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Biomedical subjects

A L Holden

Publications and source records attributed to A L Holden.

At least 19 recordsLinked to original sources
PubMed1990

Centrifugal pathways to the retina: which way does the "searchlight" point?

This commentary extends the review by Hiroyuki Uchiyama (1989) entitled "Centrifugal pathways to the retina: influence of the optic tectum" Visual Neuroscience 3, 183-206. A further scrutiny is made of the topography of the isthmo-retinal projection of the pigeon, which provides the most fully investigated example of a retinopetal projection. It is suggested that the topographical organization in this system is compatible with it's input being derived from the lower visual field (and upper retina) and it's output being directed to the horizon and lower retina. This is another form of attentional hypothesis for the centrifugal pathway, compatible with a role in ground feeding, and is testable by current tracing techniques in neurobiology.

Animals↗
PubMed1990

Ocular enlargement following infantile corneal opacification.

Congenital hereditary endothelial dystrophy (CHED) is not generally thought to be associated with other ocular abnormalities. Ultrasonography in a series of twenty eyes (ten patients) with CHED shows ocular enlargement similar to that occurring in uncomplicated axial myopia. There was an inverse relationship between the degree of enlargement and the visual acuity or visual result following penetrating keratoplasty suggesting that infantile corneal oedema sufficient to cause stimulus deprivation may result in abnormal enlargement of the globe.

Adolescent↗
PubMed1989

Binocular fields with lateral-eyed vision.

A simple geometrical model is constructed of the binocular field in animals with laterally directed eyes (such as the pigeon). Height and width are derived as a function of elevation, for eye-centred and for cyclopean co-ordinates. Stereoscopic vision in the frontal field is considered, and it is pointed out that convergence movements can facilitate stereopsis even in an afoveate periphery.

Animals↗
PubMed1988

Myopia: induced, normal and clinical.

A review is given of lid-suture myopia in monkeys, of the natural lower field myopia in the pigeon eye, and of myopias in chick eyes produced by visual occluders. The ametropias produced by ophthalmic lenses, and the pharmacology of experimental myopia, are reviewed. Human studies are reviewed which can be taken as models of the experimental myopias. Two theories of myopic growth are outlined and evaluated.

Animals↗
PubMed1988

Image size in the fundus: structural evidence for wide-field retinal magnification factor.

A simple geometrical method is described of calculating the retinal magnification factor (micrometres/degree) at the ora serrata of the human eye. At the ora the magnification factor is some 176 microns/deg., whereas at the posterior pole it is 276 microns/deg. This result may be of clinical use in examinations where dimensions of tears and holes near the ora serrata are estimated.

Humans↗
PubMed1987

The projection of the visual field upon the retina of the pigeon.

Co-ordinates in the visual field of the pigeon eye were located in the eye cup anatomically by marking the position of the trans-scleral image. The retinal horizon and vertical meridian at azimuths 0 deg (frontal field) and 90 deg (lateral field) were located. Approximately 18 deg of binocular field are available in frontal vision and the frontal meridian projects to temporal retina outside the red field. Spatial distributions of ganglion cells, displaced ganglion cells, and centrifugal terminals were located in the eye cup. Retinal magnification factors in the posterior pole and temporal periphery are close to 120 microns/deg.

Animals↗
PubMed1987

Retinal magnification factor at the ora terminals: a structural study of human and animal eyes.

A geometrical method of calculating retinal magnification factor at the limits of the retinal field, adjacent to the ora terminalis, is described. The method is applied to the eyes of 11 mammalian and avian species, using new anatomical measurements and data from the literature. In the human and monkey eye, magnification at the far periphery is substantially smaller than at the posterior pole; in cat, rabbit, rat and mouse there is lesser reduction; in pigeon, tawny owl and starling magnification is closely similar at the far periphery and posterior pole.

Adult↗
PubMed1987

Retinal dysfunction in central serous retinopathy.

Patients with acute and chronic central serous retinopathy (CSR) were studied by psychophysical and photochemical means to establish the extent of visual depression and to investigate the basis of rod dysfunction in this disorder. In acute disease with serous detachment of the retina, the loss of sensitivity attains 3 log units and parallels the height of retinal elevation as does its recovery with resolution of the episode. Immediately after resolution, there is a residual 0.5 log unit threshold elevation. In chronic disease, marked loss of function exists over areas of abnormal retinal pigment epithelium in the absence of clinically detectable serous detachment. Although rhodopsin levels are low in both acute and chronic CSR, this relative lack of visual pigment does not totally account for the functional deficits in either situation.

Fluorescein Angiography↗
PubMed1986

Stimulus contaminants in checkerboard patterns.

Variations in the mean luminance of a reversing checkerboard produced on a monitor were assessed by two methods and found to be less than 0.3 cd m-2 This quantity is much less than the amount found by other investigators.

Electricity↗
PubMed1986

A morphological analysis of experimental myopia in young chickens.

Devices that degrade vision were applied to the left eyes of 3-day old chicks. The dome device affected the entire visual field, and the arch device, only the lateral field. Control chicks wearing a circumorbital ring and untreated chicks were also examined. The dome device produced -15D and the arch device -4D of mean refractive error, while the ring and untreated chicks were emmetropic. Morphological measurements were made from macrophotographs of the intact and hemisected eyes fixed as for electron microscopy. The effects of the devices were analysed from the mean differences between the left (treated) and right (control) eyes. Nearly linear growth of the normal eye was found during the period in which measurements were taken (age 20-55 days). The ring device did not affect eye growth. The arch device significantly increased the dorsoventral equatorial diameter of the eye. The dome device had the greatest effect, and resulted in increases in both axial length and equatorial diameter during the treatment period. Dome eyes had a bulging cornea, increased anterior chamber depth, more open angle, and greater corneal diameter than controls. The axial length and equatorial diameter of the posterior segment also were increased. Two inflammatory responses of the eye were found, particularly in dome eyes; about 50% of treated eyes exhibited choroidal swelling, and vitreal clouding was found less frequently. The association between inflammation and excessive accommodation in producing the observed changes is discussed.

Animals↗
PubMed1985

Electrophysiological optometry using Scheiner's principle in the pigeon eye.

A new electrophysiological optometer has been developed, based on Scheiner's principle. Using two light-emitting diodes driven in counterphase, and grating stimuli in Maxwellian view, the system has been used to refract the photoreceptor plane of the pigeon eye. When a grating is conjugate with the photoreceptors, the electroretinographic response (e.r.g.) is minimal, and the image is stationary. As defocus is introduced, so image shift occurs, and the e.r.g. rises on each side of the refractive minimum. Two experiments were devised to test whether a derived minimum point is primary, by using gratings of bar width 3, 5 and 7 units, and by using random checkerboard stimuli. Ray tracing was used to compute the lobe width of dioptric profiles for gratings of spatial frequency 1.47, 0.88 and 0.63 cycles/deg. The computed lobe widths agree well with experimentally determined e.r.g. profiles. Examination of the lateral visual field, on the horizon, shows that the pigeon eye is emmetropic, and well corrected for astigmatism.

Animals↗
PubMed1985

Refractive sectors in the visual field of the pigeon eye.

Scheiner's principle has been used in electroretinographic optometry to refract the photoreceptor plane in different regions of the visual field of the pigeon eye. Along the horizon and in the upper visual field the eye is emmetropic, or nearly so. Below the horizon the eye becomes progressively more myopic at more negative elevations, refractive state falling to -5D at -90 deg. Lower field myopia is not an artifact of oblique astigmatism, nor of an aberration symmetrical about the optical axis. It is suggested that lower field myopia is a biological adaptation suited to keep the photoreceptors in the upper retina conjugate with the ground. Refractive state below the horizon can be fitted with a sine function by varying a parameter H (eye-ground height). The value of H agrees well with directly measured eye-ground height.

Animals↗
PubMed1985

Strategic planning in healthcare systems raises new management concerns.

Strategic planning is increasingly necessary in the current healthcare environment. Systems, as well as individual organizations, are finding the need for formalized strategic planning. In this study, data from eight systems was gathered and a number of conclusions drawn about strategic planning in healthcare systems, including: no "right" design exists, but the process should be flexible and customized; the need for formalized planning increases as the system matures; effective planning requires adequate capabilities at the organizational level; and a mix of local and corporate goals is needed to decide key issues.

Culture↗
PubMed1985

Experimental myopia in chicks: ocular refraction by electroretinography.

Application of devices that degrade the retinal image has been reported to produce enlargement of the ocular globe in young domestic chicks. Two such device types (domes and arches) were applied to 3-day-old chicks. The domes affected the entire visual field whereas the arches affected only the lateral field. A third group wore a thin circumorbital ring to control for possible mechanical impediments to growth. Untreated control chicks comprised a fourth group. At ages ranging from 3 to 7 wk, the chicks were refracted in their lateral visual fields with a Maxwellian view optometer based on Scheiner's principle, which yields an objective assessment of the refractive state of the photoreceptor image plane. One to seven measurements were taken from each of 48 urethane-anesthetized chicks. These indicated that the mean refractive states of the untreated eyes and the ring eyes were -0.20 D and -0.19 D, respectively, which did not differ significantly from emmetropia. In contrast, the mean refractive states of the arch eyes and the dome eyes were -4.11 D and -14.88 D, respectively, which differed significantly from emmetropia and from each other. The results indicate that early retinal image degradation can result in the relatively rapid development of a substantial myopia in these experimental animals.

Animals↗
PubMed1983

The distribution of displaced ganglion cells in the retina of the pigeon.

Displaced ganglion cells in the pigeon's retina, at the inner margin of the inner nuclear layer, were labelled by retrograde axonal transport of horseradish peroxidase (HRP). Large HRP injections were made in order to fill all the retinal projection sites in the thalamus and midbrain. The distribution of labelled cells was studied in retinal whole mounts incubated with tetramethyl benzidine (TMB) substrate for HRP. A maximum of 5,300 HRP labelled displaced ganglion cells was found. They were concentrated in a band of retina centred on the horizontal meridian, with high density areas (of about 110 cells/mm2) near the area centralis and in the mid-temporal retina. This is a different distribution to that of ganglion and inner nuclear layer cells; these are concentrated in the area centralis and red field. The orientation of retinal maps was checked by ophthalmoscopic measurements of the angle of the pecten to the horizontal in alert pigeons; this was found to be approximately 70 degrees. The array of displaced ganglion cells, studied by nearest neighbour distributions, was irregular and nearly random, which is consistent with a system of low spatial acuity. In the central retina only the cell bodies and not the dendrites of small displaced ganglion cells (7.5 microns diameter) were labelled; towards the periphery large displaced ganglion cells (16 microns diameter) with 2-5 radially arranged primary dendrites were found.

Animals↗
PubMed1983

The distribution of centrifugal terminals in the pigeon retina.

The terminals of the centrifugal fibres of the pigeon retina have been labelled by anterograde axonal transport or diffusion of horseradish peroxidase, and their distribution studied in whole amounts of the retina. Centrifugal terminal arborisations are concentrated in a band near the projection of the horizontal meridian on the retina. Within this band there are high density areas next to the area centralis and in the mid-temporal retina. The arborisations are largely absent from the red field and are nearly randomly arrayed. Their distribution resembles that of the displaced ganglion cells but not of other retinal neurons. We estimate that approximately 7,000 centrifugal terminal arborisations are present in each retina. Approximately two-thirds of these are convergent, with branches ending in large terminals clustered around a single cell in the amacrine sublayer; often penetrating up to 10 microns into the inner nuclear layer. The remainder are divergent endings which innervate a larger area of retina with widely spaced small terminals about 1 micron in diameter. Possible synaptic contacts between centrifugal terminals and displaced ganglion cells are occasionally seen; these contacts and the similarity in distribution suggest a link between the centrifugal and accessory optic systems.

Animals↗
PubMed1983

Vitreal and intraretinal responses to contrast reversing patterns in the pigeon eye.

The pattern electroretinogram (PERG) has been recorded vitreally and intraretinally in the pigeon eye. The amplitude of the PERG increases monotonically as pattern contrast is increased, with saturation at high levels. The PERG of the central yellow field has band-pass spatial tuning, with a high frequency cut-off at 8 c/deg. Time-to-peak is shortest at low spatial frequencies. Both PERG and local b-wave are small and positive-going close to the retinal surface, and large and negative-going in the inner nuclear layer. The PERG and b-wave show qualitatively similar depth profiles.

Animals↗