Retinal Threshold Research Articles

Laser treatment in subjects with high-risk clinical features of age-related macular degeneration. Posterior pole appearance and retinal function.

To verify that a few laser lesions in the posterior pole can cause drusen to resolve in patients with age-related macular degeneration, and to document central retinal sensitivity as drusen resolve. In a pilot study, 12 patients considered to be at high risk for sight-threatening complications from age-related macular degeneration were treated with 12 argon laser lesions in the posterior pole, with review for 12 to 24 months. Choroidal neovascularization developed in 1 patient 8 months after treatment, with consequent loss of central vision. In 9 of the remaining 11 patients, high-risk characteristics of drusen were reduced. Four patients had retinal pigment epithelial depigmentation, and all maintained 20/40 visual acuity at 12 months. One patient lost 3 lines of vision due to geographic atrophy after 12 months. Scotopic retinal threshold was elevated before treatment in 8 patients, compared with an age-matched comparison group. Of these, 4 patients underwent retesting 3 to 6 months after treatment, and all had improved thresholds, but only 1 patient sustained the improvement at 12 months. At 12 months, 3 of the 8 patients showed an improvement in their mean retinal threshold. Of those in whom the mean retinal threshold worsened, the mean elevation in threshold was not more than 0.6 log units. A few laser lesions in the posterior pole leads to resolution of drusen. There does not appear to be an increased risk for choroidal neovascularization. Retinal threshold measurements show no indication of geographic atrophy at 1 year, but cannot be excluded as a late outcome. Laser treatment may reduce the risk for profound sight-threatening lesions in age-related macular degeneration.

Stochastic retinal mechanisms of light adaptation and gain control.

Under appropriate experimental conditions, the threshold intensity of a visual stimulus varies as the square-root of the background illuminance. This square-root law has been observed in both psychophysical threshold experiments and in measurements of the thresholds of individual ganglion cells. A signal detection theory developed in the 1940s by H. L. de Vries and A. Rose, and since elaborated by H. B. Barlow and others, explains the square-root law on the basis of 'noise' due to fluctuations in the number of photon absorptions per unit area and unit time at the cornea. An alternative account of the square-root law--and also other threshold-vs-intensity slopes--is founded on the assumption of physiological gain control (W. A. H. Rushton, Proc, Roy. Soc. (London) B 162, 20-46, 1965; W. S. Geisler, J. Physiol. (London) 312, 165-179, 1979). In this paper, a neural model of light adaptation and gain control is described that shows how these two accounts of the square-root law can be reconciled by a stochastic gain control mechanism whose gain depends on the photon fluctuation level. The process by which spikes are generated in a ganglion cell is modeled in terms of a stochastic integrate-and-fire mechanism; this model is used to quantitatively fit toad retinal ganglion cell threshold data. A psychophysical model is then outlined showing how a statistical observer could analyze the ganglion cell spike trains generated by 'signal' and 'noise' trials in order to statistically discriminate the two conditions. The model is also shown to account for some dynamic aspects of ganglion cell responses, including ON- and OFF-responses. The neural light adaptation model predicts that--under the proper conditions--brightness matching judgments will also be subject to a square-root law. Experimental tests of the model under superthreshold conditions are proposed.

Preservation of retinal sensitivity in central visual field after panretinal photocoagulation in diabetics.

Changes in retinal sensitivity within central 30 degrees following panretinal photocoagulation (PRP) for more severe diabetic retinopathy were investigated. Twenty-five eyes with visual acuity of 0.4 or better and minimal maculopathy were studied prospectively. All underwent PRP in two sittings, and Humphrey field analyzer 30-2 threshold test was done before and 1 week, 1 and 3 months after the treatment. The mean retinal sensitivity threshold was obtained from each hemifield between 15 and 30 degrees and from the central 15 degree area, and the changes in the values were analyzed. Mean sensitivity threshold in the upper visual field at pre-PRP, post-PRP 1 week, 1, 3 months were 15.62, 13.81, 14.31, 14.85, respectively. Values in the lower field were 18.71, 17.25, 17.10, 18.17. Difference between pre-PRP and post-PRP was statistically significant at 1 week but no longer thereafter. Retinal sensitivity within the central 15 degrees remained stable. The data show that retinal sensitivity decreases significantly 1 week after PRP but recovers upto 95% of pre-PRP level over the following 3 months.

Contrast Sensitivity Differences Between Proficient and Disabled Readers Using Colored Lenses

Colored overlays or lenses (e.g., Irlen lenses) have been used in attempts to remediate reading difficulties. The present study included four middle-socioeconomic status (SES) adults and four middle-SES children with reading disabilities as well as an equal number of nondisabled readers of the same age groups and SES. Examined were (a) the relationship of wavelength (lens color) to visual grating performance, (b) the effect of reading disability on performance with each lens-color/luminosity-grating combination, (c) group performances on a visual detection task with the clear and chromatic lenses, and (d) peripheral retinal brightness thresholds. The spatial frequency of the gratings (and not the lens color) permitted subjects with reading disabilities to be differentiated from the proficient readers. Subjects with reading disabilities displayed significantly lower contrast sensitivity when tested with sine-wave gratings, as well as displaying higher brightness thresholds in the peripheral retina.

Age-related changes in the human lens: A comparison of Scheimpflug photography and lens density index

Two quantitative techniques, Scheimpflug photography and the lens density index (LDI), were used to measure lens density in 52 patients. In the Scheimpflug technique, a digitized slitlamp camera provides peak and average nuclear density based on a densitometric analysis of the lens nucleus. The LDI is a psychophysical measurement of the difference between retinal scotopic thresholds for two wavelengths of light. Both the Scheimpflug technique and LDI revealed an age-related increase in lens density. Peak and average nuclear density were highly correlated with the LDI.

Quantitative analysis of changes of automated perimetric thresholds after pupillary dilation and induced myopia in normal subjects.

We quantitatively analyzed the changes of the (retinal) threshold sensitivities between normal status and dilated emmetropic status, and between dilated emmetropic status and -1.0D induced myopic status. Three consecutive visual field examinations by Humphrey Field Analyzer C-30-2 threshold test and STATPAC program were performed in 36 eyes of 21 normal subjects who ranged in age from 23 to 40 years. The results were as follows: 1) There was a statistically significant decrease (0.87 +/- 0.91dB)in the mean threshold sensitivity within the central 30 degrees at the dilated emmetropic status (27.90dB) as compared with the normal status (28.77dB)(p = 0.0001). 2) There was a statistically significant decrease(0.99 +/- 0.61dB) at -1.0D induced myopic status(26.91dB) as compared with the dilated emmetropic status(27.90dB)(p = 0.0001). 3) After dilation, the mean threshold sensitivity between 20 and 30 degrees of field showed statistically significant decrease as compared with those within the central 20 degrees of visual field(p < 0.05). In case of -1.0D induced myopia, there were uniform decreases in the mean threshold sensitivities within the central 30 degrees of visual field (p > 0.05). Therefore, we emphasize the importance of consistent pupillary size and correct refractive state in performing the serial automated visual field tests.

Impairment of retinal increment thresholds in Huntington's disease.

We have investigated detection thresholds for a foveal blue test light using a Maxwellian view system in 61 normal subjects, 19 patients with Huntington's chorea, 14 patients with Tourette's syndrome, and 20 patients with schizophrenia. Ten measurements were made: The blue test light (1 degree diameter, 500 msec duration) was presented either superimposed on a yellow adaptation field (5 degree diameter) or 500 msec after switching off this field (transient tritanopia effect). In both cases five different background intensities were presented. The only abnormality found was in patients with Huntington's chorea. During adaptation these patients' thresholds are significantly higher than normal (p < 0.005). No change was found in the transient tritanopia effect. Huntington's disease causes degeneration of several different transmitter systems in the brain. Increment threshold testing allows for noninvasive investigation of patients and confirms the involvement of the retina in the degenerative process in Huntington's chorea.

Retinal increment thresholds in huntington's disease, tourette's syndrome and schizophrenia

Retinal increment thresholds in huntington's disease, tourette's syndrome and schizophrenia

Interpreting multifocal intraocular lens modulation transfer functions

The optical performance of new multifocal intraocular lens designs is frequently assessed using the modulation transfer function (MTF). We discuss the relationship between the MTF and clinical measures of human visual function, such as threshold visual acuity and contrast sensitivity. Using in vitro MTF measurements of a human eye model containing a multifocal or monofocal intraocular lens, we predict relative changes in acuity and contrast sensitivity and outline the techniques using a simple model of human retinal threshold detection. Specific concepts introduced include the visual acuity graph, predicted visual acuity graph, and predicted contrast sensitivity function.

CHROMATIC STATIC PERIMETRY IN DIABETIC CHILDREN

In order to evaluate the visual function we studied a group of 50 (26 female and 24 male) diabetic children without retinopathy. Their age ranged from 11 to 15 years with duration of the disease > 3 years, absence of fluoroangiographic signs of retinopathy, no diachromatopsia, stable glycosylated haemoglobin (HbAlc) from 6.3% to 12.1%. The patients were divided into 2 groups according to their HbAlc values of the last 12 montha:Group A1, HbAlc 3.8-9% (32 patients), Group A2, HbAlc 9.1-12.1% (18 patients). Other 2 subgroups were created according to Albumin Excretion rate (AER) values: Group B1, AER 0.1-15 mcg/min (36 patients), Group B2, AER 15.1-87.2 mcg/min (14 patients). Computerized static perimetry (CSP) evaluating retinal threshold in central visual field (24°) was performed with white and blue stimuli (Allergan Humphrey model 640 HFA). Three concentric areas of the visual field were identified: SI:0-9°, S2:10-18°, S3:>18°. The Group A2 in comparison with Group A1 showed a significant reduction of the Bensitivity to blue stimuli (S1:p=0.039, S2:p=0.043, S3: p=0.006), while no significant difference was found for the white stimuli. In Group B2 in comparison with Group B1 a significant reduction of the sensitivity to white stimuli in S3 area (p=0.043) and to blue stimuli in all areas (Si: p=0.003, S2: p=0.008, S3: p=0.005) was found. This study shows that diabetics without fluoroangiographic signs of retinopathy can have a significant impairment of the visual field. The CSP performed with blue stimuli seems to be an appropriate and suitable method to evaluate the earliest retinal functional damages.

Methodological issues in the assessment of achromatic retinal thresholds with the B.C.U.P.

Two sources of error - subject and instrumental, which are inherent in the data collected, can affect the validity and reliability of achromatic retinal thresholds in neurophthalmological research. These errors depend on the numbers of measurements, with increases in trials or subjects increasing the degree of confidence and accuracy. Errors from instrumental calibration and pupil diameter estimations are independent of wavelength but those from filter band widths are wavelength dependent - least for yellow, more for red and most for blue filters. At best an accuracy of between 0.1 and 0.2 log quanta can be expected. Abbreviated (eight filter) testing is valid for the mid-range of the visible spectrum, but variance increases for blue stimuli. Smoothing the data does not infringe on its integrity and this can be used to indicate the probable outline of the extended testing curve. When background luminance is high, difficulties in differentiating between achromatic and chromatic stimuli seem to be a problem...

Retinal Sensitivity in Acute Hypoglycemia

We measured retinal threshold sensitivity via automated computerized static perimetry in predetermined meridians in euglycemic diabetic subjects and in the same subjects during insulin-induced hypoglycemia. During periods of reduced blood glucose levels, decreased retinal sensitivity, most marked in temporal meridians, was observed.

Central visual function in patients with resolved central serous retinopathy. A long term follow-up study.

The visual acuity, central retinal sensitivity, macular thresholds, contrast sensitivity and temporal visual acuity were employed to assess the long term visual outcome in 23 patients with a history of central serous retinopathy (CSR). Twenty patients had unilateral CSR, and 3 had had bilateral disease. The unaffected eyes had significantly better visual acuities (P less than 0.014), better retinal sensitivities (P less than 0.006) and lower macular thresholds (P less than 0.008) when compared with affected eyes. The eyes with CSR whose visual acuities were mildly affected (6/9) showed lower sensitivity for the high spatial frequency (P less than 0.0072). There was no significant difference in the temporal visual acuity between eyes with CSR and the non-affected eyes.

The subjective assessment of cataract.

The medical treatment of cataract now appears to be a distinct possibility. A number of anti-cataract formulations are being clinically tested, and more clinical trials are being planned. To obtain a true assessment of a drug's efficacy, a battery of tests are needed which can accurately assess cataract progress. A clinical trial of the proposed anti-cataract drug Bendalina is being conducted. The six subjective methods used to assess cataract progression in the trial are presented. These are refractive error, LogMAR visual acuity, contrast sensitivity, glare sensitivity, retinal visual acuity and displacement threshold hyperacuity. The reasons for using each technique and their method of measurement is explained.

Rods also participate in human color vision.

In the retinal periphery, rod and cone thresholds to the same green light were measured. After the rod-cone break in the course of dark adaptation, cone sensitivity decreases gradually as rod sensitivity progressively increases. Cone sensitivity, however, increases when rod sensitivity is decreased by blue light. The emergence of the facilitatory effect of rod activation requires at least a few hundred msec after rod stimulation. It is concluded that rod activation plays a major role in sensitivity regulation of the cone system, i.e., in human color vision.

Short-term fluctuation of light discrimination sensitivity at the borders of glaucomatous visual field defects

Using profile program F-4 of the Octopus 201 computerized perimeter, retinal light sensitivity and threshold short-term fluctuations were studied in 15 eyes of 15 patients with glaucomatous field defects, with a resolution of 1.5 degrees. The scotomata were verified with programs 33 and 34 of the Octopus 201. Control profiles, either at identical locations in the healthy fellow eye or in areas showing no glaucomatous changes, were used to compare focal fluctuation in the vicinity of the scotoma. At the borders of the scotomata, between 20 degrees and 30 degrees eccentricity, a significant increase in short-term fluctuation of retinal sensitivity was found. This change in short-term fluctuation decreased with increasing distance from the field defect. Significantly greater short-term fluctuations were also detected in border-area measurements of scotomata located between 0 and 10 or 10 and 20 degrees of eccentricity.

Variability of the hill of vision and its significance in automated perimetry.

It has been assumed that the retinal threshold sensitivity profile is a standard which decreases predictably with age. On this basis the significance of relative scotomata is decided on certain types of perimeter. The hill of vision was measured in 128 healthy eyes on the Dicon 2000 autoperimeter. We found a large variation between individuals, with no relationship to age. The relevance of the findings to automated perimetry are discussed.

Effects of retinal eccentricity on displacement thresholds for unidirectional and oscillatory stimuli

Motion sensitivity was determined for line stimuli undergoing unidirectional and oscillatory displacements at various retinal eccentricities. Oscillatory motions of short duration were found to produce consistently greater motion thresholds than unidirectional stimuli at each eccentricity tested. Increasing retinal eccentricity elevated motion thresholds similarly for both forms of stimulus motion. The observed differences between unidirectional and oscillatory stimuli are attributed to differences in the temporal summation of the two forms of stimuli at short durations. These findings also suggest that differences in motion sensitivity between central and peripheral vision are quantitative rather than qualitative.

Specificity of suprathreshold computer perimetry

The problems encountered in linear suprathreshold computer perimetry are discussed in the light of results with the Fieldmaster 200 automatic perimeter utilizing different test conditions. A linear supraluminosity of the test points, especially in the central visual field, results in poor specificity of the periphery of the tested area if no substantial loss of sensitivity is acceptable. Taking the three-dimensional distribution of retinal thresholds into account, the supraluminosity of the perimetric stimuli should be adapted to the eccentricity of testing.

Computerized perimetry in infants treated with ethambutol (author's transl)

Computerized perimetry of the central visual field (Competer) was utilized in 12 eyes of children treated with Ethambutol because of tuberculosis to verify potential visual hazards of the drug. Visual acuity, visual field and the mean retinal threshold of the central field revealed no significant changes with increasing cumulative Ethambutol dose up to 166.5 g. The visual field was re-checked five times with a consecutive time lapse of 7 weeks. It can be concluded that the critical total dose of 150 g that is considered to be potential dangerous in adults does not affect the optic nerve of the infant either. Since this total dose can be exceeded in the continued treatment of the disease parents should be aware of the necessity of repeated visual field examinations or color vision testing. This concern is especially valid in infants suffering from renal tuberculosis because of higher plasma levels of the drug when excretion is prolonged.