390

Somatosensory neurons do more than convey sensation to the brain: they enable us to perceive things that we describe as pleasant or unpleasant, the shape and texture of objects, the effort required to complete tasks, and even our spatial environment. They also play a central role in movement. Figure 11-24 illustrates the two main somatosensory areas in the cortex.

The primary somatosensory cortex (S1), which receives projections from the thalamus, consists of Brodmann areas 3-1-2 (all appear red in Figure 11-24). The primary somatosensory cortex begins to construct perceptions from somatosensory information. It mainly consists of the postcentral gyrus in the parietal lobe, just behind the central fissure. Thus, S1 lies adjacent to the primary motor cortex, on the other side of the central fissure in the frontal lobe.

The secondary somatosensory cortex (Brodmann areas 5 and 7, shaded orange and yellow in Figure 11-24) is located in the parietal lobe just behind S1. The secondary somatosensory cortex (S2) refines perceptual constructions in relation to potential movement and sends information to the frontal cortex.

In his studies of human patients undergoing brain surgery, Wilder Penfield electrically stimulated the somatosensory cortex and recorded patients’ responses. Stimulation at some sites elicited sensations in the foot; stimulation of other sites produced sensations in a hand, the trunk, or the face. By mapping these responses, Penfield was able to construct a somatosensory homunculus in the cortex, shown in Figure 11-25A. The sensory homunculus looks nearly identical to the motor homunculus shown in Figure 11-6 in that the most sensitive areas of the body are accorded relatively large cortical areas.

Using smaller electrodes and more precise recording techniques in monkeys, Jon Kaas (1987) found that Penfield’s homunculus could be subdivided into a series of smaller homunculi. When Kaas stimulated sensory receptors on the body and recorded the activity of cells in the sensory cortex, he found that the somatosensory cortex comprises four representations of the body. Each is associated with a class of sensory receptors.

The progression of these representations across S1 from front to back is shown in Figure 11-25B. Area 3a cells are responsive to muscle receptors; area 3b cells are responsive to slow-responding skin receptors. Area 1 cells are responsive to rapidly adapting skin receptors, and area 2 cells are responsive to deep tissue pressure and joint receptors. In another study, Hiroshi Asanuma (1989) and his coworkers found still another sensory representation in the motor cortex (area 4) in which cells respond to muscle and joint receptors.

391

Perceptions constructed from elementary sensations depend on combining the sensations. This combining takes place as areas 3a and 3b project onto area 1, which in turn projects onto area 2. Whereas a cell in area 3a or 3b may respond to activity in only a certain area on a certain finger, for example, cells in area 1 may respond to similar information from a number of fingers.

At the next level of synthesis, cells in area 2 respond to stimulation in a number of locations on a number of fingers as well as to stimulation from different kinds of somatosensory receptors. Thus, area 2 contains multimodal neurons responsive to force, orientation, and direction of movement. We perceive all these properties when we hold an object in our hands and manipulate it.

With each successive information relay, both the size of the pertinent receptive fields and the synthesis of somatosensory modalities increase. The segregation of sensory neuron types at the level of the cortex is likely the basis for our ability to distinguish among different kinds of sensory stimuli coming from different sources. For example, we distinguish between tactile stimulation on the surface of the skin, which is usually produced by some external agent, and stimulation coming from muscles, tendons, and joints, which is usually produced by our own movements.

At the same time, we perceive the combined sensory properties of a stimulus. For instance, when we manipulate an object, we know the object both by its sensory properties, such as temperature and texture, and by the movements we make as we handle it. Thus, the cortex provides for somatosensory synthesis too. The tickle sensation seems rooted in an other-versus-us somatosensory distinction, as described in Research Focus 11-6, Tickling, on page 392.

Research by Vernon Mountcastle (1978) shows that cells in the somatosensory cortex are arranged in functional columns running from layer I to layer VI, similar to the functional columns found in the visual cortex. Every cell in a functional somatosensory cortical column responds to a single class of receptors. Some columns are activated by rapidly adapting skin receptors, others by slowly adapting skin receptors, still others by pressure receptors, and so forth. All neurons in a functional column receive information from the same local skin area. In this way, neurons lying within a column seem to be an elementary functional unit of the somatosensory cortex.

392

Damage to S1 impairs the ability to make even simple sensory discriminations and movements. Suzanne Corkin and her coworkers (1970) demonstrated this effect by examining patients with cortical lesions that included most of areas 3-1-2 in one hemisphere. The researchers mapped the primary sensory cortex of these patients before they underwent elective surgery for removal of a carefully defined piece of that cortex, including the hand area. The patients’ sensory and motor skills in both hands were tested on three occasions: before the surgery, shortly after the surgery, and almost a year afterward.

The tests included pressure sensitivity, two-point touch discrimination, position sense (reporting the direction in which a finger was being moved), and haptic sense (using touch to identify objects, such as a pencil, a coin, eyeglasses, and so forth). For all the sensory abilities tested, the surgical lesions produced a severe and seemingly permanent deficit in the contralateral hand. Sensory thresholds, proprioception, and hapsis—all were greatly impaired.

The results of other studies of both humans and other animals have shown that damage to the somatosensory cortex also impairs simple movements. For example, limb use in reaching for an object is impaired, as is the ability to shape the hand to hold an object (Leonard et al., 1991).

Nevertheless, like the motor cortex, the somatosensory cortex is plastic. Plasticity is illustrated by the reorganization of somatosensory cortex after deafferentation. In 1991, Tim Pons and his coworkers reported a dramatic change in the somatosensory maps of monkeys in which the ganglion cells for one arm had been cut, deafferentating the limb, a number of years earlier. The researchers had wanted to develop an animal model of damage to sensory nerves that could offer insight into human injuries, but they were interrupted by a legal dispute with an animal advocacy group. Years later, as the health of the animals declined, a court injunction allowed the mapping experiment to be conducted.

393

Pons and his coworkers discovered that the area of somatosensory cortex that had formerly represented the arm no longer did so. Light touches on a monkey’s lower face now activated cells in what had formerly been the cortical arm region. As illustrated in Figure 11-26, the cortical facial area had expanded by as much as 10 to 14 millimeters, virtually doubling its original size by entering the arm area.

This massive change was completely unexpected. The stimulus–response patterns associated with the expanded facial area of the cortex appeared indistinguishable from those associated with the original facial area. Furthermore, the trunk area, which bounded the other side of the cortical arm area, did not expand into the vacated arm area.

What could account for this expansion of the face area into the arm area? There is evidence for preexisting axon collaterals that are not normally active, but these collaterals would probably not be able to extend far enough to account for all of the cortical reorganization. Another possibility is that within the thalamic and brainstem relay pathways, facial-area neurons project collaterals to arm-area neurons. These neurons are close together, so the collaterals need travel only a millimeter or so (Kambi et al., 2014). Whatever the mechanism, the dramatic cortical reorganizations observed in the Pons study are helpful in understanding other remarkable phenomena, including phantom limb sensations. In humans who have lost a forelimb, touches to the face can be felt as touches to the missing forearm.

How are our abilities to move and to interpret stimulation on the body related? Our somatosensory cortex actually makes an important contribution to movement. Damage to the secondary somatosensory cortex does not disrupt the plans for making movements, but it does disrupt how the movements are performed, leaving their execution fragmented and confused. This inability to complete a plan of action accurately—to make a voluntary movement—is called apraxia. The following case highlights its symptoms.

394

A woman with a biparietal lesion [damage on the left- and right-hemisphere secondary somatosensory cortex] had worked for years as a fish-filleter. With the development of her symptoms, she began to experience difficulty in carrying on with her job. She did not seem to know what to do with her knife. She would stick the point in the head of a fish, start the first stroke, and then come to a stop. In her own mind she knew how to fillet fish, but yet she could not execute the maneuver. The foreman accused her of being drunk and sent her home for mutilating fish.

The same patient also showed another unusual phenomenon that might possibly be apraxic in nature. She could never finish an undertaking. She would begin a job, drop it, start another, abandon that one, and within a short while would have four or five uncompleted tasks on her hands. This would cause her to do such inappropriate actions as putting the sugar bowl in the refrigerator, and the coffeepot inside the oven. (Critchley, 1953, pp. 158–159)

The somatosensory cortex contributes to movement by participating in both the dorsal and the ventral visual streams. The dorsal (how) stream, working without conscious awareness, provides vision for action, as when we automatically shape a hand as we reach to grasp a cup (recall Figure 11-1). Within this stream a number of channels specify the movements—reaching, manipulating, bringing a hand to the mouth, walking—that we should make in response not only to somatosensory information but also to visual and auditory information. The ventral (what) stream, in contrast, works with conscious awareness for perception—that an object is a cup.

As Figure 11-27 illustrates, the dorsal visual stream projects to the secondary somatosensory cortex and then to the frontal cortex (Kaas et al., 2012). In this way, visual information is integrated with somatosensory information to produce unconscious movements appropriately shaped and directed to their targets, as in reaching for a cup. The secondary somatosensory area contributes perceptual information to the ventral stream by providing conscious haptic information about object identity and completed movements. From this information the frontal cortex can imagine the consequence of a movement and select the actions that appropriately follow from those already completed.

Exploring the Somatosensory Cortex

Before you continue, check your understanding.

Question 1

Question 2

Question 3

Question 4

Answers appear in the Self Test section of the book.