In addition to serial processing another principle of cortical organization is that the same information is processed differently in parallel pathways. In the visual system for example, two major parallel pathways terminate in different higher-order areas of cortex. The dorsal stream processes spatial information (position, motion, speed) and projects to parietal association cortex. The ventral stream processes information about form (color, shape, texture) and projects to temporal association cortex.
Dorsal and ventral pathways exist in other sensory systems as well (Figure 18–3). In the auditory and somatosensory systems dorsal pathways serve motor and spatial functions, whereas ventral pathways serve recognition functions. The dorsal-ventral division extends into frontal association cortex.
The dorsal and ventral systems of the cerebral cortex.
Lower-order sensory areas send their output in parallel to the parietal (dorsal stream) and temporal (ventral stream) association cortices, which in turn send their output to the frontal association cortex. The parietal cortex projects primarily to dorsal areas of frontal cortex, areas that serve motor and executive control functions for which spatial information is important. The temporal cortex projects primarily to ventral regions of frontal cortex, including the orbital prefrontal cortex, areas that mediate emotional responses to things in the environment. Emotional significance can be assigned to an object only after the object has been recognized, an ability that depends on areas of the temporal lobe.
The Dorsal Visual Pathway Carries Spatial Information and Leads to Parietal Association Cortex
The parietal cortex plays a key role in the visual guidance of motor behavior and in spatial perception and cognition (and understanding where objects are relative to each other). These two functions are related because visuomotor control requires processing spatial information. Reaching for your coffee cup while you read the newspaper requires that the brain take into account where the image of the cup is on your retina and where your eyes are pointing so as to determine where the cup is relative to your hand.
The parietal cortex is ideally suited for such computations because it is connected to visual, somatosensory, and motor areas of cortex. Parietal cortex may have initially developed the capacity to represent where things are relative to the body to guide actions such as grasping, and then developed the ability to represent where things are relative to each other without reference to the body.
As we have learned in Chapter 17, injury to the parietal cortex in humans results in a wide range of behavioral impairments, which can be classified into two broad categories. In the first category are impairments of body awareness, motor control, and visual guidance of motor behavior. These deficits result from damage to dorsal parts of the parietal cortex close to and connected with the somatosensory cortex. In the second category are impairments of spatial perception and cognition. These deficits result from damage to ventral parts of parietal cortex close to and connected with the visual cortex. Thus the parietal cortex can be thought of as having two subdivisions: a dorsal component serving primarily motor functions and a ventral component serving primarily spatial functions.
Specific impairments in the first category include asomatognosia, a disorder of body awareness in which patients deny the existence of the arm or leg contralateral to the lesion or refuse to acknowledge that it belongs to them even when they can see it. Another is ideomotor apraxia, which arises from damage to the dominant hemisphere; patients are unable to execute certain movements such as waving goodbye, either on command or by imitation, although they may spontaneously make the same movement under circumstances that evoke it habitually. A third deficit in this category, optic ataxia, results from damage to the dorsomedial parietal cortex. Patients with this deficit have difficulty reaching for an object in the peripheral visual field (as when reaching for a coffee cup while reading the newspaper). The hand may go to the wrong location, or it may be misoriented when attempting to grasp the object (Figure 18–4). Patients can, however, perform a reaching task that does not depend on vision, for example touching one's knee in the dark, and can report the locations of visible objects correctly. This collection of symptoms cannot be explained by a purely motor or purely visual mechanism but instead reflects difficulty in coordinating visual input and motor output.
Patients with damage to the dorsomedial parietal cortex have difficulty with visually guided grasping and reaching (optic ataxia).
When required to grasp an object, patients fail to shape their hand appropriately. When required to place their fingers through a slot in a plate, they reach to the wrong location and fail to orient the hand correctly. (Adapted, with permission, from Jeannerod 1986 [left panels]; and Perenin and Vighetto 1988 [right panels].)
Specific impairments in the second category include hemispatial neglect. Patients with this defect are profoundly inattentive to events in the half of space opposite the injured side (see Chapter 17). Another is constructional apraxia, an inability to appreciate the structure and arrangement of things by looking at them. Patients suffering from constructional apraxia have difficulty arranging a set of tiles or matchsticks according to a model placed in plain view. They may also be deficient in tests of writing and drawing because these require putting marks on a page in a precise arrangement (Figure 18–5).
Disorders of copying and drawing result from damage to the parietal cortex.
Drawings of complex figures are grossly inaccurate whether drawn from a model or from memory. The problem arises from an inability to perceive the spatial relations of the parts of an object. (Reproduced, with permission, from Critchley 1953; and Trojano and Grossi 1998.)
Injury to parietal cortex can impair cognitive tasks that require abstract spatial thinking. For example, patients with acalculia have trouble understanding and manipulating numbers, particularly multidigit numbers where the value of a digit depends on its place. Injury to the left angular gyrus, a region at the lateral edge of the parietal lobe, results in agraphia with alexia, a condition in which patients cannot read, write, or spell and cannot understand a word spelled out orally. Reading and writing involve spatial thinking in that they depend on the ability to perceive, remember, and reproduce the sequence of letters in a word.
Although clinical observations pinpoint the parietal cortex as important for many spatially based abilities, they do not tell us about the underlying neural mechanisms. Our understanding of these mechanisms comes in large part from studies of monkeys using single-neuron recording. Four areas in the monkey's intraparietal sulcus have been thoroughly studied: the lateral, ventral, medial, and anterior intraparietal areas. Neurons in all of these areas carry spatial information, signaling the location of an object to which the monkey is paying attention or is about to direct movement. Within a given area neurons respond to one or more specific kinds of sensory stimulation (somatosensory or visual), fire in conjunction with a specific kind of movement (looking, reaching, or grasping), and encode the location of a target relative to a specific part of the body (eye, head, or hand) or the environment.
The lateral intraparietal area encodes retina-centered information about points in the visual field that the monkey has selected for attention and is involved in visual attention and eye movements. Its neurons, like those in unimodal visual areas, have receptive fields for fixed points on the retina. Visual responses in these neurons increase when the monkey is paying attention to a stimulus in the receptive field (Figure 18–6A). This enhancement of the response occurs whether or not the monkey is planning an eye movement toward the stimulus. Neurons here also fire when the monkey is anticipating the appearance of a stimulus or remembering the location where a stimulus appeared, and some neurons fire around the time of an eye movement toward the receptive field.
Neurons in the parietal cortex of the monkey are selective for the location of objects in the visual field relative to particular parts of the body.
Each histogram represents the firing rate of a representative neuron as a function of time following presentation of a stimulus. In each diagram the line emanating from the eyes indicates where the monkey is looking.
A. Neurons in the lateral intraparietal area (LIP) have retina-centered receptive fields. The strength of the visual response depends on whether the monkey is paying attention to the stimulus. The neuron fires when a light is flashed inside its receptive field (dotted circle) (1). The visual response is increased if the monkey is instructed to attend to the location of the stimulus (2). The neuron does not fire if the stimulus is presented outside the receptive field regardless of where attention is directed (3, 4).
B. In the ventral intraparietal area (VIP) some neurons have head-centered receptive fields. This is determined by keeping the head in a fixed position while the monkey is instructed to shift its gaze to various locations. This neuron fires when a light appears to the right of the midline of the head (1, 2). It does not fire when the light appears at another location relative to the head, as on the midline or to the left (3, 4). The critical contrast is between situations 1 and 4. The retinal location of the light is the same in both (slightly to the right of the fixation point) yet the neuron fires in 1, when the stimulus is to the right of the head, but not in 4, when the stimulus is to the left of the head.
C. In the medial intraparietal area (MIP) neurons fire when the monkey is preparing to reach for a visual target and are selective for the retina-centered direction of the reach. This neuron fires when the monkey reaches for a target to the right of where he is looking (2, 3). It does not fire when he reaches for a target at which he is looking (1) or when he moves only his eyes to the target at the right (4). The physical direction of the reach is not a factor in the neuron's firing: It is the same in 1 and 3 and yet the neuron fires only in 3.
D. In the anterior intraparietal area (AIP) neurons fire when the monkey is looking at or preparing to grasp an object and are selective for objects of particular shapes. This neuron fires when the monkey is viewing a ring (3) or making a memory-guided reach to it in the dark (2). It fires especially strongly when the monkey is grasping the ring under visual guidance (1). It does not fire during viewing or grasping of other objects (4).
The ventral intraparietal area encodes head-centered spatial information about visual and tactile stimuli and is involved in multisensory guidance of head and mouth movements. Individual neurons respond to both visual and somatosensory stimuli. Most neurons respond to tactile stimulation on the face or head and to visual stimuli presented near the receptive field. The match between somatosensory and visual receptive fields is maintained when the eyes move: The visual receptive fields are shifted so as to remain at a fixed position relative to the head (Figure 18–6B).
The medial intraparietal area encodes both retina-centered and body-centered spatial information and is involved with visually guided reaching. Neurons in this area respond to visual targets and are active when the monkey is planning and executing reaching movements. They are sensitive to the direction of reaching in relation both to where the monkey is looking (right or left of the gaze) and to the body (right or left of the trunk) (Figure 18–6C).
The anterior intraparietal area encodes object-centered and hand-centered spatial information and is involved with visually guided grasping. Individual neurons are selective for objects of particular shapes and for the hand shapes required to grasp them. A neuron that fires when the monkey sees a given object will also fire when it prepares to grasp that object (Figure 18–6D).
Clinical observations in humans and electrophysiological studies in monkeys lead to two general conclusions. First, the parietal cortex is specialized for the sensory guidance of motor behavior as well as for spatial perception and cognition. Second, different regions within the parietal cortex serve different functions: Dorsal regions close to the somatosensory cortex contribute to motor control of the body, whereas ventral regions close to the visual cortex contribute to spatial perception and cognition.
The Ventral Visual Pathway Processes Information About Form and Leads to Temporal Association Cortex
The temporal association cortex, like the parietal association cortex, is a region where higher-order areas of different sensory systems share borders and are interconnected. These association areas receive information about vision, sound, and touch from lower-order visual, auditory, and somatosensory areas. For example, the inferotemporal cortex receives information about the shape, color, and texture of visual images through the ventral visual pathway. The temporal association cortex uses this information to mediate the recognition of objects in the environment and, through projections to the ventral frontal cortex, trigger appropriate emotional responses to them (see Figure 18–3).
Injury to the visual and auditory association areas of the temporal lobe in humans impairs recognition of the significance of sensory stimuli, resulting in a variety of perceptual deficits termed agnosias. Patients with visual object agnosia, a result of injury to a medio ventral part of the temporal cortex, cannot recognize things but can draw them (Figure 18–7). This deficit is a striking contrast to patients with parietal cortex injury, who can recognize things but often cannot draw them well (Figure 18–5). Patients with visual object agnosia may be unable to recognize objects in general or may be unable to make fine distinctions within a category of objects such as faces. An impairment in recognition specific to faces is called prosopagnosia.
Injury to a medioventral region of temporal cortex results in visual object agnosia.
When presented with the drawings shown in the left column, a patient with visual object agnosia was able to copy them but could not accurately identify the objects. (Reproduced, with permission, from Rubens and Benson 1971.)
Auditory agnosia has been described, although reports of it are rare, perhaps because the condition is associated with more disabling disorders of language comprehension. Patients with auditory agnosia, when asked to describe recordings of natural sounds, demonstrate that they are not deaf but that their ability to recognize the sounds is impaired.
By far the most debilitating of all conditions arising from damage to the human temporal lobe is Wernicke aphasia, a disorder in understanding spoken language. Wernicke aphasia arises from damage to the superior temporal gyrus of the left hemisphere, a region corresponding to Brodmann's area 22 (comparable in location to the auditory association cortex in the superior temporal gyrus in the monkey). In addition to the disorder of speech comprehension, the patient's own speech is severely garbled. This indicates that auditory forms of words stored in the temporal lobe serve not only as templates for speech recognition but also as guides for speech production.
Semantic dementia is a degenerative disorder typically arising from pathology of the temporal cortex. Studies of patients with this disorder indicate that this part of cortex is critical not only for object recognition but also for semantic memory. To have semantic knowledge of a thing means that one must be able to associate disparate pieces of information about it, for example, the sound, feel, appearance, and use of a telephone. These associations are forged through experience-dependent changes in the synaptic connections among the same temporal lobe areas on which recognition depends. A patient with semantic dementia shown pictures of an ostrich and a penguin may name them simply "bird" or even "animal." The loss of detailed knowledge about things in the world emerges even in tests requiring only nonverbal responses, such as placing together pictures of things that are semantically related.
Neurons in the temporal association cortex of monkeys become active under circumstances that suggest involvement in object recognition. The best understood area of temporal association cortex is the inferotemporal cortex, which occupies most of the inferior temporal gyrus and extends dorsally into the superior temporal sulcus. The activity of inferotemporal neurons, unlike neurons in the parietal cortex, is not influenced by the motor behavior of the animal. If a visual stimulus enters the neuron's receptive field and the monkey is paying attention to it, the neuron will fire at a virtually identical rate regardless of what the animal is doing or planning to do.
Inferotemporal neurons also differ from parietal neurons in that they are sensitive to the shape, color, and texture of an object in the visual receptive field. In one study individual inferotemporal neurons responded to only a few shapes out of a large test set (Figure 18–8). Because each neuron responded to different stimuli, it was possible by monitoring the activity of many neurons to determine reliably which stimulus was present on the screen. The pattern selectivity of inferotemporal neurons is largely unaffected by image size and location as long as the image falls somewhere in the neuron's typically large receptive field. This insensitivity to size and location is further evidence that the inferotemporal cortex plays a role in shape recognition (for which location and size are irrelevant) but not in motor guidance (for which they are crucial).
Neurons in the inferotemporal cortex of the monkey respond selectively to particular shapes.
Shown are responses of a single inferotemporal neuron to 14 different silhouette shapes. The histogram under each shape represents the rate at which the neuron fired as a function of time during a 2-second trial. The bar under the histogram indicates the 1-second period during which the stimulus appeared. (Reproduced, with permission, from Kobatake et al. 1998.)
Just as neurons in the inferotemporal cortex are selective for visual shapes, neurons in the auditory association cortex of the superior temporal gyrus are selective for patterns of sound. Although little studied, this region is known to contain neurons selective for particular species-specific vocalizations. Overall, the temporal association cortex plays a critical role in recognizing things and in storing some kinds of knowledge. It is not involved in the guidance of movement or in spatial perception and cognition, functions that depend instead on parietal cortex.