The Primate Visual System - Essay Example

The Primate Visual System Retinotopic Organization Retinotopic organization is the way neural information from the retina is mapped out in the visual cortex, which comprises the structures in the brain used for vision. The general idea about retinotopic organization is that for every area in the retina or the visual field where an image falls, a specific part in the visual cortex is stimulated and this is how the organism knows the location of the object. Each location on the lateral geniculate nucleus, the transmitting nucleus of the visual system, and the cortex corresponds to a location on the retina, and neighboring locations on the lateral geniculate nucleus corresponds to neighboring locations on the retina. These areas have mostly been determined using functional magnetic resonance imaging (fMRI). Typically, a participant is presented with a stimulus while his or her eyes are fixed to a central point and the wave of activity that the stimulus evokes is captured and recorded by the fMRI. Many studies employing variations of this experimental technique has led to the discovery of many areas in the brain that have a role in our vision, including areas in the cortex, the lateral geniculate nucleus, and the superior colliculus (Silver and Kastner, 2009). Studies have also found that, apart from the location of the object, the type of the stimulus presented may preferentially stimulate certain areas of the brain. For example, two visual field maps in the ventral visual cortex, named VO-1 and VO-2, have been found to respond preferentially to color and object stimuli and less to faces (Brewer et al., 2005). This kind of organization of response to neural stimulus, generally called a topographic organization, can also be seen in other sensory modalities. For example, in the auditory cortex, the part of the brain which processes sounds, there is a tonotopic map, arranged by their response to different frequencies of sound. The tonotopic map in the auditory cortex mirrors the distribution of receptors in the cochlea, the main organ of hearing. In effect, a certain frequency of sound will stimulate a specific receptor in the cochlea, which will in turn stimulate a specific area in the auditory cortex, allowing the organism to discern the pitch of the tone that he or she heard (Humphries, Liebenthal, and Binder, 2010). Another example of topographic organization is the motor cortex, which is responsible for movement. Studies with fMRI have shown the existence of a somatotopic map, with areas that correspond to movement of specific parts of the body. For example, movement of the tongue activated a ventral location in the somatotopic map, while movement of the toes activated a dorsal location (Meier et al., 2008). Mapping out these topographic organizations can contribute to the understanding of sensory systems. It can also have important clinical impact, such as in cerebral vascular events that affect certain areas of the brain, which then translate into specific deficits in the body. Division of Labor The visual system demonstrates division of labor, also called functional specialization, wherein different aspects of a visual scene, such as form, color, motion, and location, are processed by different sets of neurons. This was first seen in studies using the macaque monkey, where it was seen that there is an area, V5, that is specialized for visual motion and another area, V4, specialized for color (Zeki, 1978). Division of labor was then also demonstrated among humans, using the positron emission tomography (PET). The investigators used a technique called statistical parametric mapping to detect changes in blood flow to parts of the brain in order to determine which parts are more stimulated with certain stimuli. Presenting a colored stimulus versus a gray stimulus to a participant showed higher brain activity in a region designated the V4, located in the lingual and fusiform gyri. A second experiment showed that motion stimulation caused increased activity in a separate region, designated the V5, which was found in the junction of the parietal and occipital cortices (Zeki, 1991). These findings showed that functional specialization is one of the many ways that the nervous system organizes the many stimuli an organism is exposed to and translates it to representations that make sense. Several clinical cases have also demonstrated division of labor in the visual cortex. An example is cerebral achromatopsia, which is the inability to perceive color, usually due to a congenital defect or an acquired lesion in the fusiform gyrus or V4, where the color center is. When only V4 is damaged, people with achromatopsia can still do other visual activities, such as reading and writing, recognizing and naming objects, and perceiving distance and motion. Another example is cerebral akinetopsia, wherein the lesion is in V5, the area for perceiving motion. The patient with this lesion could only perceive objects that are stationary, but not those in motion. These examples shows functional specialization because affecting one part of the visual cortex only affects the corresponding function that it serves and spares the other functions (Zeki, 2005). Functional specialization can also be seen in the other senses, for example, the sense of sound and smell. In studies of rhesus monkeys, it has been found that neurons in the lateral areas of the auditory cortex are stimulated more than complex sounds than pure tones, and that there are areas that are more stimulated when hearing species-specific communication calls (Tian et al., 2001). In the olfactory areas of the cortex, which are the areas involved in processing the sense of smell, different are activated by different purposes. With the use of positron emission tomography, human participants are presented with different unlabeled odors. While in the process of identifying each smell, activity increased in the left cuneus and the bilateral cerebellum, implicating these areas as specialized in olfactory matching. When the participants are asked to name a particular odor, the left cuneus, right anterior cingulate gyrus, left insula, and bilateral cerebellum are activated, pointing at their role in olfactory naming (Qureshy et al., 2000). The generalizability of functional specialization once again demonstrates how organizations of the different sensory modalities are parallel to each other. Contralateral Representation The flow of information from the retina to the cortex is such that both eyes have representation in both cortices. This means that the images that pass through the left eye are processed both in the left hemisphere, which is the ipsilateral hemisphere in this case, and the right hemisphere, which is the contralateral hemisphere. The reverse is also true for the right eye. The representation of the information from the left eye in the right hemisphere is referred to as contralateral representation. This kind of organization is said to reflect a general pattern of crossing over of sensory and motor organization seen in vertebrate organisms with bilateral symmetry. There is also the observation of chiasmatic decussation wherein a portion of the fibers coming from both eyes cross over the structure called the optic chiasma and synapse with their contralateral hemispheres (Capozzoli, 1995). To demonstrate this organizational characteristic of contralateral representation, Kagan and colleagues (2010) looked at the space representation for eye movements in both monkey and human participants. They found that the contralaterality of response to cues and memory of the stimulus was stronger among monkeys than humans. The group theorizes that the organism first processes the image seen in the contralateral hemisphere, and then in higher functioning brains like the human brain, proceeds to disseminate the information to both hemispheres. The group adds that apart from contralateral representation, the human brain also exhibits hemispheric lateralization. This refers to certain cognitive functions being processed in a specific hemisphere of the brain. The right hemisphere, for example, more notably processes spatial information, while the left hemisphere processes action planning. Because of this hemispheric lateralization, it is thus important to have contralateral as well as ipsilateral representation to make sure that both hemispheres receive input from both eyes. The concept of contralateral representation outside the visual system is best demonstrated by the motor deficits seen after a person suffers a cerebrovascular event or stroke. In stroke patients with a unilateral left or right hemisphere damage, motor tasks such as grip, finger tapping, maze coordination, and pegboard tasks were performed more poorly when using the hand contralateral to the lesion than the ipsilateral hand. This shows that signals between the hand and the brain tend to cross, or have contralateral representation (Haaland and Delaney, 1981). Cortical Magnification In the retina, the area called the fovea is a small area in the center packed with the highest density of receptors. As signals are sent to the visual cortex, it has been found that though the fovea occupies a small area of the retina, its corresponding neurons in the gray matter occupies a much bigger space that is not consistent with the size of the fovea. This is termed the cortical magnification factor, wherein more cortical space is allotted to parts of the retina that send more signals to the cortex (Schira, Wade, and Tyler, 2007). Because of the cortical magnification factor, each foveal input is allotted extra cortical neurons. Compared to input from the periphery of the retina, foveal input is allotted up to six times more cortical tissue. This magnified representation is the reason why images that fall on the foveal region can be detected with higher acuity than images that fall on the periphery. This is especially useful for accomplishing tasks that require high acuity, such as reading or identifying faces (Azzopardi and Cowey, 1993). This magnification factor is also seen in other systems, most notably in the somatosensory cortex. The map on the somatosensory cortex to which each part of the body is represented is called a homunculus. The homunculus is strangely shaped, with parts of the body disproportionately represented in the cortex. In particular, the fingers, especially the thumb, are devoted a large area in the cortex. In general, parts of the body that are concerned with perceiving details are allotted a large area in the somatosensory cortex (Penfeld and Rassmussen, 1950). Differential Coupling Differential coupling is a function of receptive fields. The receptive field is the area of the retina, which, when stimulated, influences the firing rate of its corresponding ganglion cell. Stimulation outside the receptive field will not produce any reaction from the ganglion cell. Responses could be either excitatory, or an “on” response, or inhibitory or “off” response. If the response is excitatory, the ganglion cell fires when a stimulus is in the receptive field. If the response is inhibitory, the burst of firing occurs once the stimulus is turned off (Schiller, 1992). One type of receptive field is called the center-surround receptive field. This kind of receptive field has either an excitatory or inhibitory area in the center surrounded by a field that responds in the opposite way. This causes the effect called the center-surround antagonism. This effect is particularly useful in detecting sizes. For example, a ganglion cell will have an excitatory response if the stimulus hits the excitatory center. But if the stimulus becomes big enough that it also hits the inhibitory surround, its firing rate will decrease. Thus, it responds best to stimulus or spots of light of a certain size. This comprises differential coupling, wherein both excitatory and inhibitory inputs define a stimulus. Along with lateral inhibition, wherein a specific receptor would be inhibited if neighboring areas corresponding to it are stimulated, these mechanisms help the brain process sensory stimuli and turn it into a perception. Receptive fields and differential coupling are also seen in the auditory cortex. Knudsen and Konishi (1978) found neurons in the auditory cortex that respond only when the sound stimulus originates from a particular area in space, which was designated the receptive field of the cell. Furthermore, going over the excitatory area would decrease the stimulus, which shows the presence of center-surround receptive fields as well. Coarse Coding Population coding refers to the phenomenon of a group or population of neurons involved in a single task, instead of just one neuron. Population coding is especially helpful in tasks that require accuracy, such as visual discrimination. It is also seen in the primary visual cortex, used in determining orientation and spatial frequency. Population coding plays a role in judging motion direction and velocity, identifying human faces and objects, and arm movement direction, among other things. In performing such tasks, single neurons are not very informative and information from a population of neurons must be averaged to obtain more accurate information. The process by which information is gathered from population codes involves complicated neuronal computation, wherein the brain tries to decode the most accurate information from a multitude of neuronal stimuli (Averbeck, Latham, and Pouget, 2006). Face recognition is a particular task wherein sparse population coding is employed. Analysis of a population of cells in the temporal cortex of monkeys showed that the information received is carried as a population and transmitted to the anterior inferotemporal cortex, which encodes the physical properties of the face, and to the superior temporal polysensory area, which encodes other aspects of the face, such as familiarity. Information was usually sufficient if entire small populations of neurons are used to identify faces, suggesting that complex stimuli, such as faces, are encoded by sparse population code in the higher visual areas (Young and Yamane, 1992). Coarse coding and population coding is also seen in other systems, including the auditory system. Population coding helps the auditory system in coding sounds that vary enormously, which can have important survival value and have value in communication. 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