Features of Cortical Columns Within the Occipital Cortex
Like the somatosensory systems, cortical columns within the visual cortex are also arranged in terms of the similarities of the receptive properties of the neurons located in each of the columns. As shown in Figure 26-13, area 17 of the visual cortex is organized into separate vertical columns that receive magnocellular and parvocellular pathways. This means that a given interblob pathway will terminate in a column that will activate cells that will respond to a single orientation of an object in space. The orientation column contains complex and simple cells, providing the basis for higher levels of integration and abstraction to take place within the column. Orientation columns are oriented in a radial fashion, much like the spokes of a wheel. The cortex contains many orientation columns arranged in this fashion with different axes of orientation radiating outward from different columns. The center of this array is called an orientation center, and a given axis of orientation is represented only once within a given array.
As described earlier, cortical columns are concerned with color perception (blob areas) and orientation of objects (interblob areas). The cortical column can also be defined in terms of the inputs it receives from each eye. An ocular dominance column receives inputs from one eye (Fig. 26-17), and the inputs are arranged in patterns of alternate columns for each eye. Collectively, sets of orientation columns coupled with blob areas (for color vision) and sets of ocular dominance columns form a unit referred to as a hypercolumn. The hypercolumn is responsible for analyzing a single point on the retina. Thus, the primary visual cortex consists of large numbers of hyper-columns, which provide the overall basis for form perception. Moreover, adjoining hypercolumns are capable of communicating with each other by virtue of short, horizontally arranged axons. Typically, an orientation from one hypercolumn associated with a specific spatial orientation can communicate with a counterpart of a neighboring hypercolumn that is associated with the same spatial orientation. Likewise, blob areas within neighboring hypercolumns associated with the same color can also communicate with each other. Such arrangements allow for higher levels of integration of visual signals.
FIGURE 26-16 Complex cells. These neurons respond best to a bar oriented in a specific direction, but they also respond to different positions of the bar within the visual field.
FIGURE 26-17 Hypercolumn and ocular dominance. (A) Model displaying sets of orientation and ocular dominance columns together with blob areas (for color vision), which, collectively, is referred to as a hypercolumn. (B) Autoradiograph of area 17 of the visual cortex of a monkey whose right eye was removed as an adult. The left eye was injected with the tracer, 3H-proline, which was then transported through the visual pathways to area 17. The autoradiograph reveals alternating patterns of uniformly labeled ocular dominance columns with unlabeled bands. The uniformly labeled region is the optic disc (of the monocular area) covering the blind spot of the right eye that was enucleated.
Functions of the Temporal Neocortex
Inferotemporal Cortex (Inferior Temporal and Occipitotemporal [Fusiform] Gyri)
We have seen that the primary visual cortex deals with the initial analysis of visual information in terms of lines and primitive aspects of form perception. Clearly, visual perception requires higher complexities of analysis for images, such as faces and forms (shapes), to have clear-cut recognition value. Specifically, the mechanism for face and shape recognition is represented in the inferior temporal gyrus (see Fig. 1-4). This region receives inputs from tertiary (V4) parts of the visual cortex (which, in turn, receive inputs from the primary [V1] and secondary [V2] visual areas). Neurons in this region specifically respond to faces and shapes, such as the position of a hand in the visual field.
Middle Temporal Gyrus
Other parts of the temporal neocortex also contribute to a different aspect of vision—the detection of motion. Recall that certain retinal ganglion cells (type M cells) initially respond to circular fields with changing levels of contrast over time. These neurons project to the magno-cellular regions of the lateral geniculate nucleus, which, in turn, project to those regions of the primary visual cortex in which the neurons respond to specific changes in the movement of objects. Neurons located in the middle temporal gyrus receive such information from parts of the primary visual cortex (i.e., layer IV) and are thus capable of responding to the movement of the object within the visual field. Neurons in these regions of the temporal neocortex then relay signals concerning movement of the object to parts of the parietal lobe that further process the direction and speed of the object’s movement.
Superior Temporal Gyrus
On the dorsal aspect of the superior temporal gyrus lie the transverse gyri of Heschl, which face the lateral sulcus. The primary auditory receiving area for auditory signals transmitted to the cerebral cortex from the medial genicu-late nucleus is located in the middle part of the anterior transverse gyrus and corresponds to Brodmann’s area 41. Adjacent regions of the posterior transverse gyrus include area 42 and largely represent an auditory association area. While characteristics of neural processing of auditory signals within the auditory cortex are not as well delineated as those for vision, a number of similar characteristics to other sensory systems have been described. The auditory cortex, similar to other sensory regions, contains primary, secondary, and tertiary auditory receiving areas that play roles in auditory perception. The cortical neurons in the auditory cortex also respond to a higher level of complexity to auditory signals. For example, neurons located in lower regions of the auditory relay system respond to specific frequencies of stimulation and are, thus, tonotopically organized. Whereas different regions of the auditory cortex, including the primary receiving areas, may display a tonotopic organization, other neurons located in the auditory cortex respond effectively when the frequency is either increasing or decreasing, reflecting a higher level of integration of the auditory signals.
One additional feature of the primary auditory cortex should be noted. This region of cortex may be divided into two alternating zones or bands that are arranged at right angles to the axis of tonotopic organization. One zone contains neurons that respond to stimuli from either ear, although the effects of stimulation of the contralateral side are more prominent. The second type of zone contains neurons that are excited by stimuli from one side and inhibited by stimuli from the other side. Because of this arrangement, the primary auditory receiving area can respond to all frequencies and to different forms of binau-ral interaction. This kind of functional organization thus resembles the functional arrangement of the visual cortex with respect to ocular dominance columns.
Effects of Lesions of the Occipital and Temporal Regions of the Cortex Visual Deficits
A lesion restricted to the inferior bank of the calcarine sulcus will cause an upper quadrantanopia. If the lesion affects the left side of the brain, then a right upper quad-rantanopia will result. If the lesion involves the upper bank of the calcarine sulcus, then a lower quadrantanopia (i.e., loss of vision of one quarter of the visual field of both eyes) will result.
Secondary Visual Areas (in Occipital Cortex). Lesions in the secondary visual areas can produce a variety of deficits, including visual agnosia (i.e., failure to understand the meaning or use of an object) and color agnosia (i.e., inability to associate colors with objects and inability to name or distinguish colors).
Inferotemporal Cortex. Another disorder involves the loss of the ability to recognize familiar faces even though the patient is able to describe the physical features of such individuals. This disorder is called prosopagnosia and appears to be associated with lesions of the inferotemporal cortex. It is likely that such lesions disrupt the neurons that receive inputs from interblob regions of the visual cortex that mediate shapes, contours, and edges of figures.
Middle Temporal Cortex. As noted earlier, neurons in the middle temporal cortex respond to the movement of objects. Lesions of this region can produce a disorder called movement agnosia in which the patient cannot distinguish between objects that are stationary and those that are moving. A person with this disorder will recognize another individual at both a far distance and then at a closer distance but is incapable of recognizing the movement of that individual.
Auditory Deficits: Superior Temporal Cortex. Bilateral lesions of the auditory cortex do not significantly alter the capacity to discriminate tones or hear sounds. Such lesions, however, appear to interfere with the ability to interpret patterns of sounds. This deficit may extend to the inability to recognize the sounds of animals, speech of individuals, and mechanical sounds, such as horns and bells. This disorder is sometimes referred to as an acoustic agnosia.
FIGURE 26-18 Relationship of Wernicke’s and Broca’s areas. Diagram is of the left cerebral hemisphere indicating the loci of Broca’s motor speech area and Wernicke’s area. There are reciprocal connections between the two regions, which pass in a bundle called the arcuate fasciculus.
Regions Associated With Speech Deficits
Temporal-Parietal Region (Wernicke’s Area)
Lesions involving the superior temporal gyrus and adjoining parts of the parietal cortex (angular and supramarginal gyri [Fig. 26-18]) produce a disorder referred to as receptive, sensory, or fluent aphasia. In this disorder, patients are unable to understand either written or spoken language. They may be able to speak, but they do not make sense.
Frontal Lobe (Broca’s Area)
A different form of aphasia called motor or nonfluent aphasia results from a lesion of the posterior aspect of the inferior frontal gyrus (called Broca’s area). In motor aphasia, the patient is unable to express ideas in spoken words. With respect to both receptive and motor forms of aphasia, two points should be noted. The first is that it is likely that Broca’s area and Wernicke’s area are functionally interrelated because they can communicate with each other through a pathway referred to as the arcuate f asciculus (Fig. 26-18). The second point is that both forms of aphasia occur only when the dominant hemisphere is affected.
Functions of the Frontal Lobe
Motor Regions of the Cortex
Three motor regions of the cortex have been identified: a primary motor cortex (area 4), and area 6, which contains both a supplemental motor area and a premotor area. The precentral gyrus gives rise to a component of the corticospinal tract that is critical for generating voluntary control over precise movements that affect primarily the distal musculature. The precentral gyrus is somatotopically organized, and the motor homunculus is depicted in Figure 19-3. As shown in Figure 19-3, motor functions of the head region (corticobulbar tract) are represented in the ventrolateral aspect of the precentral gyrus. The hand and fingers are represented on the dorsal aspect of the cortex, and the lower limb extends onto the medial aspect of the hemisphere. Fibers arising from the precentral gyrus make synaptic contact with ventral horn cells via short interneurons within the spinal cord. Descending cortical neurons are continually modulated by other inputs. These inputs include sensory (conscious proprioceptive) signals from the region of the limb associated with the movement as well as inputs from other motor regions. The motor regions in question include the cerebellum, basal ganglia, and supplemental motor cortex.
Other components of the corticospinal tract include fibers that arise from area 6 and the postcentral gyrus. The fibers arising from the postcentral gyrus make synaptic contact with ascending sensory neurons at the levels of the dorsal column nuclei and dorsal horn of the spinal cord. The function of this descending component is to filter extraneous sensory signals that otherwise might reach the motor cortex, the net effect of which is to enhance the excitatory focus and inhibitory surround with respect to the appropriate sensory signals that affect motor neurons of the precentral gyrus. The supplementary and premotor cortices (area 6) serve to provide the programming mechanisms for the sequencing of response patterns that are critical for complex motor events such as walking, writing, and lacing one’s shoes. The outputs of these regions are directed to both the spinal cord and precentral gyrus. The actions of the neurons in area 6 are dependent upon the integrated output of the posterior parietal cortex, which receives both somatosensory and visual signals (see Figs. 19-7 and 26-19).
Two additional motor regions should also be noted. One is located in the posterior aspect of the inferior frontal gyrus and is called Broca’s area. This region, the functions of which have been described earlier, is associated with the motor expression of speech.
FIGURE 26-19 Cortical inputs to motor regions. Diagram illustrates the inputs into the posterior parietal cortex from somatosensory and visual cortices as well as the relationship of the posterior parietal cortex with the premotor area, precentral gyrus, and spinal cord. The significance of this relationship is that: (1) the posterior parietal cortex integrates different kinds of sensory information, and (2) these signals are then integrated and transmitted to the premotor area. In this manner, the posterior parietal area contributes to the programming of the sequences of movement with respect to any given motor task. When this mechanism is disrupted, motor functions are impaired, resulting in apraxia.
The second region lies rostral to area 6 in the middle frontal gyrus and is called the frontal eye fields (area 8). The frontal eye fields coordinate voluntary control of eye movements. Because there are no known projections from the frontal eye fields to cranial nerve nuclei whose axons innervate extraocular eye muscles, area 8 likely controls eye movements by projecting its axons to the horizontal gaze center located in the pontine reticular formation indirectly through projections to the superior colliculus. Specifically, area 8 projects to the ipsilateral superior colliculus, which, in turn, projects to the contralateral paramedian pontine nucleus (horizontal gaze center). Thus, stimulation of the right area 8 causes conjugate movement of the eyes to the left. If area 8 is damaged, the eyes will look toward the side of the lesion.
It should be noted that there exists an occipital eye field. Stimulation of parts of the occipital cortex similarly produces conjugate deviation of the eyes to the contralat-eral side, and lesions of this region produce conjugate deviation to the side of the lesion. It is believed that occipital control of eye movements is reflexive in nature and serves as a tracking center. These functions are mediated through descending fibers to the superior colliculus.
