Visual Information: Sensation and Perception - Assignment Example

Visual Information: Sensation and Perception 1. Cortical magnification The cortical magnification means how many neurons in an area of the visual cortex are there as a function of visual field location to process the stimulus of a given size. Corresponding to the fovea of the retina, a very large number of neurons process information from a small region of the visual field. On the other hand, a small number of neurons process the same information in the periphery of the visual field. As the number of neurons reduces in the periphery, the visual performance is affected while it is best in the center where the number of neurons is higher. The retina of our eye is composed of two types of cells: cones and rods. Cones are nerve cells sensitive to light, detail and color. Cones help us in reading letters and seeing objects especially at center. Cones are packed into the macula and help in visual details. On the other hand, rods are helpful for night vision and detection of moving objects. Rods provide peripheral vision and are insensitive to color. Thus, when we go to a dark place from light, we are unable to see objects clearly. There rods help us to distinguish objects, however, being insensitive to color, we are able to see only the colorless object. Figure 1: Log polar mapping from retina (left) to straite cortex (right). Points near the center of the visual field are represented more heavily in cortex than are points in the periphery. Source: Schwartz (1994) 2. Retina and Retinomapping Retina is the part of eye which is composed of the layer of nerve tissue and it covers the back two-thirds of the eyeball. Retina receives the light and converts it into chemical energy which activates the nerves (cones and rods). These nerves conduct the messages out of the eye into the higher region of the brain. The retina thins out in the center of the macula forming a pit called fovea. The reason of its thinness is that the light passing through fovea is scattered or observed by this thinness before it reaches the photoreceptors. Retinotopic Mapping: This is a standard procedure adopted to define the borders of early visual areas in occipital cortex. The human visual cortex is divided into several functional areas with distinct local neural properties. The positions of functionally specialized visual areas are only loosely linked to cortical anatomy and are subject to variability between individuals. Several of these areas are retinotopic, that is, their neurons respond to stimulation of limited receptive fields whose centers are organized to form a continuous mapping between the cortical surface and the visual field. The boundaries between most of the low order visual areas can be determined from their retinotopic properties: the local representation of the visual field on the cortical surface changes its orientation—the local visual field sign (VFS)—between adjacent visual areas. 3. Fovea Retina is located in the back of eye and has a complex structure. It is composed of macula which lies in the center of the retina and the macula covers up to ¼ inch diameter. The macula further contains a very small area called fovea. Fovea is responsible for very sharp vision (20/20). The fovea has the highest concentration of cone photoreceptors, ganglion cells, and horizontal cells rather than the rods. The reason for the high concentration of cones is that cones function best in the bright light. That is why fovea works best in daylight illumination. On the other hand, rods function better in dim light and are not capable of sharp vision. Thus, naturally there is high concentration of cones in the fovea in order to make it capable of sharp vision (20/20). 4. Comparison of Tactile with visual information i). Tactile Information: When we touch something, the nerves present in the skin are stimulated and carry this information to the brain where it is interpreted and processed. Thus, the tactile information comes from touching objects which is carried through the nerves ending in the skin that convey sensations to the brain via nerve fibers. Feeling a leaf or falling a rain drop in cheek or shaking hands with your friends all this is the tactile sensation and the source of gathering information through touch. Any information gathered through touching is called the tactile information. ii). Visual Information: As compared to the tactile information, the visual information we whether through seeing objects. In the visual information, light plays an important role. Our eye receives the rays of light which passes through retina to the brain where image is formed and our brain interpret it as an object. Thus, visual information works the same way as the tactile information with only difference that tactile information is carried through touching while the visual information is carried through light and the eye is the medium in conveying the information to our brain. 5. Perception and importance of Knowledge We get information about the world around us through our five senses, i.e., sight, hearing, touch, taste, and smell. All these senses convey the information to the brain which interprets it and we are able to perceive and translate objects into meaningful information about the world. This process enables us to know things around us. Knowing things and objects also depends much on our faculty of perception. Indeed, when we touch fire for the first time we come to know that it burns. Thus, the perception of fire is formed in our mind and the next time, we see any object burning our previous knowledge or experience of fire tells us that it will burn, so we refrain from touching it. This sort of knowledge forms our perception of the things around us. That is why knowledge is important for perception. 6. Knowledge and Perception The philosophers and scientists have separated perception and knowledge traditionally. Perception is related with vision and vision is passive window on the world. On the other hand, knowledge actively helps us distinguish objects and translate them into meaningful information (Gregory, 1997). Thus, our perception depends much on knowledge and the intelligent problem solving. It is rightly said that our perception is many a time misleading us, for instance, when we “see” a vast dessert shining under the sun, we perceive it to be water whereas it is in fact the sand which shines. From this, we may conclude that our perception, which depends on the senses may be wrong and misleading us at occasions. To help the perception, knowledge comes up and critically analyses the information passed by the senses and infers from our previous experiences either this information is right or wrong. Thus, the importance of knowledge is always there to help the perceptions. 7. Sensation, Perception and cognition Sensation, perception and cognition help us learning about the objects and events in the world and what benefits and/or dangers these have for us. According to Heraclitos (5th century BC) knowledge comes through the door of sensation. Thus, sensation is a process by which our sensory receptors and nervous system receive and represent stimulus energies from our environment. However, sensation is not all we require to make sense of the things we see, feel or touch. There is also a strong need of perception. In fact, through sensation, we come to know about things and objects and through perception we organize and interpret that knowledge to make it meaningful and beneficial for us. Cognition is one step farther and it we can say that it is the process or result of recognizing, judging and reasoning the information we receive through sensation.  From this we can see the significance of knowledge and experience of sensation, perception and cognition. Sensation provides us raw information which is processed and interpreted through the process of perception and cognition help us in learning through the course of time, thus adding valuable knowledge and experience to us about the world and the things and objects around us. 8. Cochlear Plant A cochlear implant is a small, complex electronic device especially designed to help for providing a sense of sound to a person who is profoundly deaf or severely hard-of-hearing. Cochlear implant consists of two portions: an external portion that sits behind the ear and the second portion that is surgically placed under the skin (see figure below) Cochlear implant is such an invention which helps a person who is profoundly deaf to provide a sense of sound. The cochlear implant is considerably different from other hearing aids and it generates the audio signals which are sent through the auditory nerves to the brain which recognizes the signals as sound. Thus the person with hearing disability can understand sounds in the environment and enjoy a conversation with the people (Cochlear Implant, 2007). 9. Animal Visual System The visual system in many species follows the same pattern like the human visual system. However, the structure may vary significantly as some animals have very complicated and developed visual system while some have just a patch of eye and consist of single optical system. There are still some species whose eyes consist just a patch of photosensitive pigment on the surface of skin which reacts to the presence of light and the creature is able to know if it is night or day. In animals, lens-containing eyes are a feature of surprisingly broad spectrum of organisms that represent a significant improvement of simple eye composed of just photoreceptor cells and pigment cells. With the passage of time, animals visual system has also developed and evaluated remarkably…… 10. P-pathway and M-pathway Our eyes break the incoming images down into three main components: color, position, and brightness. These pieces of information are communicated from eye to the brain along separate and specialized pathways. These pathways are called P-pathway and M-pathway. Through the parvocellular (P) pathway information about color and fine spatial detail is carried out while the magnocellular (M) pathway is colorblind and has poor spatial resolution. However, it is sensitive to low contrast and rapid changes. The visual cortex uses the information from these pathways to compute further details about color, shape and motion. It was believed until quite recently that only the M pathway connected to the cortical motion processing area called MT. This is because the M and P pathways remain separate as they extend through the brain to the primary visual cortex (V1). And the cells in V1 that provide input to MT appeared to receive input from only the M pathway. However the recent research show that these cells also receive input from the P pathway. 11. Signal Detection Theory Signal Detection Theory (SDT) illustrates how we analyze data coming from experiments where the task is to categorize ambiguous stimuli which can be generated either by a known process (called the signal) or be obtained by chance (called the noise in the SDT framework). For example a radar operator must decide if what she sees on the radar screen indicates the presence of a plane (the signal) or the presence of parasites (the noise). This type of applications was the original framework of SDT (Green & Swets, 1966) But the notion of signal and noise can be somewhat metaphorical is some experimental contexts. For example, in a memory recognition experiment, participants have to decide if the stimulus they currently see was presented before. Here the signal corresponds to a familiarity feeling generated by a memorized stimulus whereas the noise corresponds to a familiarity feeling generated by a new stimulus. 12. Match Bands Illusory bands of intense lightness and darkness perceived adjacent to borders of light and dark in a visual image, caused by early image-processing in the retina and optic nerve. The Mach bands are directly related to the size and the shape of on-center off-surround neural units in human vision. At low overall intensities, the dark band increase markedly in width, while the bright band does not. However, the bandwidth is more affected by the brightness slope, than by the plateau intensity per se. In this case, both bands vary approximately linearly and inversely with the log of the slope. The bright bands are slightly wider than the dark bands, for matched intensities. Both bands almost double in width with only a ±30′ para-foveal fixation. Optical blur enlarges the bands as predicted from the spread function. The apparent centers of the bright bands are positioned significantly more asymmetrically between the two edges than are the dark band centers. 13. Importance of Edge detection in Visual System Edges are often characterized by abrupt changes in intensity within an image. Consider a simple one-dimensional intensity/grey-scale profile, I(x), with each point (pixel) having a given value, I1, I2, I3…In. The change in intensity from one point to the next along the profile, is approximated by the difference in intensity, δI, between adjacent pixels, divided by their spatial separation in pixels δx. An ‘image operator’ or filter can be applied to a grey-scale image to produce a new image in which each pixel corresponds to the gradient at a given location. The presence of an edge can then be found at peaks in the gradient image (i.e. where the value of a pixel in the first derivative is larger than the value of its neighbors). According to Marr & Hildreth (1980) edges should be better located in the second derivative (δ2I/δx2) of the image, by applying the operator twice in succession. At a gradient peak in the first derivative, the slope is zero, so edges in the second derivative are located by the presence of ‘zero-crossings’. Human experiments show that people mark the positions of edges close to zero crossings of the second derivative, though it is not clear if it is something closer to the first or second derivative that is used in visual processing. 14. Cortical Magnification The cortical magnification illustrates how many neurons in an area of the visual cortex are there as a function of visual field location in a given size. Corresponding to the fovea of the retina, a very large number of neurons process information from a small region of the visual field. On the other hand, a small number of neurons process the same information in the periphery of the visual field. As the number of neurons reduces in the periphery, the visual performance is affected while it is best in the center where the number of neurons is higher. 15. True Color Blindness and Color deficiency The cone cells of the human retina see mainly the red, green and blue colors. Other colors are formed by the combination of these three basic colors. So, if the red and green cones are triggered, then the brain thinks "yellow". People who are "colorblind" partially lack these color-sensitive cones so these colors will appear darker. Few people are completely colorblind. Colorblind people are unable to see any color while people having color deficiency cannot distinguish specific colors or the mixture of basic three colors. 16. Monocular and Binocular cues Monocular cues allow us in judging the distance and depth of objects. Monocular cues include relative size, interposition, linear perspective, aerial perspective, light and shade, and monocular movement parallax. The retinal image size allow us to judge distance based on our past and present experience and familiarity with similar objects. This can be illustrated by the example of moving car when it drives away, the retinal image becomes smaller and smaller. We interpret this as the car getting further and further away.Stereopsis is an important binocular cue to depth perception. Stereopsis cannot occur monocularly and is due to binocular retinal disparity. Stereopsis is the perception of depth produced by binocular retinal disparity. Therefore, two objects stimulates disparate (non-corresponding) retinal points within Panum's fusional area. 17. LGN The lateral geniculate nucleus (LGN) is a part of the brain which primarily processes the visual information received from the retina. The LGN receives information directly from the retina and sends to the primary visual cortex. Optic nerve fibers from the eyes terminate at two bodies in the thalamus known as the Lateral Geniculate Nuclei (LGN) One LGN lies in the left hemisphere and the other lies in the right hemisphere. 18. Optic Chiasm The optic chiasm is derived from Latin which means “crossing” and X. The nerves connected to the right eye that attend to the right temporal visual field (located in the nasal portion of the right retina) cross to the left half of the brain, while the nerves from the left eye that attend to the left temporal visual field (located in the nasal portion of the left retina) cross to the right half of the brain. This process allows for parts of both eyes that attend to the right visual field to be processed in the left visual system in the brain, and vice versa. 19. Adaptation, Habitation and cessation Short-term behavioral habituation is the response decrement observed in many behaviors that occurs during repeated presentation of non-reinforced stimuli. Within a number of invertebrate models of short-term behavioral habituation, depression of a defined synapse has been implicated as the mechanism. An adaptation is a positive characteristic of an organization that has been favored by natural selection. The term adaptation is also sometimes used as a synonym for natural selection. Any change in the structure or functioning of an organism that makes it better suited to its environment is known as adaptation. Adaptation is the change in organisms that allow them to live successfully in an environment. The common factor between all these common senses is that they help living organisms to adjust them in specific environment and live in accordance with the natural process. Through habitation the organism find a suitable environment for their survival and the adaptation help them to adjust according to the surrounding environment where they adopt the environment to make it easier for living. 20. Location of Ear and Eyes The function of eyes is to see objects while that of ears to hear sound. Seeing comes through light while hearing is possible through the traveling of sound waves. As the sound waves is present in all directions, so ears is placed at both sides in order to catch the sound waves. That is why we are able to hear sound even behind us or from any direction. On the other hand, light reaches our eye in a straight direction so our eyes are placed in front side and we see any object in front of us. However, the flexibility of our eyes movement as well as the neck makes it possible for us to see objects even at sideways. 21. Mapping of the somatosensory area The sensations of pressure, flutter, and vibration are psychophysically distinct tactile modalities produced by frequency-specific vibrotactile stimulation of different mechanoreceptors in the skin. The information coded by the different low-threshold mechanoreceptors are carried by anatomically and electrophysiologically distinct pathways that remain separate at least up to and including the input stage of primary somatosensory cortex (SI) in primates. The representation of visual features in early visual cortical pathways is reasonably well characterized, but how stimulus features are represented for the other senses is largely unknown. Little is known as to whether there are functional cortical maps representing tactile features and whether such features would be mapped in a continuous or discontinuous manner. References Schwartz EL (1994). Computational studies of the spatial architecture of primate visual cortex: columns, maps, and protomaps. In: Cerebral Cortex. Vol. 10 (Peters A. ed). pp 359-411. New York: Plenum Press. Amunts, K., Malikovic, A., Mohlberg, H., Schormann, T., and Zilles, K. 2000. Brodmann’s areas 17 and 18 brought into stereotaxic space—Where and how variable? NeuroImage 11(1), 66–84. Cochlear Implant (2007), information retrieved from http://www.nidcd.nih.gov/health/hearing/coch.asp on 14 February 2008 Green D.M., Swets, J.A. (1966). Signal detection theory and psychophysics. New YorkWiley. Marr D, Hildreth E. Theory of edge detection. Proc. R. Soc. B. 1980;207:187–217 Read More