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The occipital lobe, at the back of the brain, is one of the four lobes into which each cerebral hemisphere is conventionally divided. The lobes are deﬁned purely by anatomical landmarks, not functionally. However, the occipital lobe has a certain functional unity in that, so far as we know, it is entirely concerned with processing visual information. How this processing works at the neural level, and how it is organized structurally, provides some insights about the mechanisms of visual perception. Much of this knowledge comes from studying the occipital lobe of the macaque, but the human occipital lobe appears generally closely similar. It maintains a basic spatial organization of visual information, but also extracts and organizes visual attributes such as local shape, motion, color, and depth. To understand this organization, it is important to appreciate that processing goes on in the cerebral cortex, a thin, folded sheet of tissue across which a set of distinct and specialized visual areas are laid out; but also that the cortex is layered, with different layers serving as the location of inputs and outputs of information for a particular area.
1. The Retinotopic Projection To Area V1
The ﬁbers delivering visual information to the cerebral cortex terminate in a region around the occipital pole of each hemisphere, known as striate cortex, primary visual cortex, or area V1. Each incoming ﬁber, and each individual neuron in V1, carries information about a speciﬁc small region of the visual ﬁeld (its receptive ﬁeld). These are arranged in an orderly way across the cortex, so that neurons with neighboring receptive ﬁelds are close together and overall, area V1 in each hemisphere is laid out like a spatial map of the opposite half of the ﬁeld of view. The ﬁeld is split, with the right half-ﬁeld represented in the left hemisphere, and vice versa. In this map, the vertical midline of the ﬁeld is represented furthest back, near the occipital pole, and moving forward across the cortex corresponds to moving horizontally outwards in the ﬁeld of view. This orderly ‘retinotopic’ mapping must be functionally important, presumably because it allows neurons serving nearby ﬁeld locations to interact readily. This means that simply organized, local connections between cells can serve to link the features that form spatially connected objects.
The scale of the retinotopic map is not uniform. Much more information comes from the densely packed cells of the central retina than from an equivalent area in the peripheral ﬁeld, and this is reﬂected in a correspondingly larger area of cortex devoted to the central region. In this way, cortical organization provides the greater processing power needed for the ﬁne detail and spatial precision of central vision.
2. Binocular Organization
An important function of area V1 is that it is the ﬁrst point in the visual pathway at which information from the two eyes can be integrated. The ﬁbers from each eye are segregated into alternating bands as they arrive in layer 4 of cortical area V1 (see Sect. 4). The ﬁrst real interaction between left-and right-eye signals occurs when these converge on cells outside layer 4. This interaction allows both eyes to contribute to a uniﬁed visual representation of the world. It also allows neurons in V1 and beyond to register the disparity between the two eyes’ views, which serves as the basis for stereoscopic depth perception.
3. Stimulus Selectivity Of Cortical Neurons
The number of neurons in area V1 is at least a hundred-fold greater than the number of input ﬁbers. This enormous information-processing resource means that V1 does not simply map the visual input: it also transforms it. Each local region of the visual ﬁeld is represented by an array of neurons, with each neuron responding selectively to a particular type of stimulus. Most V1 neurons respond to contours, and are selective in several, if not all, of the following respects:
strongest response to a preferred orientation of the contour, with response falling off as the angle is further from this optimum (‘orientation tuning’);
a preferred type of contour: light bar, dark bar, or edge;
a preferred spatial scale, so that the cell may respond either to the broad pattern of light and dark, or to ﬁne detail;
stronger response to one direction of motion than for the opposite direction;
differences in binocular balance: some cells respond predominantly to the right eye, some to the left, and some only give their best response when both eyes are activated together. In the latter, binocular group, cells are selective for stereoscopic depth, i.e., they respond best to objects that are either nearer, farther, or close to the distance on which the eyes are converged; and
color, with one range of wavelengths preferentially activating the cell and another inhibiting it (sometimes combined with an enhanced response to color contrast).
This array of differently tuned neurons means that the cortex can represent all these diverse properties according to which particular pattern of neurons is active (a population-based ‘place code’).
4. Columnar Organization
Retinotopic mapping (Sect. 2) implies that within a small region of V1, all the cells deal with a local region of the ﬁeld of view. The cortex also has a ﬁnergrain spatial organization, reﬂecting the differences in tuning described in Sect. 3. Connections between neurons are predominantly vertical (i.e., perpendicular to the cortical surface). Presumably as a result of these connections, neurons in a vertical column have similar response properties. In particular, there are columns of cells responding to the same orientation, and columns within which a particular eye predominates. From column to column, there is also a spatial ordering; in a horizontal track across V1, the preferred orientation changes in an orderly sequence. As with the retinotopic organization, this order means that a regular, local pattern of connectivity between cells creates communication between cells responding to contours with the same or similar orientation. Presumably, this gives a structural basis for linking together the segments of a contour that extends across many cells’ receptive ﬁelds, and so helps to establish the neural representations of shapes and objects.
Thus, at the ﬁne scale, cortical layout is not purely a retinotopic map. However, both retinotopic and orientation-based mapping mean that orderly spatial connectivity between cells can establish communication between cells responding to similar stimuli. The columnar organization has been described here for V1, but it is believed to be a general principle that applies also to the extra-striate areas described in Sect. 6.
5. Interleaved Pathways Through V1 And V2
The visual pathway leading to V1 is in fact two subpathways, and this division runs through processing in V1 and beyond. One group of cells, the magno cells, have large receptive ﬁelds, fast responses, and do not respond selectively to particular colors of light. The parvo cells, in contrast, respond less well to rapidly changing stimuli, but better to ﬁne spatial detail, and are wavelength-selective (e.g., activated by one range of colors and inhibited by others). These two classes of cell appear to separate the information useful for detecting motion on the one hand, from that required for ﬁne detail and for color vision on the other. Correspondingly, the perception of motion and of color seems to work largely independently of each other.
This division is maintained in the cortex. In V1, the ﬁbers of magno cells terminate in a distinct layer (4B) containing many motion-selective cells. Within the parvo input, color-speciﬁc information is also anatomically separated into a regular pattern of ‘blobs’ that can be revealed anatomically by staining cortical tissue. Within these blobs, cortical neurons show color-related responses and are generally not orientation-selective. Thus, in the ﬁne structure of area V1, three different kinds of processing are segregated but interleaved: motion information in layer 4B, color information in the blobs, and spatial pattern (orientation) in the ‘interblob’ regions.
V1 sends information to a neighboring area, V2, which forms a second retinotopic map. The three types of visual information interleaved in V1 are kept segregated in V2, into three parallel sets of stripes that send their output to different extra-striate areas (Sect. 6). Thus, apparently a major role of areas V1 and V2 is to marshal different kinds of information into separate cortical processing streams.
6. Specialized Extra-Striate Areas
V1 and V2 are only two of what are now recognized to be a large number of distinct cortical visual areas forming an interconnected hierarchy. Most of the occipital lobe can now be divided into a mosaic of areas, distinguished by:
mapping: each area (at least at the lower levels of the hierarchy) forms a distinct, complete retinotopic map of the visual ﬁeld;
connectivity: there is an orderly set of projection ﬁbers connecting one area to another and preserving the retinotopic mapping. Connections with ‘higher’ and ‘lower’ levels in the hierarchy are made by different cortical layers, which allow the hierarchical structure to be deﬁned; and
function: each area has a characteristic balance in the type of stimulus selectivity found in its neurons (e.g., color selectivity common and directional motion selectivity rare, or vice versa), leading to the idea that different areas have distinct functional roles in vision. This ﬁnding from macaque single-neuron recording studies is supported by functional brain imaging in humans. Imaging reveals a set of areas, each of which is activated by speciﬁc visual properties (e.g., one area by color differences, a different area by coherent motion in the visual ﬁeld), and is also most active when the subject performs speciﬁc visual tasks (e.g., identifying colors). This ﬁts with ﬁndings in some patients with brain lesions, who show a deﬁcit limited to one speciﬁc aspect of vision (e.g., a loss of just color or motion processing).
The main areas that can be identiﬁed as retinotopic maps have been labeled as V3, V3A, V4 and V5. Together they form a third tier (or possibly, third and fourth tiers) of a processing hierarchy above V1 and V2. Area V5, also known as MT, lies on the boundaries of the occipital and temporal lobes in the human brain. V5 has been especially intensively investigated because it has a very well deﬁned specialization for processing motion information. At the same level, but complementary in its specialization, is area V4, which has the highest proportion of color-selective neurons, a type of selectivity almost absent in V5. V4 also shows processing of more complex aspects of spatial pattern than those found in V1 and V2 neurons.
Areas V4 and V5 represent a further stage in the segregated streams of processing found in V1 and V2 (see Sect. 5), with V5 as the destination of the motion information derived from layer 4B of area V1, and V4 receiving the color and form information derived from the blobs and interblob regions. This division is an aspect of a broader division between ‘dorsal’ and ‘ventral’ cortical streams. The dorsal stream consists of areas carrying motion and spatial information to the parietal lobe, where it provides the visual basis for controlling actions. The ‘ventral’ stream carries pattern and color information required for the identiﬁcation of objects in the temporal lobe.
Functional anatomy, then, supports a view that the visual process is based on distinct modules with different functions; in particular, that there is a broad separation of the modules which tell us ‘what’ object we are looking at, from those that tell us ‘where’ or ‘how’ we can act upon it. In both streams, the higher levels of the hierarchy beyond the occipital lobe contain multiple, distinct areas, in which the retinotopic organization has generally been lost. It seems to be a general principle that as visual information is progressively transformed and elaborated in the brain, the less relevant is retinal location for the way in which it is represented.
While it is clear that the visual areas can be ordered into a hierarchy, the ﬂow of information is not simply bottom-up. Almost every connection from a lower to a higher area has a corresponding connection in a reverse direction. Thus, in the neural structure, there exists the potential that low-level processing, e.g., in V1 and V2, does not run simply automatically, but is controlled and directed by top-down processes. There is also the potential for iterative loops, by which information is exchanged between lower and higher areas and progressively reﬁned. And although the different modules have distinct functions, there are rich interconnections between them at the same level, including connections between dorsal and ventral streams. Occipital cortex is a modular system, but one in which the modules closely interact to produce integrated visual perception.
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