A review of research on the sensitivity of the cornea.
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Acetylcholine↗
Biomedical subjects
M Millodot
Publications and source records attributed to M Millodot.
Sensitivity and fragility in keratoconus.
38 persons with various degrees of keratoconus were examined. Using the photokeratoscope an index of the severity of the cone was established and compared to a group of normal eyes. Sensitivity was measured in the centre and periphery of the cornea. It was found to be significantly lower than in normal eyes and those eyes wearing contact lenses exhibited the lowest sensitivity. There was no correlation between peripheral CTT (Corneal Touch Threshold) and the severity of keratoconus, but there was a significant correlation between central CTT and severity, though only for the group of eyes not wearing contact lenses. Corneal fragility was also found to be greater in keratoconus than in normal eyes and more so in those eyes wearing contact lenses. There was a significant correlation between the severity and the fragility.
Adolescent↗
Electrophysiological evidence of adaptation to colored filters.
Subjects wore blue filters (Kodak Wratten 45) in front of their normal spectacle correction for a period of 4.5 hr. Visual evoked potential (VEP's) were measured (monocularly) before, in white light and through the blue filters at the start and finish of wearing the filters. Predictably, from the chromatic interval, the subjects initially showed some degree of myopia, confirmed by more negative lens power required to give maximum VEP response. However, after 4.5 hr of wear of the filters, VEP's were measured again and it was found that the maximum VEP response was obtained in each case with less negative lens power, indicating adaptation of the visual system toward emmetropia. The role of accommodation in this adaptive process is discussed.
Accommodation, Ocular↗
Effect of ametropia on peripheral refraction.
Peripheral refraction was measured in 62 eyes along the horizontal meridian from 60 degrees nasally to 60 degrees temporally, using a Topcon refractometer. The amount of astigmatism was found to increase progressively, although less than predicted theoretically, toward the periphery in 91% of the eyes. Dividing the sample into myopes, hyperopes, and near-emmetropes showed that the type of astigmatism differed significantly with each group of eyes and that this factor played a most important role.
Adolescent↗
VEP measurement of the amplitude of accommodation.
The amplitude of accommodation was measured in 44 subjects by visually evoked potentials (VEP) in response to negative lenses. It was found that about two-thirds of the subjects responded to the lenses by increasing their accommodation in order to minimise blur, while the other one-third accepted the blur and accommodated very little. The amplitude was also determined subjectively (push-up method) and it correlated very well with the VEP measurement (r = 0.91). It is therefore suggested that VEP constitutes a feasible method of assessing objectively the amplitude of accommodation.
Accommodation, Ocular↗
Accommodation responses and refractive error.
Previous investigations have reported considerable individual differences in the resting point of accommodation. Recent independent studies in Davis, Calif., and Cardiff, Wales, indicate that part of these variations are related to refractive errors. The average dioptric value of the resting point of accommodation is lowest for corrected myopes, highest for corrected hyperopes, and intermediate for emmetropes. Another part of the variation in resting point is related to individual differences in the far point accommodation. There appears to be no significant relationship between the resting point and near point of accommodation.
Accommodation, Ocular↗
Corneal Sensitivity.
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Adaptation, Physiological↗
On the artifact of retinoscopy and chromatic aberration.
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Animals↗
Effect of oxygen deprivation on corneal sensitivity.
The effect on corneal sensitivity of exposing the cornea to reduced partial pressure of atmosphere oxygen for up to 10 h was evaluated. It was found that corneal sensitivity diminished markedly with 2.1% and 3.15% partial oxygen pressure. It is suggested that lower than normal oxygen pressure at the corneal surface is mainly responsible for significant changes in corneal sensitivity.
Cornea↗
Eyelid closure causes myopia in humans.
Humans with unilateral ptosis are more myopic in the closed eye than in the normal eye. The effect seems unrelated to visual loss. In pairs of humans ranked for sleep and myopia, the more myopic tends to sleep the more. We conclude that eyelid closure causes myopia.
Adolescent↗
Loss of corneal sensitivity with lid closure in humans.
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Acetylcholine↗
Contribution of the cornea and lens to the spherical aberration of the eye.
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Adult↗
Measurement of corneal sensitivity and thickness with PMMA and gas-permeable contact lenses.
Corneal sensitivity and thickness were measured in nine subjects. Two different types of hard contact lenses were fitted to each subject; a traditional polymethylmethacrylate (PMMA) lens on one eye and a gas-permeable cellulose acetate butyrate (CAB) lens on the other eye. Corneal sensitivity did not differ significantly between the two eyes after 6 and 10 hr of wear, but PMMA lenses tended to induce slightly more corneal edema than CAB lenses. As oxygen tension behind such contact lenses is different, whereas mechanical stimulation is nearly the same, this would appear to support the view that corneal sensitivity changes are related to mechanical stimulation. However, careful scrutiny of the data does not support this view unequivocally, and other suggestions are made.
Acetates↗
Effect of long-term wear of hard contact lenses on corneal sensitivity.
The effect of years of wear of hard contact lenses on corneal sensitivity has been evaluated. By testing 91 subjects who had worn lenses for up to 22 years and a control group of 42 people, it was found that corneal sensitivity diminished exponetially. However, in five subjects who had abandoned use of their lenses, recovery occurred within four months, indicating that the process is reversible. The loss of corneal sensitivity with hard contact lens wear ought to be heeded, since the eye is thus at a greater risk of infection.
Adult↗
Hypermetropia of small animals and chromatic aberration.
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Animals↗
The discrepancy between retinoscopic and subjective measurements: Effect of age.
Comparison of the retinoscopic and subjective measurements of 1078 eyes of all ages showed that the discrepancy (retinoscopy--subjective) was positive and equal to 0.3--0.4 D in young eyes. The discrepancy diminished with age, eventually becoming negative in the 6th decade of life. The results are explained by postulating that retinoscopic light is reflected from the internal limiting membrane in young eyes and from a layer posterior to the photoreceptors in older eyes.
Accommodation, Ocular↗
The discrepancy between retinoscopic and subjective refraction: effect of light polarization.
To identify the surface from which the retinoscopic reflex originates, retinoscopy was performed on 305 eyes of various ages with polarized and depolarized light. The discrepancy between retinoscopic and subjective refraction measures in the two conditions shows that two fundus reflective layers contribute to retinoscopy: a specularly reflecting layer presumably at the retina/vitreous interface and a diffusely reflecting layer near the pigment epithelium. In young eyes the retina/vitreous interface predominates; the other predominates in older eyes.
Adolescent↗
Does the long term wear of contact lenses produce a loss of corneal sensitivity?
Corneal sensitivity was measured with the Cochet-Bonnet aesthesiometer in a control group of 42 people and in 82 people who had worn hard contact lenses for various amounts of years. Corneal sensitivity was found to diminish significantly after a few years of wear, thus placing the wearer at some risk.
Adult↗