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The visual system in the brain refers to both subcortical and cortical neural structures. In primates and other mammals these structures include various targets of the retina in the midbrain and thalamus as well as a large number of cortical areas. Several parallel visual pathways can be identiﬁed that are based on the output of diﬀerent types of ganglion cells in the retina. Visual cortex is comprised of a large number of cortical areas that can be distinguished by a number of criteria. The patterns of connections among these cortical areas allow them to be organized into a hierarchy of visual processing. Many of these cortical areas are organized into repeating modular subunits that appear to processing all the visual information within a small portion of visual space.
1. Subcortical Visual Structures
1.1 Midbrain Structures
The retina projects to a variety of nuclei in the midbrain that are involved in reﬂexive visual functions. These pathways are best understood in cats, but similar observations have been made in non-human primates. Speciﬁc classes of ganglion cells are known to project to the pretectum (PT-sparse and PT-others), where their information is used for the control of pupillary muscles (Rodieck and Watanabe 1993). Other ganglion cells project to the superior colliculus that supports reﬂexive movements of the eyes and head, especially in response to moving targets. In cats these ganglion cells are known as Y cells (alpha morphological type), but in macaque monkeys only a small proportion are parasol cells (similar to the Y cells of the cat), with the majority of cells coming from a diverse group with varying morphologies (Rodieck and Watanabe 1993).
The superior colliculus gives rise to pathways that ascend into the thalamus to innervate the pulvinar nucleus. In addition, the superior colliculus receives descending projections from a large number of visual cortical areas. These pathways linking the superior colliculus with extrastriate cortical areas describe a second cortical visual system that is distinct from the geniculo-striate system described later. This second pathway is thought to subserve the visual ability known as blindsight, where judgments about objects and especially motion can be made in the absence of true conscience experience.
1.2 Lateral Geniculate Nucleus
The vast majority of retinal ganglion cells project to the lateral geniculate nucleus of the thalamus. The lateral geniculate nucleus is a multilayered structure that receives input from both eyes to build a representation of the contralateral visual hemiﬁeld. The basic pattern of the LGN in primates consists of four layers, two each for each eye. The upper two layers become interleaved to form four layers, two each per eye. In primates, three or more anatomical classes of ganglion cells innervate the LGN. The parasol cells of the retina project to the lower two large cell containing layers, known as the magnocellular layers of the LGN. These ganglion cells and their LGN targets have large, concentrically organized receptive ﬁelds, lack color selectivity, have high contrast sensitivity, and give transient responses to stimuli. The midget ganglion cells of the retina project to the upper four small cell, or parvocellular layers of the LGN. These cells and their LGN targets have small receptive ﬁelds, which are organized with color opponent centers and surrounds, based largely on red and green cone inputs. These cells provide the majority of input to visual cortex, but their signals unambiguously represent stimulus color since diﬀerent responses are observed for diﬀerent size stimuli. The third class of ganglion cell that projects to the lateral geniculate nucleus is known as the bistratiﬁed cells which project to cells located in the intralaminar zones between the main LGN layers (Rodieck and Watanabe 1993). Physiological investigations have identiﬁed these cells to be of the blue yellow type and their receptive ﬁelds are not concentrically organized. These cells project to distinct layers in striate cortex where they are though to subserve the beginning of a cortical color processing system.
1.3 Pulvinar Complex
The pulvinar is a collection of nuclei in the thalamus that are largely related to visual processing in higher cortical areas. In phylogeny, the pulvinar nuclei have increased dramatically in size in parallel with the grown of these higher cortical areas. There remains some controversy about the number of nuclei within the visual pulvinar. On the basis of connections, a collicular recipient zone and a striate cortex recipient zone have been recognized. In addition, area MT projects to a distinct zone within the inferior pulvinar. The remaining subdivisions are less clear, but at least three subdivisions can be distinguished on the basis of their staining for the calcium binding protein parvalbumin (Cusick et al. 1993).
2. Visual Cortical Areas In Nonhuman Primates
2.1 Criteria Used For The Characterization Of Cortical Areas
Visual cortex of all mammals consists of at least two and often a much larger number of distinct cortical areas. However, the speciﬁc criteria used to identify these areas vary considerably. The earliest views of visual cortical organization were based on anatomical criteria that compared the size and distribution of cell bodies and ﬁbers across the cortical mantle. The most prominent of these architectonic studies were those performed by Brodmann (1909) and von Economo and Koskinas (1925) who subdivided the occipital cortex into three distinct zones. Primary visual cortex, also known as striate cortex, V1, area 17 of Brodmann, or area OC of von Economo and Koskinas has a very distinctive appearance, due largely to the sublamination of layer 4 and the presence of a dense ﬁber band in Brodmann’s layer 4B, the stria of Gennari. Although the architectonic method allowed the identiﬁcation of two additional belts of cortex surrounding striate cortex, subsequent studies have indicated that these belts contain more than one functional unit. More recent studies have used various histochemical and immunocytochemical markers to deﬁne distinctive anatomical features of several cortical areas.
The next most useful criterion for subdivision of cortical areas is the presence of a distinct topographic map of the contralateral hemiﬁeld. Typically, microelectrode mapping was used to deﬁne topographic maps of visual cortex and thus to delineate areas. Recently, functional imaging techniques have been used to deﬁne the topographic organization of visual areas, especially in humans. The third criterion used for the characterization of visual cortical areas has the speciﬁc pattern of intracortical and subcortical connections that a given ﬁeld makes. These patterns studied through various pathway tracing techniques that label the origins or terminations of speciﬁc pathways following the intracortical injection of various tracers. The areas that are innervated and the speciﬁc laminar patterns of their connections thus help deﬁne speciﬁc cortical areas.
The next criterion used to identify cortical areas utilizes physiological methods to describe the functional properties of their neurons. Typically microelectrode recording techniques are used to study the receptive ﬁeld properties of cells in a given area, which are then compared to other cortical areas. This approach has been most useful for the identiﬁcation of area MT in primates, whose neurons are nearly all selective for the direction of object motion. Recently, functional imaging techniques have allowed physiological investigations in the visual cortex of humans. These studies ﬁrst used positron emitted tomography (PET) and more recently functional magnetic resonance imaging (f MRI) has allowed the identiﬁcation of human cortical areas specialized for motion processing (MT), color processing (V4 or V8), or for the processing of faces or objects.
The last criterion used to study the organization of visual cortex is distinctive behavioral deﬁcits following localized lesions or transient inactivation. This criterion was ﬁrst used to identify occipital cortex as the locus of visual processing. Subsequent studies have revealed speciﬁc deﬁcits in motion or color processing following injury to area MT or ventral occipitotemporal cortex, respectively (Zeki 1990, 1991).
Although each of these criteria is useful for the identiﬁcation of visual cortical areas, most areas have only been characterized by one or two criteria. Furthermore, the results of diﬀerent studies have led to diﬀerent conclusions, so some areas are better established than others. Therefore, the described organization of visual cortex should be considered a work in progress rather than the ﬁnal word on visual cortex in primates.
2.2 Cortical Areas In Macaque Monkeys
Traditional views of the organization of visual cortex in monkeys and man were based on the idea of a primary visual area, V1 or area 17, which was surrounded by two belts containing visual association cortex and ‘visual psychic cortex.’ As additional anatomical and physiological criteria became utilized, it became clear that visual cortex occupies the whole of the occipital lobe and extends into the parietal, temporal, and frontal lobes. Figure 1 illustrates the location and extent of visual cortical areas of the macaque monkey displayed on an unfolded cortical map and on medial and lateral views of the cerebral hemisphere. As discussed above, many of these cortical areas, such as V1, V2, and MT are well characterized, appear in all primates, and are well accepted by the research community (Kaas 1997). Other areas are less well described and their precise borders and names remain somewhat controversial. As a whole, these visual areas occupy more than half of the surface area of the cerebral cortex of the macaque. They range in size from over 1100 mm2 for areas V1 and V2 to less than 40 mm2 for areas MSTd, MSTl, and V4t. Recent studies have extended this view of the organization of visual and visuo-motor areas in occipital parietal cortex (e.g., Lewis and Van Essen 2000). Speciﬁcally, this investigation used architectonic criteria to sub-divide area LIP into LIPd and LIPv, VIP into VIPl and VIPm, and to redeﬁne the extent of area PIP through the distinction of area LOP in its lateral margin.
Visual cortical areas vary in their functional specializations within perceptual and perceptual-motor categories. Overall, the visual cortex has been subdivided into dorsal and ventral processing streams (Unger-leider and Mishkin 1982). The dorsal stream begins in areas V1 and V2 and proceeds through area MT into parietal cortex. This functional stream, often called the ‘where’ stream, utilizes stereoscopic and direction speciﬁc signals in area MT to feed parietal areas that are involved in the planning of eye and limb movements to visual targets. The ventral stream also begins in areas V1 and V2 and proceeds through area V4 into temporal cortex. This functional stream, often called the ‘what’ stream, utilizes color, shape, motion, and binocular disparity information to identify objects and to enable their encoding into visual memory. Although the dorsal and ventral streams are often described as separate and parallel processing streams, there are many interconnections between them that emphasize the interdependence of these various visual functions.
3. Hierarchical Organization Of Visual Areas
Many of the visual areas described above have been identiﬁed primarily using their patterns of connections with other cortical areas. While earlier views of cortical pathways viewed each area as having a few inputs and a few outputs, current studies have greatly modiﬁed this view. Furthermore, cortical pathways diﬀer in their laminar pattern of inputs and origin of outputs. Pathways that terminate in layer 4 of a cortical area are generally recognized to be of the feed-forward type, characteristic of the thalamic input to primary visual cortex. In contrast, so-called feedback pathways tend to avoid layer 4 to terminate in layer 1 and 6. Finally, the lateral pathway has been observed which terminates in a column extending across all cortical layers. These diﬀerent laminar patterns of terminations have been correlated with diﬀerent laminar patterns of origin. Some feed-forward pathways arise exclusively from cells located in the upper cortical layers, while some feedback pathways arise exclusively from the infragranular layers. Many pathways arise from cells located in both supragranular and infragranular layers and they may be of the feed-forward, feedback, or lateral type.
A large number of cortical pathways have been identiﬁed interconnecting the 32 visual areas described in Fig. 1. As of 1990, more than 300 pathways had been recognized and since then additional areas and many additional pathways have been identiﬁed. By comparing the laminar patterns of inputs and outputs of these 32 areas it has been possible to derive an anatomical hierarchy of visual areas in the macaque (Felleman and Van Essen 1991, Maunsell and Van Essen 1983). In the hierarchy illustrated in Fig. 2, each visual area was assigned to a level based upon pairwise comparisons of each area with all other areas that it connects to. According to this scheme, nearly 90 percent of the cortical pathways can be unambiguously assigned as the feedforward, feedback, or lateral type. Most of the ambiguous connections are among visual areas making connections with frontal cortex. Although Felleman and Van Essen were able to derive a hierarchy based on the reported connections of a large number of visual areas, other investigators suggest that these data are not conclusive and thus it is not possible to derive a single cortical hierarchy (Hilgetag et al. 1996)
Recently, quantitative methods have been used to characterize the laminar patterns of origin for cortical pathways among the lower visual areas (Barone et al. 2000). This scheme introduces an anatomical distance metric between areas based on the proportion of supragranular cells in a given pathway. This analysis yields a hierarchy that in many ways is similar to the qualitative hierarchy described above, but diﬀers in the assignment of a few areas. Future work will extend these quantitative studies to additional visual areas.
4. Modular Organization And Processing Streams In Visual Cortex
4.1 Modular Organization Of Striate Cortex
A variety of anatomical and physiological evidence indicates that striate cortex is organized in modular fashion. The ﬁrst module identiﬁed was that of ocular dominance columns. In macaque monkeys, inputs from the two eyes are segregated in layer 4 into ~ 0.5 mm wide zones that appear like zebra stripes when viewed tangentially. These stripes can be identiﬁed using anatomical methods using tracer transport or through histochemical staining for cytochrome oxidase following reduction in activity in one eye through disease or lid suture.
Histochemical staining for cytochrome oxidase reveals an array of dense, metabolically active blobs separated by pale, interblobs (see Fig. 3a). Anatomical studies have revealed that these metabolically active blobs are located in the centers of the ocular dominance stripes for each eye, and they are repeated across the cortical surface. Physiological investigations indicate that cells located within the blobs tend to lack orientation selectivity and tend to be color selective. In contrast, cells located in the interblobs are orientation selective and tend to lack color selectivity (Livingstone and Hubel 1984). Furthermore, orientation selectivity is organized in a speciﬁc pattern such that the preferred orientation of cells progresses systematically around a non-oriented blob center. Studies of circuitry from the thalamus and within V1 indicate that cells in the blobs receive a direct input from the konicellular (K) cells of the LGN and receive input from the parvocellular and magnocellular streams via inputs from layer 4Cβ and 4B, respectively. The region of cortex containing one blob and the corresponding orientation ‘pin-wheel’ that surrounds it has been described as a modular hypercolumn that contains all the processing elements for a small patch of visual space.
4.2 Modular Organization Of Areas V2 And V4
Area V2 also contains a modular organization that can be revealed through staining for cytochrome oxidase. Unlike the punctate blob-like staining seen in V1, area V2 is characterized by alternating thick and thin stripes of high metabolic activity separated by pale interstripes (see Fig. 3b). Furthermore, these V2 compartments have speciﬁc patterns of connections indicating that the parallel processing streams seen in V1 continue into V2. Thus, the magnocellular stream exits V1 from cells in layer 4B to terminate within the cytochrome oxidase thick stripes. Similarly, cells in the V1 blobs terminate in V2 thin stripes, while cell in the V1 interblobs terminate in V2 interstripes. Physiological investigations of receptive ﬁeld properties using microelectrode and optical recording techniques have indicated that these diﬀerent V2 compartments contain neurons with diﬀerent receptive ﬁeld properties. Thin stripes contain a high proportion of color selective cells that lack orientation selectivity, while thick stripes and pale stripes contain a regular organization of orientation selectivity. These three V2 compartments are repeated approximately 17 times within V2 of each hemisphere and thus each can be viewed as a modular subunit that contains the processing machinery for a small patch of visual space.
The parallel cortical processing streams that begin in V1 are maintained in V2 and continue through the projections to higher cortical areas. The magnocellular stream, exits V2 to continue on to areas V3 and MT. Both of these areas have a high proportion of cells selective for the speed and direction of object motion. These areas provide input to several parietal areas of the ‘where’ functional stream. The blob and interblob streams of V1 remain largely segregated in their projections to V2 thin stripes and interstripe compartments, respectively. Both of these compartments have dense projections to area V4 (DeYoe and Van Essen 1985, DeYoe et al. 1994, Felleman et al. 1997). V4 does not contain a modular organization that can be revealed through cytochrome oxidase histochemistry. However, V2 thin stripes and interstripes project to segregated regions within V4 (Xiao et al. 1999). Furthermore, optical recording indicates that color selective and orientation selective cells are clustered in V4 in a pattern that is reminiscent of the segregated inputs from V2 cytochrome oxidase compartments (Ghose and Ts’o 1997).
5. Topographic Organization Of Human Visual Cortex
Functional magnetic resonance imaging (f MRI) has been used to identify and map the representation of visual space in the occipital cortex of man (DeYoe et al. 1996, Sereno et al. 1995). The general strategy used in these studies was to present counter phased checkerboard stimuli at the vertical and horizontal meridia and diﬀerent eccentricities, and to record the voxels that show maximum signals that are correlated with the visual stimulus. Since the visual cortex is highly convoluted in man, the visualization of activated voxels is facilitated greatly by mapping onto an unfolded cortical map. Using this approach topographically organized visual areas V1, V2, V3, VP, and V3A have been identiﬁed in human occipital cortex (see Fig. 4). As in macaque monkeys, V1 is a large area localized to the calcarine sulcus and its outer border is formed by the representation of the vertical meridian. Area V1 corresponds to area 17 of Brodmann or area OC of Von Economo and Koskinas. Area V2 adjoins V1 at the representation of the vertical meridan and nearly totally surrounds it, except perhaps at the anterior tip of the calcarine sulcus. Area V2 is split along the representation of the horizontal meridian and forms dorsal and ventral halves that represent the lower and upper ﬁelds, respectively. Area V2 only occupies the posterior portion of the architectonically deﬁned area 18 of Brodmann. Thus, cytoarchitecture is insuﬃcient and perhaps misleading, in the identiﬁcation of area V2. Areas V3 and VP border dorsal and ventral V2, respectively, at the representation of the vertical meridian. Unlike macaque monkeys in which these areas are narrow, human V3 and VP are as large as area V2. Based on position, areas V2, V3, and VP together occupy the previously deﬁned architectonic ﬁeld, area 18. Area V3A is located anterior to V3 in dorsal extrastriate cortex and unlike the adjacent areas V2d and V3, contains a representation of the full visual ﬁeld. In the ventral cortex, an additional area has been localized using topographic criteria. This area, V4v, is located anterior to area VP and adjoins VP at the representation of the vertical meridian. This region only contains a representation of the upper visual ﬁeld and no corresponding lower ﬁeld rep- resentation has been found in dorsal cortex, which would correspond to area V4d. It remains unclear whether this reﬂects a true dorsoventral diﬀerence in cortical processing, or whether area V4d has yet to be found. Finally, the human homologue of the MT complex (MT + ) has been localized in the lateral occipital sulcus, anterior to area V3A. These studies thus identify a number of topographically organized areas in humans that appear homologous to those seen in macaque monkeys.
The visual system of the brain consists of a large number of subcortical and cortical components that contribute to different aspects of visual processing. Visual processing begins in the retina where specialized
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