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1XX3_Vision Essentials.docx

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McMaster University
Joe Kim

Visual Essentials Introductions Our Visual Sense  The important role of visual information is reflected in the many visual references we use in our language. When visual information is in conflict with information from another sense, you tend to bias your trust towards your sense of vision-for most people, seeing is truly believing.  The eye is primarily an instrument to collect, focus, and senses the light stimulus. However, this is only the beginning as the heavy duty processing occurs in the brain. Case Study  John was rendered blind an occipital lobe stroke at the age of 30. Although he is now unable to perceive the visual world around him, he struggles to maintain his independence. The Stimulus: Light Amplitude  Light travels as a wave and can vary in two respects: the height of each wave, called the amplitude, and the distance between the peaks of successive waves, called the wavelengths.  Variations in amplitude affect the perception of brightness. Generally, the greater the amplitude of the light wave, the more light is being reflected or emitted by that object, and so that object appears brighter or more intense to us. Wavelength  Variations in wavelength affect the perception of colour. Wavelength is measured in nanometers, or millionth of a millimetre. o Smaller wavelengths refer to light waves with a higher frequency, because there is less distance between successive peaks. o Larger wavelengths refer to light waves with a lower frequency.  Humans are only sensitive to a tiny portion of the total range of wavelengths of electromagnetic radiation. We refer to this tiny portion that we`re sensitive to as the visible spectrum. 1 Visual Essentials  The shortest wavelength that we can see is around 360 nanometers, which looks violet, and the longest wavelength that we can see is around 750 nanometers, which looks red.  There are other species which can see the light outside our visible spectrum. o Insects like bees can see the wavelengths shorter than 360 nm in the ultraviolet spectrum, and perceive differences in the colours of flowers that all look the same colour to us. o Some species like snake can see light made up of wavelengths longer than 750 nm in the infrared spectrum, which allows them to find prey in the dark by being able to see the body heat that is emitted by the prey. Purity  The two physical characteristics of light, amplitude and wavelength translate into our perceptions of brightness and colour, respectively.  A final physical characteristic of light we will consider is purity, which affects the perception of the saturation, or richness of colours.  A light that is made up of a single wavelength is a pure light, and the perceived colour would be described as completely saturated.  However, natural light you experience will more likely be a combination of many wavelengths. This light would be described as desaturated. o Most of the colours we see in our everyday life are not pure but a mixture of wavelengths and thus are less intense than pure colours. The Eye The Eye  Light first passes through the curved cornea, which begins the focusing process. The cornea is a transparent window at the front of the eye. The rest of the eye is covered by the white part of the eye called sclera, a tougher membrane.  After the cornea, light passes through the pupil, which is the round window that you see as a black dot in the middle of your eye.  The iris, or the coloured part of your eye, controls the size of the pupil. The iris consists of a band of muscles that is controlled by the brain; if not enough light is reaching the retina, these muscles cause the pupil to dilate into a larger opening, whereas if too much light is entering they eye then these muscles cause the pupil to constrict into a tiny opening. 2 Visual Essentials  After passing through the pupil, light passes through the lens, a transparent structure that does final focusing of light onto the retina at the back of the eye. The Lens  The curvature of the lens causes images to land on the retina upside-down and reversed from left to right. However, the final perceived image is a product of brain activity. o Thus, rather than seeing everything upside-down and reversed, there is a correction that allows us to see a properly oriented image.  The lens is a flexible piece of tissue, the shape of which can be altered by surrounding muscles, allowing it to focus on objects that are close or far away.  If the object is close, the lens of your eye gets fatter or rounder to produce a clear image, but if the object is far away, the lens of your eye gets elongated to focus the image on the back of your eye. o This change in the shape of the lens to focus on objects that vary in distance is called accommodation. The Retina  After travelling through the lens, light passes through the vitreous humour, which is the clear jelly-like substance that comprises the main chamber inside the eyeball. 3 Visual Essentials o The light finally lands on the retina, which is the neural tissue that lines the back of the eye. The Retina Retinal Layer 1: Photoreceptors  The retina is a paper-thin sheet that covers that back of the eye, and is made up of a complex network of neural cells arranged in three different layers.  The organization of these layers may seem counter-intuitive; the layer at the very back of the eye, farthest away from the light is where the photoreceptors are located.  Photoreceptors are cells in the retina that are responsible for translating the physical stimulus of light into a neural signal that the brain can understand. To reach the photoreceptors, light must pass through the other 2 layers of retinal tissue which are transparent.  The reason for this inside-out arrangement in the retina has to do with where the photoreceptors get their nutrients from, which is a layer of cells at the very back of the eye called the retinal pigment epithelium (RPE). The photoreceptors would die without access to the RPE cells, and if the photoreceptors were located at the front of the retina, facing the light, then they would not have access to the RPE that they need to survive. Photoreceptors: Rods and Cones  Cones are designed to operate at high light intensities and are primarily used for day vision. The cones provide us with the sensation of colour and provide good visual acuity, or sharpness of detail. Cones become more concentrated towards the fovea, a tiny spot in the middle of the retina that contains exclusively cones.  Rods are designed to operate at low light intensities, and are primarily used for night vision. They provide no colour information and offer poor visual acuity. There are no rods in the fovea itself, with an increasing concentration in the region just surrounding the fovea.  When you`re trying to see an object in an environment that is dimly lit, you’re better off looking slightly to one side of the object as opposed to trying to stare right at it. o When you stare right at it, the image is focused on the cone rich fovea which doesn’t work well in a dimly lit environment. 4 Visual Essentials  By staring at one side of the object, you’ll be using your rods and increasing the chance that you’ll see it. Visual Pathways Introduction to Visual Pathways  The place where visual perception all comes together is the brain. You can think of the visual system as being comprised of a set of assembly lines. o Areas along the visual pathways process parts of the visual pathways process parts of the visual input before sending those partially processed bits of information on to the next set of areas down the line for further processing. Main Pathway: Occipital Lobe  After the optic chiasm, the information from each visual field arrives in the opposite hemisphere, at which point the optic nerve fibres split and travel along two pathways.  Most of the retinal of ganglion cell axons travel along the main pathway and synaplateralhe geniculate nucleus (LGN), which is a part off the thalamus that receives visual information. o After being processed here, the visual signals are sent to areas in the occipital lobe that make up the primary visual cortex.  There are over 20 cortical areas that process visual information, but most of the research done on visual processing has concentrated on area V1 of the occipital lobe, otherwise known as the primary visual cortex. o Collectively, the visual processing areas in the occipital lobe outside the striate cortex are known as extrastriate cortex. Primary Visual Cortex 5 Visual Essentials  Just as many photoreceptors may synapse onto a single ganglion cell in the retina, the receptive field of the LGN is made up of many ganglion cells.  The receptive field of a single V1 cell is a combination of the receptive fields of many LGN cells. So again,
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