CHAPTER 11 THE SOMATOSENSORY SYSTEM AND TOPOGRAPHIC ORGANIZATION 11.1. MAPPING STIMULUS SPACE The brain is organized spatially in three dimensions as is the environmental space in which we operate. It is often possible to find a systematic correlation between the responses of neurons to a given stimulus parameter and the locations of the neurons within a 2- or 3- dimensional array in a specific area of the brain. The somatosensory and visual systems are particularly straightforward examples of systems in which a stimulus parameter (location on the skin or in the visual field) is represented systematically across an array of neurons. 11.1.1. Stimulus parameters and neural filtering. A stimulus parameter refers to one particular aspect of the stimulus. In the chemical senses examples of stimulus parameters are taste or odor quality, and intensity. In the visual system, stimulus parameters include color, brightness, and location in the visual field. In the somatosensory system, stimulus parameters include quality (e.g., pressure, vibration, heat, tickle, etc), intensity, and location on the skin. Within each sensory organ, different receptor cells and neurons respond best to different values of a given stimulus parameter. In the eye, for example, different receptors respond best to different wavelengths (colors) of light. In addition, different receptors respond best to different locations within the visual field. In the skin, different types of receptors respond best to different physical stimulus types and different body locations. Note that in any sensory system the same cell may be sensitive to more than one stimulus parameter. "Filtering" refers to the fact that each receptor cell passes on information about a specific set of stimulus properties, but excludes or reduces information about other properties. The filter properties of sensory receptor cells may result from their position within the receptor array (e.g., position of a visual receptor cell on the retina, or the location of a touch receptor in a particular place on the skin). Filter properties may also result from the membrane properties of the receptor cell (e.g., different receptor sites for specific molecules in taste or olfactory cells). Filter properties may also result from various non-neural, non-receptor structures associated with the excitable cell (e.g., different types of connective tissue structures associated with certain types of touch and vibration sensitive receptors in the skin). 11.1.2. Neural tuning. The process of filtering results in a "tuning" of the receptors and neurons in sensory systems to specific ranges of values for specific stimulus parameters. Receptor cells in the visual and somatosensory systems are generally "tuned" to a particular region of the visual field or the skin surface; so are neurons at higher levels of these systems. Different types of cutaneous receptors are tuned to different rates of vibration. As we 68
will see later, receptor cells in the auditory system are "tuned" to a particular range of air vibration, corresponding to sound frequency (pitch). Cells in the gustatory and olfactory systems are typically "tuned" to one or more classes of chemical substance. If we know the levels of activity in the different receptor cells or neurons within a population, we can gain much information about the properties of the stimulus eliciting that pattern of activity. 11.2. FORMATION OF SPATIAL REPRESENTATIONS The fact that different receptors or neurons within an array, or population, are tuned to different values of a parameter results in a spatial representation, or "map" of that parameter. For example, each level of the visual system contains a "map" of the retina corresponding to visual space. Each level of the auditory system contains a "map" of the inner ear corresponding to sound frequency, which we perceive as pitch. Each level of the somatosensory system contains a map of the body surface. A systematically organized representation of a stimulus parameter (e.g., visual space, body surface, sound frequency) within a neural population (array) is called a topographic representation. The topographic representation may originate at the peripheral level or through neural processing at higher levels. Within a given system, each level projects to the next level in an organized way so that the topographic representation is preserved. Once created, the map may undergo various transformations (e.g., expansion, compression, sharpening or blurring). The basic organization of each sensory map is generally established early in neural development, but the detailed organization of sensory maps remains somewhat plastic throughout life. 11.3. SPATIAL MAPS IN THE SOMATOSENSORY SYSTEM 11.3.1.Receptive fields and topographic maps. Various kinds of sensory receptors are distributed over the entire body surface as well as in the muscles, joints, and internal organs. Because each receptor has a specific location in space, it transmits information about what is happening at its particular location. The region of space within which a stimulus will affect the response of a receptor cell or sensory neuron is called the spatial receptive field of that receptor or neuron. Stimuli within the receptive field may either excite or inhibit the receptor or neuron. The receptive fields of individual sensory cells often overlap somewhat with one another so that different cells may provide information about the same location in space, just from a slighly different vantage point. If there is convergence of inputs from multiple sensory receptors (or neurons) onto one neuron (target cell), then the receptive field of the target cell will be the sum of the receptive fields of the input cells. Convergence of excitatory inputs results in large receptive fields. (Divergence, on the other hand, would result in multiple target cells all with the same small receptive field). If there is convergence from excitatory neurons with receptive fields in one area 69
and inhibitory neurons with receptive fields in another area, the target cell s activity will be increased by activity in the excitatory inputs and decreased by activity in the inhibitory inputs. 11.3.1.1. Cutaneous receptive fields and sensory maps of the body. Information from the cutaneous receptors is transmitted from the spinal cord to the ventral posterior nucleus of the thalamus, and from there to the somatosensory cortex. As in the case of the visual system, each level of the somatosensory system contains a topographic representation or "map" of the body surface, and this map is maintained from one level to the next. 11.3.2. Receptive field size and the magnification factor. Because of convergence and divergence in connections from receptors or sensory neurons to their target cells, some parts of sensory space (e.g., some regions of the body) are expanded relative to other parts. The result would be comparable to your taking a photograph of your friend and greatly enlarging his or her hands or face so that you can see them in more detail. If the enlarged picture were displayed on your computer screen, more pixels would be used to represent a square inch of your friend s nose than would be used to represent a square inch of his or her arm. At the same time, a pixel on the nose would represent a smaller area than a pixel on the arm. In the nervous system, the situation is similar. In an area in which cells have small receptive fields (e.g., the lips), a large number of cells (large expanse of cortex) will be required to represent a given surface area. In a region where cells have large receptive fields, few cells (small expanse of cortex) will be required to represent a given area. The amount by which a point on the skin is expanded in a central representation is called the magnification factor. 11.3.2.1. The magnification factor and acuity. Different parts of the body representation have different magnification factors, starting at the spinal cord and thalamus, and continuing on to somatosensory cortex. As would be expected, neurons in the somatosensory cortex have receptive fields that vary in size depending on body location. Areas of the body with smaller receptive fields occupy larger cortical areas and provide greater acuity, as measured by 2-point discrimination thresholds. The two-point discrimination threshold is a measure of the smallest detectable distance between two points at which pressure is applied. Figure 11-1. Two-point discrimination refers to the ability to distinguish two small points, such as the points of the compass shown here, from a single large point. 70
Figure 11-2. 2-point discrimination thresholds vary greatly across the body surface. The bars indicate the minimum separation at which two points are perceived as separate. The small bars indicate fine discrimination, the large bars indicate poor discrimination. 2-point discrimination ability varies greatly across the body surface. The smallest separations (i.e., highest acuity) can be detected on the lips and fingertips. The distance between the two points is large (i.e., low acuity) on parts of the body such as the back and legs. The relative importance of different body regions is reflected in the cortical representation of the body surface of every animal species. In humans and monkeys, the fingertips are one of the most sensitive parts of the skin, so their representation in the cortex is large. In animals with whiskers, the whisker representation occupies a large part of somatosensory cortex. Each whisker is represented by a large cortical module called a "barrel". Because the layout of the somatosensory map in cortex roughly reflects the shape of the body, the map is sometimes referred to as a "homunculus", which in Latin means "little man". Figure 11-3. Receptive fields on different parts of the body vary in size with small receptive fields in areas of good 2-point discrimination (fingers, left), and large receptive fields in areas of poor 2-point discrimination (forearm, right). 71
Figure 11-4. The representation of different parts of the body in somatosensory cortex (left) is not proportional to their actual size. Some are greatly magnified (lips, hand), while others are greatly reduced (trunk). The drawings at right show the "homunculi" as they appear in the cortex of four different species. 11.4. RELATIONSHIP BETWEEN THE SOMATOSENSORY AND MOTOR SYSTEMS. The motor cortex lies next to the somatosensory cortex. The motor representations of different body areas also vary in magnification factor, just as do the somatosensory maps. Figure 11-5. In the motor cortex, there is also a "homunculus" in which motor activity in a specific part of the body is correlated with neural aactivity or stimulation of a specific area of the motor cortex. Note that the magnification factor in motor cortex is somewhat different from that in somatosensory cortex with an even larger representation of the hand and speech organs. 72
11.5. RECEPTIVE FIELD ORGANIZATION IN THE SOMATOSENSORY SYSTEM. In the thalamus, somatosensory receptive fields have a roughly circular center-surround organization. This means that a stimulus applied to the part of the skin shown as the center causes activity in the thalamic neuron to increase above baseline. A stimulus applied to the surrounding part of the skin causes activity in the same thalamic neuron to decrease below baseline. If one were to observe the activity of a different thalamic neuron, it might have a center located in the same part of the skin as the surround of the first neuron. The inhibitory surrounds are produced by a pattern of neural connections known as lateral inhibitory connections. Lateral inhibition in the somatosensory system helps sharpen the tactile perception of contrasts and edges; it sharpens acuity and aids in 2-point discrimination by creating an area of depressed activity in the neurons whose centers represent the region between the two points. Figure 11-6. The receptive field of a neuron in the somatosensory thalamus or primary cortical somatosensory area typically has an excitatory center somewhere on the body (in this case the monkey's arm). Stimulation of adjacent areas of the skin, in the surround area, causes inhibition of the same neuron s activity. Figure 11-7. A hypothetical network that originates in an array of receptors in the skin, indicated by the top row of whole circles and partial circles. The partial circles are meant to depict the degree to which the receptors are compressed by the punctate stimulus (thick arrow) applied to the surface of the skin. Each receptor has an unbranched straight-through excitatory connection (thin arrows) to a neuron in the first array (middle row of open circles). The number next to each axon indicates the change in firing rate evoked by the stimulus relative to a 73
baseline level of spontaneous activity. The top graph on the right plots the magnitude of the change in firing for each neuron in the first array. Each neuron in the first array has a straight-through excitatory connection (thin vertical arrows) to a neuron in the second array (bottom row of open circles), but each axon sends branches (short angled arrows) to innervate two inhibitory neurons (black circles), one on either side. Each inhibitory neuron sends an axon (angled black line ending in a bar) to the neuron corresponding to its nearest neighbor in the second array. If we assume that the inhibition provided by each inhibitory neuron is the same as the amount of excitation it receives, the net change in firing rate for each neuron in the second array will be equivalent to the straight-through excitation minus the sum of the inhibition from either side. The net change in activity is indicated below each neuron in the second array and plotted in the graph at the bottom right. Notice that the biggest difference between adjacent neurons in the first array is 3, but in the second array it is 5. Thought Question: What would happen if you added another lateral inhibition network to the system shown in Figure 11-7 such that each neuron in layer 2 projected straight-through to excite a corresponding neuron in a third layer and also sent inhibitory projections to adjacent neurons in the third layer? Assume that the decrease of -4 does not cause the neuron to completely stop firing, but to go to an output value of 1. Figure 11-8. The 3-dimensional graph on the left shows the hypothetical responses across a two-dimensional array of somatosensory neurons (with lateral inhibition) representing two dimensional space on the skin. The stimulus is pressure at a single point on the skin. There is a peak in the response at the point in the array that represents the point of stimulation; surrounding the peak is a trough of inhibition. The stacked graphs on the right compare activity evoked by 2-point stimulation of the skin across one dimension of the 2-D neuronal array under two different sets of conditions - with lateral inhibition (left-front) and without lateral inhibition (right-back). 11.6. ORGANIZATION OF SOMATOSENSORY CORTEX In the somatosensory cortex, neurons are arranged in columns. Cells in each column respond to a particular type of input (e.g., vibration, pressure, temperature, etc.), There is a "primary" area that receives input directly from the thalamus and other "higher-level" areas where response properties are more complex. Neurons in these non-primary areas respond to stimuli that are oriented on the skin in a particular way or that move across the skin in a particular direction. Some neurons respond best to 3-dimensional ojects held in the hand. There are also polysensory areas where somatosensory information comes together with visual, chemosensory, and other types of information. 74
The posterior parietal area is an example of a higher-level polysensory processing area in which somatosensory information is coordinated with visual information. It is thought that the posterior parietal cortex plays a role in directing attention to selected objects. Parietal cortical lesions can cause a "neglect" syndrome in which the patient becomes unaware of a part of the body or a part of the visual environment. 11.7. PLASTICITY AND REORGANIZATION OF SOMATOSENSORY CORTEX The somatosensory cortex provides an opportunity for scientists to study the degree to which the organization of the cortex can change in response to various manipulations of its sensory input. It is known that in monkeys who have lost a digit, the representation of adjacent digits eventually "takes over" at least part of the area formerly devoted to the representation of the missing digit. Similarly, intensive stimulation of one digit increases the cortical area devoted to representation of that digit. Figure 11-9. Organization of receptive fields and representation of the digits on a monkey's hand before any experimental stimulation (left) and after intensive stimulation of digits 2, 3 and 4 in a task that required contact with a rough, moving surface (right). Notice that the area of somatosensory cortex devoted to the representation of these three digits has expanded (shaded area, top right) and the number of receptive fields recorded in somatosensory cortex is disproportianately large for the tips of digits 2, 3, and 4. Thought question: Assuming that no new neurons are produced in adults, how do you think cortical reorganization takes place? 75