Extrastriate Visual Cortex

What is extrastriate visual cortex?

The term “extrastriate” refers to all visually responsive cortex other than primary visual (striate) cortex, and does not receive strong direct projections from the lateral geniculate nucleus (LGN) (see Chapter 30 ). A central hypothesis in studying extrastriate cortex is that it is composed of discrete cortical areas which can be identified by histology, retinotopic mapping, patterns of connections with other areas, and the unique contribution each area makes to visual processing. Extrastriate areas that receive strong inputs directly from primary visual cortex can be thought of as a lower tier within a processing hierarchy, compared to a higher tier of areas that receive their main visual inputs from relays through other extrastriate areas. The more relays between an extrastriate area and primary visual cortex, the higher its position in the processing hierarchy and, generally speaking, the more complex is its visual processing. Superimposed on this hierarchy is the concept of parallel streams of visual areas that are more strongly interconnected with each other than with areas of different streams, and that are all concerned with some common aspect of visual processing, such as object recognition (ventral stream) or object location (dorsal stream). At least 25 cortical areas predominately or exclusively engaged in vision have been described in monkeys; the number of visual areas identified in humans is rapidly approaching, and will probably surpass, that total ( Fig. 31.1 , Box 31.1 and see also Box 31.6 ). ^,

Figure 31.1, Human ( A , B , C ) and monkey ( D ) visual cortical areas. Visual cortical areas are shown on a schematic diagram of the human brain from the posterior aspect (A) and mid-sagittal plane (B). Many of the areas cannot be appreciated on the surface view of the brain, and a flat map of the human (C) visual cortical areas is also shown. Areas involved in vision are colored and labeled. The depths of the sulci are shown in black and gyri in white to give a perspective to location relative to the sulcal patterns. For comparison, a flat map of visual areas in the macaque monkey is shown in (D). Homologous areas, insofar as they can be identified, are given the same color and nomenclature, but many more areas have been studied in monkey brain than in human. In the occipital cortex of humans, there are: the second visual area (V2); the third visual area, broken into a dorsal (V3d) and a posterior half (VP); V3 anterior (V3A); the ventral (V4v) and dorsal (V4d) subdivisions of V4; and the sixth, seventh and eighth visual areas (V6, V7 and V8). Visual responsive areas in the parietal lobes include: the middle temporal area (MT); the medial superior temporal area (MST); the lateral occipital area (LO); and the extrastriate body area (EBA). Several visual areas in the intraparietal sulcus have been recently studied and named (IPS1, IPS2, IPS3, IPS4). In the occipitotemporal cortex are found the fusiform face area (FFA) and the parahippocampal place area (PPA). Monkey extrastriate areas shown in (D) include, in the temporal lobe: the posterior inferotemporal area, with dorsal (PITd) and ventral (PITv) subdivisions; the central inferotemporal area, with dorsal (CITd) and ventral (CITv) subdivisions; the anterior inferotemporal area, with dorsal (AITd) and ventral (AITv) subdivisions; the superior temporal polysensory area, with anterior (STPa) and posterior (STPp) subdivisions; the floor of the superior temporal sulcus (FST); and temporal areas F (TF) and H (TH). In the parietal lobe are found: the medial and superior temporal area, with dorsal (MSTd) and lateral (MSTl) subdivisions; the parieto-occipital area (PO); the posterior intraparietal area (PIP); the lateral intraparietal area (LIP); the ventral intraparietal area (VIP); the medial intraparietal area (MIP); the medial dorsal parietal area (MDP); the dorsal prelunate area (DP); and Brodmann's area 7a (7a).

Box 31.1

Organization of extrastriate visual areas

“Lower tier” extrastriate visual areas receive predominantly from primary visual cortex.

“Higher tier” extrastriate areas receive less from primary visual cortex, and more from both lower and higher tier areas and generally have more complex visual processing.

Lower Tier Areas

Higher Tier Ventral Stream Areas (Object Recognition)

Higher Tier Dorsal Stream Areas (Motion and Location of Objects)

Box 31.6

Summary of major points

Extrastriate visual cortex contains a constellation of richly interconnected areas comprising large portions of the occipital, parietal, and temporal lobes and contributes to a diverse array of visual information-processing tasks
Extrastriate visual areas are identified by a combination of features including receptive field mapping, histology, patterns of connections with other brain regions, and functional specificity
In both monkeys and humans a dorsal visual pathway dealing with object location extends through V2, V3, MT, PO/V6, and into parietal cortex
In both monkeys and humans a ventral pathway dealing with object recognition extends through V2, V4, and into temporal cortex

Methods used to identify extrastriate areas in monkeys and humans

Identifying cortical areas and their borders is neither straightforward nor completely resolved, and different schemata for parceling extrastriate cortex into areas exist, depending upon how the criteria of histology, retinotopy, connectivity, and physiology are weighted. There is most consensus regarding areas lower in the hierarchy, nearer to V1, and most divergence of opinion for higher-level areas in the temporal and parietal lobes.

Histology

Cortical areas can be expected to have a uniform and distinct histology, such that anatomical borders can be recognized by changes in the laminar organization, density, or size of neurons, myelinated fibers, enzyme activity, neurotransmitter levels and their receptors, and so forth. However, not all borders can be recognized with all stains. Early parcellation schemes based on classical histological techniques are still in wide use, although containing many known inaccuracies. Based on Nissl staining of neuronal cell bodies (cytoarchitectonics), Brodmann divided most of extrastriate occipital cortex, in both human and monkeys, into only two areas, 18 and 19, each of which includes multiple areas defined by other criteria such as retinotopy and connectivity. Other histologic stains not available to Brodmann, such as cytochrome oxidase (CO; a metabolic enzyme that serves as a marker for long-term activity levels of neurons) histochemistry, reveal the borders of the many visual areas within the cortical territory occupied by Brodmann's areas 18 and 19. Even with improved staining techniques, however, it is likely that some areal boundaries may escape detection by any histologic methods.

Retinotopic mapping

Each visual area comprises a single map of receptive field locations, with V1 demonstrating the most complete map of visual space (see Chapter 30 ). These maps can be studied with electrophysiologic recordings in monkeys and with functional magnetic resonance imaging (fMRI) or positron emission tomography (PET) in humans. However, none of the extrastriate maps are as regular as in striate cortex (V1). Extrastriate areas have larger receptive fields and more scatter in the position of each receptive field, resulting in “cruder” maps. Between extrastriate areas, there is generally a smooth transition of receptive field locations. Contiguous visual areas can have “mirror-image” representations, where the topography of the visual field map is reversed at the border of the two contiguous areas. When a mirror-image area like V1 borders a non-mirror-image representation like V2, the vertical meridian representation forms the border of the two areas and allows continuity of receptive field locations across the border. Visual field sign (mirror image or non-mirror image) can be computed from the local gradient of receptive field position, and can be used to infer borders of extrastriate areas. Extrastriate maps also tend to show breaks in the retinotopy (split representations) along the horizontal meridian representation ( Fig. 31.2 ). These breaks often form the borders of visual areas, allowing smooth transitions of receptive field locations from one area to the next, at the expense, in this case, of splitting the receptive field map within a single area. The splitting of maps within a single area and the continuity of receptive field positions at the borders of different areas make it difficult to identify areal boundaries on the basis of receptive field maps alone.

Figure 31.2, Maps of human visual areas, generated by functional imaging using visual stimuli that vary in retinotopic locations. ( A ) Schematic view of the retinotopy in V1, V2, and V3 (including upper and lower visual fields). The vertical (dashed line) and horizontal (solid line) meridians and foveal representations (stars) are shown. The numbers represent corresponding retinotopic locations in visual space in the three areas. The maps in (B) and (C) are flattened views of a right hemisphere, as if the cortex had been removed from the brainstem and flattened. Dorsal is to the top of each map and ventral is towards the bottom. In making the reconstruction, a cut (visible to the left in B and C) was introduced along the horizontal meridian representation of V1 to relieve the curvature of the cortical mantle and reduce distortion. ( B ) A map of cortical response plotted as a function of the polar angle of an annular stimulus (see text). The dashed and solid lines show the representations of the vertical and horizontal meridians, which form the boundaries of several cortical areas. The asterisks mark the location of the foveal representation. ( C ) A map of the local visual field sign, where yellow is a mirror image representation, like V1, and blue is a non-mirror image representation like V2. The locations and extent of V1, V2, V3d/VP, V3A, and V4v can be identified in this fashion.

Connection patterns

Each visual area is interconnected with a subset of the other visual areas and subcortical visual structures, giving it a unique connectional signature. Neuroanatomic tracing methods in monkeys involve injecting tracers into one area either to label cells projecting to the injected area or to label axons projecting from the injected area to other areas. Tracing connectivity in human visual cortex was classically limited to postmortem staining for degenerating axons after cortical lesions. Two new techniques suitable for studying connectivity in human extrastriate areas have recently been developed. Connections can be traced between cortical areas with an MRI technique that analyses the anisotropy of diffusion of water within oriented bundles of axons (diffusion tensor analysis; DTI). ^, Currently, DTI is not, and may never be, sensitive enough to demonstrate all connections among visual areas, ^, but it has proved useful in demonstrating pathological changes. ^, Connectivity can also be inferred by imaging a rise in activity of target areas following noninvasive activation of a projecting cortical area with transcranial magnetic stimulation (TCMS). TCMS induces a brief electrical current inside cortical neurons by external application of a large, rapidly fluctuating magnetic field, and, with proper stimulation paradigms, can either activate or inactivate cortical areas. It is important to note that connections are between retinotopically corresponding points in the visual field representation of the interconnected areas. Thus connectivity information has been used to infer receptive field maps. This is especially true for interhemispheric connections, which, especially for areas in the lower tier of the processing hierarchy, are concentrated in representations of the vertical meridian.

Functional specificity

The functional properties of neurons are often characteristic of each extrastriate area. A range of visual stimuli, selected to emphasize, for example, motion versus form and color information, can be used to selectively activate specific areas. These studies, like those detailed above on retinotopy, can be performed in humans using functional imaging techniques (fMRI, PET) or in monkeys using imaging or electrophysiological techniques. In vivo imaging in humans can reveal functional deficits due to cortical lesions caused by stroke, and localize the site of the damage to particular cortical areas. Repetitive TCMS can be used to reversibly inactivate extrastriate cortical areas in humans, allowing deficits in function to be associated with the inactivated area. ^,

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