Imaging human touch

• Dr. Philip Servos, Wilfrid Laurier University, Waterloo, Ontario, Canada

Since the early 20th century researchers have used techniques to visualize the neural organization of the human touch system. Imaging human touch covers research from the earliest known work involving electrical stimulation mapping to more recent techniques such as event-related potentials, magnetoencephalography, positron emission tomography, and functional magnetic resonance imaging. Topics include somatotopy, the cortical processing of perceptual attributes such as shape, texture, and hardness, the neural bases of tactile illusions as well as the time course of somatosensory processing.

Early Work Localizing Touch to the Postcentral Gyrus

As with other sensory modalities early work in the 1870s investigating the cortical representation of human touch focused on the effects of brain damage (typically tumours and vascular accidents). Much debate centered on the degree to which the pre- and post-Rolandic gyri (respectively, the precentral gyrus and postcentral gyrus) were involved in the cutaneous senses (Dana, 1888).

Electrical Stimulation Mapping

The work of Harvey Cushing

Subsequent work (in particular that of Harvey Cushing) at the turn of the 20th century localized the cutaneous senses to the postcentral gyrus. The electrical stimulation studies in humans by Harvey Cushing ushered in the modern era of mapping out the human touch system (Cushing, 1909).

Figure 1: Diagram of stimulation sites along the central sulcus that resulted in cutaneous sensations published by Cushing in 1909 (Figure 2). Region to the right of the ‘central fissure’ represents the postcentral gyrus.

The work of Wilder Penfield

Cushing’s methods were refined in the 1930s at the Montreal Neurological Institute by Wilder Penfield (Penfield & Boldrey, 1937). By this point Brodmann’s areas were commonly used to describe cortical regions of the brain and Brodmann’s areas 3b, 1, and 2 – all within the postcentral gyrus – were collectively known as primary somatosensory cortex (S1). Penfield did not attempt to differentiate between these subregions within S1. Working mainly with epileptic patients in whom pre-surgical explorations were being made Penfield used the electrical stimulation mapping technique to map out many aspects of brain function including the touch system. Small sterile tags were used to keep track of the stimulation points on the surface of the brain (see figures 2 & 3). Penfield’s work on the cortical representation of human touch is widely known, in part for his much reproduced somatosensory homunculus.

Figure 2: Lateral view of exposed cortex – anterior regions to right of image – posterior regions to left. The superior portion of the postcentral gyrus is indicated by tag ‘2’ and tag ‘3’ corresponds to a relatively inferior region of the postcentral gyrus. Tag ‘1’ corresponds to a superior portion of the precentral gyrus and tag ‘7’ corresponds to a relatively inferior region of the precentral gyrus. Figure 11 from Penfield (1958).

Figure 3: Diagram from Penfield and Boldrey (1937) depicting the overwhelming number of stimulation sites located within the postcentral gyrus (as opposed to the precentral gyrus) that led to cutaneous sensations. Note the relative overlap of the digits at the various stimulation sites.

Figure 4: Somatosensory Homunculus as depicted in Figure 17 of the 1950 Penfield and Rasmussen book, The Cerebral Cortex of Man.

An additional insight from Penfield’s work is that there 
might be multiple somatotopic maps for certain regions of the body. For example, he found evidence for two separate cortical representations of the hand (Penfield & Rasmussen, 1950). One of these, area S1, was located in the postcentral 
gyrus whereas the other representation, was located in the upper bank of the Sylvian fissure. Much work since then has confirmed the role in touch of the secondary somatosensory cortex (S2) located in the parietal operculum (Brodmann’s areas 40 & 43).

There were, however, limitations to Penfield’s methods. He examined primarily epileptic patients – it is possible the cortical organization of such patients may differ from that of neurologically intact research subjects (Maegaki et al., 1995; Maldjian et al., 1996; Weiller et al., 1993). Consider too the possibility of the spread of charge in the electrical stimulation mapping method and that the method was restricted to the cortical surface. A large proportion, probably 30-40% (Geyer et al., 1999, 2000) of primary somatosensory cortex, including the entire extent of area 3b (Geyer et al., 1999, 2000; White et al., 1997) is located in the fundus and posterior bank of the central sulcus. The presence of blood vessels within the most superior aspects of the central sulcus would not have allowed Penfield to stimulate these regions of the postcentral gyrus. Nevertheless, much has been learned from this work and Woolsey and colleagues have subsequently confirmed these findings and extended them by not only using electrical stimulation mapping techniques on exposed cortex of humans but also recording the electrical activity from these regions (Woolsey et al., 1979).

Noninvasive Methods of Somatotopic Mapping

Positron emission tomography (PET)

The advent of non-invasive imaging techniques in humans in the 1980s – positron emission tomography (PET) and the 1990s – functional magnetic resonance imaging (fMRI) – allowed for further advances in mapping human touch. Much of the human neuroimaging work has been inspired by discoveries of the neural bases of touch in the non-human primate brain by Jon Kaas and his colleagues (Kaas et al., 1979). Perhaps the most telling finding from this work is that four different cortical maps of the body surface exist in the primate – in a rostral-caudal gradient (from areas 3a, 3b, 1, and 2). At least two of these maps (3b and 1) involve cutaneous receptors and would play a critical role in human touch.

Figure 5: Summary diagram from Figure 1 of Kaas et al. (1979) depicting multiple somatotopic maps in S1 of the owl monkey.

The earliest PET work imaging human touch was published in 1978 (Reivich et al., 1978). Early PET studies involved stroking of the hand with a brush. Increased PET signal occurred in what appeared to be the contralateral post-central gyrus (Greenberg et al., 1981; Reivich et al., 1979). Subsequent work in the 1980s refined this work and showed in greater detail the activation produced by contralateral hand stimulation (Fox et al., 1986, 1987; Hagen & Pardo, 2002) and when subjects haptically identified textured tiles (Ginsberg et al., 1987). Distinct regions within the contralateral postcentral gyrus were also identified for the lips, fingers, and toes (Fox et al., 1987). Stimulation of the fingers also produced activation within posterior parietal regions (Hagen & Pardo, 2002) as well as within S2 both contralaterally and ipsilaterally (Burton et al., 1993; Hagen & Pardo, 2002).

Functional magnetic resonance imaging (fMRI)

Finger somatotopy

On/off paradigms

With the increasing popularity of fMRI in the 1990s, due in part to its higher spatial resolution (PET voxels typically in the 1-1.5 cubic cm range as opposed to fMRI voxels in the 5 cubic mm range), the field shifted away from PET studies of touch. fMRI work has focused primarily on the somatotopic organization of the arm (Servos et al., 1995, 1998), face (Eickhoff et al., 2007, 2008; Huang & Sereno, 2007; Iannetti et al., 2003; Kopietz et al., 2009; Moulton et al., 2009; Servos et al., 1999) and with a particular emphasis on the fingers (Blankenburg et al., 2003; Kurth et al., 1998; Martuzzi et al., 2014; Overduin & Servos, 2004, 2008; Polonara et al., 1999; Puce et al., 1995) or the tips of the fingers (Francis et al., 2000; Gelnar et al., 1998; Maldjian et al., 1999; McGlone et al., 2002; Nelson et al., 2008; Sakai et al., 1995; Schweizer et al., 2008; Stippich et al., 1999). The bulk of these studies involving the fingers have consistently shown activation within contralateral S1 (Eickhoff et al., 2008; Francis et al., 2000; Gelnar et al., 1998; Jang et al., 2013; Kurth et al., 1998; Maldjian et al., 1999; Martuzzi et al., 2014; McGlone et al., 2002; Moore et al., 2000; Nelson et al., 2008; Overduin & Servos, 2004, 2008; Polonara et al., 1999; Puce et al., 1995; Sakai et al., 1995; Schweizer et al., 2008; Stippich et al., 1999), and S2 (Burton et al., 2008; Eickhoff et al., 2007, 2008; Francis et al., 2000; Gelnar et al., 1998; Jang et al., 2013; McGlone et al., 2002; Polonara et al., 1999) whenever the fingers or palm are stimulated.

The relatively high spatial resolution made possible by fMRI has allowed some researchers to identify somatotopic representations within subregions of S1 such as area 3b (Eickhoff et al., 2008; Gelnar et al., 1998; Kurth et al., 1998; Martuzzi et al., 2014; McGlone et al., 2002; Moore et al., 2000; Nelson et al., 2008; Overduin & Servos, 2004), area 1 (Eickhoff et al., 2008; Gelnar et al., 1998; Kurth et al., 1998; Martuzzi et al., 2014; McGlone et al., 2002; Moore et al., 2000; Nelson et al., 2008; Overduin & Servos, 2004), area 2 (Eickhoff et al., 2008; Gelnar et al., 1998; Kurth et al., 1998; Martuzzi et al., 2014; McGlone et al., 2002; Moore et al., 2000; Nelson et al., 2008), and area 3a (Gelnar et al., 1998; McGlone et al., 2002; Moore et al., 2000). Moreover, progress has been made in delineating four possible subregions within S2 namely OP1, OP2, OP3, and OP4 (Burton et al., 2008; Eickhoff et al., 2007, 2008, 2010). Three of these subregions, OP1, OP3, and OP4, have somatosensory functions and correspond respectively to macaque S2, VS, and PV whereas OP2 appears to be involved in vestibular processing (Eickhoff et al., 2007). The subregions sensitive to somatosensory stimuli appear to differ in their patterns of cortical connectivity. Eickhoff and colleagues examined the cortical connectivity of OP1 and OP4 using diffusion tensor imaging. Relative to OP4, OP1 was found to have stronger connectivity with parietal regions (presumably involved in tactile perception) whereas OP4 had stronger connections to frontal motor areas – regions presumably involved in sensorimotor processing and action control (Eickhoff et al., 2010).

Parietal regions caudal to S1 have also been implicated (Francis et al., 2000; Gelnar et al., 1998; Jang et al., 2013; Kurth et al., 1998; McGlone et al., 2002). Some work has also demonstrated the role of ipsilateral somatosensory cortex in processing tactile inputs from the fingers or hand. Ipsilateral activations have been observed in Brodmann’s areas 1 and 2 as well as S2 (Burton et al., 2008; Eickhoff et al., 2008; Kurth et al., 1998).

fMRI work (Maldjian et al., 1999; Martuzzi et al., 2014; McGlone et al., 2002; Nelson et al., 2008; Schweizer et al., 2008) has also confirmed the mediolateral ordering of the digit representations from little finger to thumb along the postcentral gyrus as originally observed with electrical stimulation mapping (Penfield & Boldrey, 1937) and recording from surgically exposed cortex (Woolsey et al. 1979). It should be noted that earlier MEG work also demonstrated this somatotopic ordering of the digits (Baumgartner et al., 1991) [although it is not entirely clear given the limits of the spatial localization of MEG if the activation was specifically localized to the postcentral gyrus] and a more recent optical imaging study as well (Sato et al., 2005).

Phase paradigms

In most of the above-mentioned studies, the BOLD (blood oxygenation level dependent) signal that occurs as a result of cutaneous stimulation is typically compared to a control condition in which no cutaneous stimulation occurs allowing investigators to delineate a somatosensory organization on the basis of distinct activation ‘hotspots’ in response to the stimulation. Another approach is to use a sliding window of stimulation and then analyze the phase characteristics of the BOLD signal. This technique was pioneered for retinotopic mapping studies of visual cortex (Engel et al., 1994; Sereno et al., 1995) and has subsequently been applied to the touch system (Besle et al., 2013; Huang et al., 2007; Overduin & Servos, 2004, 2008; Sanchez-Panchuelo et al., 2010, 2012; Servos et al., 1998). The basic principle of this technique (using an example in which a single finger is stimulated) is that the phase lag of the BOLD signal should be relatively short for cortical regions representing portions of the finger that were stimulated early on in the stimulus paradigm (e.g., the distal pad of the finger) whereas the phase lag of the BOLD signal should be relatively long for cortical regions representing portions of the finger that were stimulated later in the paradigm (e.g., the more proximal portions of the finger). One advantage of the phase-delay technique is that it is a much more efficient method to investigate the relative position of the finger within its cortical representation or the relative position of each finger within for example, S1. Such investigations involving on-off paradigms would require an unmanageable number of on/off experiments at many different locations on the fingers. Early work imaging the arm representation in S1 determined the relative position of the arm within its cortical representation i.e., the relative position of the distal and proximal portions of the arm (Servos et al., 1998). The relative position of the index finger (i.e., its distal and proximal aspects) in areas 3b and 1 has also been imaged using the phase-delay technique (Blankenburg et al., 2003). The phase mapping technique has also been used to confirm the mediolateral ordering of the digit representations from little finger to thumb in S1 (Overduin & Servos, 2004). Later studies have refined these maps (Besle et al., 2013; Huang et al., 2007; Sanchez-Panchuelo et al., 2010). Some fMRI work involving phase mapping has demonstrated a rostral-caudal gradient in area 3b for the distal-to-proximal portions of the finger and a reversal of this gradient in area 1 (Blankenburg et al., 2003). As promising as these results are they are a bit puzzling because one would have expected the gradient in 3b to be in the caudal-rostral direction given the neurophysiological recordings made in monkey S1 (Kaas et al., 1979). More recent fMRI work has demonstrated the expected caudal-rostral gradient in area 3b for the distal-to-proximal portions of the finger. Moreover, the phase reversals between adjacent regions within S1 that are predicted on the basis of unit recording work in the monkey (Kaas et al., 1979) were also confirmed for areas 3a, 3b, 1, and 2 (Sanchez-Panchuelo et al., 2012). In the non-human primate somatosensory cells sensitive to cutaneous inputs within the postcentral gyrus are known to project to the precentral gyrus (Strick et al., 1978). Consistent with this, quite a few human neuroimaging studies of somatosensation have observed activation within regions in the precentral gyrus (Besle et al., 2013; Francis et al., 2000; Huang et al., 2007; McGlone et al., 2002; Moore et al., 2000; Overduin & Servos, 2008). Some fMRI evidence based on the phase mapping technique involving the finger representations in S1 suggests that there may be a relatively high degree of symmetry between the digit representations in S1 and corresponding regions in primary motor cortex, that is, in the precentral gyrus (Overduin & Servos, 2008).

Figure 6: Activation tuning and symmetry of the index finger in flattened representations of pericentral cortex. In each plot the cortex is shown as a 2D surface, oriented in approximate rostrocaudal and lateromedial directions. The plot shows functional data, representing phase lag according to the reference bar at top, superimposed over M1 (rostral – left portion of each plot) and S1 (caudal – right portion of each plot). Figure 4 from Overduin and Servos (2008).

Face somatotopy

Some fMRI work involving the somatotopic organization of the face representation has challenged the original Penfield work depicting the face as being organized in a superior-to-inferior direction along the postcentral gyrus (Penfield & Rasmussen, 1950). Servos et al. (1999) provided evidence that the face representation is in an inverted position relative to the postcentral gyrus consistent with the orientation of the face in S1 in the monkey (Kaas et al., 1979). Earlier MEG work also suggests that the face representation is inverted (Yang et al., 1993). However, other research involving human intra-operative optical imaging studies that electrically stimulated different parts of the face (Sato et al., 2005; Schwartz et al., 2004) as well as an fMRI study (Huang & Sereno, 2007) have supported to some degree the original Penfield work. Some fMRI studies have suggested other organizational possibilities for the face in S1, such as a representation featuring spatial overlap (Iannetti et al., 2003; Kopietz et al., 2009; Nguyen et al., 2004 (note that this is a MEG study)) or a segmented ‘onion-like’ organization that follows the three branches of the trigeminal nerve (Moulton et al., 2009). Non-human primate work is consistent with the latter studies suggesting a relative degree of spatial overlap for the various portions of the face representation in human S1. These studies suggest that there is a relatively complex, interdigitated organization of the face representation in S1 not only in the squirrel monkey (Manger et al., 1995) but also in the macaque (Manger et al., 1996). Interestingly, these latter two studies provide some evidence for a posterior to anterior gradient of the face representation with more superior portions of the face corresponding to more posterior regions in area 3b and more inferior portions of the face corresponding to more anterior regions – rather than the superior inferior gradient for respectively the more inferior and superior portions of the face in the owl monkey as described by Kaas et al. (1979).

Other factors affecting somatotopy

In addition to the standard methods of stimulating the touch system to investigate somatotopic organization (e.g., brush strokes, piezoelectric actuators, air jets, and electrical stimulation) other factors have been investigated namely the interpersonal aspects of the tactile stimulus and the effects of disuse.

Gazzola et al. (2012) observed enhanced activation in S1 when the legs of male subjects were manually caressed by a female relative to when a male did the caressing. Malinen et al. (2014) administered three types of touch to a subject’s hand: poking, holding, and a smoothing movement. The latter two stimuli were considered pleasant as compared to the poking stimulus. Responses within area 3b differentiated between the three types of stimuli although the pleasantness rating of the various stimuli did not correlate with the characteristics of the area 3b response.

Even relatively short periods of limb disuse can have an effect on the nature of the somatotopic maps in somatosensory cortex. Lissek et al. (2009) studied individuals who had their limbs immobilized for around six weeks (their arms were in a cast as a consequence of bone fracture). Following removal of the cast the subjects’ tactile sensitivity of the fingers was lower than control subjects. Moreover, the BOLD activation within the somatotopic maps corresponding to the fingers was also decreased. These effects persisted for two to three weeks

Mapping Studies of Touch-Related Perception and Cognition

Tactile perception of shape, texture, and hardness

In addition to the above-mentioned PET and fMRI studies examining the cortical representation of the body surface, these techniques have also been used to examine the perceptual and cognitive aspects of touch.

PET work by O'Sullivan et al. (1994) provide some of the earliest work dissociating cortical regions for the processing of shape (in this instance, length) and roughness. Roughness discriminations activated primarily the contralateral postcentral gyrus whereas the length discrimination task not only activated the contralateral postcentral gyrus but also contralateral and ipsilateral regions within the angular and supramarginal gyri. Subsequent work, however, suggested that shape and length discriminations activate a region within the intraparietal sulcus (IPS) whereas roughness discrimination activates the parietal operculum (Eck et al., 2013; Roland et al., 1998; Sathian et al., 2011; Stilla & Sathian, 2008). Other work (Bodegård et al., 2001) suggests a hierarchical processing of object features such that tactile motion, length, curvature and roughness are processed within areas 3b and 1, curvature (see also Bodegård et al., 2000) and shape discrimination activate area 2, and the IPS and supramarginal gyrus are activated by shape discrimination (Bodegård et al., 2001). Additional work by Servos et al. (2001) observed contralateral postcentral gyrus activation following haptic texture, hardness and shape discriminations and an additional region within the parietal operculum during hardness discriminations. A more recent study implicates areas 3b, 1, and 2 in roughness perception (Carey et al., 2008). It should be noted that unlike the previously described studies involving discrimination tasks this study involved a simple stimulation task so direct comparisons with these other studies are problematic.

As is evident from the above-cited studies multiple cortical regions appear to be involved in the processing of haptic shape, texture, and hardness. However, currently there is not a clear consensus about exactly which regions are specialized for a given process and which brain regions may play a role in the processing of multiple stimulus attributes.

A recurrent finding in the touch-related perceptual and cognitive imaging literature is that compared to simple tactile stimulation such as that used in most somatotopic mapping studies, when more complicated perceptual and cognitive processes are involved one observes activation not only in S1 and S2 but also in additional cortical regions not classically associated with somatosensation. For example, haptic shape identification tasks activate the striate visual cortex (Deibert et al., 1999), the lateral occipital complex (LOC) (Deibert et al., 1999; Stilla & Sathian, 2008) and inferior parietal cortex (Deibert et al., 1999; Stilla et al., 2007; Stilla & Sathian, 2008). Yang et al. (2012) observed IPS and LOC activation when subjects performed a tactile angle discrimination task and visual cortex activity has been observed during tactile texture perception (Eck et al., 2013; Sathian et al., 2011; Stilla & Sathian, 2008). Tactile localization and spatial acuity are associated with IPS activation (Sathian et al., 2011; Stilla et al., 2007). A further insight from the Stilla et al. study is that IPS activity predicted individual differences in tactile acuity. Additionally, Kilgour and colleagues observed activation within the fusiform gyrus and parahippocampal region during haptic exploration of face masks (Kilgour et al., 2005) and increased BOLD in the fusiform gyrus when subjects haptically scanned familiar faces as compared to unfamiliar faces (James et al., 2006). Taken together the quite common observation of neural activity within brain regions usually associated with visual processing during tactile perception tasks suggests that subjects may be using visual strategies such as visual imagery to help them solve such tasks.

Tactile illusions

The tactile motion aftereffect has been investigated with fMRI (Planetta & Servos, 2012). During the inducing tactile stimulation (a rotating ridged drum in contact with the volar skin of the hand) contralateral activity is observed in S1 and S2. Interestingly, during the period in which subjects experience the illusion i.e., when the drum is stationary and the subjects report feeling as if the drum is rotating in the opposite direction, the BOLD effect is only observed in the contralateral S1 not the contralateral S2.

The neural substrates of the cutaneous rabbit have also been studied. In this illusion a series of rapid taps are delivered to a particular skin location and then immediately delivered at another location 5-10 cm away. Subjects often feel illusory taps along the skin between the two locations as if a small rabbit were hopping along the skin (Geldard & Sherrick, 1972). Blankenburg et al. (2006) used fMRI to identify the somatotopic representation of the forearm in S1 following tactile stimulation to three locations along the forearm. They then examined the neural response to the illusion following stimulation by the two lateral locations along the forearm. Remarkably, during the illusion the central portion of the arm representation was also activated even though tactile stimulation had been restricted to only the two lateral forearm locations.

Work examining tactile-visual illusions such as the rubber hand illusion (Botvinick et al., 1998) have implicated regions posterior to S1 in the IPS that play a role in binding the synchronous visual and tactile stimulation necessary for this illusion and a role of the premotor cortex in establishing body ownership of the rubber hand (Ehrsson et al., 2004). Other work involving the touch-induced visual illusion (Violentyev et al., 2005) has also implicated a role for the IPS as well as the lingual gyrus in binding the tactile and visual streams (Servos & Boyd, 2012).

Tactile-visual interactions

The neural bases of tactile-visual interactions have also been investigated. In research involving cross-modal priming of haptically examined novel objects and visual images of those objects, regions previously considered to be largely visual in nature such as the LOC (Amedi et al., 2001; James et al., 2002) as well the striate cortex and lingual gyrus (James et al., 2002) are also active during haptic exploration of objects. Other work has investigated the relative contribution of non-tactile stimuli (in this case visual) to processing in somatosensory cortex. In a tactile-visual integration task Dionne et al. (2010) observed greater BOLD signals in S1 as compared to when the task involved only tactile stimuli. Consonant with this, Eck et al. (2013) presented subjects with either visual or tactile textures and observed a surprisingly high degree of activation overlap for both texture types in primary somatosensory and primary visual areas.

Mapping the Time Course of Touch: Event-Related Potentials (ERP) and Magnetoencephalography (MEG)

Although fMRI has excellent spatial resolution with many fMRI studies of touch acquiring images with in-plane resolutions of 2 mm or less, it is challenging to obtain second to sub-second temporal resolution using this method. Some questions, such as the time course of information flow within and between brain regions are better answered with techniques that have sub-second temporal resolution such as magnetoencephalography (MEG) and event related potentials (ERPs).

The bulk of ERP and MEG investigations of touch have focused on the fingers. Various ERP components (collectively known as somatosensory evoked potentials (SEPs)) or MEG components (collectively known as somatosensory evoked magnetic fields (SEFs)) associated with finger stimulation (either by direct stimulation of peripheral receptors or by median or ulnar nerve stimulation) have been investigated. The human ERP and MEG work on touch complement and parallel each other quite well so will be discussed jointly.

Early work by Stohr and Goldring on SEPs set the stage for subsequent research (Stohr & Goldring, 1969). Our current understanding of the neural significance of the various SEPs and SEFs is as follows: after the onset of a tactile stimulus on the fingers several event-related components can be seen such as the N25 (Baumgartner et al., 1991; Hari et al., 1993; Schubert et al., 2006), P50, N80, P100, and N140 (Allison et al., 1989a, 1989b; Eimer & Forster, 2003; Forss et al., 1994; Hari et al., 1993). The P50 and N80 components are generated in contralateral S1 and are related to the processing of the physical attributes of the stimulus. About 100 ms after stimulus contact, additional cortical regions are activated such as S2 marked by a parietal P100 and frontal cortices marked by a frontal N140 (Allison et al., 1989b; Eimer & Forster, 2003; Forster & Eimer, 2004). Such mid-latency components are usually ascribed to processing within a frontoparietal network contributing to conscious perception and attention (Allison et al., 1992; Auksztulewicz et al., 2012; Forster & Eimer, 2004; Schubert et al., 2006). Late components such as the P300 are typically related to novelty, change detection, and response inhibition (Auksztulewicz et al., 2013; Nakata et al., 2010) and are observed in contralateral S1, ipsilateral and contralateral S2 as well as frontal regions. Intriguingly, there is evidence suggesting that neural activity in the gamma range (30-70 Hz) may be phase locked between S1 and S2 for the early component SEFs (up to 100 ms post-stimulus onset) but not for the later components (Hagiwara et al., 2010).

SEP and SEF studies have also shed light on the nature of connectivity between S1 and S2. Consistent with neurophysiological work in the monkey time course analyses have demonstrated the flow of sensory signals from S1 to S2 in a serial fashion (Hu et al., 2012; Inui et al., 2004; Schnitzler et al., 1999).

ERP work investigating the neural bases of tactile illusions such as the Aristotle and Reverse illusions has demonstrated that S1 activity is associated with experience of the illusions whereas activity in more posterior parietal regions are associated with the instances in which subjects resist these illusions (Bufalari et al., 2014).

Finally, there is a growing ERP literature related to the active scanning of tactile patterns. Using a simple geometric shape discrimination task, Lucan et al. (2010) observed LOC activation about 150 ms after stimulus presentation (a timeline that is consistent with the time course of visual shape recognition). An ambitious recent ERP study by Adhikari et al. (2014) examined the time course of multiple cortical regions during a tactile acuity task. The earliest cortical signals arose in S1 at 45 ms post-stimulus with subsequent LOC activity at 130 ms, IPS activity at 160 ms, and dorsolateral prefrontal cortex (dlPFC) activity at 175 ms. Strikingly, between 130-175 ms after stimulus onset ERP signals on correct trials differed from those generated during incorrect trials. Adhikari and colleagues analyzed beta band activity (12-30 Hz) and identified a feedforward network involving all four regions. Additionally, they analyzed gamma band activity (30-100 Hz) and identified a recurrent network involving S1, IPS, and dlPFC. Most interestingly, measures of network activity within both bands was correlated with accuracy of task performance.

Conclusion

With the development of non-invasive imaging tools such as PET and fMRI having a relatively high degree of spatial resolution and tools with high temporal resolution such as ERP and MEG much has been learned about the brain regions involved in human touch. Future work will no doubt delineate further the somatotopic maps involved in touch as well as shedding light on the network of brain regions and their timing relationships involved in higher-order touch processes such as tactile object recognition. New imaging techniques such as optical imaging (Habermehl et al., 2012) complement existing methods and inevitably will also move the field forward.

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Sponsored by: Prof. Tony J. Prescott, Dept Psychology, Univ of Sheffield, UK

Reviewed by: Prof. Krish Sathian, Department of Neurology, Emory University

Reviewed by: Prof. Ehud Ahissar, Department of Neurobiology, The Weizmann Institute, Rehovot

Accepted on: 2015-03-17 15:43:21 GMT