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P E King-Smith

Publications and source records attributed to P E King-Smith.

At least 19 recordsLinked to original sources
PubMed2000

The thickness of the human precorneal tear film: evidence from reflection spectra.

PURPOSE: Interferometric methods have considerable potential for studying the thickness of layers of the human tear film and cornea because of their ability to make noninvasive, accurate, and rapid measurements. However, previous interferometric studies by Prydal and Danjo yielded tear thickness values near 40 and 11 microm, respectively, considerably greater than estimates made by invasive methods of 4 to 8 microm. Using a modified version of Danjo's method, interference effects from the tear film and cornea were studied, with the aim of correlation with known structure and optical properties of the cornea and hence determining the most probable value of tear film thickness. METHODS: Reflectance spectra from the human cornea were measured at normal incidence. These spectra show oscillations whose maxima correspond to constructive interference between light reflected from the air surface and from some deeper surface. The frequency of these spectral oscillations is proportional to the thickness of the layer between the air surface and the second surface. Therefore, Fourier analysis of reflectance spectra can be used to determine the thickness of layers of the tear film and cornea. In the main experiment, 36 low-resolution spectra were obtained from six normal eyes for measuring thickness up to 100 microm. Control experiments included measurements of the time course of thickness changes and high-resolution spectra for measuring thickness up to 1000 microm. RESULTS: For the main experiment, in the thickness range 1 to 100 microm, the strongest peak in the Fourier transform was near 3 microm (range, 1.5-4.7 microm) beneath the air surface. In the range 20 to 100 microm, the strongest peak was near 55 microm (range, 50-59 microm) for all 36 spectra; none were in Prydal's range near 40 microm. This 55-microm peak is consistent with a reflection from the basement membrane of the epithelium. Time course measurements after a blink show that the 3-microm peak is not an artifact. High-resolution spectra gave a peak near 510 microm, corresponding to the complete thickness of the cornea (plus tear film). This peak had a contrast similar to that of the 3-microm peak. CONCLUSIONS: These studies did not confirm Prydal's estimate of approximately 40 microm. Nor were there prominent peaks near Danjo's value of approximately 11 microm, except in cases of probable reflex tears. Because the reflection at the aqueous-mucus boundary would be expected to be weaker than that from the epithelial surface, the 3-microm peak is unlikely to correspond to the aqueous layer (rather than the complete tear film). The proposal that the 3-microm peak corresponds to a reflection from the front of the cornea is supported by the demonstration of a peak of similar contrast from the back of the cornea. Thus, the current evidence consistently supports a value of approximately 3 microm for the thickness of the human precorneal tear film.

Adult↗
PubMed1999

Three interferometric methods for measuring the thickness of layers of the tear film.

The thickness of different layers of the tear film has been measured by three types of interference method, namely, wavelength-dependent fringes (WDFs), thickness-dependent fringes (TDFs), and angle-dependent fringes (ADFs). This review begins with a discussion of characteristics which are common to all these methods--high-, intermediate-, and low-index layers, phase, optical path difference, and contrast. For each of the three methods, we present a figure showing constructive and destructive interference, derive equations for calculating tear layer thickness, describe a typical optical system, and show representative results. The particular advantages and limitations of each method are discussed. Given the clinical importance of the tear film in dry eye syndrome and contact lens wear, it is unfortunate that there are considerable discrepancies among the results of interferometric and other methods for measuring tear film thickness; further development of these noninvasive, interferometric methods should help to provide a clearer picture of the thickness of different layers of the tear film, in normal and dry eyes, and in contact lens wear.

Diagnostic Techniques, Ophthalmological↗
PubMed1998

Interferometric measurement of tear film thickness by use of spectral oscillations.

A method of measuring the tear film thickness is described in which interference causes oscillations in the reflectance spectrum from the tears. Strong oscillations were usually observed when a contact lens was worn. Measurement of modulation and phase of these oscillations confirmed that they were associated with the tear layer in front of the contact lens. Calculated thickness of this layer averaged 2.7 microns. In one out of five subjects, weak oscillations were sometimes observed without a contact lens. These oscillations probably arose from the aqueous layer of the tears with a thickness of approximately 3 microns. The relative merits of three interference methods of measuring the tear film are discussed.

Adult↗
PubMed1997

Principles of an adaptive method for measuring the slope of the psychometric function.

Recent developments in the efficient estimation of threshold are here extended to the problem of how best to estimate the slope of the psychometric function. An adaptive method is described for selecting stimulus intensities that are optimal for slope estimation. A two-dimensional array of probabilities of different thresholds and slopes is used to calculate the stimulus intensity for the next trial; this array is updated after the trial, using Bayes' theorem to incorporate information from the subject's response. The practical implementation and efficiency of the method are demonstrated and discussed.

Bayes Theorem↗
PubMed1997

Comparison of red-green, blue-yellow and achromatic losses in glaucoma.

Achromatic losses in glaucoma would be expected to be greater than, or equal to, red-green chromatic losses if the following assumptions are made: (1) the function of the remaining axons is either unchanged or non-selectively reduced; (2) red-green chromatic information is signaled by the midget ganglion cell system; and (3) the function of the magnocellular system is reduced at least as much as that of the midget ganglion cells. This prediction was tested by measuring red-green (along with blue-yellow) mixture thresholds for 1 deg, 0.2 sec test spots presented on a color monitor on a white background of 50 cd/m2. Ellipses were fitted to plots of green contrast as a function of red contrast (or yellow as a function of blue), and major and minor axes of these ellipses were taken as measures of chromatic and achromatic thresholds, respectively. The study population consisted of 29 eyes in 29 patients with early glaucoma; control data were derived from a data bank of 83 normal eyes. Red-green losses were significantly (P < 0.05) greater than achromatic losses in 6 out of the 11 eyes which showed significant losses of either chromatic or achromatic sensitivity (or both). It is concluded that, for these eyes, at least one of the above three assumptions is incorrect.

Adult↗
PubMed1994

Efficient and unbiased modifications of the QUEST threshold method: theory, simulations, experimental evaluation and practical implementation.

QUEST [Watson and Pelli, Perception and Psychophysics, 13, 113-120 (1983)] is an efficient method of measuring thresholds which is based on three steps: (1) Specification of prior knowledge and assumptions, including an initial probability density function (p.d.f.) of threshold (i.e. relative probability of different thresholds in the population). (2) A method for choosing the stimulus intensity of any trial. (3) A method for choosing the final threshold estimate. QUEST introduced a Bayesian framework for combining prior knowledge with the results of previous trials to calculate a current p.d.f.; this is then used to implement Steps 2 and 3. While maintaining this Bayesian approach, this paper evaluates whether modifications of the QUEST method (particularly Step 2, but also Steps 1 and 3) can lead to greater precision and reduced bias. Four variations of the QUEST method (differing in Step 2) were evaluated by computer simulations. In addition to the standard method of setting the stimulus intensity to the mode of the current p.d.f. of threshold, the alternatives of using the mean and the median were evaluated. In the fourth variation--the Minimum Variance Method--the next stimulus intensity is chosen to minimize the expected variance at the end of the next trial. An exact enumeration technique with up to 20 trials was used for both yes-no and two-alternative forced-choice (2AFC) experiments. In all cases, using the mean (here called ZEST) provided better precision than using the median which in turn was better than using the mode. The Minimum Variance Method provided slightly better precision than ZEST. The usual threshold criterion--based on the "ideal sweat factor"--may not provide optimum precision; efficiency can generally be improved by optimizing the threshold criterion. We therefore recommend either using ZEST with the optimum threshold criterion or the more complex Minimum Variance Method. A distinction is made between "measurement bias", which is derived from the mean of repeated threshold estimates for a single real threshold, and "interpretation bias", which is derived from the mean of real thresholds yielding a single threshold estimate. If their assumptions are correct, the current methods have no interpretation bias, but they do have measurement bias. Interpretation bias caused by errors in the assumptions used by ZEST is evaluated. The precisions and merits of yes-no and 2AFC techniques are compared.(ABSTRACT TRUNCATED AT 400 WORDS)

Adaptation, Ocular↗
PubMed1991

Correlation of chromatic, spatial, and temporal sensitivity in optic nerve disease.

Spearman rank-order correlations (R) were made between the color-mixture threshold, spatial contrast sensitivity, and flicker sensitivity measurements of 38 patients with a variety of optic nerve disorders. Patients had to satisfy the following criteria: greater than 0.5 log unit loss of chromatic or achromatic sensitivity (compared to age-matched normals), central fixation, no congenital color defects, and no ocular media abnormalities. The results of the analysis show a significant correlation between selective losses of high spatial frequency sensitivity (relative to low) and selective losses of red/green and blue/yellow sensitivities [R = -0.680 (P less than 0.001) and R = -0.439 (P less than 0.01), respectively]. A mild correlation was found between selective spatial and selective temporal losses [r = -0.399 (P less than 0.05)] (ie, low temporal frequency losses correlate with high spatial frequency losses and vice versa). A stronger correlation was found between selective red/green and selective blue/yellow sensitivity losses [R = 0.657 (P less than 0.001)]. No correlation was found between selective temporal losses and selective chromatic losses. These findings can be explained in terms of differential losses of three types of fibers: (1) fibers that are particularly sensitive to red/green color, high spatial and low temporal frequencies; (2) fibers signalling blue/yellow color; and (3) fibers that are relatively sensitive to high temporal frequencies and low spatial frequencies.

Adolescent↗
PubMed1990

Chromatic, spatial, and temporal losses of sensitivity in multiple sclerosis.

Chromatic, spatial, and temporal losses of sensitivity were measured in 15 eyes of 10 patients with recovered optic neuritis. Chromatic sensitivities (for both red-green and blue-yellow) were measured using color-mixture thresholds; the chromatic sensitivity loss was classified as "selective" if it was significantly greater than the achromatic loss. Spatial and temporal sensitivities were measured with contrast sensitivity functions and flicker modulation sensitivity, respectively; these losses were classified as selective if the losses at high (spatial or temporal) frequencies were significantly greater (or significantly less) than losses at low frequencies. All patients had central fixation and were optically corrected carefully. In 1 eye, selective losses of sensitivity for red-green and blue-yellow were combined with a selective loss of sensitivity at high spatial (but not temporal) frequencies. This type of loss may indicate a selective loss of small axons in the optic nerve. The 8 other eyes that showed significant losses were generally nonselective in their chromatic, spatial, and temporal losses; this may indicate a nonselective loss of small and large axons.

Adult↗
PubMed1988

A quantitative scoring technique for panel tests of color vision.

Panel tests of color vision (eg FM100-Hue test) lack a common quantitative method for the scoring of cap arrangements. We describe a scoring method applicable to all panel tests that makes use of a novel technique to analyze test cap data, namely the calculation of a moment of inertia from the Color Difference Vectors (CDVs) of any arrangement pattern. Using the Farnsworth D-15 panel, as an example, we specify how to determine CDVs and demonstrate the benefits of calculating a moment of inertia for the analysis of these vectors. Moment of inertia analysis yields three factors which quantify cap arrangements: the first is the confusion angle which identifies the type of color defect; the second is the Confusion index (C-index) which quantifies the degree of color loss relative to a perfect arrangement of caps; and the third is the Selectivity index (S-index) which quantifies the amount of polarity or lack of randomness in a cap arrangement. A retrospective study on the result of 53 normal and 66 congenitally color defective observers is reported and provides normative data. We show that the technique differentiates between different types of color defect and provides useful clinical information regarding a loss of color vision. Likewise, a similar observation is made on a smaller sample of FM100-Hue results. A BASIC computer program is provided for anyone wishing to use the technique.

Adolescent↗
PubMed1986

Red-green mixture thresholds in congenital and acquired color defects.

A color television display was used to measure thresholds for mixtures of red and green on a white background; red and green components could be either incremental, decremental or zero. Ellipses are fitted to a plot of green contrast as a function of red contrast, and it is argued that the length of the ellipse is a measure of red-green color discrimination and the width of the ellipse is a measure of luminance discrimination. It is shown that the technique reliably distinguishes normals from congenital color defectives and also protan from deutan subjects. For some cases of acquired color defects (e.g. optic neuritis), there is a roughly equal loss of color and luminance discrimination whereas, in other cases (e.g., hereditary optic atrophies), the loss of color discrimination is much greater than the loss of luminance discrimination.

Adult↗
PubMed1986

Rod and cone ERGs and their oscillatory potentials.

Normal human ERGs were recorded from a dark-adapted subject using white and colored test flashes. Oscillatory potentials (OPs) were studied after high-pass digital filtering. When blue and red responses were compared at equivalent photopic intensities, OPs were visible at much lower intensities for the blue flashes. As the intensity was reduced from maximum, the first (negative) wave for red flashes maintained a latency of 20-25 msec before being lost in noise, whereas the first wave for blue flashes increased its latency progressively from 25 to 60 msec. These differences between photopically matched red and blue responses are interpreted to be due to rod-generated responses. When blue, orange, and white responses were compared at equivalent scotopic intensities, the latency of the largest negative wave was found to be similar for all three colors. The authors interpret this wave to be the beginning of the rod-generated OPs, so that the preceding waves (particularly evident for orange flashes) are cone-generated OPs, and they propose that the existence of separate rod and cone OPs should be borne in mind when investigating clinical changes in OPs.

Action Potentials↗
PubMed1985

Color mixture thresholds measured on a color television--a new method for analysis, classification and diagnosis of neuro-ophthalmic disease.

A color television display can be used to determine color and brightness discrimination thresholds using identical adaptation conditions and experimental technique. The color discrimination threshold is measured by using an equiluminous test spot--i.e. one which differs in color from the surrounding screen but has the same luminance. Because there is no brightness clue, the subject is forced to detect such a spot by using color discrimination. It is shown how color and brightness thresholds may be determined from threshold measurements of different color-mixtures even though it is not known beforehand which stimulus will be equiluminous for the subject. Results are shown for normal subjects, congenital color defectives and for two patients having optic nerve disease who show respectively non-selective and selective loss of color discrimination compared to brightness discrimination. Normal control data are presented, illustrating the effect of eccentricity, optical blur, viewing distance, pupil size and age. It is concluded that the technique is relatively insensitive to moderate variations in these factors and that it is more sensitive in detecting selective color loss than a spectral sensitivity technique which has been described previously.

Adult↗
PubMed1984

Selective damage to chromatic mechanisms in neuro-ophthalmic diseases I. Review of published evidence.

Acquired color deficiencies may correspond to a general, non-selective loss of visual sensitivity. We summarise evidence for the opposite view that, in some cases, chromatic sensitivity can be more (or less) reduced than achromatic sensitivity. This evidence is based on: (1) Disproportion between chromatic and achromatic isopters; (2) Differential damage to red-green and blue-yellow color vision; (3) Detection static perimetry; (4) The foveal photochromatic interval; (5) The two color threshold technique; (6) Spectral sensitivity on a white background; (7) Single unit and histological studies of the retina and lateral geniculate nucleus; (8) Lesions of the prestriate color area; (9) Selective damage to achromatic processes. Possible problems of interpretation are considered and a new technique for comparing chromatic and achromatic sensitivity is briefly described.

Animals↗
PubMed1984

Dichotomy of psychophysical responses in retrobulbar neuritis.

Thirty cases of retrobulbar neuritis (RBN) with varying degrees of myelin loss have been analyzed in terms of loss of chromatic function, loss of spectral flicker detection and reduction in visual acuity. We suggest, from the preliminary results of this study, that there may be two distinct types or stages in RBN. Both stages include moderate to severe impairment or damage to the colour-opponent (and most likely the fine-acuity) system. The second type or stage also involves a conduction block or failure, severely reducing sensitivity to fast flicker, although other luminance functions not involving fast-flicker detection may be spared.

Adolescent↗
PubMed1984

What can colour thresholds tell us about the nature of the underlying detection mechanisms?

Two-colour increment thresholds, which have traditionally been analysed in terms of pi-mechanisms, were subjected here to a different form of analysis. Detection thresholds were measured for monochromatic test flashes and 10 bichromatic mixture combinations containing various proportions of two out of the five primaries: white, blue, green, yellow and red. The 1 degree, 0.5-s duration, foveal flashes appeared on a 1000-td blue background. All the thresholds were simultaneously fitted by assuming probability summation from a limited number of independent linear mechanisms. The assumption of two detection mechanisms provided a reasonable fit for all the bichromatic mixture thresholds. One mechanism received a positive contribution from both M and L cone types, whereas the other received an excitatory contribution from the L cone type, and an inhibitory contribution from the M cone type. The predictive power of the model was tested and compared to the pi-mechanism concept by fitting a spectral-sensitivity curve measured under identical conditions. It was concluded that parts of pi-mechanism curves probably reflect the responses of colour-opponent processes rather than those of single cone types.

Color Perception↗
PubMed1983

Spectral threshold: measurement and clinical applications.

Photopic spectral sensitivities for a foveal target on a white background are measured for 18 normal eyes, and the results are explained in terms of the function of retinal ganglion cells. Averaged results for diseases such as glaucoma, optic atrophy, tobacco amblyopia, and retrobulbar neuritis are reviewed, and an analysis of the change in shape of the spectral sensitivity curve in these diseases is presented. It is shown how the location of disease sites may be related to characteristic changes in spectral sensitivity.

Adult↗
PubMed1983

Spectral thresholds in macular degeneration.

Spectral sensitivities were measured in 18 normal eyes, 9 eyes in patients with senile macular degeneration, 4 patients with Stargardt's juvenile macular degeneration (JMD), and 2 patients without conclusive signs--that is, genetic or morphological abnormalities--to indicate the cause of loss of central vision. Spectral sensitivity, testing for which included measurements on white, yellow, purple, and blue backgrounds, is here used as an aid in differential diagnosis for cases of macular degeneration.

Adolescent↗