S1 somatotopic maps
|Stuart Wilson and Chris Moore (2015), Scholarpedia, 10(4):8574.||doi:10.4249/scholarpedia.8574||revision #150585 [link to/cite this article]|
S1 somatotopic maps refers to spatial patterns in the functional organization of neuronal responses in the mammalian primary somatosensory cortex (S1/SI). Here the term ‘map’ refers to a population of neurons that respond selectively to the presence of stimuli that collectively sample from an underlying stimulus space. Maps are referred to as ‘somatotopic’ when that space is related to locations on the body, such that adjacent neurons in the neural tissue respond selectively to stimuli presented to adjacent locations on the body.
SI refers to a somatosensory neocortical area that responds primarily to tactile stimulation of the skin (and/or hair). Among neocortical areas, neurons in SI representations typically have the smallest receptive fields and receive the shortest-latency input from the sensory periphery. This area is properly defined as containing a single map of the periphery (for example, as found in Brodmann area 3b of the monkey and human), though in some cases several areas in the post-central gyrus of the primate are referred to as SI, in historical deference to early mapping methods that did not recognize the presence of multiple maps in this gyrus as discussed below.
Discovery of somatotopic maps
The modern concept of systematic body maps in the brain is most directly attributable to the ideas of British physician John Hughlings Jackson, who in 1886 published the observation that epileptic seizures progress across the body in somatotopic sequence, i.e., with spasms progressing from the hands to the arms to the shoulders, or originating from the feet and progressing up the leg to the trunk, in what is now commonly referred to as a Jacksonian march. Based on these observations, Hughlings Jackson inferred the presence of orderly representations of the body in the brain, and particularly in the cortical hemispheres (see York and Steinberg, 2011, for a recent review). In 1870 German medical professors Gustav Fritsch and Eduard Hitzig published a seminal study showing that twitches of specific muscles could be caused by electrical stimulation of specific sites in the frontal cortex of dogs (see Taylor and Gross, 2003). In 1874 physician David Ferrier published The Functions of the Brain, a book dedicated to his mentor John Hughlings Jackson, detailing numerous lesion and electrical stimulation experiments on the cortices of birds, cats, guinea-pigs, rabbits, rodents, dogs, jackals, and monkeys. Ferrier used long trains of electrical stimulation, which often yielded more complex and coordinated movements of muscle groups than the local twitches of individual muscles elicited by the shorter electrical pulses delivered by Fritsch and Hitzig (see Graziano, 2009, for a review). This book introduced some of the first illustrations of the localization and systematic mapping of motor function to specific cortical sites, as drawn by the brother-in-law of Ferrier, painter Ernest Waterlow (Sandrone and Zanin, 2014). However Ferrier was not convinced that maps for touch in the cortex were also topographically organized; “...I am inclined to think that the experimental evidence is against any absolute differentiation of centers of tactile sensation for special regions [. . . ] though probably the various motor centres are each anatomically related by associating fibres with corresponding regions of the falciform lobe. This association would form the basis of a musculo-sensory localisation.” (page 344-345; Ferrier, 1886). Some of the earliest images of high-resolution motor maps in primates originate in Leyton and Sherrington (1917) (see Lemon, 2008 for a historical perspective).
The first evidence for localization of cortical responses to somatosensory stimuli (rather than motor responses) came from two case studies on epileptic human patients (a 15 year old boy and a 44 year old man) reported by Cushing (1909). Cushing found that weaker electrical stimulation of the cortex could elicit the subjective experience of somatosensation localized to specific parts of the hand, without eliciting muscle movements; “. . . in both of these patients stimulation of the post-central convolution gave definite sensory impressions which were likened in one case to a sensation of numbness, and in the other to definite tactual impulses.” (p53). These observations paved the way for the discovery and investigation of SI somatosensory maps in the decades to follow.
The somatosensory homunculus
The prototypical SI somatotopic map is the human somatosensory ‘homunculus’, meaning ‘little man.’ In the context of somatotopic maps, the term homunculus was introduced by Wilder Penfield in what remains one of the most influential articles in neurology, summarizing data from electrical stimulation of the cortex in 126 operations performed on conscious epileptic patients (Penfield and Boldrey, 1937).
The somatosensory homunculus is now synonymous with cartoon drawings of the little man commissioned by Penfield to artist Mrs H. P. Cantile, however Penfield and Boldrey (1937) employed a variety of formats to depict their original data, including; i) detailed dictations of patients’ subjective reports of tactile sensation and movements elicited by stimulation at each cortical site; ii) accompanying photographs of the exposed cortex overlaid with paper tickets labeling the corresponding stimulation sites; iii) drawings of the cortex marked with the location of stimulation sites corresponding to each body part; iv) drawings in which stimulation sites reliably correlating with specific body parts are enclosed by (often largely overlapping) boundaries; v) bar charts for motor and sensory responses indicating the proportion of responses evoked by stimulation of each major body part and drawn in the sequence in which they tend to appear along the cortical surface; and vi) a cartoon corresponding to the bar charts drawn with familiar outlines of the shape of the body parts to depict the now famous homunculus. In the words of Penfield and Boldrey (1937); “The homunculus gives a visual image of the size and sequence of cortical areas, for the size of the parts of this grotesque creature were determined not so much by the number of responses but by the apparent perpendicular extent of representation of each part” (p431).
The accuracy and usefulness of Cantlie's homunculus cartoons, which were further developed in the book of Penfield and Rasmussen (1950), have been questioned by many, notably by Schott (1993) who highlights the mismatch between the sharp topological borders suggested by the clean lines of the drawings, and a considerable overlap of projections from the body surface as depicted in format iv by Penfield and Boldrey (1937). Nonetheless, these images have been highly influential in suggesting two features of the human SI map organization: First, an exaggerated cortical territory dedicated to the somatosensory and motor representation of the hands and face; and, second, somatotopic discontinuities at the junction between the hands and face representations and at the junction between the feet and genital representations.
Soon after the publication of Penfield’s homunculus, Woolsey (1952) published an image of the somatosensory homunculus for the primate, depicting a single continuous map spanning across the four cytoarchitectonic bands of the anterior parietal cortex, Brodman areas 3b, 1 (posterior cutaneous field), 2, and 3a (for details see the Scholarpedia article by Kaas, 2013). At a finer-scale resolution, Kaas et al. (1979) found that a full representation of the primate body is repeated approximately four times within the post-central gyrus, such that four homunculi lie in parallel to each other. Woolsey (1952) and Kaas et al. (1979) derived their somatotopic maps not by stimulating the cortex, but instead by measuring electrical responses to the delivery of cutaneous stimulation (i.e., touch) to the body surface. Measured in this way, virtually all mammals tested have revealed similar systematic body maps in their respective SI, and a corresponding cartoon and “unculus” suffix has been used to describe many of them. For example, the term ‘simculus’ was used by (Woolsey, 1952) to describe the organization in monkeys, the term ‘ratunculus’ was used by Woolsey and LeMessurier (1948) for the organization in rats (see also Welker, 1976), and the terms ‘molunculus’ for moles and ‘platypunculus’ for platypus have also been used colloquially. SI somatotopic maps thus appear to be a highly conserved organizational principle across the mammalian lineages (see Krubitzer, 1995, 2007).
The somatotopic discontinuities at the feet/genital and hand/face representations of the original homunculi were celebrated in a song by Penfield’s colleague Kershman, which includes the verse “The sensory type he was leering/With hand neatly balance on thumb/His happiness founded on things near his toes/That need not always be numb” (Feinsod, 2005). Penfield and Boldrey (1937) were more candid in describing the foot/genital discontinuity, noting; “Presumably rectum and genitalia should be placed above feet, that is within the longitudinal fissure, but our evidence is not sufficient for conclusion and they seem to be somewhat posterior to feet.” (p433). Despite this observation, the co-localization of the feet and genitals in the original homunculus images has persisted as part of neuroscience “folklore” (see Parpia, 2011 for a comprehensive review). For example, Ramachandran has speculated that invasion of the genital region of the cortex by adjacent territory otherwise dedicated to representing the feet may explain the mis-localisation of sexual pleasure to the feet reported by some lower limb amputees (e.g., Ramachandran and Hirstein, 1998).
A number of more recent fMRI imaging studies have restored the tactile representation of the genitals to their somatotopically consistent position in the map adjacent to the trunk, e.g., see Kell et al. (2005) for males, Michels et al. (2010) for females. A possible explanation for the hands/face discontinuity is given by Farah (1998), who hypothesized that the organization reflect the position of the fetus in the womb with its hands and face (and feet and genitals) often touching at the same time. The coincident activation arising from their spatial proximity was predicted to drive Hebbian plasticity, and therefore adjacency within the map. Computer models later showed how such mapping could emerge, reproducing continuous somatotopic maps in self-organizing neural networks, with selective discontinuities between co-stimulated areas of the simulated body surface (Stafford and Wilson, 2007). This model also demonstrated that Hebbian learning mechanisms alone cannot account for the consistent medial-lateral ordering of somatotopic body maps. Models of this nature have been criticized for failing to incorporate subcortical contributions to the discontinuities that may originate from divergent afferent projections at the spinal cord (Parpia, 2011).
Studying images of the somatosensory homunculi can give the impression that SI somatotopic maps remain fixed, but there is strong evidence that maps are highly plastic in the proportion and ordering of representations on a variety of timescales, ranging from the trajectory of development and maturation of cortical circuitry to the rapid allocation of attention. Florence et al. (1996) showed that cutting and then repairing the nerves of the hand in monkeys, so as to destroy the topology of its projections into the brain, can still yield somatotopic maps for the hand in SI, thus demonstrating the ability of the cortex to rewire itself during development to reflect the functional (rather than strictly anatomical) relationships between adjacent body parts. Braun et al. (2001) found that tactile representations of the fingers were localized to different positions in SI when human subjects were using the hand to write, as compared to when the hand was at rest, thus suggesting that somatotopic maps dynamically re-organize according to specific motoric tasks. Using similar methods Elbert et al. (1995) found that tactile representations of the fingers in the left hand of string players were larger than in controls (and larger than for the right hand), and they report a strong correlation between the age at which musicians began playing and the magnitude of the exaggerated cortical representation, again suggesting a functional remapping of SI due to motor experience. Similar methods have implicated distortions in somatotopic maps of the hand as an underlying factor in dystonia (Bara-Jimenez et al., 1998).
In humans, Weiss et al. (2000) reported that tactile representations of fingers left intact after others of the same hand were amputated became expanded in under ten days post-amputation, as compared to representations in the other hand of the patient or of either hand in controls. SI representations expand in owl monkeys for tactile stimulation of digits receiving tactile input during behavioral tasks in areas 3b (Jenkins et al., 1990) and 3a (Recanzone et al., 1992). In owl monkeys Merzenich et al. (1984) measured a dramatic functional reorganization of the receptive field properties of SI neurons and in the overall somatotopic map of intact digits following digit amputation, supporting a model in which cortical territories dedicated to spared digits invade those no longer receiving input from the missing digits. These data have also been well captured by self-organizing Hebbian models of somatotopic map formation (e.g., Ritter et al., 1992).
The evidence for somatotopic remapping considered in this section was focussed on primate S1, and we direct the reader to important further studies on primate S1 plasticity by Moligner et al., (1993), Moore et al., (2000), and Tegenthoff et al., (2005). The next sections focus on somatotopic maps in other species.
Cortical magnification of highly relevant tactile representations
A key feature of SI organization that is preserved across mammalian species is that different areas of the somatosensory surface are magnified depending on the behavioural relevance of the corresponding sensors. As one example, the representation of the face and hands in human SI occupy disproportionately larger cortical territory relative to their dermatopic size than other body representations such as the trunk. Krubitzer et al. (1995) similarly describes an exaggerated representation of the electro-tactile bill of the platypus, thought to help it localize underwater prey (Pettigrew et al., 1998). Similarly Catania and Kaas (1997) describe the SI representation of the tactile appendages on the face of the star-nosed mole, proposing that the particularly magnified representation of the central appendage constitutes a foveal system in much the same way as the high-resolution sampling of the center of the retina is exaggerated in retinotopic maps (see Catania, 2012 for a recent review).
A consistent corollary of expanded representation size for a given representation relative to its spatial occupation of the skin surface is a relative decrease in the size of receptive fields of individual neurons within that SI representation (Sur et al., 1980). This principle of cortical magnification and receptive field restriction can be preserved when the elaboration of cortical representation is driven by plastic changes in the adult (Merzenich et al., 1984).
The barrel cortex
The vibrissae or ‘whiskers’ in many species have a particularly highly magnified representation. Since it was first comprehensively characterized by Woolsey, Welker and colleagues, the cortical representation of the vibrissae in rodent S1 has become one of the most important models in modern neuroscience, due to the precise correspondence between the individual sensor (the vibrissa or ‘whisker’) and the cortical column, called a ‘barrel’, with which it correlates most strongly (Woolsey and van der Loos, 1970; Welker and Woolsey, 1974, Welker 1976). In mice the barrel cortex comprises around 70% of SI, and 13% of the entire cortical surface (Lee and Erzurumlu, 2005).
The rodent barrel cortex contains a grid of discrete architectonic units, one per vibrissa, which after staining are visible to the naked eye. The pattern of barrels directly corresponds to the layout of the whiskers on the face, such that adjacent whiskers are principal to adjacent barrels. In layer 4 of the mouse, each unit is delineated in the plane tangential to the surface of the brain by a perimeter of cell bodies that is shaped like a barrel. The pattern of delineations between barrels has been described as Dirichlet domains (Senft and Woolsey, 1991). Within each barrel, changes in synaptic contact density reveal regular geometric patterns (Land and Erickson, 2005; Louderback et al., 2006; see Ermentrout et al., 2009).
When the coresponding vibrissa is deflected, neurons of a given barrel respond with action potential firing at shorter latency and with greater magnitude than the others. Thus, the approximately 10,000 neurons comprising each barrel column (Beaulieu, 1993) tend to be mapped primarily to a specific vibrissa in their action potential output, called the principal whisker. Important nuances exist in this assertion, however: Neurons within the barrel column often very large ‘subthreshold’ fields driven by vibrissae across the face (reviewed in Moore et al., 1999), and a significant number of individual neurons will respond best to non-aligned vibrissae, particularly when the more global pattern of stimulation is altered (Jacob et al., 2008).
The map for vibrissa identity amongst the barrels of granular layer 4 barrel cortex reflects a similar organisation between nuclei known as barrelettes in the brainstem (Ma and Woolsey, 1984), and as barreloids in the thalamus (van der Loos, 1976). Upon vibrissa deflection, a rapid pathway for excitation propagates along the neuraxis from barrelette, to barreloid, to barrel, to regions of the supragranular and infragranular cortical layers that are aligned to the corresponding barrel, and laterally into adjacent supra- and infra- granular barrel regions (Armstrong-James et al., 1992; Lefort et al., 2009). It had been suggested that during development the organization of topographic vibrissal maps unfolds in sequence along the pathway, with each vibrissal identity map inheriting the organisation from the antecedent layer (Killackey, 1980).
Species with vibrissae that have been found to have barrels include the mouse, rat, hamster, gerbil, muskrat, chipmunk, grey squirrel (although less pronounced), prairie dog, guinea pig, chinchilla, porcupine, mole, rabbit, ferret, wallaby, and the Australian opossum. Species with vibrissae that have been found not to have barrels include the beaver, tree shrew, cat, dog, raccoon, squirrel monkey, rhesus monkey, and the American opossum (summary based on Table 1.1 of Fox, 2008; see also Rice, 1995).
For a comprehensive review of the anatomy and physiology of the barrel cortex see Fox, 2008, see related Scholarpedia article on laminar specialization in barrel cortex by Ahissar and Staiger (2010), and see the recent article by Meyer et al., (2013). For a comprehensive review on plasticity in the barrel cortex see Feldman and Brecht (2005) and see related Scholarpedia article on S1 long-term plasticity by Shulz and Ego-Stengel (2012).
Somatotopic feature maps in the barrels
Within the vibrissal SI representation of rats, additional maps have been discovered that are organized in a somatotopic fashion. Barrel cortex neurons respond selectively depending on the direction in which the corresponding whisker is moved (Simons and Carvell, 1989; see also Hellweg, Schultz and Creutzfeldt, 1977). A feature map representing the direction of vibrissal movement is somatotopically aligned within a single barrel column in a pinwheel type fashion (Andermann and Moore, 2006; see also Bruno et al., 2003). Neurons preferentially responsive to the direction of vibrissal deflection towards a given neighboring vibrissa are represented in the somatotopic portion of the barrel column more proximal to that neighboring vibrissa (see also Tsytsarev et al., 2010). Evidence suggests this map is present only in older animals (Kerr et al., 2007), suggesting it may emerge through activity-dependent plasticity (Kremer et al., 2011, see also Wilson et al., 2010). A recent model proposed that the large size of the barrels and relatively slow signal propagation speeds could render neurons at different locations with respect to the barrel centers sensitive to the relative timing of adjacent-whisker deflections, yielding a somatotopic map for the relative timing of tactile stimuli (Wilson et al., 2011; based on evidence from Shimegi et al. 2000 and Jacob et al. 2008). Neimark et al. (2003) have also proposed that the increasing length of the vibrissae from the snout to the ear renders neurons in barrels for the longer vibrissae sensitive to lower resonant frequency vibrations, yielding a somatotopic map of vibration frequencies that spans the barrel cortex in rodent SI, similar to the tonotopy observed in auditory representations.
As the barrel cortex becomes an increasingly important model system in neuroscience, and as efforts become focused on simulating its microciruitry in exquisite detail (Markram, 2006), it is becoming increasingly important to develop complementary models that address how constraints imposed by the overall somatotopic maps in which these circuits are embedded might impact on cortical function.
An important and open question is whether spatial patterning and map organization in the brain is important for neural computation, or whether somatotopic maps, and topological mapping in the brain more generally, are epiphenomena of efficient developmental processes (see Purves, Riddle and LaMantia, 1992, and Wilson and Bednar, 2015 for further discussion).
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