Optic Flow Essay Example for Free

Optic Flow Essay The study of optic flow becomes much more complicated when the movement of an observer towards a surface or object is not straight on. In fact, even in the more straightforward condition of straight ahead condition, Gibsons mathematical analysis was wrong. In situations when, for example, we are walking or driving, it is commonly necessary to estimate the chances of collision with an object or surface towards which we are moving obliquely. Similarly, we dont always look straight ahead when moving. The resulting oblique relationships between angle of gaze, direction of movement, and converging paths are much more difficult to analyze. Cutting (1996; Cutting et al. , 1999), has made major contributions to the study of perception during motion by enriching knowledge of the complexities of optic flow. Sensory stimulation is a chemical process which occurs between the human body and brain. When we experience sensation, body is exposed to some type of sensory stimulation. The sensory receptors then receive this information, and transmit it to the brain using neural impulses, or neurotransmitters. There it is interpreted into the correct sensation. Sensations are the basic building blocks of perception. Perception is the process of organizing and making sense of sensory input. Perception allows us to better interpret the information sensory receptors receive, and form images of the world around us. Transduction is what happens when the receptors transform the energies they receive into a form that can be interpreted and utilized by the nervous system. Human beings are able to experience various sensations because the nervous system encodes the messages we are receiving and interpreting. In his doctrine of specific nerve energies, Johannes Muller theorizes that different sensory modes exist because the information received by sensory receptors stimulates different synapses when traveling back to the brain. This is known as anatomical encoding. This type of encoding determines which sensory organ is activated by a certain type of stimulation, according to which specific neural pathway it travels. (Wade, Tavris, 2006). Anatomical Encoding and Sensory Modalities Anatomical encoding does not totally explain how or why different sensory modalities exist independently. Scientists found it difficult to link specific skin senses to individual neural pathways; neither does the doctrine of specific nerve energies explain variations of stimulus within a certain sensory experience, i. . the softness of an animals fur versus the coarseness of sandpaper, or the distinction between the colors light blue and dark blue. A second type of encoding, called functional encoding, is therefore required to make sense of these differentials in sensory perception. According to Wade and Tavris, functional codes rely on the fact that sensory receptors and neurons fire, or are inhibited from firing, only in the presence of specific sorts of stimuli (2006). This means that any given time, some neurons are firing, and some are not. The information regarding the rate of, number of, and patterning of each cells firing is what forms a particular functional code. Sensory adaptation occurs when continual exposure to the same sensory stimulus results in decreased sensitivity to the presented stimulus. Basically this means that given enough time, senses learn to become accustomed to the stimulation receptors gather. The sensory system brings information regarding environment to brains. They help us to interact with environment. The sensory system is made up of five senses, which correspond to five sense organs. The five senses are vision, hearing, taste, touch, and smell. These correspond with the eyes, ears, tongue, skin, and nose. Each of these contains sensory neurons, which transmit impulses to the central nervous system. The information is processed and from that we receive a perception which we interpret and which may change behavior accordingly. This is called Transduction which is the process of the receptors changed the information they receive into a form which the nervous system can utilize. Spatial Projection and The Surface Of The Body Up To The Cortex The visual, auditory, and somatic systemsindeed, all the sensesseem to maintain a spatial projection from the surface of the body up to the cortex. Moreover, connections in each system must be very precise for the signals from the various receptors to be kept straight as they pass up the system. The precision of arrangement is remarkable. (Lappe, M. 2000) It should be in order to ask how such an arrangement came about. What factors are at work as the organism develops to make all the connections come out right? To this question we now have an answer. First, Weiss, then L. S. Stone, and more recently Sperry have gone through a series of ingenious experiments to pin down the factors that control how connections are formed in the nervous system. Sperry, for example, has crossed the sensory and motor nerves in the legs of the rat, and from that has picked up some clues. He has also cut the optic nerve, rotated the eyeballs in various degrees, allowed the nerves to regenerate and then tested animals for the return of spatial vision. There are many details to his experiments, and they prove somewhat confusing, but the upshot of them all is this: Nerve fibers grow back to make the same connections that they made in the first place. To put the matter in another way, the nerve cells along the sensory pathways have some sort of biochemical tags that keep them straight when connections are being laid down. One might say that each nerve cell has a name and that other nerve cells know what that name is. It is still a mystery what these names are and how the cells know each others names and that will be a subject for future research. At any rate, nerves can be badly cut, mangled, and twisted, but somehow or other nerve fibers get back where they belong. For us, it is interesting to know that biochemical factors are at work in laying out the spatial arrangements of the nervous pathways. Lateral dominance has also been a serious problem in getting at the anatomical basis of cognition. That one of hands or feet or eyes is the major one and the other the minor one is a fact not easily disputed. We know, too, that in some affairs one side of the brain is dominant; that is to say, it plays a major role in perception or action, while the other side is minor. Although people have often argued about how important lateral dominance is and how many of the worlds ills it accounts for, few deny altogether that it exists. We must, in fact, believe that some parts of the brain, like the speech area, show very strong one-sidedness and that, in the case of others, the sides share about equally in the functions that concern them. If that be true, how can we tell where to look for a particular function? If one kind of cognition belongs to one side of the brain and we make a lesion in the other side, we will completely miss the point. Or if a type of cognition shares equally corresponding areas on both sides, it takes a perfect bilateral lesion in the areas to make the localization known. We ought to consider, too, the matter of individual differences. We find it natural to say that people are different in the measurements of personality, intelligence, or some other aspect of behavior, but we often seem to assume that brains are standard products turned out on an assembly line so that they look as much alike as new cars. The fact is that brains vary a lot in their size and shape. Lashley has been going into that matter lately, and he assures us that there are individual differences in brain anatomy. It appears that any speculation in this respect is restricted by conditions which are inherent in research experiments. According to observations, only objects or patterns cause any demonstrable satiation. Hence, we must find a process which accompanies object or pattern vision rather than the perception of homogeneous surfaces. The alpha rhythm of the human brain is much more seriously disturbed by visual objects or patterns than it is by a bright homogeneous field. Adrian suggests that it is attention to which the alpha rhythm is so sensitive. But there remains the other possibility that, quite apart from this factor, the rhythm is strongly disturbed by a visual process which accompanies the perception of objects or patterns. Vision Optic Flow and Perception It seems safe to say that, in terms of stimulation, an object is an area (or a volume) which differs from its environment either as a whole or along its boundary. We see things of any kind only when a relation of inequality obtains between the stimulation in one area and that in another, surrounding, and area. Thus it seems plausible to assume that the process which goes with object or pattern vision is a relationally determined process, and that satiation is established in regions in which this process takes place for some time. Relational determination is not a familiar term. Relationally determined processes are extremely common in physics. For instance, if temperatures differ in two parts of a system, a current of heat energy is established which tends to equalize the temperatures. The direction of the flow depends upon the direction of the difference, and in the absence of any difference there is no flow. Merchant, H. , Battaglia-Mayer, A. , Georgopoulos, A. P. 2001) Similarly, if a solution which contains certain molecules is surrounded by a second solution which contains these molecules in a different concentration, a current of diffusion will be observed, unless the solutions are separated by an impermeable barrier. The current flows as long as the concentrations differ. Thus, it is again a relation of inequality between the two parts of the system which maintains the process. Incidentally, examples exhibit relational determination in more than one sense. As the currents of heat or diffusion spread, their distribution in space depends upon the shape of the boundary at which the parts of the systems are in contact. This shape is defined in terms of geometrical relations among parts rather than of merely local conditions, and the distribution of the flow adapts itself to such relations. Therefore, not only the flow as such is relationally determined, but the same holds also for its pattern in space. Some such processes cause obstructions in the medium in which they occur, and that in this fashion after-effects are established when later further processes spread in the same medium. The relationally determined process which underlies pattern vision is a direct electric current and that such a current flow when conditions of excitation in one part of the visual cortex differ from those in an adjacent part. An attempt was also made to explain how the electromotive forces originate which drive the current from one part to the other, and back again to the former. The explanation involved no hypothesis which is at odds with available knowledge of nerve impulses and their influence upon cortical tissue. Vaina, L. M. , Rushton, S. K. 2000) Rather, those forces were derived from concepts which play a great role in present neurophysiological discussions. Nevertheless, this particular part of the theory need not now be described, because there may be various ways of deriving electromotive forces which would drive a direct current through the tissue. It seems that, whatever choice may be, the distribution of the flow as such would always be about the same. It is this flow which we will now consider. The flow would spread through the tissue as a volume conductor, which is to say that, in this connection, the brain must be regarded as a continuous medium to which principles of continuity physics apply. In this respect, there is a possibility which is implicit in present neurophysiology, even though its consequences have, until recently, not been explicitly considered. Surely, if the potentials of the alpha rhythm as well as those of on and off effects can spread through the skull, there is nothing in the brain to prevent such potentials from spreading through this medium as a continuum. As a result, it can hardly be a disturbing thesis that a steadier flow would do the same. (Sherk, H. , Fowler, G. A. 2001) In flowing through a continuum, a direct current assumes a distribution which is relationally determined by the shape of given boundaries. In object or pattern vision, the boundaries in question would be those between cortical areas in which retinal stimulation establishes different kinds or degrees of excitation. It will suffice if we consider a fact which concerns only the distribution of the current as such, and is quite independent of further theorizing. If excitation within a circumscribed cortical area differs from that in its environment, the resulting current must circle around the boundary at which the two areas are in contact. Moreover, unless the surrounded area is very large, the current must be denser in this area than it is in the environment in which it can spread widely. This is true whether or not local excitation is higher in the circumscribed area. Thus, if a black object is shown on a white background, the density of the flow must be maximal within the area of the black object, just as it is maximal within the area of a white object surrounded by black. Satiation and Proximal Energy We can now turn to the problem of satiation. The present theory has no difficulty in solving this problem. Any direct currents which flow through the nervous system polarize the surfaces of cells, and also change their polarizability. Generally speaking, this effect, the so-called electrotonus, has the character of an obstruction. Further currents which afterwards flow through the same medium are weakened. At the same time, they suffer changes of their distribution in space. We are not introducing a special hypothesis if we assume that the currents of theory are also electrotonically active, and that the resulting obstructions follow the same rules as hold for electrotonus in general. For instance, the degree to which the various parts of the medium are electrotonically affected is directly related to the density of the current in those parts. Now we know that the density of the currents postulated in theory must be maximal within the area of a circumscribed object, still great in adjacent parts, and progressively lower at greater distances. Perception is treated throughout as the representation to the individual of real, physical states of affairs of both the external environment and of the self. The former include objects, persons, scenery events, and extended terrain and spaces. States of the self include postures, movements, and activities such as reaching, standing, running, and speaking. Although it is obvious that own physical states are represented to uswe readily and immediately perceive what we are doingthis aspect of perception is usually overlooked in the theoretical treatments, with notable exceptions (Gibson, 1979) Physical states of both the environment and the individual give rise to patterns of energy at the sensory receptors. These patterns, which usually vary over time as well as space, are the proximal stimuli that initiate the chain of neural activity and culminate in a perceptual representation. Certain features of these proximal energy patterns correlate with particular properties or attributes of a physical state of affairs. For example, both the size of the retinal image and the degree of convergence of the eyes correlate with the size of an external object. Likewise, a pattern of stimulation in the joints, tendons, and muscles of an arm correlates with its position. A point to be emphasized is that the features of the proximal stimulus pattern are not simply replicas or necessarily even rough copies of the physical properties that give rise to them. Rather they correlate with them. For example, binocular disparity is in no sense a replica of observer-object distance; it is a correlate of it. This view was first adumbrated clearly by Gibson (1950, 1959) and is emphasized here.