The yellow spot, also called the macula lutea, is a small area on the retina through which the visual axis passes. Within the macula lutea is the zone of sharpest vision (fovea) and also of color vision, because the approximately 6 million cone-shaped M, L, and S color sensors are concentrated almost exclusively in the fovea. The lenses of the eye can change their refractive power (accommodation) within certain limits so that, depending on requirements, near or distant objects in the yellow spot, or fovea, are brought into focus.
What is the yellow spot?
The yellow spot or macula lutea is a defined small area in the retina in extension of the visual axis. Depending on the definition, the yellow spot in humans has a diameter of 3 to 5 mm. For vision, there are 120 million highly light-sensitive and motion-sensitive rod-shaped light sensors and about 6 million less light-sensitive cone-shaped sensors in the three designs S-, M- and L-cone, with which color vision is possible due to their different sensitivity for certain wavelengths of the incident light. The macula lutea contains the zone of sharpest vision, the fovea centralis, in its center. It exclusively contains the cone-shaped light sensors. The fovea centralis has a diameter of about 1.5 mm and contains the foveola, also called the visual dimple. Our central vision is concentrated on this tiny area, which is about 0.35 mm in diameter. With its fovea centralis, the yellow spot achieves its greatest importance in color and focus vision in the central visual field at relatively high light intensity (daylight). At low light intensity, peripheral vision via the rod sensors comes to the fore, but at the price of very low resolution and loss of color vision.
Anatomy and structure
The macula lutea is a defined area in the central portion of the retina, up to 5 mm in diameter with the addition of the peripheral areas. The name yellow spot is based on the fact that the retina in this area is more pigmented with the carotenoids lutein and zeaxanthin. Anatomically, the macula differs from the other areas of the retina in the accumulation of the three different color receptors, S, M and L cones, which have different sensitivities to the different wavelengths of light, thus allowing color discrimination within the visible color spectrum. In the central area of the macula there is a small funnel-shaped depression, the fovea centralis, in which the three color receptors are exclusively located, about 140,000 per qmm. While in the outer areas of the fovea centralis all three types can be found, the foveola (dimple), which with a diameter of only 0.35 mm separates the absolutely central area of the fovea, contains only color receptors of type M and L (green and red). In the marginal areas toward the outside, the macula increasingly includes the light-intense rod sensors.
Function and tasks
The macula lutea is the most important area of the retina for sharp, high-resolution vision and for color vision. The yellow spot embodies the central visual field. When objects are to be “eyed,” the eyes involuntarily adjust themselves to image the object in the foveola, the tiny depression in the fovea centralis. To do this, the lens of the eye accommodates in such a way, also unconsciously, that a high-resolution “image” can be created depending on the distance of the object. However, the image is not created as on a projection screen, but each individual color receptor (M and L cones) reports its received stimulus via its own ganglion cell to the visual center. This compiles an image that is not necessarily an exact 1:1 reproduction of reality, but has undergone several “image processing” processes in which sensory feedback from other sensors, such as vestibular stimuli, also have an influence. Also, in stereoscopic vision, the brain can complete the images of both eyes to a certain extent, as in the example of the blind spot (exit point of the optic nerve from the retina). Actually, we should see in our visual field two black dots each corresponding to the blind spot of the right and the left eye. However, the visual center is able to replace the black dots with visual material, which is seen by the right and the left eye respectively.
Diseases
One of the most common diseases of the macula lutea is age-related macular degeneration (AMD). The disease affects men and women equally from about age 50. Damage to the macula initially leads to a decrease in visual acuity and contrast perception. A high sensitivity to glare sets in and visual field loss in the central visual field may occur. Even if the exact causes of the disease are not yet sufficiently clarified, it is certain that the starting point of the disease lies in the supporting and supplying layers of the retina. Certain genetic defects are also considered risk factors. Macular degeneration has also been associated as a consequence of malaria prophylaxis with cloroquine. Advanced diabetes can cause diabetic maculopathy, which results from fatty deposits around the macula and edema that can form in the choroid due to damaged vessels. Relatively rare is retinopathy centralis serosa (RCS). It is caused by a leakage of fluid from the choroid due to a leak in the Bruch’s membrane. This can cause the retina to detach in places, leading to symptoms such as a “gray spot” in the central visual field, image distortion, and disturbances in color perception.
Typical and common diseases
- Age-related macular degeneration
- Light sensitivity
- Visual field loss
