Primary Somatosensory Cortex
In the early twentieth century, eminent scientists like Brodmann and Campbell postulated that the well-developed outer surface of the brain can be viewed as identifiable areas performing discrete functions. They had also observed notable histological differences between areas beholding motor functions and their sensory counterparts. Out of the various granular cortical areas, the region which lies directly behind the central sulcus in the parietal lobe, i.e., the pre-central gyrus, is the primary somatosensory cortex. Woolsey C.N. and his colleagues (Woosley and Van Der Loos 1970), after their painstaking endeavors related to this area in Macaque monkeys, found that there is a single large representation of the contralateral body surface area in the anterior parietal cortex. The profoundness of the high density points in this strip of area produced localized sensations upon stimulation. Juxtaposing the previous knowledge, it was evinced that this representation was spanning four distinct cyto-architectonic fields, Brodmann areas 3a, 3b, 1, and 2. Unlike the lesion studies, which can be successfully applied to study the functional localization of certain areas, studies related to somatosensory cortex had predominantly used electrical stimulation evoked potentials.
Gross Connectivity of Primary Somatosensory Cortex (S1)
The principal function of the area is to receive the inputs which originate from the mechanoreceptors. These information conveyed are relayed in two synapses and modulated in the thalamus. By virtue of processing, it guides the detection of mechanical stimuli such as tactile sensation and proprioception. The stimuli are gauged in terms of magnitude and precise location from body surface. Additionally, in humans, the sensations could also be perceived in cognitive dimensions which help in recognizing the shapes and size (O’Leary et al. 2007). In clinical settings, testing of these coherent perceptions is used to discriminate the level of lesion.
Based upon the inputs received, S1 can further be subdivided as areas 3a, 3b, 1 and 2. The afferents from the muscles project predominantly to area 3a and to perform the sensorimotor integration, it retains continuity with primary motor area. This sensorimotor projection, which is more pronounced in rodents, acts as a feed-forward circuit when their whiskers get stimulated and helps in navigation while moving (Ferezou et al. 2007). Few studies (Sherman and Guillery 2013) have postulated the trans-thalamic pathway via which the projections from the layer V of the somatosensory cortex travel to the primary motor cortex to serve this function. The proprioceptive inputs are processed mainly in area 3a and 2. From the peripheries, cutaneous afferents project to areas 3b (Sherman 2016). The inputs from area 3b are usually projected to areas 1 and 2. Area 1 processes the information for texture and area 2 processes regarding size and shape. Thus, it can be ascertained that the loss of function pertaining to this area depends upon the sub-parcellation of sensory functions (Kaas et al. 1979). Furthermore, clusters of neurons are compartmentalized in the form of columns, which are the proprioceptive units of the S1. Each column, spanning across the histological layers receive inputs, process them, and send outputs. Inputs, usually from the thalamus are received by layer IV of the cerebral cortex and outputs, going to various regions of the brain emanate from the pyramidal neurons of layer V (Welker and Woosley 1974). The neurons occupying particular column is committed to respond when a particular type of receptor in the stipulated body area is stimulated. Similar to visual processing, if one cluster with larger receptor fields responds to all types of stimulation in a particular area, their counterpart clusters tries to process the complex aspects of stimulation.
How Submodalities of Touch Are Segregated in the Sensory Cortex?
Mountcastle VB and colleagues (Mountcastle 1998) proposed that the cluster of neurons can be segregated further based on their capability to respond to sustained indentations. His experiments, involving penetrations of vertical microelectrodes in the S1 of monkeys, found out neurons that were responding to the same sort of mechanical stimulus were aligned perpendicularly to the cortical surface and this paved the way for coining the threshold concept related to columnar organizations (Mountcastle et al. 1990). When we receive such stimuli, neurons which respond throughout the stimuli could be deemed as slowly adapting ones and neurons which respond only to the initiation or caseation of the stimuli could be deemed as rapidly adapting. Romo and colleagues (Romo et al. 1998) found that when animals are presented with pulses resembling vibratory flutter, only rapidly adapting neurons could sense them. There exists controversy regarding what exactly determines the specifications of the cortical neurons with two school of thoughts. One is grounded on the fact that differential inputs to the cortex deciding the specifications and the other pointing the role of functions related to touch such as texture, intensity, and motion of the stimuli.
Underpinnings Regarding the Sensory Homunculus
The representation of the various areas is usually organized and this can be represented in the form of a disproportionate and discontinuous caricature of the body, the sensory homunculus. The homunculus is oriented in such a way that regions of the head occupy the inferior part; upper limb occupies superior part of the post central gyrus. The sensations from the lower limb and perineum are represented in the medial portions of the gyrus, near the para-central lobule. The reason for disproportionate representation can be rooted back to evolutionary functionality of the particular species. For example, the vibrissae of the rats play the key function in executing the navigation and thus, they are densely represented in the S1. In fact, the sensory signal from each of the vibrissae goes to the committed collection of neurons and they are arranged in the form of barrels. A typical barrel could be described as the oval shaped dense collection of neuronal cell bodies with a hollow in the centre, made of neuropil. The representation of the rat vibrissae or human digits is determined by the combination of genetic factors and inputs relayed by afferents, which determines the specification within the area.
With evolution, the limbs of primates evolved to act as the tactile sensing organs. This change has increased the representation of the hand and digits to a remarkable extent. In addition, this helps humans in performing precise, fine motor activities in association with cerebellum and basal ganglia. The rich sensitivity of the lips and vague tactile feelings in back of the body can also be explained by this differential representation. Differences in interpretation of the stimulus also depend upon whether it is active touching, otherwise known as haptics, which usually activates multitude classes of mechanoreceptors or passive touching.
Implications Regarding Somatosensory Cortex in the Clinics
The above discussed knowledge plays an important role in understanding the wide spectrum of clinical presentations in clinical neurology. Sensory discrimination is a complex phenomenon which requires the multitude processing of the entire somatosensory unit. Lesions resulting in compromised functioning of the S1 present with abnormal sensations and feeling of numbness on the contralateral side of the body. The patient could sense the pain and temperature sensations only in a crude form. This is due to the inability to evaluate the precise form, intensity, and location of the presented stimuli. Higher cortical functions such as stereognosis require assimilation of prior inputs, usually the stored associated memories from other parts of the cortex. For example, when two coins of different weight are placed in the hands of blindfolded person, he tries to associate the texture with the memories and identify the object. Semmes and colleagues (Semmes et al. 1960) proposed that the effect of lesion differs depending upon the side of lesion and this can be attributed to the cerebral dominance. Left-sided lesions tend to impair the sensations of both sides whereas the right-sided lesions have impairments confined to left side alone. On the other hand, right-sided lesions tend to impair the ability to detect the direction of the provided stimuli and tactile maze learning (Corkin et al. 1965). Contrastingly, stimulation of S1 often elicits numbness and tingling sensation in the contralateral body surface which are not accompanied by pain and temperature sensations. This could be evidenced as a part of “sensory march” during the aura phase of seizures related to this area. Head and Homes (Head and Holmes 1911) suggested that dysfunction of parietal sensory cortex leads to minor impairments which are mostly missed out during the clinical examination. For example, in such conditions, sensory perception attains fatigue and often, patients find it difficult to distinguish subsequent contacts at same time. Similarly, patients try to disregard the stimuli presented on the impaired side when two stimuli are given simultaneously on both sides (Carmon and Benton 1969). Presumably, this feature could be used to discriminate the type of lesion during routine neurological examinations.
Additional Pearls Regarding S1 from the Laboratories
An underemphasized role of S1 is related to the precision grip generation which is mediated via its connection with primary motor cortex. Studies (Davare et al. 2011; Hikosaka et al. 1985) involving inactivation of S1 in monkeys have demonstrated impaired prehensile activities such as difficulty in opposition and grip forces. Even though isolated inactivation could not be studied in humans, it has been observed that when the digits are anesthetized, coordination became impaired (Brochier et al. 1999). Zainos and colleagues (Zainos et al. 1997) found that after removal of S1, an animal could detect the stimuli but could not categorize stimulus speed. Few studies also have corroborated abnormalities in S1 in fraction of patients suffering from focal dystonia of hand. Bara-Jimenez and colleagues (Bara-Jimenez et al. 1998) documented degradation of the conserved organization of S1 in these patients along with expansions of finger representations in corresponding cortical centers. Studies (Harrison et al. 2013; Winship and Murphy 2009) in which stroke was experimentally induced in rodents, a shift in the somatosensory map was observed. This was largely because neurons which were previously involved in integrating motor commands shift their roles and take minor roles in sensory processing (Wolpert et al. 1995).
A significant feature related to the S1 is the visual enhancement of touch, which potentially modulates the visual acuity (Taylor-Clarke et al. 2002). When the subject views the location where stimulus is being provided, then, the tactile acuity tends to improve and this might be due to some under emphasized cross modal interaction between visions and touch (Saal and Bensmaia 2014). The purported reason could be that viewing of the stimuli location improves the degree of representation of tactile space for that prescribed body part and this can be compared to visualizing using magnifying lens (Schaefer et al. 2005a). Thus, we could infer that S1 could function beyond just being a veridical station for peripheral inputs because of its ability to converge tactile perception and bodily representation, modulated by other inputs such as vision. Clubbing the sensorimotor integrative functions of the S1 along with the concept of widening of “body space” purported by the visual enhancement of touch, we could infer that a potential feed-forward model operates over there by combining the sensory consequences of the intended motor command and predicted motor outcomes (Haggard et al. 2003; Schaefer et al. 2005b). Indeed the motor learning, which involves identification of the mismatch between the intended and real sensorimotor states and recalibrating by using necessary motor commands, is facilitated using S1 (Desmurget et al. 2009). Furthermore, integration of somatosensory information from both hands helps the primates to perform coordinated tasks and tactile based explorations. Few studies (Tommerdhal et al. 2006) have shown that finger stimulation of one hand might lead to altered perception of the contralateral side.
Somatosensory cortex also has a role in recognizing emotions. Adolphs and colleagues (Adolphs et al. 2000) documented that patients with lesions involving S1 had impaired ability in categorizing and rating the intensity of emotions. The role related to somatosensory processing might presumably be due to the connection with amygdala. The empathic responses, which get induced on observing others’ states (Straube and Miltner 2011), have associated sensory dimension (Keysers et al. 2010). Upon recognizing the facial expressions (Adolphs et al. 1996) and self-generated emotions (Damasio et al. 2000), there is an associated activation of somatosensory cortices. In addition to the discriminatory component, the affective component which involves emotional processing involves synchronized action of orbitofrontal and primary somatosensory cortices (McGlone et al. 2007). For example, nociceptive stimuli usually evoke negative reactions such as punishment, whereas pleasant stimuli like caressing touch tend to evoke positive rewarding reactions (Rolls et al. 2003). When the stimuli are given subsequently, we try to pre-empt the outcome by comparing with previous experience. Sometimes, a mere thought regarding the experience is enough to make us imagine the prior outcome. To sum up, somatosensory cortex, containing conserved representations of the body, maintains the internal and external awareness states (Critchley et al. 2004) and this can be equated with interoceptive attention. Studies (Braun et al. 2005) using functional imaging have documented higher activation of somatosensory cortex when an individual starts bothering about the existing emotional state.
Plasticity of Somatosensory Cortex
The somatotopic maps, even though being conserved for respective body surfaces, have mutable degrees of plasticity. This can be observed in conditions such as amputation, where the input coming to a restricted area gets lost. In due course of time, the restricted area becomes responsive to neighboring responsive surfaces. Ramachandran VS and colleagues (Ramachandran et al. 2010) did significant amount of research to found the cross activation of neighboring areas after a hand is being amputated. This induces a form of tactile illusion which is referred as phantom limb (Ramachandran and Hirstein 1998). Amputees in such cases experience the touch to their face as local and referred sensation on the phantom hand which can be deemed as altered reference fields. In one of the models (Flor et al. 2006) developed for pain due to phantom-limb, it was observed that activation of afferents from the face and stump of missing limb elicited sensations in the perceptual areas of missing limb. But these sensations felt in the amputated limb is mostly painful (McCabe et al. 2005), which is plausibly due to the incongruent mismatch happening at different levels of sensorimotor pathway. In addition, expression of genes aiding in synesthesia helps in cross-activation throughout the brain, and in professionals such as pianists the expression is significantly higher.
In addition to the limited cortical reorganization, cortical neurons demonstrate significant degrees of axonal plasticity. After subcortical lesions, it was found that ascending projections from thalamus to sensory cortex were bypassing the lesion areas (Staudt et al. 2006) and reaching their destination in the post-central gyrus. This suggests that the sensory neurons which convey sensations from periphery to specified sensory areas are “committed” much earlier in developmental stages.
- Mountcastle, V. B. (1998). Perceptual neuroscience: The cerebral cortex. Cambridge, MA: Harvard University Press.Google Scholar
- Ramachandran, V. S., Brang, D., & McGeoch, P. D. (2010). Dynamic reorganization of referred sensations by movements of phantom limbs. Neuro Report, 21, 727–730.Google Scholar
- Semmes, J., Weinstein, S., Ghent, L., & Teuber, H. L. (1960). Somatosensory changes after penetrating brain wounds in man. Cambridge, MA: Harvard University Press.Google Scholar
- Sherman, S. M., & Guillery, R. W. (2013). Thalamocortical processing: Understanding the messages that link the cortex to the world. Cambridge, MA: MIT Press.Google Scholar