Understanding of the Vision Process - Research Proposal Example

How do we see? How do we see? It has been said that the eyes are the window to the soul. What is it about the eyes and how and whatwe see? In the following pages the question ‘How do we see?’ will be discussed. First we will examine the actual process of sight exploring the various parts of the eye and the brain and how they inter-relate. After an understanding of the vision process is presented we will discuss the psychology of sight and the various aspects of human behavior the affect and are acted upon by what we see or what we perceive we see. The human eye is an intricate natural ‘camera’ comprised of many parts working in tandem. Rather than utilizing film as does a mechanical camera the eye uses light to process the objects within our field of vision and through this process allows us to see. Figure 1 below displays the various parts of the human eye which we will briefly discuss in the following paragraphs as a general overview before more fully discussing the relationship between the eyes and the brain and how this intricate process allows us to see. Figure 1: Diagram of the Human Eye1 As seen in the above diagram the human eye is made up of several components which when working properly allows us to see. The cornea is found on the outside area of the eye which aids in the focusing of light entering the eye. The cornea itself is a transparent covering which overlays the iris. Between the iris and the cornea lies the aqueous humor which is a clear liquid. The iris is the coloured portion of our eye that results in the categorisation of a person’s eye colour. Although many people view the iris as just that, a colour, it possesses a vital function in the sight process. The iris houses the pupil which is the dark inner circle visible in the centre of the iris. The pupil acts as a ‘shutter’ of the eye and expands or contracts as necessary to allow the proper amount of light to flow into the eye itself (Than, 2005). Behind the pupil is found the crystalline lens which is held in place by ciliary muscles. The movement of these muscles plays an important part in how we see. As the muscles contract, they push the lens closer together causing it to thicken. This movement allows us to see objects that are very close to us. When we squint that is our attempt to further concentrate the crystalline lens. Conversely, as the ciliary muscles relax, the crystalline lens is able to expand and thin which allows us to see objects at a further distance (Than, 2005). The Vitreous humor comprises the majority of space in the interior portion eye. Almost jelly like in consistency; the vitreous humor further filters the light entering into the interior of the eye and acts to protect the retina at the innermost portion of the three layers of the human eye. The outside layer which is comprised of the cornea also contains the schlera which gives our eyes their white colour. The second layer contains the choroids. “The choroid contains blood vessels that supply the retina with nutrients and oxygen and removes its waste products” (Than, 2005: screen 1) The retina itself is made up of millions of cells comprised of two types: rods and cones each of which performs unique functions. Cones are concentrated in the central part of the retina which is called the fovea. Cones process colour and also the fine details of what we see whilst the rods are process our vision in poor lighting in “monochrome” (Than, 2005: screen 1). In simplistic terms, light is filtered through the pupil into the crystalline lens and further through the vitreous humor until reaching the retina where the cones and rods convert the light that reaches them into electrical energy or impulses which are transmitted to the brain via the optic nerve. When these electrical impulses reach the brain, it decodes the impulses and translates them into what we see, notably referred to as our vision (Than, 2005). Having a basic understanding of how the eye works we will now concentrate on the relationship between the eyes and the brain. The pathway to from the eyes to the vision center in the brain has been clearly mapped out and functions in a very systematic, well laid out manner as demonstrated in Figure 2 below. Figure 2: The Visual Pathway2 As seen in Figure 2 above according to Hubel (2001) the pathway from the eye to the brain is very organized and mapped out. Through the retina of the eye, via the transformation of light into electrical impulses, specific sets of impulses are transmitted through the optic nerve fibres through the geniculate cells in the geniculate body to the visual cortex in the brain. The electrical impulses sent from each retina pass through the optic chiasm. It is at this junction point that the data received from each eye is channelled to both sides of the brain. For example the electrical impulses from the right retina through the optic nerves are filtered at the chiasm with roughly one half passing to the right side of the brain and the remainder to the opposite. These impulses are sent to several different areas within the brain (as example areas; however, the majority of these impulses are sent to the lateral geniculate bodies. These geniculate bodies which are comprised of millions of cells primarily receive input for the optic nerve cells although they also send back impulses from the cerebral cortex. Although the entire process which occurs in the lateral geniculate bodies is not fully understood as yet, it appears that the information received from the optic nerves are passed through these series of cells virtually unaltered. Figure 3 below displays a cross sectional view of the optic nerve as it leaves the eye. According to Hubel (2001) the picture displayed is approximately 2 millimetres wide which offers a scale to understand how minute in size the visual pathway is. Figure 3: Cross Section of the Optic Nerve Opening as it leaves the Eye3 The method by which the optic fibres disperse the information to the geniculate bodies is unique. Specifically, according to Hubel (2001: Ch. 4, p. 5): Fibers from the left half of the left retina go to the geniculate on the same side, whereas fibers from the left half of the right retina cross at the optic chiasm and go to the opposite genicu­late…; similarly, the output of the two right half-retinas ends up in the right hemisphere. Because the retinal images are reversed by the lenses, light coming from anywhere in the right half of the visual environment projects onto the two left half-retinas, and the information is sent to the left hemisphere. As humans, it is important to consider that the field of vision of each eye as primarily the same, that is to say our eyes follow the same path, so virtually what we see from one eye is the same as that seen by the other. Therefore, the information received in both sides of the brain is virtually identical. According to Hubel (2001) the lateral geniculate body itself is comprised of six layers of cells with each of these layers further housing layers of tightly compacted cells as shown in Figure 4 below which is a cross-sectional cut of the lateral geniculate body of a primate. Figure 4: The Six Layers of the Lateral Geniculate Body4 The geniculate as shown above is layered and it is important to note that there are separate plates for right eye and left receipt of transmissions. The plates according to Hubel (2001) are further broken down into layers which receive input from one eye or the other. “Any single point in one layer corresponds to a point in the layer implies movement in the visual field along some path dictated by the visual-field-to-geniculate map” (Hubel, 2001: ch. 4, p. 8). The layers are consisting of two types of cells called magnocellular and parvocellular. The six layers form a broad band called the optic radiations. It is the optic radiations which travel to the primary visual cortex. The cerebral cortex which houses the primary visual cortex is made up of many layers each with different primary functions. The primary visual cortex displayed in Figure 5 below is houses over 200 million cells in approximately a 2 millimetre thickness (Hubel, 2001). Figure 5: The Primary Visual Cortex5 Information is transmitted within the visual cortex both up and down throughout the layers. It has been found that there is also some side to side transmission although it is not nearly as extensive as the primary up/down transmission. Whatever the cortex is doing, the analysis must be local. Information concerning some small part of the visual world comes in to a small piece of the cortex, is transformed, analyzed, digested—whatever ex­pression you find appropriate—and is sent on for further processing some­where else, without reference to what goes on next door. The visual scene is thus analyzed piecemeal. The primary visual cortex cannot therefore be the part of the brain where whole objects—boats, hats, faces—are recognized, perceived, or otherwise handled; it cannot be where "perception" resides. (Hubel, 2001: Ch. 5, p. 7) Figure 6 below displays how the information received by the eye and passed through to the primary visual cortex is processed. Figure 6: The Primary Visual Cortex Functioning6 A tilted line segment shining in the visual field of the left eye (shown to the right) may cause this hypothetical pattern of acti­vation of a small area of striate cortex (shown to the left). The activation is con­fined to a small cortical area, which is long and narrow to reflect the shape of the line; within this area, it is confined to left ocular-dominance columns and to orientation columns representing a two oclock-eight oclock tilt. Cortical representation is not simple! When we consider that the orienta­tion domains are not neat parallel lines, suggested here for simplicity, but far more complex Figure 7 further displays how the visual cortex is divided into layers in two manners, one for left and one for right eye dominance. Hubel (2001) notes that the illustration is for visual purposes only obviously the cortex is much more complex and the various layers and the dominance are interwoven amongst each other, but the principles set for the when viewing the figure demonstrate how the functioning occurs within the visual cortex. Figure 7: The Primary Visual Cortex layers and eye domination7 According to Hubel (2001) the corpus callosum is comprised of the fibres that join the left and right hemispheres of the brain as shown in figure 8below. Figure 8: The Corpus Callosum8 The purpose of the corpus callosum at least in part appears to be to enable the brain to process data at the midpoint of our vision, the area of overlap in the field of vision between the left and right eye. Having viewed the process of how what we see is transmitted to the brain in order for us to receive the visual signals is a complex process that we are just beginning to understand. In the following paragraphs we will discuss sight from a psychological perspective. Gratton et al. (2003: 487) note that vison is a rudimentary means by which we observe changes. According to their theory: The idea that changes in the way living tissue absorbs, reflects, or scatters light may provide information about psychological states or processes is very old (e.g., Darwin, 1872). We can all easily recognize signs of emotional states (sudden pallor or blushing) or of exertion (redness and perspiration) in other people just observing their faces -perhaps the simplest way to make optical observations. How does this occur? In the following section we will explore the psychology of sight. Blake (1997) states that in order to understand the relationship between vision and consciousness we need to examine the operations of sight itself and determine how the brain processes what we see. Through the neurological process of eye sight the eyes process two individual images in the form of light through the retina to the optical cord which transmits the electrical impulses to the brain which then translates them into one unified scene. What happens, however, when the eyes detect two dissimilar sights? According to Blake when this occurs each eye sends a contradictory message to the brain about the object within a relative field location. “Faced with this physical impossibility, the brain lapses into the unstable state called binocular rivalry, which is characterized by alternating periods of left-eye dominance and right-eye dominance that continue as long as the eyes view rival stimuli” (Blake, 1997: 157). In short what occurs is that there is a shifting back and forth between one eye and the other in an attempt to harmonize what is being viewed. Figure 9 below displays examples of stimuli that cause binocular rivalry. Figure 9: Binocular Rivalry 9 Blake (1997) questions what happens with the stimulus (or image) received in the brain while the focus is on the other opposing object from the temporarily dominant eye. In order to understand processes may be disrupted during periods of binocular rivalry, Blake (1997) draws attention to several illusionary occurrences that occur. The first illusion experience is termed visual aftereffects. “Prolonged exposure, or adaptation, to a visual stimulus can briefly alter the appearance of other stimuli viewed immediately afterward” (Blake, 1997:158). This can easily be described as it is a common phenomenon we have all experienced. After staring at a particular object for a long period of time, and then looking away and viewing another object the new object’s appearance is momentarily altered, almost as if the eye has to readjust to the new image. Prolonged exposure to one particular image can produce neural fatigue. That is the greater the concentration on a particular image and the greater the length of time on which it is concentrated lengthens the visual aftereffect. Another type of visual aftereffect that Blake (1997) explored is referred to as global looming. In this multiple small images are viewed by the eye. The various points seem to move out from the center of the image when prolonged viewing occurs. Figure 10 below displays Global Looming. Figure 10: Global Looming 10 Blake (1997) concludes that investigating such visual rivalry and aftereffects can better help us understand the hierarchy with which we process information within our consciousness and further adds insight into how we send and receive feedback. Giusberti et al. (1998: 281) examined the differences between visual perception and imagery stating: visual imagery and visual perception do not share the same processes and the effects related to this first phase. In fact, we found that, as far as vividness ratings of perceived and imagined patterns is concerned, visual perception may involve automatic and pre-attentive processes that are very far removed from the controlled and non-automatic processes involved in the creation of visual images. To test their theory Giusberti et al (1998) examined in depth optical illusions which are, according to the authors, lower level, uncontrolled processes within the normal eye functioning process. In their estimation the use of optical illusions become a valid means of investigating the inherent differences in visual mental imagery and visual perception. They note that unlike other images, optical illusions, by their very nature, cannot be confounded by outside experiences or knowledge. Simply put, even though a person views an optical illusion, knows what it is and understands the knowledge base of optical illusions, they still see them. Understanding does not remove the visual picture seen by the observer. As an example, Giusberti et al (1998) draw a parallel with same sized circles. If the subject is asked to trace to identical circles on a piece of paper, they know they are the same size. However, when told to draw circles around the outer side of one circle and then asked to draw much larger circles around the other original circle, even though they know the original circles are identical, the one with the larger outside circles will appear to be larger. Giusberti et al (1998) demonstrated via use of the Ebbinghaus illusion how varying the size of the outside circles could affect the perceptions much more than the mental imagery by the subject. The Ebbinghaus illusion is demonstrated in Figure 11 below. As seen in the figure as the outside size of the various examples are altered the perceived difference in the actual size of the inside circle becomes apparent. Figure 11: The Ebbinghaus Illusion 11 Additionally, Giusberti et al. (1998) tested for differences in imagery and perception using the Ponzo Illusion which is demonstrated in Figure 12 below. In their experiment the authors altered the angle used thereby: Producing a condition working against the illusion. In other words, the manipulation of the induction conditions in the Ponzo illusion implied two concurrent variations having opposite effects on the optical illusion. Therefore, we expected that in the perceptual modality the two situations would have similar effects. (Giusberti et al., 1998: 283) The authors theorized that as imagery is more conceptual in nature the Ponzo illusion would have a great influence on the visual display as seen by the subject, as in Blake’s (1997) theory binocular rivalry would be present as two opposing images would be simultaneously viewed. Figure 12: The Ponzo Illusion 12 Results of the study revealed that: the experiments seem to confirm that there are asymmetries between perception and imagery, and that such differences mainly concern specific perceptual processes which differ from those involved in the generation of a mental image. From the present results, and from others mentioned in the literature, these processes seem to be in the most peripheral phases of information processing. We can only have optical illusions at the perceptual level, but not at the imagery level. This is due to the differences between the two processes: Optical illusions involve the initial phases of visual information processing, whereas visual imagery is based on higher-level processes. Low level perceptual processes, that is, the initial phases of visual information processing, are automatic, rapid, and pre-attentive (Giusberti et al, 1998:287). Rocca et al (2005) in order to investigate the effect of optic nerve damage commonly associated with Multiple Sclerosis (MS) conducted a study to determine if modern advances in magnetic resonance imaging (MRI) might capture optic nerve damage. The optic nerve has proven to be very difficult to image based several technical difficulties associated with MRI use in the optic nerve imaging. The first problem lies with the small size of the nerve and the need to obtain a high spatial resolution. Secondly, according to Rocca et al (2005: 537), “Secondly, the surrounding fat, bone and cerebrospinal fluid (CSF) might produce artifacts and compromise the final quality of images. In addition, eye movements should be taken into account.” Although the study was concerned primarily with MS patients the ability to accurately image the optic nerve for lesions and damage will benefit not only MS patients. The benefit according to the authors is that brain imaging in conjunction with optic nerve imaging will enable medical personnel to rule out other possible causes for sight impairment including Optic Neuritis (ON). The development of the MRI technique to image the optic nerve has been the discovery that lesion size in length not depth slows recovery of the optic nerve. The successful attainment of MRI images also allows for more rapid, accurate diagnosis of disease when several have similar symptoms initially. Diba, Prasanna, and Yorio (2005) in order further understand the effects of glaucoma on the optic nerve head conducted research on bovines whose optic nerve and subsequent makeup mimics that of humans. In glaucoma high levels of endothelin-1 (ET-1) were observed in the aqueous humor of patients with open-angle glaucoma, as well as animal models of glaucoma” (Diba et al, 2005: 288-289) which enables the researchers to conduct this study using bovine eyes obtained from a slaughterhouse. Western blotting was used to determine the ph levels and effects when proteins were introduced to the area surrounding the optic nerve. Findings of the study revealed that “the ETB receptor, but not the ETA receptor, was present in the optic nerves, suggesting it plays the major role for endothelin-induced optic nerve degeneration” (Diba et al, 2005: 292). Figure 13 below displays the findings of the Big ET-1 in the Bovine research. Figure 13: Diba, Prasanna, and Yorio Bovine Optic Nerve Research Findings13 Although additional research is need the authors concluded that study has shown that contrary to what was previously believed the retina at the plasma membrane contains the Big ET-1 which could be useful in measuring and diagnosing early various optic diseases including hypoxia and increased intraocular pressure which is a common symptom of glaucoma in adults. Throughout this paper we have examined the biological function of the eye and its many intricate parts to understand how the brain translates the light waves entering the eye are translated into the images we see. We also discussed the differences between visual images and perception as well as discussed the mechanics of such phenomenon as optical illusions. Further a discussion into various ‘tricks’ the brain can play on us with regard to such things as binocular rivalry. Lastly, several recent studies into the optic nerve functioning and current research methodology and emerging technology can help us to understand the miracle of sight. References Blake, R. 1997. What can be ‘perceived’ in the absence of visual awareness? Current Directions in Psychological Science, 6 (6), 157-162. Diba, A, Prasanna, G, and Yorio, 2005, November 4. Localization of endothelin-converting enzyme in bovine optic nerve and retina. Journal of Ocular Pharmacology and Therapeutics, 21, 288-297. Giusberti, F, Cornoldi, C, De Beni, R and Massironi, M. 1998. Perceptual illusions in imagery. European Psychologist, 3 (4), 281-288. Gratton, G, Fabiani, M, Elbert, T and Rockstroh, B. 2003. Seeing right through you: applications of optical imaging to the study of the human brain. Psychophysiology, 40, 487-491. Hubel, D. (2001). [online] Eye, brain, and vision. Harvard University Website. Online book. Available from http://neuro.med.harvard.edu/site/dh/index.html [Accessed 23 April 2006] Rocca, M A, Hickman, S J, Bo, L, Agosta, F, Miller, D H, Comi, G, and Filippi, M. 2005. Multiple Sclerosis, 11, 537-541. Than, K. 2005, November 28 [online] How the human eye works. Live Science Website. Available from http://www.livescience.com/humanbiology/051128_eye_works.html [Accessed 23 April 2006] Read More