Fisiologia da visão
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Fisiologia da visão
- 1. INSTITUTO DE FISIOLOGIA AEROESPACIAL BRIGADEIRO MÉDICO ROBERTO TEIXEIRA INSTITUTO DE FISIOLOGIA AEROESPACIAL BRIGADEIRO MÉDICO ROBERTO TEIXEIRA INSTITUTO DE FISIOLOGIA AEROESPACIAL BRIGADEIRO MÉDICO ROBERTO TEIXEIRA INSTITUTO DE FISIOLOGIA AEROESPACIAL BRIGADEIRO MÉDICO ROBERTO TEIXEIRA INSTITUTO DE FISIOLOGIA AEROESPACIAL BRIGADEIRO MÉDICO ROBERTO TEIXEIRA INSTITUTO DE FISIOLOGIA AEROESPACIAL BRIGADEIRO MÉDICO ROBERTO TEIXEIRA INSTITUTO DE FISIOLOGIA AEROESPACIAL BRIGADEIRO MÉDICO ROBERTO TEIXEIRA INSTITUTO DE FISIOLOGIA AEROESPACIAL BRIGADEIRO MÉDICO ROBERTO TEIXEIRA
- 2. FISIOLOGIA DA VISÃO
- 8. A RETINA FÓVEA (centro da mácula, só cones) DISCO ÓPTICO (entrada e saída de nervos e vasos) MÁCULA (centro da retina)
- 11. RETINA E DISCO ÓTICO VISTA LATERAL
- 18. VISÃO NOTURNA
- 21. CURVA DE ADAPTAÇÃO NOTURNA CONES BASTONETES Tempo de exposição ao escuro (min). Limiar de luminância (escala log). 7 6 5 4 3 2 0 10 20 30
- 23. Comprimento de onda (nm) Resposta Relativa (%) AZUL VERMELHO FENÔMENO DE PURKINJE 100 80 60 40 20 0 350 400 450 500 550 600 650 AZUL - VERDE (Escotópica) AMARELO - VERDE (Fotópica) 510nm 560nm
- 32. INSTITUTO DE FISIOLOGIA AEROESPACIAL BRIGADEIRO MÉDICO ROBERTO TEIXEIRA INSTITUTO DE FISIOLOGIA AEROESPACIAL BRIGADEIRO MÉDICO ROBERTO TEIXEIRA INSTITUTO DE FISIOLOGIA AEROESPACIAL BRIGADEIRO MÉDICO ROBERTO TEIXEIRA INSTITUTO DE FISIOLOGIA AEROESPACIAL BRIGADEIRO MÉDICO ROBERTO TEIXEIRA INSTITUTO DE FISIOLOGIA AEROESPACIAL BRIGADEIRO MÉDICO ROBERTO TEIXEIRA INSTITUTO DE FISIOLOGIA AEROESPACIAL BRIGADEIRO MÉDICO ROBERTO TEIXEIRA INSTITUTO DE FISIOLOGIA AEROESPACIAL BRIGADEIRO MÉDICO ROBERTO TEIXEIRA INSTITUTO DE FISIOLOGIA AEROESPACIAL BRIGADEIRO MÉDICO ROBERTO TEIXEIRA
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- During this presentation, you will review some key information relating to visual function and night vision. You will also progressively dark adapt as the room lighting is gradually dimmed.
- We have now reviewed the essential elements of visual function, particularly at night. You are also now fully adapted to the dark.
- We have now reviewed the essential elements of visual function, particularly at night. You are also now fully adapted to the dark.
- The cornea is the clear outer layer at the front of the eye through which light enters the eye. The cornea provides approximately 75% of the focusing ability of the eye. The lens is located within the eye. The lens provides the other 25% of the eye’s focusing ability. It fine-tunes the visual image.
- The retina is the light-sensitive inner layer of the eye, and is made up of 2 types of photoreceptors called cones and rods. The cornea and lens act to focus incoming light onto the retina. The cones and rods then convert the light into electrical impulses via a photochemical reaction. There are approximately 17 rods for every cone in the retina. The diagram at the bottom left shows the distribution of the rods and cones in the retina.
- The Iris and the pupil control the amount of light that enters the eye. The iris is the coloured part of the eye, while the pupil is the central hole in the iris. The iris is composed of muscles, which can contract and expand automatically to adjust the size of the pupil. This then controls how much light enters the eye. In bright light the iris contracts the pupil, restricting the amount of light that enters the eye. At night, the iris allows the pupil to widen in order to allow as much light as possible to enter the eye.
- This image shows the central part of the retina. The two major areas of the retina, the fovea and the optic disc, are shown. These areas are only approximately 3 millimetres apart. The fovea is the dark area to the left. This area is located in the geometric centre of the eye, along the visual axis. Light entering the middle of the eye will eventually fall on this part of the retina. The fovea is composed exclusively of cones (about 7 million). The optic disc is located approximately 10-15 0 to the nasal side of the retina. This disc is the end-on view of the optic nerve as it leaves the eye. Blood vessels can be seen entering and leaving the eye through this area. The optic disc contains no cones or rods. We will see the importance of this later in relation to the physiological blind spot.
- Cones are photoreceptors that are located in the fovea, where there are approximately 47,000 cones per square millimetre. Best visual ability (also known as acuity) occurs in this part of the retina. There are three different types of cones (red, blue and green) which all have different peak spectral sensitivities. Each cone connects to a single optic nerve fibre. This 1:1 ratio results in an image that has high resolution and detail.
- Rods are photoreceptors that are predominantly in the peripheral part of the retina. The retina contains approximately 120 million rods. They are associated with poor visual acuity, and provide little in the way of image detail and resolution. They are not sensitive to colour at all. Functionally rods are connected into large groups. Many thousands of rods connect with a single optic nerve fibre, in a ratio of up to 10,000:1. This very high ratio explains the poor image quality.
- Cones are photoreceptors that are located in the fovea, where there are approximately 47,000 cones per square millimetre. Best visual ability (also known as acuity) occurs in this part of the retina. There are three different types of cones (red, blue and green) which all have different peak spectral sensitivities. Each cone connects to a single optic nerve fibre. This 1:1 ratio results in an image that has high resolution and detail.
- The physiological blind spot is an important concept. The optic disc contains no rods and cones, and as such cannot convert light into electrical energy. This area of the retina covers 2 - 6 0 of the visual field. As such, it is sufficiently large that an object 18 metres tall 200 metres away can be invisible to the eye. The blind spot for each eye is close to the nasal side of the retina. As such, light falling on the blind spot of one eye will fall on corresponding normal light-sensitive retina in the other eye. The object will therefore be seen, and the blind spot will not be apparent to the viewer. If, however, one eye is closed, the blind spot can be made obvious. The fact that we have two eyes with blind spots in different relative areas means that under normal binocular conditions the blind spots are not apparent. O ponto cego anatômico é um importante conceito. O disco óptico não contém cones e bastonetes. Essa área da retina abrange uma angulação de 2 a 6 graus do campo visual, que é suficientemente ampla para tornar invisível aos olhos um objeto de 18 metrros na distância de 200 metros. O ponto cego de cada olho é fechado para o lado nasal da retina, ou seja, a Luz incidindo no ponto cego de um olho, corresponderá a uma incidência normal de luz na retina do outro olho. O objeto séra portanto visto e o ponto cego não estará evidente para a pessoa. Se, todavia, um olho está fechado, o ponto cego pode tornar-se evidente. O fato que temos 2 olhos com pontos cegos em áreas relativamente diferentes significa que sob condição binocular, o ponto cego não é percebido.
- What is meant by visual acuity? It is the term used to describe visual ability, and is measured via a standard eye chart viewed at a distance of 6 metres. Normal visual acuity is described as 6/6, which means that you can see at 6 metres what the rest of the population can also see just as well at 6 metres. Poor visual acuity would be 6/60, which means that you can see at 6 metres what the rest of the population can also see just as well at 60 metres.
- The human eye can function over a wide range of light levels, from faint starlight right through to bright sunlight on snow. The detection of light requires both rods and cones to function, depending on the amount of available light. The detection of light involves the conversion of light energy into electrical energy. This conversion process occurs in the rods and cones via a photochemical reaction. The cones contain 3 photopigments called opsins that are used in these reactions, while rods contain only 1 pigment called rhodopsin. Three different types of vision have been described which depend on the available light levels. These are: Photopic vision Scotopic vision Mesopic vision
- Photopic vision is used during conditions of high light levels such as day-time. It involves the cones, and has good visual acuity and form sense. Scotopic vision is used during low-light levels such as at night. It involves the rods, and has poor acuity and form sense. Mesopic vision is a transitional stage of vision, in which light levels are such that both cones and rods can contribute to visual function. This corresponds to twilight or dusk, dawn, or full moonlight conditions. The use of night vision goggles depends on mesopic vision. It is a common misconception that only cones operate during the day and only rods operate during the night. In reality, both cones and rods operate over a wide range of luminance. Indeed, during mesopic vision they operate simultaneously.
- Colour vision is an important element of visual function. The human eye can detect millions of subtle variations in colour. Colour vision is a function of the cones, and as such involves focal vision. There are three different types of cone that are specific for three primary colours. Blue, red and green cones are present in a ratio of 1:10:10. Using these three cone types, any colour can be represented by varying the proportion and saturation of these three primary colours. The three cone types have different peak spectral sensitivities; Red cones are maximally sensitive to wavelengths of 564 nm. • Blue cones are maximally sensitive to wavelengths of 420 nanometres (nm) • Green cones are maximally sensitive to wavelengths of 534 nm.
- The night blind spot is an important concept. The fovea contains cones, which at night are effectively not used. The fovea thus becomes a functional blind spot at night, since it contains no rods. At night, therefore, each eye has two blind spots: The physiological blind spot (optic disc) The functional night blind spot (fovea)
- Having considered the physiology of normal vision in some detail, the next series of slides will consider fundamental aspects of night vision.
- Night vision is predominantly a function of the rods, and is mediated largely by peripheral or ambient vision. Visual function at night is generally poor, in terms of acuity, resolution and detail. Colour vision at night is also poor. These features are shown in this mountain scene at night, taken from a small aircraft. The night environment consists of degraded visual cues. Worsening atmospheric conditions, such as fog, clouds and rain can degrade what visual cues there are at night even further. These factors can all increase the risk of spatial disorientation at night.
- Dark adaptation is an important concept. The human eye takes time to adjust to varying ambient light levels. On going from dark conditions to well-lit conditions, the eye adapts almost instantaneously. However, a much slower adaptation takes place when the opposite occurs, i.e., on going from well-lit conditions to dark conditions. In addition, each eye adapts at its own independent rate.
- Here we see a graphical representation of dark adaptation. On entering darkness, the cones take less than 10 minutes to achieve full adaptation. However, not much change in visual function occurs. The rods take longer to adapt, and once they have completed this process the change in visual function is much greater. Threshold luminance is much lower, indicating much better night vision.
- The Purkinje phenomenon is also an important concept. It relates to the fact that rods and cones have different spectral sensitivities. Rods are maximally sensitive to blue-green light in the 500 nm range, and are thus relatively insensitive to red light. Cones are maximally sensitive to yellow-green light in the 560 nm range.
- The graph shown here demonstrates this Purkinje phenomenon. It shows that there is a shift in visual sensitivity in changing from photopic or day vision to scotopic or night vision. In practical terms, the Purkinje phenomenon is the relatively greater brightness of blue- green light compared with yellow or red light upon changing from photopic to scotopic vision.
- Cockpit lighting is clearly very important for safe and effective night flight. Flight at night relies on both cones and rods. The rods are used for external vision, while the cones are used for seeing the interior of the cockpit (instruments and controls). Red lights used to be common in cockpits, as the rods are insensitive to red wavelengths. As such, red light does not interfere with dark adaptation of the rods.
- Red light does tend to interfere with colour discrimination, which can be an important task with modern cockpit displays and map reading. This effect is made worse for aircrew whose eyesight is deteriorating due to age. The best solution for cockpit lighting is a compromise, using dim white lighting. This allows cone function to be preserved, yet if not too bright the rods will still be able to continue functioning with minimal interference.
- Keeping the eyes moving in a scan pattern will ensure that more of the light-sensitive peripheral retina will be stimulated. This increases the chances of detecting an object, whether it is stationary or moving. You should never fixate on an object for more than 2 or 3 seconds. You should also ensure that there is a 10-15 0 overlap between areas of scan, to minimize the effect of the night blind spot.
- Keeping the eyes moving in a scan pattern will ensure that more of the light-sensitive peripheral retina will be stimulated. This increases the chances of detecting an object, whether it is stationary or moving. You should never fixate on an object for more than 2 or 3 seconds. You should also ensure that there is a 10-15 0 overlap between areas of scan, to minimize the effect of the night blind spot.
- There are some important steps for maximizing unaided night vision prior to flight. A balanced diet is very important. Adequate levels of vitamin A will help ensure that the photopigments (which contain vitamin A) are at correct levels. An adequate, well-balanced diet will ensure an optimal supply of vitamin A. Plenty of rest is important, as fatigue will degrade visual performance. Avoiding bright lights prior to flight will help preserve dark adaptation by minimizing photopigment depletion. In the same way, wearing sunglasses during the day will also help. Smoking leads to problems with dark adaptation and visual performance, as well as hypoxia that will also interfere with vision. Alcohol will impair eye coordination and judgement. Various drugs may have an adverse effect on reaction times, awareness levels and visual performance.
- There are some important steps for maximizing unaided night vision prior to flight. A balanced diet is very important. Adequate levels of vitamin A will help ensure that the photopigments (which contain vitamin A) are at correct levels. An adequate, well-balanced diet will ensure an optimal supply of vitamin A. Plenty of rest is important, as fatigue will degrade visual performance. Avoiding bright lights prior to flight will help preserve dark adaptation by minimizing photopigment depletion. In the same way, wearing sunglasses during the day will also help. Smoking leads to problems with dark adaptation and visual performance, as well as hypoxia that will also interfere with vision. Alcohol will impair eye coordination and judgement. Various drugs may have an adverse effect on reaction times, awareness levels and visual performance.
- We have now reviewed the essential elements of visual function, particularly at night. You are also now fully adapted to the dark.
- We have now reviewed the essential elements of visual function, particularly at night. You are also now fully adapted to the dark.