Biomedical subjects

Barbara K Pierscionek

Publications and source records attributed to Barbara K Pierscionek.

7 recordsLinked to original sources

PubMed2007

The effect of human in vivo accommodation on crystalline lens stability.

AIM: To determine the effect of human in vivo accommodation on the stability of the crystalline lens. METHODS: Using a dual Purkinje image (DPI) eyetracker, the phase-time difference and amplitudes of Purkinje images I (P(I)) and IV (P(1V)) were measured in 37 normal emmetropic subjects (34 women and 3 men; mean age 19.8, range 18-22 years) when they changed focus from 70 to 15 cm and simultaneously rotated their heads horizontally from side to side or made horizontal saccades between two targets 6.8 degrees apart. RESULTS: When the subjects changed focus from 70 to 15 cm and rotated their heads or made eye saccades, the phase-time difference between P(I) and P(IV) decreased. During saccades, the amplitude of both P(I) and P(IV) overshoots significantly increased with focus at 15 cm; however, their ratio (P(IV) overshoot amplitude/P(I) overshoot amplitude) significantly declined. CONCLUSIONS: The lens is stable during accommodation. The implications of these findings on the mechanism of accommodation are discussed.

Accommodation, Ocular↗

PubMed2006

The relationship between accommodative amplitude and the ratio of central lens thickness to its equatorial diameter in vertebrate eyes.

AIM: To determine the relationship between accommodative amplitude and central lens thickness/equatorial lens diameter (CLT/ELD) ratio in vertebrates. METHODS: Midsagittal sections of lenses from fixed, post mortem eyes from 125 different vertebrate species were photographed. Their CLT/ELD ratios were correlated with independently published measurements of their accommodative amplitudes. Using the non-linear finite element method (FEM), the efficiency of zonular traction (the absolute change in central radius of curvature per unit force [|DeltaCR|/F]) for model lenses with CLT/ELD ratios from 0.45 to 0.9 was determined. RESULTS: Vertebrates with CLT/ELD ratios < or =0.6 have high accommodative amplitudes. Zonular traction was found to be most efficient for those model lenses having CLT/ELD ratios < or =0.6. CONCLUSIONS: Vertebrates with lenses that have CLT/ELD ratios < or =0.6--i.e. "long oval" shapes--have the greatest accommodative amplitudes; e.g. primates, diving birds and diurnal birds of prey. Vertebrates that have oval or spherical shaped lenses, like owls and most mammals, have low accommodative amplitudes. Zonular traction was found to be most efficient when applied to model lenses with CLT/ELD ratios < or =0.6. The implications of these findings on the mechanism of accommodation are discussed.

Accommodation, Ocular↗

PubMed2006

Hypermetropia-succeeded myopia after hyperbaric oxygen therapy.

A 58-year-old man presented with a change in vision during hyperbaric oxygen (HBO) therapy. Subsequent follow-up visits showed a hypermetropic shift, which succeeded the myopic shift after each of two series of HBO treatments. The maximal refractive amplitude was 3.00 D (range -1.37 D to +1.62 D) in the right eye and 2.75 D (range -1.25 D to + 1.50 D) in the left eye. Refraction stabilized after 1.5 years at +0.62 D and +0.50 D to pretreatment values in the right and left eye, respectively. The findings are discussed with regard to possible changes in the structure of the lens.

Follow-Up Studies↗

PubMed2005

Use of the Hermann grid illusion in the measurement of contrast perception in dyslexia.

We measured contrast thresholds for perception of the Hermann grid illusion, using different contrast polarities and mean luminances, in dyslexics and non-dyslexics. Both groups of subjects gave significantly lower thresholds with grids having dark squares and light paths, but there was no significant threshold difference between groups. Perceived strength of illusion was also measured in grids at suprathreshold contrast levels. Dyslexics perceived the illusion to be significantly stronger than non-dyslexics when the grid had light paths and low luminance.

Adolescent↗

PubMed2005

Orthokeratology for myopic children: wolf in sheep's clothing?

Orthokeratology attempts to reduce myopia by remoulding the corneal shape with contact lenses. A recent resurgence is predicated on new contact lens designs with a prefigured back contact surface and higher oxygen transmissibility. This Clinical Controversy presents an analysis of the risk factors associated with orthokeratology and its suitability for children, followed by commentaries from specialists who have an interest in the method. Some state that there is a lack of data on relative risks of corneal infection and that there is a need for large-scale randomized controlled studies; however, opinion is expressed by others that orthokeratology is a clinically safe procedure using modern lenses. It is noted that the physiological and biophysical bases of orthokeratology are virtually unknown, and further research on the human cornea is indicated to scientifically establish the safety of orthokeratology. Prospective patients, and their parents in the case of children, should be fully informed of the risks.

Child↗

PubMed2004

Schematic eye with a gradient-index lens and aspheric surfaces.

A new schematic eye with aspheric surfaces and a radially varying refractive-index distribution lens is proposed. Image quality and spherical aberration are determined by use of ray tracing, and the results are presented as spot diagrams and compared with five existing model eyes. The proposed model provides the best image quality and lowest spherical aberration.

Computer Simulation↗

PubMed2003

Optical development in the foetal bovine lens.

The refractive index variations along the optic axes of nine foetal bovine lenses were measured using an optic fibre reflectometer. The device measures refractive index directly, using the principle of Fresnel reflectance. These results show that lenses with a wet weight >0.7 g (corresponding to 4.5 months gestational age), have a refractive index profile which approximates a parabolic shape, akin to that found in post-natal and adult bovine lenses. In lenses of wet weight below 0.7 g, the refractive index distributions show irregularities. These findings suggest that the refractive index profile in the bovine lens starts to form into a parabolic distribution, as seen in adult lenses, about half-way through gestation. The trigger for this formation is not known.

Animals↗