Intro | Visual Cortex | Lateral Geniculate Nucleus | Optic Nerve | Retina | Thalamus
Part 1: Image-Mapped Tutorial
Part 2: Matching Self-Test
Part 3: Multiple-Choice Self-Test
The complexities of Visual Cortex are simplified by understanding that the neurons of this region are distinguished by the stimulus features that each detects. The three major groups of so-called feature detectors in visual cortex include simple cells, complex cells, and hypercomplex cells. Simple cells are the most specific, responding to lines of particular width, orientation, angle, and position within visual field. Complex cells are similar to simple cells, except that they respond to the proper stimulus in any position within their receptive field. In addition, some complex cells respond particularly to lines or edges moving in a specific direction across the receptive field. Hypercomplex cells are responsive to lines of specific length. It is believed that the information from all feature detectors combine in some way to result in the perception of visual stimulation.
As discussed previously with Figure 14, the encoding of stimulus color begins in the retina, as different wavelengths are transduced by the trichromatically responsive cone cells (Trichromatic Theory of color vision). Color vision, however, is considerably more complicated than this and includes higher processing along the visual pathway. Research indicates that the perception of approximately one million colors involves the process of additive color mixing, in which light of varied wavelengths are combined or mixed. The Opponent Process Theory of color perception best explains the contribution made by the LGN and visual cortex to color perception, although this processing also occurs at the retinal level. In this process, three types of neurons respond antagonistically to pairs of colors. For example, cells respond in opposite ways to blue versus yellow, red versus green, or black versus white. This sequential encoding of light wavelength (hue), saturation (purity), and amplitude (brightness) ultimately results in the perception of color.
The ability to perceive form, patterns, and objects not only results from the encoding of stimulus features by visual neurons (feature analysis), but is also under the influence of top-down processes. This top-down influence is known as perceptual set, and is the effect of an individual's unique experiences on her expectations of the world. The now relatively inactive subfield of Gestalt psychology focuses on processes that affect the perception of "whole", those that defy an explanation based on a simple combination of the elements that compose the "whole". These contributions have greatly enhanced the understanding of visual perception. A phenomenon known as figure and ground reversal is a classic example of the effect of perceptual set on perception. An ambiguous visual stimulus may be perceived in two different ways depending on one's perceptual set. Both perceptions, however, cannot be seen simultaneously.
Other principles of Gestalt perceptual organization include: 1) Proximity, whereby elements that are close together tend to be grouped together, 2) Closure, whereby missing elements are supplied to complete a familiar object, 3) Simplicity, whereby elements are organized in the simplest way possible, 4) Continuity, whereby elements are seen in a way to produce a smooth continuation, and 5) Similarity, whereby similar elements are grouped together.
Depth or distance perception is provided by a number of cues both monocular (based on an image in either eye alone) and binocular (based on the differing views of each eye). Monocular cues for depth perception include: 1) Linear perspective provided by parallel lines that appear to merge with increased distance, 2) Texture gradient such that a texture is finer for more distant objects, 3) Relative size with closer objects appearing larger than distant objects of the same size, 4) Interposition of closer objects which overlap or mask more distant objects, 5) Light and shadow patterns which create three-dimensional impressions, and 6) Height in plane cues with closer objects appearing lower in the visual field than more distant objects.
Two additional and important forces at work in visual perception are perceptual constancy and optical illusions. Perceptual constancy refers to the tendency to experience a stable view of the world in spite of a continuously changing sensory environment. Without this allowance for constant change, our world-view would be chaotic and as confusing as optical illusions, visual stimuli that appear to us quite differently than they occur in reality. Classic examples of optical illusions include the Muller-Lyer, Ponzo, and Poggendorff. See the world wide web links in the Suggestions for further study section to explore some of these illusions.
| Advanced |
Visual cortex is divided into 5 separate areas, V1-V5. Primary visual cortex or V1 (often called striate cortex because of its striped appearance under a microscope) receives its information directly from the lateral geniculate nucleus. Following processing in this region, the visual neuronal impulses are directed to secondary visual cortex or V2. V2 then projects to V3, V4, and V5. Each of these areas is further subdivided and sends information to any of 20 or more other areas of the brain that process visual information. This general arrangement is subdivided into three parallel pathways. Although each pathway is somewhat distinct in function, there is intercommunication between them.
In the first and completely parvocellular pathway, neurons in the interblobs of V1 project to the pale stripes of V2. The pale stripes of V2 project to the inferior temporal cortex. This is the pathway composed of feature detectors (simple, complex and hypercomplex cells) as described in the basic information section. Parvocellular neurons show a low sensitivity to contrast, high spatial resolution, and low temporal resolution or sustained responses to visual stimuli. These cellular characteristics make the parvocellular division of the visual system especially suited for the analysis of detail in the visual world. Neurons found in the inferior temporal cortex respond to very complex stimulus features of a specific nature regardless of size or position on the retina. Some neurons in this region respond selectively to faces of particular overall feature characteristics. It is not surprising, therefore, to learn that this region is intimately involved in visual memory. Damage to the parvocellualar pathway will induce disorders of object recognition. Common examples of such disorders include visual agnosia, or the inability to identify objects in the visual realm, and prosopagnosia, a subtype of visual agnosia that affects specifically the recognition of once familiar faces. This division of the visual system tells us to identify what we see.
In the second visual cortical pathway, the neurons in lamina (layer) 4B of V1 project to the thick stripes of V2. Area V2 then projects to V3, V5 (or MT, middle-temporal cortex), and MST (medial superior temporal cortex). This pathway is an extension of the magnocellular pathway from the retina and LGN, and continues the processing of visual detail leading to the perception of shape in area V3 and movement or motion in areas V5 and MST. Cells in V5 are particularly sensitive to small moving objects or the moving edge of large objects. Cells in dorsal MST respond to the movement (rotation) of large scenes such as is caused with head movements, whereas cells in ventral MST respond to the movement of small objects against their background. Magnocellular neurons show a high sensitivity to contrast, low spatial resolution, and high temporal resolution or fast transient responses to visual stimuli. These cellular characteristics make the magnocellular division of the visual system especially able to quickly detect novel or moving stimuli, the abilities that allow us to respond quickly and adaptively to possible threatening stimuli. Perhaps this is why this division was the first to evolve.
Finally in the third and mixed visual cortical pathway, neurons in the "blobs" of V1 project to the thin stripes of V2. The thin stripes of V2 then project to V4. Area V4 receives input from both the parvo- and magnocellular pathways of the retina and LGN. The parvocellular portion of (V4) is particularly important for the perception of color and maintenance of color perception regardless of lighting (color constancy). The V4 neurons associated with the (non-color) magnocellular pathway appear to be involved somehow in the control of visual attention to less noticeable, subtle stimuli in the environment.
The question of how these different areas work together to result in our final perception of the visual world is often referred to as the "perceptual binding problem". This issue is discussed in considerable detail at one of the links provided below.