Fovea centralis
The fovea centralis (Latin for “central pit”) or pit of sight , a depression in the retina ( retina ) located in the center of the so-called yellow spot ( macula lutea ), is the area of sharpest vision in mammals .
The fovea centralis of the adult human has a diameter of approx. 1.5 mm and lies approx. 4 mm towards the temporal side of the optic disc ( discus nervi optici ) or 5 ° temporal to the anatomical axis of the eye. This area of the retina does not contain rods for twilight vision, but only cones as light receptors for color perception, which are densely packed here at around 147,000 per mm², while the density of the total of 3.3 to 7 million cones in the peripheral retina is around 5,000 per mm² (that of the rods is over 35,000 / mm²). The cone cells of the fovea are particularly slim and here arranged closely in a regular mosaic. These are predominantly M-cones for the green area of visible light and L-cones for the red area. In the middle of the fovea, the foveola with a diameter of about 0.33 mm in humans , there are only M and L cones, but no S cones (for the blue area).
The foveola (Latin for “dimple”) is the reference point of the relative localization , it represents the subjective center and the oculomotor zero point and at the same time represents the main direction of vision “straight ahead”.
By moving the body, head and eyes, an object of interest is usually gazed into and fixed by the eye in such a way that the projected images of the parts of the object that are to be visually more precisely come to lie one after the other in the visual pit (see illustration under peripheral vision ). Due to the lack of rod cells in the fovea centralis and the associated inability to perceive finer structures in poor light conditions, it is difficult, for example, to read a text in advanced twilight . With increasing darkness, the less bright stars can then also be seen, but not recognized in color; after the transition from photopic to scotopic vision , faint stars are more noticeable if you just look past them.
In the central area of the fovea, the neuronal interconnection is such that a bipolar cell is assigned to each photoreceptor cell and a retinal ganglion cell is assigned to each photoreceptor cell, i.e. in a ratio of 1: 1: 1 without convergence . The receptive field of a ganglion cell in the foveal system is thus minimal. As a result, the highest resolution and therefore the greatest visual acuity (visual acuity) is achieved in the fovea centralis . The remaining (extrafoveal) areas of the retina are used for peripheral vision. In this system, many receptors are connected to one ganglion cell via several bipolar cells. As a result of the strong convergence, the photosensitivity is higher, but the resolution is lower. This interconnection provides a blurred overall impression in which objects are less noticeable through their outlines than through movement.
However, the actual images of vision do not arise either on the retina or in circumscribed visual centers of the brain. Only together with acquired memory images and associated associations can the many individual visual impressions of both systems be processed into a coherent perception without delay .
See also
Individual evidence
- ↑ Walther Graumann, Dieter Sasse: Compact textbook of the entire anatomy. Volume 4: Sensory systems, skin, CNS, peripheral pathways. Schattauer, Stuttgart et al. 2005, ISBN 3-7945-2064-5 , p. 43.
- ^ Theodor Axenfeld (founder), Hans Pau (ed.): Textbook and atlas of ophthalmology. With the collaboration of Rudolf Sachsenweger and others 12th, completely revised edition. Gustav Fischer, Stuttgart et al. 1980, ISBN 3-437-00255-4 .
- ↑ Herbert Kaufmann (Ed.): Strabismus. 3rd, fundamentally revised and expanded edition. Thieme, Stuttgart et al. 2004, ISBN 3-13-129723-9 .
- ↑ Hans-Werner Hunziker : In the eye of the reader. From spelling to reading pleasure. Foveal and peripheral perception. Transmedia Stäubli Verlag, Zurich 2006, ISBN 3-7266-0068-X .