The Thalamus and Cerebral Cortex (Integrative Systems) Part 3

Localization of Function Within the Cerebral Cortex

Our understanding of cortical localization is derived from two main sources: basic laboratory investigations and clinical studies. For each of the functional regions that will be considered in the following sections, the discussion begins with a brief analysis of the basic functional properties of that region and then uses information derived from clinical studies to evaluate the region’s overall functions.

The Parietal Lobe

The principal unit of organization is the cortical column (Fig. 26-11). Within each column, neurons located in each of the layers within a given column respond best to a single class of receptors, and all of the neurons respond to stimulation applied to the same local region of the body surface (Fig. 26-11). Adjoining columns representing the same body surface have different physiological properties, such as being rapidly adapting or slowly adapting to sensory stimulation.

 Columnar organization of somatosensory cortex. Diagram illustrates that different fingers (D1-D3) are represented on adjoining regions of somatosensory cortex. Within the area represented by each finger, there are alternating columns of neurons that are rapidly adapting (green) and slowly adapting (red). The inputs for each type of receptor for each digit are organized into separate columns.


FIGURE 26-11 Columnar organization of somatosensory cortex. Diagram illustrates that different fingers (D1-D3) are represented on adjoining regions of somatosensory cortex. Within the area represented by each finger, there are alternating columns of neurons that are rapidly adapting (green) and slowly adapting (red). The inputs for each type of receptor for each digit are organized into separate columns.

Other characteristics concerning somatosensory neurons should also be noted. For example, as indicated earlier, different regions of the somatosensory cortex may respond to different modalities of stimulation, but they also have a preference for responding to a single type of stimulus. Like other sensory modalities, certain classes of somatosensory neurons appear to respond to complex aspects of a given form of stimulation. For example, although one class of neurons may respond to any form of tactile stimulation applied to a certain region of the body surface, other more complex neurons may respond to the orientation and direction of stimulation. Such response properties suggest that these neurons have larger receptor fields and are capable of integrating the activities of other more basic neurons located in a slightly different region of somatosensory cortex. In fact, it is suggested that the neurons that respond to the more simple aspects of stimulation lie in the region of area 3 and the neurons that respond to the more complex properties of stimulation lie in area 2. It is the increased complexity of the response properties of these neurons that likely provides the neuronal basis for spatial perception that is essential for recognition of three-dimensional objects (i.e., stereognosis). It is important to note that such perceptual discriminations are essential for complex skilled movements to be made.

Effects of Lesions

Complex sensory discriminations require the presence of an intact somatosensory cortex of the parietal lobe, which is organized along lines of complexities of hierarchies of neurons that integrate different kinds of sensory stimuli. Thus, it is not difficult to appreciate how damage to different cell groups in the somatosensory cortex can produce different types of disorders. Lesions of the primary somatosensory cortex produce paresthesias consisting of abnormal tingling sensations and numbness on the side of the body opposite to the lesion. In addition, there is loss of ability to characterize the specific types of stimuli impinging on the individual as well as an inability to localize or evaluate the intensity of the stimulus.

Other parts of the parietal and frontal lobes, which receive inputs from sensory regions of the cortex, require the engagement of neuronal networks processing the appropriate memories to function properly. The arousal of such memories represents the basis of understanding for carrying out specific tasks and is referred to as "gnosis." When this process is disrupted, several different kinds of disorders occur, including agnosia, apraxia, and aphasia.

Regions Associated With Visual Functions

In fact, some processing of visual information actually involves adjoining regions of the temporal lobe as well.As we pointed out for the somatosensory systems, the cortical column plays a very important role in the processing of sensory information and in providing the basis for the perception of somatosensory signals. Likewise, for the visual system, the cortical column is critical for the processing of visual signals and for the establishment of form perception.

Projection Patterns From the Lateral Geniculate Nucleus

Both the retina and lateral geniculate nucleus contain neurons that have concentric receptor fields with surround inhibition. Some of the cells are "on-center" and "off-sur-round," whereas others are "off-center" and "on-surround." Each of these cell groups gives rise to distinct pathways that reach the primary visual cortex (referred to as VI; the secondary visual area is called V2). Several of the pathways are associated with magnocellular neurons, and other pathways are associated with parvocellular neurons of the lateral geniculate nucleus. The pathways arising from mag-nocellular neurons are associated with identifying the location of the visual image, whereas the parvocellular pathway is associated with form and color.

When a special stain (cytochrome oxidase) for identifying mitochondrial enzymes is applied to this region of cortex, the visual cortex reveals two types of staining patterns. One pattern includes intense staining of small, ovoid areas called blob areas. The surrounding areas stain less intensely and are called interblob areas (Fig. 26-12).

Retinal projections to the visual (striate) cortex. (A) Projections of the magnocellular pathway, which is associated with identification of the visual image, to upper parts of layer IV. (B) Parvocellular pathway, which is associated with form and color, to lower parts of layer IV. LGN = lateral geniculate nucleus; IVB = layer IVB of visual cortex; IVC = layer IVC of visual cortex.

FIGURE 26-13 Retinal projections to the visual (striate) cortex. (A) Projections of the magnocellular pathway, which is associated with identification of the visual image, to upper parts of layer IV. (B) Parvocellular pathway, which is associated with form and color, to lower parts of layer IV. LGN = lateral geniculate nucleus; IVB = layer IVB of visual cortex; IVC = layer IVC of visual cortex.

The important point to note is that color processing takes place in the blob areas, whereas the interblob areas contain orientation columns (i.e., vertically oriented columns that include neurons that respond to bars of lines having a specific spatial orientation). Thus, one pathway arising from the parvocellular portions of the lateral geniculate nucleus projects to blob regions of V1 and terminates in specific parts of layer IV (Fig. 26-13).1 This pathway is associated with the processing of color vision. But completion of the process for color vision and form requires the activation of additional pathways and regions of the visual cortex. These include a pathway with connections from V1 to V2 (area 18), then to V4 (area 19), and ultimately to the inferotemporal cortex (Fig. 26-14). This pathway is specifically concerned with color perception and is sometimes called the parvocellular blob stream pathway. A second pathway originating from the parvocellular region of the lateral geniculate also projects to V1, which in turn, projects to V2, then to V4, and finally to the inferotemporal cortex. This pathway, however, is concerned with the processing of form perception (i.e., the outline and orientation of images critical to form perception) and is sometimes referred to as the parvocel-lular interblob pathway. A third pathway originates from the magnocellular region of the lateral geniculate nucleus and projects through the interblob region, terminating, in part, within layer IV of V1. From this region, fibers project to V2, V3, and the middle temporal gyrus (V5 [Fig. 26-14]). This pathway is associated with neurons that detect directional movement of objects. In addition, this pathway also projects to an adjoining region of the parietal lobe, called the medial superior temporal gyrus. Neurons in this region respond to a wider variety of motion, which is linear, circular, or spatial in direction (Fig. 26-14). Overall, this pathway is referred to as the magnocellular stream pathway. It is not known to what extent each of these systems may interact with each other in producing visual perception of objects. It has been suggested that the processing of color information may be independent of the systems mediating depth, form, and general figure-ground relationships. On the other hand, one parvocellular pathway (the parvocellular interblob pathway) may use such stimuli as orientation cues to produce fine visual discriminations, whereas the magnocel-lular stream pathway may use the same information for generating an impression of the location of the object in space.

Analysis of Form Perception

Several of the basic questions that have confronted investigators are: (1) how do neurons in the visual cortex respond to visual stimuli, and (2) how do such changes relate to our capacity to recognize objects? Answers to these basic questions were provided following the discovery of different classes of neurons in the visual cortex that respond, not to a point of light (as do geniculate and retinal ganglion cells), but to bars (linear properties) of light and their orientations.

Processing of visual information outside the visual cortex. (A) Fibers mediating visual information pass both dorsally and ventrally from the primary visual cortex to both the parietal and temporal lobes. (B) The dorsal pathway also supplies neurons of the temporal and neighboring parietal cortices (labeled as MT [middle temporal gyrus] and MST [medial superior temporal gyrus]) that respond to specific directional properties of movement of objects. The ventral pathway supplies neurons of the inferotemporal (IT) aspect of the temporal lobe, which respond to faces.

FIGURE 26-14 Processing of visual information outside the visual cortex. (A) Fibers mediating visual information pass both dorsally and ventrally from the primary visual cortex to both the parietal and temporal lobes. (B) The dorsal pathway also supplies neurons of the temporal and neighboring parietal cortices (labeled as MT [middle temporal gyrus] and MST [medial superior temporal gyrus]) that respond to specific directional properties of movement of objects. The ventral pathway supplies neurons of the inferotemporal (IT) aspect of the temporal lobe, which respond to faces.

One class of cell is called a simple cell. It is located near the region of sublayer 4C0. Within this sublayer, stellate cells, which have circular receptive fields, receive inputs from the lateral geniculate nucleus. The simple cells receive converging inputs from groups of stellate cells. Thus, the receptive fields of the simple cells are considerably larger than stellate cells. Because the inputs to the simple cell from the stellate cells are arranged at slightly different retinal positions, the integrated response of the simple cell is to a group of points of light (i.e., a beam of light). The key feature here is that the simple cells are rectangular in form and have specific excitatory foci and inhibitory surrounds. For example, in the illustration shown in Figure 26-15, a simple cell responds most effectively to a bar of light that is presented in a vertical position. This occurs because the neuron from which the recording was taken has a receptive field whose excitatory zone is oriented vertically, with parallel inhibitory zones located on either side of the excitatory zone. Thus, when the bar of light is reoriented from a vertical to a horizontal position, the neuron ceases to discharge. Note that the cell could also respond to a variety of changes in the vertical position of the bar of light (Fig. 26-15). In this manner, different groups of cells can respond to various orientations of light. Ultimately, any possible orientation of light will excite some neurons in the visual cortex.

Another class of cells has been identified that responds to somewhat different characteristics of light beams. Complex cells differ from simple cells in that complex cells have larger receptive fields. In addition, the "on" and "off" zones are not clearly defined. The complex cell responds to a specific orientation of the beam of light, but the precise position of the image within the receptive field is not critical (Fig. 26-16). In addition, movement of the image across the receptive field is also an effective stimulus for exciting this (complex) cell. Complex cells, which receive inputs from simple cells, are located in other layers of the primary visual cortex than are simple cells (mainly in layers II, III, V, and VI).

Orientation-sensitive simple cell. Illustration demonstrates that a simple cell in the primary visual cortex responds maximally to a bar of light oriented approximately 45° to the vertical. Bars of light oriented differently evoke a much weaker neuronal response, especially if the orientation is opposite of that which evokes a maximal response. When spots of light are presented, the response of the cortical neuron is much weaker and diffuses light. The basic concept involves the notion of a convergence of similar center-surround organizations that are simultaneously excited when light falls along a straight line in the retina and, thus, strikes the receptive fields of these cells. These cells then converge upon a single cell in the visual cortex, thereby establishing an excitatory region that is elongated.

FIGURE 26-15 Orientation-sensitive simple cell. Illustration demonstrates that a simple cell in the primary visual cortex responds maximally to a bar of light oriented approximately 45° to the vertical. Bars of light oriented differently evoke a much weaker neuronal response, especially if the orientation is opposite of that which evokes a maximal response. When spots of light are presented, the response of the cortical neuron is much weaker and diffuses light. The basic concept involves the notion of a convergence of similar center-surround organizations that are simultaneously excited when light falls along a straight line in the retina and, thus, strikes the receptive fields of these cells. These cells then converge upon a single cell in the visual cortex, thereby establishing an excitatory region that is elongated.

The perception of edges, which provides the basis for form perception, is achieved by the combined actions of the simple and complex cells. Recall that the simple cell will respond to a beam of light within a specific receptive field, whereas the complex cell will respond to the same orientation of the beam but will extend to different receptive fields. If one’s eye is focused on an edge of a line or figure, then certain simple and complex cells will respond to that edge. If the eye then shifts to a different aspect of the figure, another group of simple cells associated with the new receptive field will discharge, but the complex cells that responded to the first aspect of the figure may continue to discharge even though the focus is shifted slightly to another aspect of the figure. In this manner, the complex cell is capable of recognizing a certain aspect of the image at any point within its receptive field. Therefore, the combined actions of the various simple cells coupled with those of the complex cells likely provide the neuronal basis for perceiving edges, borders, boundaries, and contrasts.

Other evidence suggests that there is even a third type of cell, which is a cell that responds like a complex cell in that it increases its firing frequency as the bar (held in a certain orientation) is extended. However, these cells differ from other complex cells because, as the beam of light is extended beyond a certain length, the neuron ceases to discharge. This type of variant of a complex cell is referred to as end-stopped. End-stopped cells are believed to receive inputs from groups of complex cells and possibly signal the length of a line of an object as well as its borders and curvatures. Thus, when viewed in its totality, a general principle appears to emerge from our analysis of the visual system. This principle is that, as the visual signals pass to higher levels within the visual pathway, the level of complexity governing the discharge patterns of the neurons increases. The levels of complexity are heavily dependent on the converging inputs from different groups of neurons located at lower levels within the visual pathway.

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