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Biomedical subjects

Vaegan

Publications and source records attributed to Vaegan.

48 records · Page 3Linked to original sources
PubMed1983

An automated statis perimeter/adaptometer using light emitting diodes.

An automated static perimeter/adaptometer is described which measures thresholds with lights of 2 wavelengths. The instrument uses light-emitting diodes to produce the stimuli and is controlled by a small computer, making it very suitable for clinical testing of large numbers of patients. The use of 2 LEDs with different peak emission wavelengths (530 and 660 nm) permits an assessment of the relative state of rod and cone mechanisms in a particular region of the retina either during dark adaptation or when the eye is fully dark adapted.

Dark Adaptation↗
PubMed1983

Artefacts of intrusion in the pattern electroretinogram.

The pattern electroretinogram (PERG) was recorded intravitreally. Comparison of responses recorded in both eyes and at the optic tectum shows that the PERG does not have an origin in distant CNS generators, but rather, that it has an ocular origin.

Animals↗
PubMed1982

Clinical and experimental evidence that the pattern electroretinogram (PERG) is generated in more proximal retinal layers than the focal electroretinogram (FERG).

A TV monitor was used to evoke either a pattern ERG to a contrast-reversing checkerboard (PERG), or a focal ERG to alternate increases and decreases of luminance of the blank screen within a bright surround (FERG). Both responses are small (approx 2 microV) and fast (approx 50 msec to peak) and are similar in several other properties. However, they differ in timing and respond differently to changes in contrast. Each frame of a TV picture evokes a "raster ERG," even though the screen is blank. The response is focal and specific to a small central strip of the screen. It is simpler to record than the FERG, where the whole screen is flashing. Because the FERG summation area is about 4 deg, small squares (checks) reversing in contrast produce little luminance response. In 5 of 7 cases where the PERG is unilaterally reduced, the FERGs or raster responses were not affected. Thus clinical evidence also suggests that the PERG may be a separate phenomenon to the FERG and produced at a different site. Toxic, traumatic, congenital, and degenerative diseases of the optic nerve reduce the PERG. The comparison is most easily made in unilateral disease. Ten weeks after an optic nerve insult, the PERG becomes reduced in the affected eye as if retrograde degeneration was occurring. In 27 amblyopes of various types, the PERG was reduced in 23 where orthoptic treatment had failed. In 4 patients responding to treatment, PERGs of the amblyopic eye were as large as, or larger than, those of the fellow eye. The loss is greater with smaller checks. Retinal changes do occur after age 4 but so slowly that responses in heavily occluded eyes are not reduced. An additional level in the visual pathway is thus accessible to evoked potential investigation.

Amblyopia↗
PubMed1982

A forced-choice test improves clinical contrast sensitivity testing.

Decreased contrast sensitivity has been demonstrated in early glaucoma, but the deficit in not regularly observed. We designed a prototype for a forced-choice printed test and evaluated it with several other measures of contrast sensitivity. The results also bear on the pattern of loss and the variables which effect performance. Mildly glaucomatous patients show at 6 db (50%) loss of contrast sensitivity at all spatial frequencies tested compared with age matched controls. Moving gratings give the same information as stationary ones, and practice effects are negligible. Contrast sensitivity at or below 2 cycles/degree is poorly correlated with visual acuity and does not change with age in the forced-choice test. Subjective judgment made the apparent contrast threshold higher, age dependent, and more variable, particularly at higher spatial frequencies. The pattern of variability can explain some reports of insignificant effects and why low spatial frequency contrast sensitivity detects glaucoma better than visual acuity. Methods correlated so poorly, despite high reliabilities, that uncontrolled biases must be suspected in subjective measures. Our new forced-choice format was superior to all other tests on at least one formal criterion and always at least equal to them. Improvements in contrast sensitivity screening tests are thus indicated.

Adolescent↗
PubMed1980

Pattern ERGs are abnormal in many amblyopes.

We have used a pattern-reversing chequerboard (1 degree squares, or smaller: 84 per cent contrast, or less: 2 to 16 reversals/sec: field size 22 degrees x 16 degrees: mean luminance 50 cd/m2) to evoke responses from the eyes of young adults and children. ERGs were recorded with gold foil electrodes, which did not interfere with the normal optics. All patients were affected accurately. The fixation point was adjusted so that squinters received the stimulus on corresponding areas of each retina. The normal ERG obtained is 1.5 to 3 mu V in amplitude, and in thirteen normal subjects right and left eyes gave equal responses. The extreme asymmetry detected was c. 10 per cent. In all amblyopes, including children of 6 years old and upwards, satisfactory recordings were obtained. In most amblyopes, the response from the affected eye was 50 per cent or less of the response from the fellow eye. In all cases of failure to respond to treatment, the asymmetry was highly significant. In adults, occluding the screen area corresponding to the area of amblyopic suppression, reduces the ERG in the fellow eye, but does not reduce the ERG in the amblyopic eye. We conclude that amblyopic retina produces very little electrical activity that we can record. The timing, response to frequency of stimulation, and other parameters in the pattern ERG are dissimilar from those in the ERG produced by small changes of luminance. We have little evidence about the class of cell generating the response we observe, but evidently there is peripheral abnormality in the visual system even of amblyopic children.

Adolescent↗
PubMed1979

Convergence and divergence show large and sustained improvement after short isometric exercise.

There are conflicting reports on the effect of vergence training. In two studies using push up and prism vergence exercises the conflict is shown to result from differences in the constancy of effort. Improvement only occurs in exercises involving sustained effort in the direction being trained. The effect of alternating movements in both directions is small and inconsistent. Substantial longlasting gains in either convergence or divergence prism vergence scores can result from just 5 min of sustained effort at an angle halfway between the break and recovery points. Results for adults and children on motor-driven instruments are comparable and the scores obtained discriminate better than those from hand-turned instruments. The most likely physiological model involves potentiating processes which are specifically maximized by isometric exercise. Potentiation results in a positive feedback process, the effects of which are strong enough to precipitate strabismus and which might be deliberately manipulated in therapy of poor vergence control, if responses in the required direction can be induced.

Adult↗
PubMed1979

Amblyopia in bilateral infantile and juvenile cataract. Relationship to timing of treatment.

Contact lenses were fitted for 51 aphakic infants. From this group data were obtained that support the hypothesis that much of the visual defect in cases of congenital cataract is due to functional amblyopia, and that it is therefore largely preventable and even treatable. The implications of the results for the management of these patients and for human amblyopia are that early treatment and optical correction are essential, since the effects of deprivation start at about 4 months of age and continue to a cumulative but decreasing degree throughout the first decade of life.

Amblyopia↗
PubMed1979

Critical period for deprivation amblyopia in children.

The critical period in which human visual acuity may be lost after visual deprivation can be deduced by studying children with amblyopia which follows uncomplicated unilateral cataract and the subsequent aphakic blur. The visual acuity at the first contact lens fitting increases linearly on a log scale as a function of the age at onset between 3 and 10 years. Correction before 4 months of age produces less visual loss but we cannot specify the function within this period. If the eye is deprived between 6 and 30 months of age finger counting is the best visual acuity achieved. Deprivation commencing between the ages of 3 and 10 years differs only in that vision reduces at a slower rate and is more likely to respond to total full-time occlusion. Similarly, some unilateral high myopes responded well and three unilateral aphakes who are blind in the other eye slowly achieved good vision. Part-time occlusion and Cam treatment were seldom effective. Children appear to have a critical period which continues for longer than expected from animal models.

Age Factors↗
PubMed1979

Independence of convergence and divergence: norms, age trends, and potentiation in mechanized prism vergence tests.

Motor-driven prism stereoscope vergence tests, base in break, base in recovery, base out break, and base out recovery, repeated three times (BIB, BIR, BOB, BOR X 3) were given to three separate groups. Two instruments with different speeds were used. Repeated testing shows a slight trend to convergence in slower machines. The results from both machines are still comparable. Means are similar to previously published norms from hand-turned instruments, but the variance of scores is about twice as large and there is an eso trend at near. The motor drive aids discrimination, since Ss who are capable can now follow with smooth vergences over a wider range. In general, there is no substantial trend with age between 3 and 25 years other than those which may be due to decreasing response times. Vergence scores can be adequately reduced to two independent factors, loading on convergence and divergence, respectively, which suggests that there are two processes varying independently in maturation and therapy as well as assessment.

Adolescent↗