Visual Perception

Perception is described by medical scientists as a knowledge driven cognitive process in which the individual acquires, interprets, selects and organizes sensory information from the external environment. Although the physical process of perception appears to be quite straightforward, in actuality there have been a number of definitions that have been proposed to describe the activities that occur during the process of perception. This is because, as many scholars argue, the process of perception is one that often occurs outside of the cognitive sphere. Scholars examining the process of perception have noted that perception can have unconscious, philosophical or psychological underpinnings that are not widely understood in the context of the biological explication that is presented above. Given the need to better understand the process of perception, a wide range of theories about visual perception have been developed in recent years. While many of these theories demonstrate a unique method for examining the process, some overlap melding both the biological and the psychological (Gordon, 2004).

With the realization that theories of visual perception can be so dynamic and integrative, there is a clear impetus to consider these issues in understanding theories of visual perception. To this en this investigation considers the perceptional consequences of neural organization according to the indirect realist approach. Specifically, this investigation considers the indirect realist approach to visual perception, the role of neuronal structure in perception, and a comparison/contrast of the indirect realist approach of visual perception to theories developed by Marr, Triesman, and Biederman. Through a careful consideration of what has been written on these subjects a clear understanding of the indirect realist approach to visual perception will be garnered. Further, by examining the current literature on perception a more integral comprehension of the extent of theories on visual perception will be elucidated.

Indirect Realist Approach-An Overview

In order to begin this investigation, it is helpful to first consider what has been written about the basic definition of indirect realism. Research on the development of indirect realism seems to suggest that the development of this theory extends from direct realism. Whereas direct realism argues that the senses provide the individual with a direct understanding of the external world, indirect realism argues that individuals are only directly aware of internal representations of the world. Thus, the individual is not directly affected by what he or she sees in the external world; rather the individual is impacted by the indirect images that are created internally as a result of the presentation of visual stimuli in the external world. Indirect realism is also known as epistemological dualism or representationalism (Indirect realism, 2006). Further examining the definition of indirect realism, another author makes the observation that perception occurs through a "perceptual intermediary" that is placed between the object in the external environment and the individual's perception. "When looking at an everyday object it is not that object that we directly see, but rather, a perceptual intermediary" (O'Brien, 2006).

When one thinks about how an object is actually perceived in practice, the true nature of indirect realism becomes more evident. The images that are presented to the individual are often translated into emotions, feelings or understandings. As such, the object in and of itself is not what is perceived by the individual. Instead, the image presented is translated by the mind and assigned a certain meaning by the brain. While this process is indeed quite complex, the logical clearly demonstrates why there is so much controversy over how the process of visual perception occurs. Objects, by themselves have little meaning, unless the objects are effectively "translated" by the individual to give them meaning. As such, the plausibility of indirect realism is indeed quite salient.

Neuronal Structure-A Review of How Perception Occurs

Although mammalian neuronal organization can be directly related to the process of direct realism in a concrete manner, linking this organization to the context of indirect realism is a more difficult challenge. With this in mind, it is first helpful to consider the process of perception in the context of neuronal organization. Through a careful consideration of the structures of this system it will be possible to provide a more integral understanding of how neuronal organization can contribute to the development of indirect realism.

Looking first at the process of perception from the physiological standpoint of how visual stimuli enters the eye, Kaiser (1996) observes that when light is comes in contact with the eye, photopigments in the outer structures of the eye trigger neurons in the back of the eye to send an electrochemical from the eye, though the optic nerve to the brain. The image is then interpreted by the brain and perception occurs. The neurons that are responsible for effectively translating visual information into electrochemical signals in the brain are known as ganglion cells. Like most neurons, ganglion cells have dendrites-which bring visual information in-and axons-which send information away from the neuron. In addition, ganglion cells also communicate with one another via electrochemical processes. Through these cells, the critical information about what is being "seen" is translated to the brain.

When placed in this context, the process of both vision and perception appear to be quite straightforward. However, the ganglion cells that comprise the eye have a number of unique properties that can significantly impact the process of perception. According to one author, "The ganglion cell may also receive inputs from other receptors. So there is an area out in the visual field, which includes the receptive field of the original receptor, that is the receptive field for the ganglion cell. That is changes in the light stimulus in the receptive field will modulate the activity of the ganglion cell" (The receptive field, 2006). What this effectively suggests is that various stimuli presented to the individual can modulate the response that is garnered by the individual. Thus, when an subject looks at an object in bright light, the ganglion response to that object that is produced will be different than if the same object were seen in lower light. When placed in this context, the nature of indirect realism becomes more apparent.

With the realization that various ganglion cells in the visual field respond differently given various conditions, researchers have furthered their understanding of these structures by classifying them into five distinct types, each of which is influenced by a different stimulus change. These types of ganglion cells include:

· Parvocellular: Almost 80 percent of the ganglion cells in the retina are parvocellular. These ganglion cells receive data from only small numbers of rods and cones. As such, these cells have slow condition velocity and typically respond to changes in color. Although these cells show some receptivity to contrast, the difference in contrast must be significant in order for these cells to detect a difference (Ganglion cell, 2006).

· Magnocellular: These ganglion cells comprise 10 percent of the ganglion cells found in the retina. Magnocellular cells garner input from a large number of cells and have high conduction velocities. For this reason, these cells respond to "low contrast stimuli" but not to changes in color (Ganglion cell, 2006).

· Koniocellular: Koniocellular cells also comprise approximately 10 percent of the ganglion cells found in the retina. These cells have a moderate conduction velocity and are sensitive to "moderate contrast stimuli." In addition, these cells have been noted to be involved in color vision (Ganglion cell, 2006).

· Other Retinal Ganglion Cells: These cells are involved in controlling the papillary light reflex and the giant retinal ganglion cells.

· Photosensitive Ganglion Cells: These ganglion cells contain their own photopigment, giving them the ability to respond directly to light.

While it is evident that all of the ganglion cells play a role in mitigating the image that is photochemically transmitted to the brain, it is evident that the parvocellular, magnocellular and koniocellular cells clearly have some control over the message that will be transmitted to the brain. These cells are stimulated based on the degree to which certain properties of the object are translated to the ganglion. Thus, if a bright red object is presented the parvocellular may respond more actively than if a dull red object is presented. The manner in which these ganglion cells respond when presented with a stimulus will impact the way in which the brain perceives the object being examined.

Neuronal Structure and Indirect Realism

When what is known about the basic physiology of the ganglion cells in the retina is placed in the context of indirect realism, the manner in which this can be grounded in physiology becomes more evident. The individual looks at an object and, based on the properties of that object that are translated to the ganglion cells, the brain is provided with a visual image. If the object is bright red, this will induce higher rates of activity in certain ganglion cells of the retina, changing the signal that is transmitted to the brain. As such, the final image that is produced will be predicated not just upon the actual attributes of the object, but rather on how the physiological implements of the neuronal structure interpret what is seen. What this clearly suggests is that visual perception is not as straightforward as originally hypothesized in the direct realism approach. Rather, perception is mitigated by a host of physiological phenomenon that can, and do, impact what the person actually sees.

Examined in this manner it becomes evident that the process of visual perception is not just a philosophical explication for understanding this phenomenon. Rather, when the biology is developed and examined, there are actual attributes of the neuronal structure that support the theory of indirect realism. When an image is examined by the individual, there are a host of factors that will precipitate how the ganglion cells in the eye respond to the object. When all of this data is aggregated in the brain, the final image will be produced. Given that ganglion cells in the retina can have a wide range of responses to the various attributes of the object-i.e. color, contrast and light-it is not surprising to find that two people, supposedly looking at the same object can produce notably different perceptions of the same object. While the theory of indirect realism is philosophical in nature, the virtue of the philosophical argument has clearly provided a means for a physiological explication of perception using this theory.

Perception Theories of Marr, Triesman, and Biederman

With the basic context of indirect realism clearly elucidated it is now possible to compare and contrast theories of perception offered by Marr, Triesman and Biederman. Looking first at the theory of perception supported by David Marr, Glennerster (2002) observes that under Marr's theory, vision was viewed as "nothing more than an information processing task." With this in mind, Marr developed a pathway to describe the manner in which information traveled from the eye to the brain. The stages of this process included: the retinal image; the "primal sketch, where lines edges and junctions" were made explicit; the 2-D sketch; a 3-D model representation; and "a space frame centered on the observer" (p. 1). Examining the context of Marr's theory, it seems reasonable to argue that Marr's theory is one that defines indirect realism. Marr described the development of various stages of visual representation that could be used for creating the final image for vision. In this context, it is evident that the representations that Marr describes are inclusive of the indirect realism approach to visual perception.

Triesman described the development of perception from the standpoint of two specific processes: pre-attentive and attentive. In the pre-attentive stage, the individual assesses the object presented from the standpoint of gross features such as color, curvature, line orientation, and size. During pre-attention the most notable features of the object are encoded by the individual. In the attentive stage, the specific features of the object are translated into a feature map that allows the individual to decipher what the image actually is (Healey, 2005). This process of visual perception indicates a strong element of translation occurring in the context of perception. The individual sees an object and then must access a feature map to accurately identify this object. What this suggests is that the individual must figure out what it is. As such, past experiences with the same or similar objects will serve as the basis for forming perception in this case. While the process of indirect realism must be deduced in this case, it is evident that this theory could fall into this philosophical category.

Biederman, in his explication of visual perception "proposes that objects are broken up into parts based on geometrical properties of occluding contours in the image, in particular that parts are defined in relation to sharp concavities on contours" (Johnston, 2005). Thus, perception occurs when the individual breaks down the object being visualized based on the geometrical properties of the object. When this occurs the individual collects data on the object based on the forms observed. This data is then transmitted to the brain, were the information is translated into an object that provides the individual with a clear picture of what is being observed. Much like the theory offered by Triesman, this theory also appears to loosely fall into the classification of indirect realism. Through the structures of the eye, the individual perceives shapes that enable the brain to formulate an image of the object. In this case as well the object formed for perception will be based on the past experiences of the individual. As such, the final perception will be unique to the individual and thus an experience in which no pure transmission of visual data has occurred.

When the three theories are compared, it is evident that each indicates some element of translation which makes it difficult for direct perception of the image to have occurred. Regardless of the visual stimulus presented each individual will translate and reconfigure the image in the mind in a different manner. What this suggests is that the process of translation is one that obfuscates the final perception that is garnered by the individual. This is why two people looking at the same object will often have notably different perceptions of the same visual stimulus. Even when light, color and all other visual variables are removed, the perception that occurs for each individual will be different. As such, the theory of indirect realism clearly has some application to understanding visual perception.

Conclusion

Any tow individuals looking at the same object at the same time will ultimately produce two different perceptions of that object. Although there have been a number of theories espoused to explicate these differences, it is evident that there are a myriad of ways in which visual stimuli is translated into perception. Given that the process of visual perception does not appear to occur in a direct manner-such as what is proposed under the theory of direct realism-defining how visual perception occurs will remain a challenging task for psychologists. Clearly, indirect realism provides a critical piece of the puzzle by demonstrating that perceptions are more than just pure visual images.

References

Ganglion cell. (2006). Wikipedia. Accessed March 27, 2006 at: http://en.wikipedia.org/wiki/Ganglion_cell.

Glennerster, A. (2002). Computational theories of vision. University of Oxford. Accessed March 27, 2006 at: http://www.physiol.ox.ac.uk/~ag/pub/g2002/myg2002.pdf..

Gordon, I.E. (2004). Theories of Visual Perception. Hove, England: Psychology Press.

Healey, J. (2005). Perception in visualization. North Carolina State University. Accessed March 27, 2006 at: http://www.csc.ncsu.edu/faculty/healey/PP/.

Indirect realism. (2006). Wikipedia. Accessed March 27, 2006 at: http://en.wikipedia.org/wiki/Indirect_realism.

Johnston, A. (2005). Object and face perception. University College London. Accessed March 27, 2006 at: http://www.psychol.ucl.ac.uk/alan.johnston/Object.html.

Kaiser, P. (1996). The Joy of Visual Perception. Accessed March 27, 2006 at: http://www.yorku.ca/eye/thejoy.htm.

O'Brien, D. (2006). Objects of perception. The Internet Encyclopedia of Philosophy. Accessed March 27, 2006 at: http://www.iep.utm.edu/p/perc-obj.htm#Indirect%20Realism.

The receptive field. (2006). Rochester Institute of Technology. Accessed March 27, 2006 at: http://www.cis.rit.edu/people/faculty/montag/vandplite/pages/chap_10/ch10p2.html.