The color center is a region in the brain primarily responsible for visual perception and cortical processing of color signals received by the eye, which ultimately results in color vision. The color center in humans is thought to be located in the ventral occipital lobe as part of the visual system, in addition to other areas responsible for recognizing and processing specific visual stimuli, such as faces, words, and objects. Many functional magnetic resonance imaging (fMRI) studies in both humans and macaque monkeys have shown color stimuli to activate multiple areas in the brain, including the fusiform gyrus and the lingual gyrus. These areas, as well as others identified as having a role in color vision processing, are collectively labeled visual area 4 (V4). The exact mechanisms, location, and function of V4 are still being investigated.
Primary visual cortex
The primary part of the visual cortex, (V1), is located in the calcarine sulcus, and is the first cortical area involved in visual processing. It receives visual input from the lateral geniculate nucleus, which is located in the thalamus. V1 sends the visual information received from the LGN to other extrastriate cortex areas for higher order processing. This higher order processing includes the recognition of shapes, motion, and color.
V1 has multiple areas that are color sensitive, which indicates that color processing is not limited to one area. According to a paper by Dr. Robert Shapley, V1 has an important role in color perception. fMRI experimental results showed that V1 has two kinds of color sensitive neurons: single-opponent and double-opponent cells. These cells are integral in the opponent process of interpreting color signals. Single-opponent neurons respond to large areas of color. This is advantageous for recognizing large color scenes and atmospheres. In comparison, double opponent cells respond to patterns, textures, and color boundaries. This is more important for perceiving the color of objects and pictures. The double-opponent cells are receptive to opposite inputs from different cone cells in the retina. This is ideal for identifying contrasting colors, such as red and green.  Double-opponent cells are particularly important in computing local cone ratios from visual information from their receptive fields.
Single opponent color sensitive neurons can be divided into two categories depending on the signals they receive from the cone cells: L-M neurons and S/(L+M) neurons. The three types of cone cells, small (S), medium (M), and long (L), detect different wavelengths across the visible spectrum. S cone cells can see short wavelength colors, which corresponds to violet and blue. Similarly, M cells detect medium wavelength colors, such as green and yellow, and L cells detect long wavelength colors, like red. L-M neurons, also called red-green opponent cells, receive input from long wavelength cones opposed by input from medium wavelength cones. S/(L+M) neurons receive input from S-cells and is opposed by a sum of the L and M-cell inputs. S/(L+M) neurons are also called blue-yellow opponent cells. The opposition between the colors allows the visual system to interpret differences in color, which is ultimately more efficient than processing colors separately.
Higher order visual processing
The primary visual cortex V1 sends visual information to the extrastriate cortical areas for higher order visual processing. These extrastriate cortical areas are located anterior to the occipital lobe. The main ones are designated as visual areas V2, V3, V4, and V5/MT. Each area can have multiple functions. Recent findings have shown that the color center is neither isolated nor traceable to a single area in the visual cortex. Rather, there are multiple areas that possibly have different roles in the ability to process color stimulus.
Visual area V4
Anatomical and physiological studies have established that the color center begins in V1 and sends signals to extrastrate areas V2 and V4 for further processing. V4 in particular is an area of interest because of the strength of the color receptive fields in its neurons. V4 was initially identified in macaque monkey visual cortex experiments. Originally, it was proposed that color was selectively processed in V4. However, this hypothesis was later rejected in favor of another hypothesis which suggested that V4 and other areas around V4 work together to process color in the form of multiple color selective regions. After identification of V4 as the color selective region in macaque monkeys, scientists began searching for a homologous structure in the human cortex. Using fMRI brain imaging, scientists found three main areas stimulated by color: V1, an area in the ventral occipital lobe, specifically the lingual gyrus, which was designated as human V4, or hV4, and another area located anteriorly in the fusiform gyrus, designated as V4α.
The purpose of V4 has changed dynamically as new studies are performed. Since V4 responds strongly to color in both macque monkeys and humans, it has become an area of interest to scientists. The V4 area was originally attributed to color selectivity, but new evidence has shown that V4, as well as other areas of the visual cortex, are receptive to various inputs. V4 neurons are receptive to a number of properties, such as color, brightness, and texture. It is also involved in processing shape, orientation, curvature, motion, and depth.
The actual organization of hV4 in the cortex is still being investigated. In the macaque monkey, V4 spans the dorsal and ventral occipital lobe. Human experiments have shown that V4 only spans the ventral portion. This led to distinguishing hV4 from the macaque V4. A recent study from Winawer et al. analyzing fMRI measurements to map the hV4 and ventral occipital areas showed variances between subjects used for hV4 mapping was at first attributed to instrumentation error, but Winawer argued that the sinuses in the brain interfered with fMRI measurements. Two models for hV4 were tested: one model had hV4 completely in the ventral side, and the second model had hV4 split into dorsal and ventral sections. It was concluded that it was still difficult to map the activity of hV4, and that further investigation was required. However, other evidence, such as lesions in the ventral occipital lobe causing achromatopsia, suggested that the ventral occipital area plays an important role in color vision.
The search for the human equivalent of V4 led to the discovery of other areas that were stimulated by color. The most significant was an area anterior in the ventral occipital lobe, subsequently named V4α. Further fMRI experiments found that V4α had a different function than V4, but worked cooperatively with it. V4α is involved in a number of processes, and is active during tasks requiring color ordering, imagery, knowledge about color, color illusions, and object color.
The V4 and V4α areas are separate entities, but because of their close proximity in the fusiform gyrus, these two areas are often collectively called the V4-complex. Research into the V4-complex discovered that different chromatic stimulations activated either the V4 or the V4α area, and some stimulation parameters activated both. For example, naturally colored images activated V4α more powerfully than V4. Unnaturally colored images activated both V4α and V4 equally. It was concluded that the two sub-divisions cooperate with each other in order to generate color images, but they are also functionally separate.
An interesting study from Nunn et al. on the activation of the V4-complex in people with visual synesthesia from hearing spoken words was used to predict the location of the color center. Synesthesia is the phenomenon where a sensory stimulus produces an automatic and involuntary reaction in a different sensation. In this study, people who would see colors upon hearing words were studied to see if the color reaction could be traced to a specific cortical area. fMRI results showed that the left fusiform gyrus, an area consistent with V4, was activated when the subjects spoke. They also found a simultaneous activation of V4α. Interestingly, there was little activity in areas V1 and V2. These results validated the existence of the V4-complex in humans as an area specialized for color vision.
V2 prestriate cortex
V2, also called the prestriate cortex, is believed to have a small role in color processing by projecting signals from V1 to the V4-complex. Whether or not color selective cells are present in V2 is still being investigated. Some optical imaging studies have found small clusters of red-green color selective cells in V1 and V2, but not any blue-yellow color selective cells. Other studies have shown that V2 is activated by color stimuli, but not color after images. V4 also has feedback on V2, suggesting that there is a defined network of communication between the multiple areas of the visual cortex. When GABA, an inhibitory neurotransmitter, was injected into V4 cells, V2 cells experienced a significant decrease in excitability.
Functional magnetic resonance imaging, or fMRI for short, has been key in determining the color selective regions in the visual cortex. fMRI is able to track brain activity by measuring blood flow throughout the brain. Areas that have more blood flowing to them indicates an occurrence of neuronal activity. This change in blood flow is called hemodynamic response. Among the benefits of fMRI includes dynamic, real-time mapping of cortical processes. However, fMRI cannot track the actual firing of neurons, which happen on a millisecond timescale, but it can track the hemodynamic response, which happens on a seconds timescale. This method is ideal for tracking color selective neurons because color perception results in a visual after-image that can be observed in the neurons, which lasts about 15 seconds.
Sakai et al. used fMRI to observe whether activation of the fusiform gyrus correlated with the perception of color and the after image. The subjects in the Sakai study were placed in the fMRI machine and were subsequently subjected to various visual stimuli. A series of three images were shown to subjects while fMRI was used to focus on the hemodynamics of the fusiform gyrus. The first image was a pattern of six colored circles. The next two images were achromatic. One of the images had a grey cross, and the other image had the same six circles as the first image, except they were six shades of grey that correlated with the colored images. The subjects were cycled between the circle and cross images. During the cross images, the subjected perceived an after-image. The results of the experiment showed that there was a significant increase of activity in the fusiform gyrus when the subject viewed the color image. This provided more evidence to the existence of the color center outside of the primary visual cortex.
Cerebral achromatopsia is a chronic condition where a person is unable to see color, but they are still able to recognize shape and form. Cerebral achromatopsia differs from congenital achromatopsia in that it is caused by damage to the cerebral cortex as opposed to abnormalities in the retinal cells. The search for the color center was motivated by the discovery that lesions in the ventral occipital lobe led to color blindness, as well as the idea that there are area specializations in the cortex. Many studies have shown that lesions in the areas commonly identified as the color center, such as V1, V2, and the V4-complex lead to achromatopsia. Cerebral achromatopsia occurs after injury to the lingual or fusiform gyrus, the areas associated with hV4. These injuries include physical trauma, stroke, and tumor growth. One of the primary initiatives to locating the color center in the visual cortex is to discover the cause and a possible treatment of cerebral achromatopsia.
The extent of the symptoms and the damage is different from person to person. If a person has complete achromatopsia, then their entire visual field is devoid of color. A person with dyschromatopsia, or incomplete achromtopsia, has similar symptoms to complete achromatopsia, but to a lesser degree. This can occur in people who had achromatopsia, but the brain recovered from the injury, restoring some color vision. The person may be able to see certain colors. However, there are many cases where there is no recovery. Finally, a person with hemiachromatopsia see half of their field of vision in color, and the other half in grey. The visual hemifield contralateral to a lesion in the lingual or fusiform gyrus is the one that appears grey, while the ipsilateral visual hemifield appears in color. The variance in symptoms emphasizes the need to understand the architecture of the color center in order to better diagnose and possible treat cerebral achromotopsia.
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