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Psych 1XX3 Vision I Lecture Notes.pdf

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McMaster University
Joe Kim

Psychology 1XX3 Notes – Vision I – Mar 1, 2010  Nearly 1/3 of the brain is devoted to processing visual information.  If our visual sense is giving us information that is in conflict with information from another sense, we tend to bias our trust towards our sense of vision. Our Visual Sense  The eye is primarily an instrument to collect, focus, and sense the light. Although there is some initial processing done on the information collected, the heavy duty processing occurs in the brain. The Stimulus: Light Intro to Light  Properties of light: There are three physical characteristics of light that translate into the three psychological perceptions of our visual world. Amplitude:  Light travels as a wave that moves at about 300 000 km/sec. Light waves can vary in two respects: the height of each wave, called the amplitude, and the distance between the peaks of successive waves, called the wavelength.(See image below).  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. (See image below, left.) Wavelength:  Variations in wavelength affect the perception of colour. Wavelength is measured in nanometers. Smaller wavelengths refer to light waves with a higher frequency, because there is less distance between successive peaks. Larger wavelengths refer to light waves with a lower frequency. (See image above, right.)  Humans are only sensitive to a tiny portion of the total range of wavelengths of electromagnetic radiation, and this tiny portion that we're sensitive to is called the visible spectrum. The shortest wavelength that we can see is around 360 nanometers, which looks violet to us, and the longest wavelength that we can see is around 750 nanometers, which is red.  However, other species can see light outside our visible spectrum. For example, insects like bees can see wavelengths shorter than 360 nm in the ultraviolet spectrum, and may perceive differences in the colours of flowers that all look the same colour to us.  Other species like snakes 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 final physical characteristic of light is purity, which affects the perception of the saturation, or richness, of colours.  A light that is made up of a single wavelength is said to be a pure light and the perceived colour would be described as completely saturated.  At the other extreme we could have a light that is a combination of all wavelengths this light would be perceived as white and would be described as completely desaturated.  Most of the colours we see in everyday life are not pure but a mixture of wavelengths and are less intense than pure colours. 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 of the eye, 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 is basically a band of muscles that is controlled by the brain, so that if not enough light is reaching the retina, these muscles cause the pupil to constrict into a tiny opening.  After going through the pupil, light passes through the lens, a transparent structure that does the final focusing of light on the retina.  See image on next page. 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.  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.  This change in the shape of the lens to focus on objects that vary in distance is called accommodation.  After travelling through the lens, light passes through the vitreous humor, which is the clear, jelly-like substance that comprises the main chamber inside the eyeball, until it finally lands on the retina, which is the neural tissue that lines the back of the eye. (See image on next page.) The Retina:  Neural processing of visual information: the retina, because this is where the physical stimulus of light is first translated into neural impulses. The 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, or 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 live. (See image below.) Photoreceptors: Rods and Cones  There are two different kinds of photoreceptors, called rods and cones, each named for their respective shapes. (See image below)  Humans have about 125 million rods but only 6 million cones.  Cones are designed to operate at high light intensities and are 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. When you want to see something in detail, we move our eyes so that the image falls directly onto the fovea.  Rods are designed to operate at low light intensities, and so are used for night vision. They provide no information from which colour can be determined, and offer poor visual acuity. There are no rods in the fovea itself, with increasing concentration in the region just surrounding the fovea. This arrangement make rods very useful for peripheral vision.  This  explains  why,  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.  When you stare right at it you are using your cones, which do not work in a dimly lit environment. But if you stare to one  side  of  the  object,  you’ll  be  using  your   rods and increasing the chance that you will see it. Photoreceptors: Response to Light  How do photoreceptors actually translate the physical stimulus of light into a neural signal that the brain can read?  Photoreceptors contain a photo pigment, which is a complex molecule that is sensitive to light.  The human eye has 4 different kinds of photo pigments, one for rods and three for cones, but they all basically work the same.  When a photon of light is absorbed, it changes the chemical state of the photo pigment and splits into its two component molecules which sets off a biochemical chain reaction leading to an electrical current flowing across the membrane.  The original light stimulus is now in a currency that can be understood and processed by the brain.  On
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