The visual system is the part of the BRAIN that receives information from the eyes and processes that information to facilitate actions and decisions. The information received and processed can be used to control directly undertaken actions, such as walking or grasping an object, but can also be used to facilitate the planning of future actions. The first stage of the visual system is to be found in the EYE itself. An image of the scene is focused by the lens of the eye on to the RETINA, the layer of photosensitive cells or receptors which lines the back of the eye. The light causes a pattern of electrical activity in the receptor cells. This pattern is converted into a pattern of neuronal impulses at the stage of the retinal ganglion cells. These cells send axons up the OPTIC NERVE and into the brain proper. The information first gets processed at the LATERAL GENICULATE NUCLEUS (LGN) and is then passed on to the VISUAL CORTEX. These early stages of the visual system are generally concerned with ensuring that the best quality of information is recovered from the image as possible. For example, the range of luminances in a scene can be extremely high—much beyond the capacity of the bandwidth of the visual system. Some parts of the image are very dim and the luminance differences are very small, but at the same time, other parts of the image can be very bright and have relatively large luminance differences. The early stages of the visual system are concerned with making sure that, whatever the local image luminance is, the important contrast information is re-ceived at the visual cortex, where the bulk of the visual processing occurs.
After the lateral geniculate nucleus, some of the visual information is passed on to the SUPERIOR COLLICULUS, where it is responsible for controlling eye movements. The majority of the information however, gets passed up into the visual cortex. In the visual cortex, the information from the retinal image is analysed in a variety of different ways to extract information about spatial luminance pattern (mainly edges and texture), motion, depth and colour. The visual cortex is split up into a number of different visual areas, each with its own retinotopic mapping of the visual field (see AREAS V1–V5).
Within each visual area, there is a considerable degree of specialization of function. A major feature of most visual areas is that the cells in these areas tend to have receptive fields that are considerably smaller than the whole visual field. This means that each cell is processing information from a small part of space. The whole visual field is covered by arranging for a large number of cells to process the information at each stage, each covering a slightly different part of the visual field. In the first visual area, V1, cells have receptive fields that indicate that they are each individually sensitive to a bar or edge of a particular size and orientation light and moving in a particular direction. These cells are organized into a column of cells, within which all orientations and spatial scales from the one retinal region are processed. The output of these cells in V1 can be thought of as comprising a register of the local structures within the image. There are various classes of cells, differing in the extent to which the pattern they are optimally sensitive to is contrast specific (so-called simple cells, are tuned to an edge or bar of a particular contrast direction, whereas so-called complex cells are tuned to an edge or bar of any contrast direction—see SIMPLE, COMPLEX AND HYPERCOMPLEX CELLS).
Later stages of processing in the visual pathway are involved in processing higher level aspects of the visual stimulus. One of these, the sensation of colour, has been studied to some considerable extent (see COLOUR VISION). The colour of a surface is a property of that surface that does not vary significantly with changes in how the surface is illuminated, even though the intensity of the light at different wavelengths will change. Thus a red surface looks red whether it is illuminated with light that has a reddish or a bluish tinge to it. This phenomenon is referred to as COLOUR CONSTANCY. Cells in V1 are typically sensitive to the wavelength of the light that they receive and are thus not colour constant, whereas cells at a later stage of processing in V4 tend to show responses that are related to the colour of the surface that falls within their receptive field, irrespective of the wavelength composition of the light involved. Other visual areas at this stage of processing are involved in similar complex computations, such as the MIDDLE TEMPORAL REGION (area MT), which is concerned with spatial aspects of motion processing and spatial form.
The visual system projects information, after processing to this level, onwards in at least two different directions (see TWO CORTICAL VISUAL STREAMS). One projection is up through the TEMPORAL LOBE towards sites in the brain where memory information of various types is held. This projection is thought to be much involved in object recognition. Another projection is forwards in the brain towards the MOTOR CORTEX areas, where the information from vision is used to control actions, such as LOCOMOTION.
ROGER J.WATT
This is the complete article, containing 880 words
(approx. 3 pages at 300 words per page).
