From there, it is carried in a tract often called the optic radiation, which curves around the wall of the lateral ventricle in each cerebral hemisphere and reaches back to the occipital lobe. The axons included in the optic radiation terminate in the primary visual cortex in what is called a retinotopic manner, meaning that axons carrying information from a specific part of the visual field terminate in a location in V1 that corresponds to that location in the visual field.
For example, axons carrying information about the inferior portion of the visual field terminate in areas of V1 that lie above the calcarine sulcus, while those that carry information about the superior portion of the visual field project to areas below the calcarine sulcus.
These projections to the primary visual cortex from the thalamus travel along at least three distinct pathways. One pathway arises from large neurons in the retina called magnocellular , or M, cells; another pathway projects from smaller neurons called parvocellular , or P, cells; and a third pathway travels to V1 from small neurons called koniocellular , or K, cells.
These different types of neurons preferentially respond to different types of visual stimuli, thus it seems these pathways are each somewhat specialized for specific categories of stimuli. M cells, for example, seem to be specialized to detect movement e. P cells appear to be important for spatial resolution e. The functions of K cells are still not fully understood, but they are thought to be involved with some aspects of color vision.
Neurons in the primary visual cortex are arranged into columns of neurons that have similar functional properties. The secondary areas of these various cortexes then converge onto what are called associative areas, which perform a more global level of information processing. They progressively associate signals from other sensory modalities to create an integrated, multisensory representation of the world. To date, researchers have discovered nearly 30 different cortical areas that contribute to visual perception.
From all this complexity, however, a general pattern does emerge. There appear to be two major cortical systems for processing visual information: a ventral visual pathway that extends to the temporal lobe, and a dorsal visual pathway that projects to the parietal lobe. The basic function of the ventral visual pathway seems to be to let us consiously perceive, recognize, and identify objects by processing their "intrinsic" visual properties, such as shape and colour. In the cortical areas that contribute to the ventral system , increasingly complex, specialized representations of the outside world are elaborated.
Many of the neurons in area V3 have properties similar to those in V2. For example, most of them are selective for orientation. But much remains unknown about area V3, and it also has some cells with more complex properties. For example, some of these cells are sensitive to colour and movement, traits more commonly analyzed in subsequent stages of visual signal processing in the brain. After passing through areas V1, V2, and V3, part of the visual information continues ventrally to area V4 on its way to the temporal cortex.
Area V4 receives information from the blobs and interblobs of the striate cortex , via a relay in V2. Like the cells in all of the other visual areas besides V1 also known as the "extrastriate areas" , the cells in area V4 have larger receptive fields than those of the striate cortex. Also, the receptive fields of V4 are often sensitive to both colour and orientation.
The exact role of area V4 is still under debate, but it is probably involved in recognizing shapes, and it appears to be essential for perceiving colours. The cells of area IT receive many connections from area V4 and respond to a very wide range of colours and simple geometric shapes.
These cells appear to play an important role in visual memory, in addition to being a key locus for object recognition. Neurons have been found in area IT that respond specifically to images of faces. Initially discovered through intracellular recordings in monkeys, the existence of these cells has been confirmed in human beings through functional magnetic resonance imaging MRI studies.
This discovery is of some interest to neuropsychologists, who have long known of a rare syndrome called prosopagnosia, in which patients have difficulty in recognizing faces, even though the rest of their vision is normal. One such distortion is magnification, which favors the central visual field at the expense of the periphery Figure 4A. This emphasis is only partially inherited from the retina Adams and Horton, Another distortion is geometrical, and it transforms concentric circles and radial lines in an image into vertical and horizontal lines in V1 Figure 4A.
These effects can be summarized by a simple mathematical rule based on the complex logarithm Schwartz, Figure 4B. A consequence of this rule is that scaling or rotating an image simply translates its representation in V1, potentially helping subsequent stages to recognize images regardless of distance and orientation.
Neurons in area V1 are classically divided into two types: simple and complex Hubel and Wiesel, , , based on the structure of their receptive field. ON and OFF subregions differ in their responses to the onset of stimuli on a gray background: ON subregions respond white bars, and OFF subregions respond to black bars.
When thinking about the responses of simple and complex cells, it helps to consider two descriptive models Figure 7. A simple cell Figure 7A operates weighted sums linear filtering on an image, with weights defined by the profile of the receptive field; the output of this filtering can be positive or negative, but only the portion that exceeds a threshold results in a response Movshon et al.
A complex cell Figure 7B , in turn, can be thought of as integrating the output of multiple simple cells with overlapping receptive fields but different arrangements of ON and OFF regions Movshon et al. V1 cells are commonly classified as simple or complex based on their responses to drifting visual gratings. In simple cells the responses are periodic, whereas in complex cells they are steady in time. When this assay is applied to the spike responses of the neurons, it classifies simple and complex cells as distinct groups Skottun et al.
However, in terms of the underlying synaptic inputs V1 cells seem to fall along a continuum, suggesting that the distinction between simple and complex may be one of degree rather than kind Priebe et al. In addition to stimulus position, V1 neurons are selective for a number of attributes, including orientation, direction of motion, spatial and temporal frequency.
In many species they are also selective for binocular depth and color. The selectivity for orientation, spatial frequency, direction, and temporal frequency can be viewed in an integrated fashion by thinking of receptive fields in frequency space reviewed in Mante and Carandini, Figure Frequency space has three dimensions: two of spatial frequency Fx and Fy, and one of temporal frequency, Ft. In this space, a receptive field in space-time e.
Figure 9 is simply represented by a ball Figure 10, left. The height of the ball from the ground indicates preferred temporal frequency, the distance from the middle vertical indicates spatial frequency, and the angle on the ground plane indicates preferred orientation and direction of motion.
Different V1 neurons have different preferences, and their preferences are thought to tile the frequency space Figure 10, right.
Many of the forms of selectivity exhibited by V1 neurons are novel, in that they are not inherited from LGN. The mechanisms and circuits creating this selectivity are in most cases not known. For instance, we know little about the mechanisms by which V1 achieves direction selectivity. Conversely, a rich literature investigates the circuits that generate orientation selectivity. The mechanisms of orientation selectivity are typically studied in simple cells that receive direct LGN input.
These cells are thought to obtain their orientation selectivity through appropriate summation of LGN inputs Hubel and Wiesel, Figure Connectivity from LGN to V1, however, may not be the only factor contributing to orientation selectivity. Other factors that are thought to contribute include spike threshold, intracortical excitation, and intracortical inhibition Douglas and Martin, ; Ferster and Miller, ; Finn et al.
V4 receives information from V2 and is part of the ventral processing stream. Cells in V4 are very responsive to color. The inferotemporal cortex is located along the lower inferior portion of the temporal lobe. This area of the brain is part of the ventral processing stream and seems to respond best so simple shapes circle, square, etc.
Over Vivid Vision Providers prescribe virtual reality alongside patching and vision therapy to treat your lazy eye. Sign up through our doctor locator to see if Vivid Vision is right for you. Visual Cortex The visual cortex is located in the occipital lobe of the brain and is primarily responsible for interpreting and processing visual information received from the eyes. Primary Visual Cortex V1, striate cortex, Brodmann area 17 The brain is filled with depressions or grooves sulci and elevations gyri.
Visual Area Three V3 V3 communicated directly with the respective dorsal and ventral subsystems of V2. Visual Area Four V4, extrastriate cortex V4 receives information from V2 and is part of the ventral processing stream. Visual Area Five V5, middle temporal cortex V5 is part of the dorsal processing pathway and contains cells highly sensitive to motion.
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