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Biomedical subjects

R A Crone

Publications and source records attributed to R A Crone.

At least 19 recordsLinked to original sources
PubMed1992

The history of stereoscopy.

Ptolemy (127-148 AD) studied physiological diplopia, correspondence and the horopter. He had all the data to build a theory of depth perception through disparity detection, but left that undone. Alhazen (1000 AD) associated depth perception with the sensation of binocular convergence, just as Kepler (1611) and Descartes (1637). With the development of the concept of retinal correspondence and the fusion of the retinal images in the brain (Huygens 1667, Newton 1704) a cerebral mechanism of disparity detection became thinkable. The rise of Empiricism (Molyneux' Premise, the case of Cheselden) postponed the solution of the problem, finally reached by Wheatstone (1838). Physiological proof of Wheatstone's theory came from the experiments of Barlow et al. (1967).

Depth Perception↗
PubMed1988

A noncyclopean diagram of binocular vision in strabismology.

Most diagrams concerning binocular vision and squint contain the cyclopean eye, which is a vestige of the 2000-year-old projection theory. The author's previously published diagram is based on a motor theory of spatial localization which has been formulated by Descartes, Lotze, and Roelofs.

Eye↗
PubMed1988

[Greek anatomy of the eye in Pakistan today].

A Pakistani wall plate depicting the anatomy of the eye is described and illustrated. The picture is partly Greek (the hollow optic nerve), in part medieval Arab (the optical image on the disk), and in part modern (e.g., the refraction anomalies). A brief account is given of how Greek science passed to the Arabs and then to India. The wall plate bears witness to the pluralism in Pakistani medicine. Side by side with cosmopolitan medicine there exists the Unani Tibb, based on the Ionic-Arabic tradition.

Eye↗
PubMed1985

Bielschowsky head-tilt test--I. Ocular counterrolling and Bielschowsky head-tilt test in 23 cases of superior oblique palsy.

We have measured the amplitude of ocular counterrolling (OCR) and the change in vertical deviation in the Bielschowsky head-tilt test (BHT) in 23 cases of unilateral superior oblique palsy. OCR was measured with a photographical method, using limbal, conjunctival vessels as landmarks. Average OCR of the healthy eye was 5.4 +/- 2.4 (SD) deg either side, at 45 deg of body-tilt. BHT and OCR (of the healthy eye) were not related in the group as a whole. An important perturbing factor was the duration of the palsy. To clarify the relation between BHT, OCR and duration of palsy, the BHT/OCR ratio was calculated in each patient. Six cases with a palsy of presumed recent onset had BHT/OCR ratio of 0.57 +/- 0.09, while twelve cases of long-standing palsy had a BHT/OCR ratio of 1.04 +/- 0.71. This means that in cases of recent onset, the relation was relatively fixed. All high BHT/OCR ratio's occurred in long-standing palsies, whether acquired or congenital. In our opinion, disproportionately large amplitudes in the Bielschowsky head-tilt test are caused by secondary innervational changes or contractures.

Adolescent↗
PubMed1984

The accuracy of fixation in lateral gaze and the problem of retinal slip.

The mean accuracy of foveal fixation in lateral gaze was measured by means of an after-image method used previously in the study of autokinesis. During physiological excursions of gaze a lag between target image and foveal image could not be demonstrated. It must be smaller than 1/1000 of the excursion. Thus there is a striking difference between the mechanism of voluntary gaze on the one hand and the mechanism of vergence on the other hand. During sustained divergence the eyes lag almost 1/10 in respect to the required vergence.

Afterimage↗
PubMed1984

Measurement of the mechanical stiffness in cyclotorsion of the human eye.

We have measured the stiffness in cyclotorsion of the human eye using a scleral suction contact ring mounted on a shaft fitted with an eddy current motor to provide the torque to turn the eye and a shaft-position-encoder to register the torsion. The relation proved to be almost linear within the measuring range in four subjects with constant slope values of about 0.75, 0.60, 0.55 and 0.45 g . cm/deg. The average, 0.6 g . cm/deg, equals 0.5 g if applied at the radius of the globe. This value is rather low compared to the values obtained by other authors for horizontal eye movements. This may be due to different length-tension characteristics of oblique muscles compared to recti muscles.

Adult↗
PubMed1980

Treatment of (recurrent) pterygium oculi by lamellar keratoplasty.

Apart from cosmetic problems, pterygium oculi can cause functional disturbances, especially when it recurs after (repeated) operative removal: diplopia and/or reduced visual acuity. The results we obtained with the treatment of these functional disturbances by various types of lamellar keratoplasty in 20 patients (22 eyes) are good.

Adult↗
PubMed1980

Total occlusion of the left main coronary artery: clinical and catheterization findings.

In this report clinical and angiographic data on three patients with total occlusion of the left main coronary artery is presented. Two of our cases demonstrated rich collateralization with good preservation of left ventricular (LV) function. The third case, with initial subtotal occlusion and no collaterals, sustained a severe anterolateral myocardial infarction (MI) responding to the use of the intra-aortic balloon pump (IABP). This report is consistent with the findings of others who suggest the possible beneficial effect of collaterals in preserving myocardial contractility in this unusual situation. We suggest that the IABP may be of benefit in patients with total occlusion of the left main coronary artery and sparse collaterals who remain unstable and cannot be operated on immediately.

Adult↗
PubMed1980

Anomalous and normal motor fusion in esophoria and microesotropia.

Certain cases of esophoria and microesotropia have common characteristics: a small manifest deviation (called fixation disparity in esophoria), increase of the angle after occlusion, and convergent motor fusion in the presence of a persistent esodeviation. Convergent motor fusion with conservation of the esodeviation is anomalous motor fusion in which the zero point is situated at the binocular pseudofovea. The increase of the angle after occlusion, however, points to a rudiment of normal motor fusion with its zero point at zero disparity. The findings in esophoria and microstrabismus can be explained from sensory-motor factors. The hypothesis of anatomical factors is superflous.

Adult↗
PubMed1979

What is normal binocular vision?

The vergence position of the eyes is determined by the near fixation-accommodation-miosis synkinesis and the fusion mechanism. The contribution of both systems was analysed in 30 normal subjects and 16 subjects with abnormal binocular vision. Prism fixation disparity curves were determined in three different experimental situations: the routine method according to Ogle, a method to stimulate the synkinetic convergence (Experiment I, with one fixation point as sole binocular stimulus) and a method to stimulate the fusion mechanism (Experiment II, with random dot stereograms). Experiment I produced flat curves and Experiment II steep curves. The mean diameter of the horizontal Panum area was 5 minutes of arc in Experiment I and 2 degrees in Experiment II. On the basis of these findings, it was postulated that the synkinetic system operates in the absence of fixation disparity and the fusion system in the presence of fixation disparity. In Experiment II, esodisparities of 100 minutes of arc occur in a number of normal subjects. The dividing line between normal and abnormal binocular vision therefore is blurred. Normal persons can display disparities, the order of magnitude of which is equal to that of the angle of squint in micro-strabismus.

Accommodation, Ocular↗