Physiological Processes Involved in our Spatial Vision - Literature review Example

Physiological Processes Involved in our Spatial Vision Introduction Eye is an image forming organ and like a camera. In front of the eye, a small hole is present which is called pupil. The light enters through this hole and is focused on retina, which is a camera film like structure. Lens of the eye focuses the images from different distances on the retina. A color ring is present on eye which is known as iris. The front white sheet of the eye, sclera covers the outside structure. Coroid is the vascular layer of eye. The optic nerve sends image to the brain. (Gregory & Richard L., 1997) In a nonscientific context, the vision means seeing. Seeing is a process which enables a person to see things of this world around him/her. To see is a natural God gifted phenomenon. One gets benefits and enjoys the environment. During this phenomenon, the patterns of light are projected on our retinas by the objects. From the scientific view, vision is a complex process. The image is encoded in the retina. The retina goes through some important processes and then reaches to the stage of recognizing an object or personality or anything else. Actually it is the object that reflects light from its background and the luminant boundaries in the retinal image are formed. This is a useful starting point in the spatial vision and is performed by an important group of retinal cells, the ganglion cells. (Jenkin, 1994) Spatial Vision Spatial vision is that branch of psychology/biology that deals with the study of interpretation of light patterns on the retina by the visual system. This is a new interdisciplinary approach and is also called as vision sciences. This field of science integrates psychological, computational, neuro-scientific perceptions and the visual brain knowledge. Rods and cons of the eye are photoreceptor in nature and transduce light into electrical impulses inside our nerve fibers. Both types of cells are interdependent to each other. Not only a discrete area of human cognition is involved in spatial vision, but various other areas are also taking part in this two sided/faceted relation. The spatial perception influences and is influenced. (DeValois & Russell L., 1990) The field of visually perceived space introduces the knowledge of scientific psychology. In the current era, its importance is at a great rise. A keen interest is being shown by the biologists and psychologists. They are busy conducting an active research in this most important scientific knowledge. The visual space is usually described in terms of conscious experiences. Wagner (2006) stated that visual space is exposed by psychophysical judgments of an observer. (De Valois & Karen K., 2002) Performance Eye is an organ of human organization that is generally responsible to extract all the information. Specifically, these are the retinal ganglion cells of the eye that represent the extracted information in their responses. The retinal ganglion cells are found in a great number that is about one million in each eye. At the level of primary visual cortex some of the extracted information is represented in the responses of about 200-500 million neurons. The information which is contained in ganglion cell responses is exhibited in the primary visual cortex. This means that during the visual processes, the information is lost in the physiological channels (mechanisms) which occur between cornea and optic nerve. (McIlwain, 1996) General Physiological process The physical space is objectively defined by measuring devices, while the visual space is subjectively defined. The initial physical events in the visual systems are the absorption of light by a photopigmentation and transduction of this light into neural activity. Statistical comparisons reveal that old subjects showed more activation of occipitotemporal cortex during the spatial task and more activation of superior parietal cortex during the object task than did the young subjects. These results show less functional separation of the dorsal and ventral visual pathways in older subjects and may reflect age related reduction in the processing efficiency of these visual areas. If one knows the cognitive operations applied at a given location in physical space to produce a corresponding location in visual space, one could establish the mapping between these spaces. (C.J. Owsley & R. Sekuler, 1980) This knowledge could produce advances in various areas of human activities such as 1- in robotics conceiving the visual analysis of the camera’s image to produce computerized visual space, 2- in neurosciences, developing man-machine interfaces, allowing us to stimulate some neural circuits (by transcranial magnetic stimulation), eliciting spatial sensations. Scientists believe in the notion that the physiological processes are carried out in the human retina which are described by means of Fourier’s transformation. In the early phases of the visual system’s functioning the light variations contained in the stimulation projected on the retina are analyzed by means of a series of filters that are systemized with spatial frequency orientation etc. Photographic space is more an isotropic than visual space (hyperbolic) whereas the geometrical structure of stereoscopic space is similar to that of visual space. (Jenkin, 1994) Receptive Field Results of tedious researches have revealed that each ganglion cell shows its input from the receptive field. Cell’s receptive field is a specially localized region of the retina which performs the duty of bringing a change in the response of retinal cells. Receptive fields of retinal ganglion cells have distinct ON and OFF regions. These are also called as two antagonistic organizations namely centre and surround. This type of organization forms ganglion cells, which respond to contrast as compared to the simple light intensity. The receptive fields of light intensity of different ganglion cells with different sized receptive fields overlap in the retina. Due to the presence of different sized receptive fields, the ganglion cells selectively respond to the patterns of different scales. In other words, a cell with an ON centre receptive field, reacts intensively to a bright spot on a dark background. A cell with an OFF centre receptive field, reacts strongly to a dark spot on a bright background. The receptive fields respond differently to bright boundaries. In this type of case, visual neurons distribute the responses in to two dimmentional way (neural image). The responses towards the boundaries show deviation. According to Indow (2004), the long series of physical, physiological and psychological process produce the space we see around us. ((De Valois & Karen K., 2002) Spatial Tuning The scientists use animals for testing to know the anatomy and physiology of human beings. The same is true in the case of spatial vision of the humans. They observed the eyes of different animals and concluded that the retinal ganglion cells are characterized by their spatial frequency selectivity. This selectivity depends upon the size of their receptive fields that have an excitatory centre and inhibitory surround. Cells with small receptive fields will show a response towards high ranges of spatial frequencies and those with large fields will respond to lower ranges. In this modern age, scientists by the use of electrophysiological techniques have come to know that the receptive field organization was observed for the first time in the retina of the eye. This information is accepted throughout the visual system. (Pinna, 2006) Orientation Tuning As a result of electrophysiological studies on animals like cats, Hubel and Wiesel (1962, 1968) have contributed a lot in evaluating the physiology of our spatial vision. They performed experiments on the visual cortical cells of the cat. By utilizing their researches, we come to know that in the cortex of our retina there are specific cells that are sensitive to motion. Interestingly they respond to motion in one direction and not in the other (bringing this selectivity to the temporal domain). Similarly, there is another type of cells that respond to edges and not to bars. This is selectivity towards symmetrical patterns. Majority of the cells exhibit maximum responses to patterns at a certain orientation. If the pattern is changed, the response quickly declines thus the cells are characterized by a physiological process the orientation tuning. (De Valois, 2002) Responses in a visual mechanism are actually the light patterns in the image of the retina. These patterns are produced by the objects in the visual fields. Light patterns represented as a set of responses in the visual mechanism, exhibit selectivity for contrast, spatial frequency and orientation. Human physiology and psychology researchers agree on a phenomenon known as stimulus selectivity which plays a fundamental role in spatial vision. Visual system in human is selective as regards to many stimulate dimensions, like orientation, size, position, wavelengths, direction of motion and speed of motion etc. Mechanisms (channels) are selective to different areas of brain according to the stimulate dimensions. Neorophsiological and anatomical studies reveal that these are the neurons in the visual path ways that perform the function of selectivity along stimulus dimension. The intensity of selectivity is lower in the retina, and then gradually it increases towards the primary visual cortex. Both photoreceptors as well as cortical neurons are selective towards stimulus dimensions. The photoreceptors are selective towards spatial, wavelengths (color) and temporal frequency etc. The cortical neurons are selective towards spatial, position, wavelength, temporal frequency, orientation, spatial frequency, direction of motion and disparity etc. All the cone cells show similar intensity response functions, while the cortical neuron cells differ from cell to cell as regards ro contrast response functions. (Snowden, 2006) According to the neuron doctrine (Barlow, 1972. 1995) the stimulus selectivity along the visual path way selectivity belongs to a hierarchical process that in the end leads to individual neurons that shows the response to specific objects. This is the selectivity which critically segregates the objects of the world from their context. This physiological process of object recognition and scene interpretation is also known as object segregation. The responses of the individual neurons fall under the knowledge of single neuron electro physiology, while response of the whole individual is psychophysics. Visual system of human cannot transmit and use all the information because the nerves system will use some information and rest will be lost. Now question arises how we can know about the used and lost information. This is the behavior by which one can guess that what of the information is utilized and what is wasted. The physiology described in the above, can be understood with the help of an example which quotes the task of color discrimination. A human observer comes across an innumerable number of different wavelength distributions. The scientists dealing with behavioral researches inform us that majority of these wavelength distributions remain in-distinguished for the human being. Much of this chromatic information (wavelengths), disappears in the nervous system at the level of photoreceptors. The reality described in this physiological example, opens the pathways for neuron-physical scientists. They, by using this reality have designed the chromatic stimuli as well as pointed out the area of nervous system specific for color discrimination. Spatial Frequency Selectivity The images may be simple as well as the complex ones. The visual system, deals differently with these two different types of images. Different visual information is being passed on forward at different spatial scales. When we see a cat for example, our visual system, functions in such a manner that a coarse spatial scale is conveyed to perceive the general shape of the cat but a fine scale for the hair of the cat. Different scales have different sized spatial fields. These visual fields contain superimposed receptive fields. The receptive fields are different in sizes but they are tuned to different spatial frequency. That is how they collect even the discrete information of the image and convey them at different spatial scales. If the stimulation from the excitatory region of the retinal ganglion cell matches to that from its inhibitory surround, the effects are cancelled out. During the synthesis of square wave, the sine waves combine in many different patterns, the result is the production of different types of images. The square waves have sharp edges and flat plateaus, while the sine waves don’t have sharp edges and they have ripples. As the sine waves with an increasingly higher spatial frequency join the summing process, and image profile is resulted and this image profile has sharp edge and flat plateaus. This looks like the square waves. In the case, if this process is continued for a very long time and gradually higher spatial frequencies are added in it, a perfect square wave is obtained. It means that a square wave is produced only when the combination of sine waves is correct. This combination should be at a correct spatial frequencies and amplitudes. Different combinations of different sine waves will show the result of the production of different images. (R. J. Watt, 1982) Primary visual Cortex The spatial vision pathway, moving from the retina through lateral geniculate nucleus to the primary cortex makes the process a refined one. The cells in the lateral geniculate nucleus are similar to the cells of retina. Some receptive fields of the cortical cells differ in their shape from the rest ones. They are elongated in shape and their elongated shape helps them coding the orientation of image structure. These cells are called as simple cells. There is a class of cortical cells which is very similar to retinal cells. They can be generated by wiring together. (DeValois, 1990) Variability Visual performance is not a uniform physiological process, but a changeable one. The amount of noise may change it, for example, if the noise increases in the responses of neurons, the visual performance is lowered down. The cortical neurons responses show a greater change/variability with a great increase of the responses. Stimuli falling outside of the classical receptive field affect the responses of cortical neurons. Neuralistic viewing condition When the eyes are observed from the physiological point of view focusing on spatial vision, it is concluded that the eyes remains moving under viewing conditions. This means that the low temporal frequencies are not common. Viewing stationary events, the eyes jump from location to location and the frequency is very common. The fixation time duration is approximately 200-500 ms (according to Carpenter, 1991). This can be interspersed with saccadic correction. (Kentridge, 1996) Conclusion As the science advanced, new modern techniques in different branches of sciences have been evolved. Same is true with biology/psychology of human beings. Neuro-image techniques have been immerged that are much useful to know the physiologies of spatial vision. By this technology the experimenters can perform experiments directly on human beings instead of animal testing. They can perform the measurements of huge populations of the most important cells involved in spatial vision, the neurons. (De Valois, 2002) Visual perception means that we can perceive, detect and understand the objects, living organisms and everything all around us in this vast world. All this depends upon the neural processes in our eye. The starting point of these processes is our retina. During this process, the retinal image is decomposed into a group of parallel neural images. These images have multiple scales, in other words spatial frequency bands of different types. The neural images also possess different polarities. An imperfect optical system blurs the image. The blurring occurs due to the reason that high spatial frequency components are removed. If the retinal receptors show an under-sampling, the result is the appearance of visual distortion. The overall effect of the blurring by retinal ganglion cells links with contrast sensitivity function. At the level of the primary visual cortex, spatial frequency tuning is tighter as compared to starting processes and the representation of visual information is again branched into neural images. The branched neural images are responsible for multiple orientations. The spatial splitting of the image in the visual cortex produces two effects; 1- Images convey different information at different spatial scales, so independent representation of different spatial frequency bands in the image are seen. 2- It provides a direct means by which individual stimulus features. (McIlwain, 1996) References C.J. Owsley, R. Sekuler (1980).Human aging and spatial vision. Science, Vol. 209. 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