Visual Perception

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href="/bb/neuro/neuro98">Biology 202
1998 Second Web Reports
O n Serendip

Visual Perception

Gungsadawn Katatikarn

Any one given experience an organism perceives must incorporate several sensory systems, that involves numerous number of organs , that further more are comprised of millions upon millions of firing cells. Perception is not a direct mirroring of stimulus, but a complex chaotic patterns dependent on the simultaneously activity of neurons. This essay deals primarily with neurons from the optical sensory system. The outer ridge of the brain, known as the cerebral cortex begins the analysis of sensory messages. (1) Nevertheless, visual perception is possibly more widespread than one area of the cerebral cortex and like ly over various subcortical structures and number of different systems as well. (2) One of the many ways for the "perception process" to begin, is vision. Vision is dependent on the interaction between light input and the eye. The visual input is seen through lens that takes different light outside, refract and bend into points of light that focus on specific places on the retina. This light-sensitive tissue that lines the back of the eye consists of interconnected neurons. The three diff erent types are receptor cells, bipolar cells and ganglian cells. When photoreceptors are stimulated, they change in structure of photopigments in the receptors and transduce light input into neural activity. (2) Electrical stimulus trave ls down the axon of bipolar cells to the ganglian cells. The ganglian cells are activated through nerve impulses or action potentials and travel down the optic nerve. This activity conducts along the optic nerve to the geniculate nucleus that then travels to the mid- brain. (2) Finally the firing neurons activity travels to the cortex at the back of the brain, known as the striated cortex and occipital cortex. (2) The above description of the light transducation in the re tina through to the occipitial and striated cortex seems to be a straightforward perception process. However, there are many gaps within the course, which cannot possibly fully formulate the sense of perception. Not only are there missing intermediatary, but there is outside activity happening simultaneously along this path. As discussed in lecture, the retina sends (5 to 10) different information activities to different parts of the brain. Neural data may be taken to the midbrain instead to the dicephalo n. The dicephalon consists the lateral geniculate, which consists the visual occipital cortex. The complex midbrain is made up of other complex parts such as the superior collicus and the optic tectrum. The optic tectrum is then made up of more complex in tricate parts, such as the protectal nucleus and the basal optic nerve. Once again, the organization and specificity of complex within complex come into play. As seen here there are parallel simultaneous paths that are activated or inactivated due to spec ific retina information.

Neurologists, Sheinberg and Logothetis studied macaque monkey responses to different visual imputs. (3) They tested the difference between one image fixed on one eye and one image fixed on one eye accompanie d with a flash of a second image on the other eye. Tests show that former test did not always evoke effective stimulus, while the latter test yields 90% effective stimulus. Activity is taken from superior temporal sulcus (STS) and the inferior temporal co rtex (IT). (Please refer to Figure 1.) This test provides prevalent information on patterns of retinal neurons as well as perception. The unfamiliar sunburst pattern shows no neural response, while the same test done with a familiar monkey portrait yields neuron activity. Why does the macaque monkey perceive the monkey picture, but not the sunburst pattern? It may be due to the idea that the monkey instantly recognize the monkey face and not familiar with the pattern. Maybe the macaque monkey has a point of reference, a persistent memory, which causes it to instantaneously recall the image. Or rather the parts of the brain tested, (STS)&(IT), are not the sites where sunburst type patterns are perceived. The "Flash Suppression" tests support this analysis because when the sunburst pattern is flashed during the fixed monkey image the neural effective stimulus decreases greatly. While in contrast, when the monkey picture is flashed before the fixed sunburst pattern, the neural activity is activated. When tes ted with sunburst pattern, all neural activities are suppressed and ineffective. (3)

The property of selective data for specific pathways is prevalent in all many cells, organs and systems. Also, these cells, organs or systems work p arallel to other cells, organs or systems. Milner and Perrit (1991) did a study of two parallel active systems. (3) The patient studied has diffuse brain damage. She visualizes color and texture but cannot comprehend form and orientation. Although she claims consciously that she cannot report the latter two, she can catch a ball well, "post " her hand and slip a card into a slot without any problems. (3) In another study, the "Classical blindsight," a patient has damage t o cortical area VI, can track direction, intensity and speed of a specific light source without ever perceiving it consciously. (3) These well-known studies suggest that there are multiple cortical streams in conjuction with motor streams . In other words the systems work in parallel to each other. Fuster's diagram demonstrates the sensory and motor parallel systems. (Refer to Figure 2.) Each part of the sensory and motor cortex input/output pathways may project in several directions, fro m intermediate levels to prefrontal systems, to motor outputs. (3) There is not one process; it is not necessary for lower levels of the prefrontal system to the higher levels of the prefrontal system and then moves down the motor system out to the environment. (3) At first glance the actions from the previous experiment and following graph seem random, but the actions are rather specialized. The neural information is sent to different locations of two parallel systems si multaneously to cause specific activity. These chaotic pathways fill in the gaps mentioned earlier and therefore add to the viewer's perception.

Some may think that chaos is randomness. However, there is a distinct difference between chaos and random ness. To illustrate the difference, one can make the following analogy: Chaos is rush hour traffic, automobile dash from various destinations and directions to different locations. Although, it seems hectic fashion, it is not. The mass of aggressive autom obiles creates its own organization or underlying pattern. (5) Every week day, at nine in the morning and five in the afternoon, the stream of cars moving simultaneously from different locations to others form organization. ( 5) Where as in terms of randomness, it may be at any other time of the day. The stream of automobiles is erratic. Each automobile races from different locations to other locations. Instead of a controllable or rather predictable process as seen before , there is an unorganized process.

Is a chaotic process an advantage for perception? As complex thought propagates more complex thoughts and ideas. These chaotic processes provide vivid experiences. To handle coexisting, elaborate processes the brai n must construct a multilevel, explicit, symbolic network that is responsible for visual perception. (3) The information necessary to represent an object is contained in the firing ganglian cells in the retina, there is no explicit repres entation of the object there. How does the brain differentiate signals from the same stimulus? (1) How are these different depictions brought together to orient the image seen? (3) The activity itself must not only deal w ith input signals, but must primarily deal with internal conditions, such as the discriminating activity of neurons. (1) Therefore activity is not from inputs, but from internal inhibition. Lateral Inhibitory feedback mechanism is essenti al for the stabilization of perception. Ganglian cells' lateral inhibition filters and monitors information taken in finds the contrasting edges rather than entire objects. (4) The outline of object is enough to orient the object in time and space. Without such mechanisms like receptive fields, sensory inputs would be erratic or overpowering that render incoherent perception.

Only a minimal amount of information is brought into the most densely packed photoreceptor center on the retin a. This small region is known as the forvea and is where the most precise light input is received. The information given is not sufficient to the broad view one perceives. That is why chaotic methods are taken that evokes firing neurons from different reg ions of different systems together instantaneously. Chaotic processes provide advantages, because complexity yields complexity. Yet perhaps, systems' chaotic methods are more reasonable than linear, straightforward procedures. Is it reasonable that all of what one perceives is dependent exclusively on a specific set of procedures, rather than the interaction of cells, organs and systems? The former process seems to be improbable, while the latter process proves to be a chaotic, massive and cooperative.

It was alluded to in class that one does not need the formulized "picture" or the visual perception. Nonetheless, I think that this added perplexity is a necessity. The visual perception of the "picture" encourages our understanding of ourselves as we ll as our surroundings.

Cited References

1."The Physiology of Perception," by Walter J. Freeman

2.The Joy of Visual Perception: A Web Book, by Peter K.Kaiser

3. "Consciousness and Neuroscience," by Francis Crick

4. Ratlif f, Floyd. "Contour and Contrast," Scientific American, June 1972, pp. 91-101

5. Crutchfield, James. "Chaos," Scientific American, December 1986, pp. 46-57

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