evoked cortical potential over each hemisphere on monocular stimulation. Such a technique should produce evidence of responses of either increased amplitude or shorter latency over the contralateral hemisphere. Twenty-five human albinos (twenty-three oculocutaneous, two ocular) have been examined. Pattern appearance-disappearance visual evoked cortical potentials were used, but only on bioccipital derivations did these show clear lateralization. With the flash response the P2 component has a consistently shorter latency over the contralateral hemisphere to the eye stimulated. The visual evoked subcortical potential shows contrasting lateralization. The implications of these findings are discussed.

Adolescent↗
PubMed1985

Lateralisation of the flash visually evoked cortical potential in albino babies.

Flash VECPs have been reported in 3 albino babies ranging in age from 1 to 7 months. The responses were dominated by a large positive component preceded by an earlier negative. The amplitude of this positive component measured from the peak of the preceding negative wave had a greater amplitude over the contralateral hemisphere on monocular stimulation indicative of the misrouting present in the albino visual system.

Albinism↗
PubMed1985

Source derivation of the brain-stem auditory evoked potential.

This study described the results of applying source derivation to the brain-stem auditory evoked potential. Unilateral stimulation gives rise to current sinks, associated with waves I and III, which are located in the vicinity of the ipsilateral mastoid. Waves II and V are associated with a pair of current sinks which appear bilaterally in the vicinity of each mastoid.

Adult↗