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Comparative Anatomy and Physiology of the Mammalian Eye
Published in David W. Hobson, Dermal and Ocular Toxicology, 2020
Certain species possess a highly specialized area of the retina, the macula, which contains the fovea (Figure 20). This is found in humans, primates, birds, and some fish and reptiles. In addition, an area centralis is described in several other laboratory animals such as the cat, but these animals do not possess a true fovea. In the center of the macula is an area, the fovea, which is the area of highest visual acuity. It is completely rod free, containing only cone photoreceptors.4 The fovea is an avascular area and lacks all retinal layers except the photoreceptors and their nuclei.4
Medical and Mathematical Background
Published in Arwa Ahmed Gasm Elseid, Alnazier Osman Mohammed Hamza, Computer-Aided Glaucoma Diagnosis System, 2020
Arwa Ahmed Gasm Elseid, Alnazier Osman Mohammed Hamza
The amount of light that enters the eye is controlled by the iris, which has the ability to enlarge or contract, while the lens converges the incoming light rays to a sharp focusing point onto the retina by lengthening and shortening its width. The retina, situated at the back of the eye, is approximately 0.5 mm thick, see Figure 2.2. In the center of the retina is the optic nerve, an oval white area measuring about 2 mm (height) × 1.5 mm (width). At approximately (4.5–5 mm), or 2.5 disc diameters away from the optic nerve, lies a blood vessel-free reddish region, known as the fovea, Figure 2.2. The center of the fovea is also called the macula.
Geometric Transformations in the Visual-Motor Interface for Saccades
Published in Michael Fetter, Thomas Haslwanter, Hubert Misslisch, Douglas Tweed, Three-Dimensional Kinematics of Eye, Head and Limb Movements, 2020
When the eye fixates a target, light from this target stimulates the fovea. Light from another target would converge on a different point on the retina. As mentioned above, oculomotor physiologists call the displacement between these two points of stimulation “retinal error”. In three-dimensions, the following two definitions of retinal error are convenient for all but the very closest targets: (1) Each point on the retina specifies a potential rotation of gaze direction through a certain angle about an orthogonal axis (convenient for the displacement view) or (2) Each point on the retina specifies a potential gaze direction relative to current gaze direction (convenient for the spatial view). It should be noted that in any definition, retinal error has two important attributes. First, it is defined relative to the eye (the oculocentric reference frame), irrespective of the position of the head or any other part of the body in space. As we shall see, this imposes a reference frame problem in generating eye movements relative to the head. Second, because the retinal map is two-dimensional, it only specifies two-dimensional gaze directions, not the orientation of the eye about the gaze axis. Since the eye muscles must be controlled by a 3D signal, this imposes a degrees of freedom problem on the brain (Tweed and Vilis, 1990a; Crawford and Vilis, 1995), i.e. the arbitrary choice of eye orientation about final gaze direction and the trajectory that takes it there. We shall consider each of these compuational problems in turn.
Retinochoroidal vascular plexuses in patients with iron deficiency anaemia
Published in Clinical and Experimental Optometry, 2022
Eyüp Düzgün, Nazan Demir, Abdurrahman Alpaslan Alkan, Ceylan Uslu Doğan, Akın Çakır
We also evaluated the FAZ metrics provided by the software. They included the FAZ area, FAZ perimeter, acircularity index and foveal vessel density in a 300-µm wide region around FAZ (FD-300). FAZ was defined as the area surrounding the fovea that is devoid of retinal capillaries. The device outlined the margin of the FAZ and measured the FAZ metrics. FAZ parameters were based on full retina vasculature, including both SCP and DCP. The choriocapillaris was segmented from 9 µm above to 30 µm below the Bruch membrane. The choriocapillaris flow area was defined as the area occupied by the vasculature in a circle of 1 mm radius centred on the fovea. The retinal thicknesses in the foveal and parafoveal regions also were measured. The foveal retinal thickness was defined as the average retinal thickness within the central 1 mm diameter ring.
An Overview of Preferred Retinal Locus and Its Application in Biofeedback Training for Low-Vision Rehabilitation
Published in Seminars in Ophthalmology, 2022
Shengnan Li, Xuan Deng, Jinglin Zhang
Additionally, Schumacher etal.1811 used fMRI to measure brain activity in the calcarine sulcus while visually stimulating peripheral retinal regions in patients with macular disease. Their results showed that visual stimulation (a fixation cross) of PRL in the retina increased brain activity in the cortex, normally representing central vision relative to the visual stimulation of the perifovea and relative to stimulation in the periphery of age-matched control participants. In brief, the activation of the “fovea” cortex by peripheral stimuli was observed. These data directly combined cortical reorganization with behavioral adaptations adopted by macular damaged patients. fMRI, also used in Baker’s study,15 showed a similar significant activation of peripheral stimuli relative to the fixation baseline at the occipital pole of all participants with complete loss of foveal visual input. All the above-mentioned evidence clearly indicates that cortical plasticity in the visual system remains in patients suffered CVL in life and that the functionalization of PRL is based on visual cortical reorganization which lies on complete foveal visual input loss.
Optical Coherence Tomography Angiography in Prematurity
Published in Seminars in Ophthalmology, 2021
Mireille N. Jabroun, Bilal K. AlWattar, Anne B. Fulton
In conclusion, children with preterm birth appear to have changes of the vascular and neurosensory retina. At the fovea, a shallower pit, an increase in the central foveal, inner and outer retinal thicknesses is observed. While the foveal vessel density is noted to increase in some studies, a decrease in the foveal avascular zone was universal among the reviewed papers. Future longitudinal studies that can evaluate these changes in the growing preterm using handheld OCTA may provide further insight into the pathophysiology of foveal maturation in the preterm. Because these changes correlated with gestational age and birth weight, it remains unclear whether those changes are due to prematurity alone or related to the effects of ROP and its treatments. Larger studies comparing different groups of preterm children while controlling for gestational age and birth weight would be helpful in elucidating any effect of ROP and its treatment. While some studies demonstrated a correlation between foveal structural changes and a worse visual outcome, studies able to capture good quality images in the more severe cases with low visual acuity are still needed to demonstrate its clinical significance.