Illustration of the distribution of cone cells in the<unk>  of an individual with normal color vision (left), and a color blind (protanopic) retina. Note that the center of the<unk>  holds very few blue-sensitive cones. In the primate<unk>  (including humans) the ratios of ganglion cells to photoreceptors is about 2.5; almost every ganglion cell receives data from a single cone, and each cone feeds onto between one and 3 ganglion cells. Therefore, the acuity of<unk> l vision is limited only by the density of the cone mosaic, and the<unk>  is the area of the eye with the highest sensitivity to fine details. Cones in the central<unk>  express pigments that are sensitive to green and red light. These cones are the 'midget' pathways that also underpin high acuity functions of the<unk> . The<unk>  is employed for accurate vision in the direction where it is pointed. It comprises less than 1% of retinal size but takes up over 50% of the visual cortex in the brain. The<unk>  sees only the central two degrees of the visual field, (approximately twice the width of your thumbnail at arm's length). If an object is large and thus covers a large angle, the eyes must constantly shift their gaze to subsequently bring different portions of the image into the<unk>  (as in reading).<unk> l fixation is also considered as a overt form of attention which allows to focus sensory processing resources on the most relevant sources of information. Also,<unk> ted vision may allow speeding up learning of specific visual tasks by disregarding not relevant context and focusing on the relevant information only with lower dimensionality. blind spot of a human eye Since the<unk>  does not have rods, it is not sensitive to dim lighting. Hence, in order to observe dim stars, astronomers use averted vision, looking out of the side of their eyes where the density of rods is greater, and hence dim objects are more easily visible. The<unk>  has a high concentration of the yellow carotenoid pigments lutein and zeaxanthin. They are concentrated in the Henle fiber layer (photoreceptor axons that go radially outward from the<unk>  and to a lesser extent in the cones. They are believed to play a protective role against the effects of high intensities of blue light which can damage the sensitive cones. The pigments also enhance the acuity of the<unk>  by reducing the sensitivity of the<unk>  to short wavelengths and counteracting the effect of chromatic aberration. This is also accompanied by a lower density of blue cones at the center of the<unk> . The maximum density of blue cones occurs in a ring about the<unk> . Consequently, the maximum acuity for blue light is lower than that of other colours and occurs approximately 1° off center. Angular size of<unk> l cones On average, each square millimeter (mm) of the<unk>  contains approximately 147,000 cone cells, or cones per millimeter. The average focal length of the eye, i.e. the distance between the lens and<unk> , is 17.1 mm. From these values, one can calculate the average angle of view of a single sensor (cone cell), which is approximately arc seconds. The following is a table of pixel densities required at various distances so that there is one pixel per 31.5 arc seconds: Example object Distance from eye assumed PPI (PPCM) to match avg.<unk> l cone density (20/10.5 vision) 42" () 16:9 HDTV, ° Peak cone density varies highly between individuals, such that peak values below 100,000 cones/mm and above 324,000 cones/mm are not uncommon. Assuming average focal lengths, this suggests that individuals with both high cone densities and perfect optics may resolve pixels with an angular size of arc seconds, requiring PPI values at least times those shown above in order for images not to appear pixelated. It is worth noting that individuals with 20/20 (6/6 m) vision, defined as the ability to discern a 5x5 pixel letter that has an angular size of 5 arc minutes, cannot see pixels smaller than 60 arc seconds. In order to resolve a pixel the size of 31.5 and 21.2 arc seconds, an individual would need 20/10.5 (6/3.1 m) and 20/7.1 (6/2.1 m) vision, respectively. To find the PPI values discernible at 20/20, simply divide the values in the above table by the visual acuity ratio (e.g. 96 PPI / (20/10.5 vision) = 50.4 PPI for 20/20 vision). Entoptic effects in the<unk>  The presence of the pigment in the radially arranged axons of the Henle fiber layer causes it to be dichroic and birefringent to blue light. This effect is visible through the Haidinger's brush when the<unk>  is pointed to a polarized light source. The combined effects of the macular pigment and the distribution of short wavelength cones results in the<unk>  having a lower sensitivity to blue light (blue light scotoma). Though this is not visible under normal circumstances due to "filling in" of information by the brain, under certain patterns of blue light illumination, a dark spot is visible at the point of focus. Also, if mixture of red and blue light is viewed (by viewing white light through a dichroic filter), the point of<unk> l focus will have a central red spot surrounded by a few red fringes. This is called the Maxwell's spot after James Clerk Maxwell who discovered it. Bifoveal fixation In binocular vision, the two eyes converge to enable bifoveal fixation, which is necessary for achieving high stereoacuity. In contrast, in a condition known as anomalous retinal correspondence, the brain associates the<unk>  of one eye with an extrafoveal area of the other eye.
Fovea centralis
Caput Medusae
Corpora quadrigemina
Caput medusae
Vesical tenesmus
Cutis verticis gyrata
Pterygium inversum unguis
Glia limitans