Displacement Thresholds Research Articles

Correlation of peripheral displacement thresholds and optic disc parameters in ocular hypertension.

Both peripheral displacement thresholds and measurement of optic disc parameters have been advocated in the early diagnosis of glaucoma at the stage before scotomata are detected on conventional visual field assessment. The peripheral movement displacement thresholds were measured in 50 eyes of 50 patients with ocular hypertension and a significant correlation was found with neuroretinal rim areas measured using a computerised image system. There was also a significant correlation with optic disc diameter but no correlation with cup-disc ratio. The results support the proposition that measurement of peripheral movement displacement thresholds may be a useful tool in the detection of early glaucomatous optic nerve damage.

Effects of posterior segment disorders on oscillatory displacement thresholds, and on acuities as measured using the potential acuity meter and laser interferometer.

Oscillatory displacement thresholds (ODTs) were measured in 67 subjects with clear ocular media, but known posterior segment disorders. The ability of these thresholds to assess reduced visual function, as quantified by visual acuity deficits, was compared with that of the Potential Acuity Meter (PAM) and the Rodenstock Retinometer (RR). ODTs were found to be sensitive to acuity deficits which were retinal/neural in origin. Despite the fact that interferometric acuities are, unlike ODTs, a measure of resolution, the RR proved no more accurate in assessing the level of letter acuity which existed than did ODT measurement. As would be expected, the PAM provided for the most accurate means of assessing non-optical visual acuity deficits. However, the PAM proved to be less useful in eyes with: very poor levels of acuity, and certain anomalous conditions. The results have implications for the relative ability of each of the three tests to identify reduced visual function behind cataract.

Motion displacement thresholds for compound stimuli predicted by the displacement of centroids

Minimum displacement thresholds were measured using a compound motion stimulus, in which a single bar split into oppositely moving components, having different luminances and absoute displacements. The results fit a simple model in which observers detect the movement of the centroid of the combined luminance pattern of the two components. This was so even when the two components were separated by more than the static spatial resolution limit of 30 sec arc.

Near-threshold ion-induced defect production in graphite.

Subplantation of the noble-gas ions ${\mathrm{He}}^{+}$, ${\mathrm{Ne}}^{+}$, ${\mathrm{Ar}}^{+}$, and ${\mathrm{Kr}}^{+}$ into graphite in the energy range of 10--150 eV with doses in the range of 1--15\ifmmode\times\else\texttimes\fi{}${10}^{14}$ ions/${\mathrm{cm}}^{2}$ has been studied by Auger electron spectroscopy (AES) and computer simulations. A technique based on AES line-shape analysis has been employed to describe quantitatively the ion-induced damage to the lattice. The carbon KLL AES line shapes and the AES spectra from subplanted Ne, Ar, and Kr were used to determine the ion penetration thresholds ${\mathit{E}}_{\mathit{p}}$, ion displacement thresholds ${\mathit{E}}_{\mathrm{th}}$, and the lattice displacement energies ${\mathit{E}}_{\mathit{d}}$. The ${\mathit{E}}_{\mathit{p}}$'s scale linearly with the atomic radius of the projectiles. Defect production begins at ${\mathit{E}}_{\mathit{p}}$, although these energies are below ${\mathit{E}}_{\mathrm{th}}$. A mechanism for defect production at energies below ${\mathit{E}}_{\mathrm{th}}$ based on noble-gas interstitials and lattice strain and distortion is developed. This process is modeled through the charmm molecular modeling program and the trim classical trajectory simulation.

Spatio-temporal characteristics of human motion perception

A bi-local detector array model was assumed to describe the functional performance of monocular motion perception. Distributions of model parameters were measured in human vision at several positions in the visual field. The stimulus paradigm was designed to measure directional motion perception thresholds for individual combinations of spatial displacement and temporal delay in random dot apparent motion stimuli. The resulting data support previous results on perceivable spatial displacement limits in human vision but also indicate that both minimum and maximum perceivable spatial displacement thresholds in human observers have a similar dependence on temporal delay. This dependence changes with eccentricity in the visual field in a qualitatively similar manner but by quantitatively different factors. A description of possible biological properties of the bi-local detector population is presented that may explain how detection of spatio-temporal pattern displacements can be performed by a single system. Such a system also predicts that minimum and maximum perceivable spatial displacement thresholds should scale with visual field eccentricity in a manner consistent with our results.

Pit organs in axolotls: a second class of lateral line neuromasts.

The lateral line system of axolotls (Ambystoma mexicanum) consists of mechanoreceptive neuromasts and electroreceptive ampullary organs. All neuromasts in salamanders are located superficially and are organized into lines that are homologous to canal neuromasts in fishes. Ampullary organs are confined to the head and generally are located adjacent to the lines of superficial neuromasts. Axolotls, however, also possess a third class of receptors; these form restricted patches on the head and are possibly homologous to the superficial pit organs in fishes. In order to test this hypothesis the morphology of the suspected pit organs was examined with scanning electron microscopy, and a number of their physiological properties were determined. Pit organs are approximately half the size of neuromasts and have fewer hair cells, although these hair cells do possess kinocilia and stereocilia like those of neuromasts. Pit organs also possess cupulae and exhibit a pattern of innervation identical to that of neuromasts. Pit organs and neuromasts also exhibit similar rates of spontaneous activity, are excited by weak water currents but not weak electric stimuli, and are not inhibited by magnesium ions. Pit organs appear to have slightly lower rates of spontaneous discharge than neuromasts, however, and have slightly lower displacement thresholds to low frequency wave stimuli. These data support the contention that the pit organs of axolotls constitute a second class of neuromasts homologous to the pit organs of fishes.

A neural network model for visual motion detection that can explain psychophysical and neurophysiological phenomena

This paper proposes a new neural network model for visual motion detection. The model can well explain both psychophysical findings (the changes of displacement thresholds with stimulus velocity and the perception of apparent motion) and neurophysiological findings (the selectivity for the direction and the velocity of a moving stimulus). To confirm the behavior of the model, numerical examinations were conducted. The results were consistent with both psychophysical and neurophysiological findings.

Comparison of displacement thresholds in auditory and visual modalities

Previous research of moving sound sources show an increase of minimal displacements as a direct function of velocity. The current experiment attempted to establish upper limits for this function. Five subjects were tested, three naive and two experienced. MAMA thresholds for velocity ranging from 10 to 1500 deg per second were obtained using a three-up one-down adaptive procedure. For comparison purposes, visual targets were also employed in the paradigm. For velocities in excess of 100 deg per second, auditory displacement thresholds were substantially lower than those obtained for vision. However, the superiority of the visual modality was evident at lower rates of travel. These results will be discussed relative to our hypothesis that the auditory system may have a significant role in oculomotor processes.

Visual dysfunction in type II diabetic patients revealed by a hyperacuity test.

Displacement thresholds for an oscillating bar, which fall into the hyperacuity range, were determined in 21 subjects with non-insulin dependent diabetes mellitus and 19 age-matched visually normal controls. The diabetic subjects were classed as either having minimal or no retinopathy. Whilst thresholds for the diabetic group were significantly raised above those of the normal group, there were no significant differences in thresholds between the diabetic subgroup with retinopathy and the subgroup without. Greater thresholds tended to be found at higher frequencies of oscillation as the known duration of the diabetes increased.

Displacement thresholds in sapphire

The displacement threshold for the aluminium ion in sapphire has been determined by comparing measurements of the colour centre concentration induced by electron bombardment with values calculated ...

Variations in hyperacuity performance with age

The term hyperacuity has been applied to a group of stimuli which produce spatial thresholds smaller than those expected given the relatively large receptor spacing and the retinal image quality of the human eye. It is not yet firmly established whether hyperacuity performance declines with increasing age in the same way as most other measures of visual ability. This is perhaps due to the use of varying task configurations and criterion-dependent psychophysical techniques. The present study examines age-related performance in three different hyperacuity tasks using a criterion independent forced-choice method. Both displacement and bisection thresholds were found to increase with age, but there was no significant change in vernier acuity. This indicates that age has a differential effect on thresholds for various hyperacuities depending upon the task requirements. No significant age-related trend was observed in hyperacuity bias, which represents the difference between subjective and true physical alignment.

Hyperacuity Thresholds for Oscillatory Movement are Abnormal in Strabismic and Anisometropic Amblyopes

The hyperacuity performance of amblyopic individuals is known to be abnormal, particularly on vernier tasks. Oscillatory movement displacement thresholds (OMDT's) a form of hyperacuity, were investigated over a range of temporal frequencies (1, 4, 7, 10, and 13 Hz) in 8 normal controls, 5 strabismic amblyopes, and 4 anisometropic amblyopes to see if this form of hyperacuity was also affected by amblyopia. OMDT's were found to be significantly raised in all of the strabismic amblyopes and three of the four anisometropes over all temporal frequencies investigated when compared to the control group. In the fourth anisometrope, OMDT's were raised at low temporal frequencies only. The findings are interpreted as evidence that magnocellular and parvocellular channels are affected in the amblyopic visual system. The functional loss in amblyopia cannot be described completely unless both temporal and spatial thresholds are investigated.

Spatial summation determines the contrast response of displacement threshold hyperacuity

The effect of line length on displacement threshold hyperacuity at various levels of contrast was investigated. At high contrasts there was no significant effect of line length, but as contrast was reduced thresholds for shorter line lengths increased rapidly. Thresholds at longer line lengths demonstrated a form of contrast saturation which differed from vernier acuity whose thresholds increased consistently with a reduction in contrast. As line length in the displacement threshold task was reduced, the amount of contrast saturation became less and displacement thresholds therefore resembled the contrast response of vernier acuity. Results are explained in terms of spatial summation along the length of the lines, the effects of which depend upon the spatial configuration of the hyperacuity stimulus under consideration.

A horizontal stripe of displacement sensitivity in the human visual field

Displacement thresholds of peripheral sites in monocular human vision were obtained. The average of 12 directional thresholds at different visual field sites was used to define isometric lines of average displacement threshold about central vision. Isometric lines extended further into the temporal visual fields along the horizontal meridian than along other meridians. At any single site in the peripheral visual field the thresholds were not the same in all directions; they were larger toward and away from central vision. These two psychophysical findings vary in a qualitatively similar manner across the retinal field, as does the average size and the collected orientation bias of dendritic fields of retinal ganglion cells.

The effect of spatial parameters on oscillatory movement displacement thresholds

Earlier work has established that oscillatory movement displacement thresholds (OMDT) are a form of hyperacuity. There is speculation that the mechanism determining OMDT, like motion perception in general, involves direct motion sensing at high temporal frequencies of oscillation and spatial localization processes (from which motion is inferred) at low temporal freqeuncies, which are both hyperacuities in their own right. OMDT were determined, for three experienced observers, over the temporal frequency range 1–15 Hz, for three stimulus lengths and three stimulus widths. Both decreasing stimulus length and decreasing stimulus width increased OMDT at all temporal frequencies. Furthermore, the resulting functions consistently exhibit a “kink” in the temporal frequency midrange. The results are interpreted as evidence that there are two subsystems involved in the analysis of visual motion with the kink indicating the transition where one system begins to predominate over the other.

Radiation effects in high-temperature superconductors: A brief review

A brief review of the published results on radiation effects in high-temperature superconductors is given. The focus is on six observations: lack of ionization damage, displacement damage, displacement thresholds near 20 eV, effects on transition temperature and critical current, amorphization, and atomic site- and sublattice-specific damage processes. It is concluded that these new ceramic superconductors have a very complex interaction with radiation fields. For ambient-temperature, fast-neutron irradiation, they show effects similar to those observed for A-15 superconductors. However, their total response is far more complex.

Displacement thresholds for various types of movement: Effect of spatial and temporal reference proximity

Displacement thresholds for unidirectional stop-go-stop and continuous types of movement were measured as a function of duration of movement. A circular stationary reference of varying radius surrounding the stimulus was either present throughout the whole duration of movement, disappeared just before movement began, or was totally absent. In the absence of the reference, thresholds increased with duration according to a constant velocity prediction. When present continuously, thresholds for all durations of movement were reduced. For references of close proximity, displacement thresholds became independent of movement duration. Results are discussed in terms of direct mechanisms of movement detection and also spatio-temporal hyperacuity analysis mediating the detection of relative positional change.

Use of displacement threshold hyperacuity to isolate the neural component of senile vision loss

Displacement threshold hyperacuity is a visual task which should remain unaffected by the optical changes within the aging human eye. As such, it represents a method by which the neural component of senile vision loss may be studied. Displacement thresholds were measured in eighty subjects in the 18-85-yr age range. Results demonstrate a marked reduction in sensitivity with increasing age with the rate of deterioration being proportional to the age of the subject. This can only be attributed to senile changes within the neural system.

1.3-μm light-emitting diode from silicon electron irradiated at its damage threshold

We report 77-K electroluminescence from an irradiated carbon-rich silicon diode that has an internal quantum efficiency more than 103 times higher than that of band-to-band recombination in an unirradiated, but otherwise identical diode. This is achieved by creating optically active Cs-SiI-Cs complexes with room-temperature electron bombardment at an energy between the displacement thresholds for single vacancy and divacancy formation. Under these irradiation conditions, it is possible to create a high concentration of radiative defects without gross degradation of the diode’s electrical characteristics. The technique could provide very large scale integration-compatible silicon light-emitting diodes for 1.3–1.6 μm all-silicon integrated optics.

MOVEMENT IN DECLINE? OSCILLATORY MOVEMENT DISPLACEMENT THRESHOLDS INCREASE WITH AGEING*

Displacement thresholds for continuous oscillatory movement were determined for ten subjects in each of three age groups--20-23, 40-55 and 60-80 years. Oscillation frequencies of 1, 2, 3, 5, 8, 13 and 20 Hz were used, all subjects having a corrected visual acuity of 6/5. A randomized block factorial design of analysis of variance revealed significant effects of both temporal frequency of oscillation (F6,189 = 4.10, p less than 0.001) and age (F2,189 = 46.98, p much less than 0.001). The deterioration in movement sensitivity occurs equally at both high and low temporal frequencies--no significant interaction effects being revealed (F12,189 = 0.45, p = not significant). The frequency effects are ascribed to underlying mechanisms mediating movement perception. Reasons for decreased movement sensitivity with increasing age are discussed.