Touch and the body: The role of the somatosensory cortex in tactile awareness

This review addresses the role of early sensory areas in the awareness of tactile information in humans. The results of recent studies dealing with this important topic are critically discussed: In particular, we report on evidence from neuropsychology, neurophysiology, neuroimaging, and behavioral experiments that have highlighted the crucial role played by the primary somatosensory cortex (SI) in mediating our awareness of tactile information. Phenomena, such as tactile hallucinations, tactile illusions, the perception of supernumerary limbs, and synaesthesia are also discussed. The research reviewed here clearly shows that the activation of SI is necessary, but not sufficient, for the awareness of touch. On the basis of the evidence outlined here, we propose a neurocognitive model that provides a conceptual framework in which to interpret the results of the literature regarding tactile consciousness. Alberto Gallace Department of Psychology University of Milan-Bicocca Charles Spence Department of Experimental Psychology Crossmodal Research Laboratory Oxford University "everything occurs on the skin" (Helmholtz, 1821-1894). * ADDRESS FOR CORRESPONDENCE: Dr. Alberto Gallace, Room 3138, Dipartimento di Psicologia, Universita’ di Milano-Bicocca, P.zza dell’Ateneo Nuovo 1, 20126 Milano, Italy. Email: alberto.gallace1@unimib.it Psyche, Volume 16, number 1 Touch and the Body 31 Theoretical Framework Traditionally, studies of the awareness of sensory information in humans have focused on visual awareness (e.g., Baars, 1997; Singer, 1998; VanRullen & Koch, 2003). This is rather surprising when one considers that touch, the first sense to develop in the womb in humans, might be the matrix upon which the awareness of ourselves as individuals, separated from the external world, starts to form (e.g., Barnett, 1972; Gottlieb, 1971; cf. Bermudez, Marcel, & Eilan, 1995; Montagu, 1978). Our awareness of touch needs to incorporate the stimulation of a much larger receptor surface as compared to vision (see Montagu, 1971). It has been estimated that our skin and its tactile receptors account for 18% of our body mass (e.g., Montagu, 1971). The sense of touch continuously informs our consciousness about areas of the body that are currently out of view (i.e., a person’s back) and, in contrast with vision, it can do this regardless of the position or status of our head or eyes. As far as this point is concerned, one cannot fail to note that we can close our eyes and prevent visual stimuli from entering consciousness, but perhaps as an ultimate form of bodily protection against injury, we can never voluntarily shut down our sense of touch (see Gregory, 1967; though see Marx et al., 2003, for evidence that the overall neural level of activation in the somatosensory system varies as a function of whether the eyes are open or closed). It is for reasons such as these that developing an understanding of how our awareness of tactile sensations comes about should be considered of great importance. If one considers the number of differences between the processing of tactile and visual information, one might reasonably expect there to be differences in terms of the mechanisms underlying the awareness in these two sensory modalities. Indeed, although vision is the last of our senses to develop, by the time we reach adulthood, nearly half of the cerebral cortex (i.e., the outer layer) of the brain is involved in some way or other in the processing of visual information (Sereno et al., 1995). Certainly, it is only via the study of tactile awareness that we can hope to understand whether or not touch and vision differ in terms of the cognitive and neural mechanisms leading to conscious sensations. There are, however, a number of important reasons why the awareness of tactile information has thus far received far less attention than the same topic in other sensory modalities (especially with respect to vision). The first reason is mainly philosophical. After Aristotle and Plato, vision has always, with very few exceptions (e.g., Berkeley, 1732), been considered the most important of the human senses (see Classen, 1997). The greater importance given by philosophers to the study of visual awareness seems to be, at least in part, related to the historical discussion regarding the relationship between “appearance” and “reality”. In fact, a number of epistemological scenarios have been designed across the centuries in order to understand the nature of these two concepts (e.g., Pastore, 1971). From a more empirical point of view, this might also be related to the fact that vision typically “dominates” or “captures” touch when the two modalities conflict (e.g., see Bertelson & de Gelder 2004; Rock & Harris, 1967, for reviews; though see Heller, 1992). The second reason is that touch is often considered to represent a rather complex sensory modality, even though it is considered to be a “primitive” sense (e.g., Gregory, 1967). In fact, what we commonly call the sense of touch, actually Psyche, Volume 16, number 1 Touch and the Body 32 comprises the processing of pressure, temperature, pleasure, pain, joint position, muscle sense, and movement (see Berkley & Hubscher, 1995; Iggo, 1977; Löken, Wessberg, Morrison, McGlone, & Olausson, 2009; Lumpkin & Caterina, 2007; Olausson et al., 2008; Rolls et al., 2003; Spence, Bentley, Phillips, McGlone, & Jones, 2002). Whether pain should be considered to be part of the sense of touch is still a matter of some debate (e.g., Berkley & Hubscher, 1995). Currently, there is also little agreement as to whether all of the other forms of processing should be considered to be separate sensory modalities or sub-modalities of touch (see Auvray, Myin, & Spence, 2010; Durie, 2005). This latter point may be less relevant, considering that visual information has also been shown to be analyzed by a different subset of processing systems (e.g., those involved in color, movement, and orientation perception) and receptors (e.g., rods, cones and photoreceptors; see Zeki, 1993, for a review). A third reason is related to the difficulties associated with designing experiments to study touch empirically. This is, in part, also due to the lack of proper technological devices for delivering controlled and reliable tactile stimuli (see Ingeholm et al., 2006). To a certain extent, the latter point has been addressed by recent technological advances, making the experimental study of touch somewhat less challenging than it used to be (see Gallace, Tan, & Spence, 2007). A final reason for the lack of attention given by researchers to the neural and cognitive mechanisms of tactile awareness is related to the lack of a proper language to classify tactile/haptic sensations (see Spence & Gallace, 2008, for a discussion on this point). Although the contents of visual awareness can easily be discussed, it is rather difficult to discuss the contents of tactile consciousness if we lack a shared vocabulary for talking about touch, and the contents of tactile consciousness. This latter point, unfortunately, still remains valid, although a few attempts have recently been made to develop a lexicon for touch (see Sonneveld, 2007, cited in Sonneveld & Schifferstein, 2008). In the last few decades, we have seen a number of studies that have tried to investigate the role of primary sensory areas in the awareness of visual information (e.g., Cowey & Walsh, 2000; Kleiser, Wittsack, Niedeggen, Goebel, & Stoerig, 2001; Lamme, 2001, 2006; Lamme, Supèr, Landman, Roelfsema, & Spekreijse, 2000; Lee & Blake, 2002; see Rees, 2007; Tong, 2003, for reviews). The results of these studies indicate that neural activation across early sensory areas (such as V1) is not in-and-ofitself sufficient to elicit awareness. By contrast, whether or not V1 is necessary to elicit visual awareness of incoming visual stimuli is currently still a somewhat controversial topic (e.g., Kleiser et al., 2001; Lamme et al., 2000; see also Rees, 2007). Perhaps because of the reasons listed above, the role of sensory areas, specifically of the somatosensory cortex in people’s awareness of incoming tactile information, has only relatively recently been addressed by cognitive neuroscientists (e.g., Palva, Linkenkaer-Hansen, Näätänen, & Palva, 2005; Preissl et al., 2001; though see Libet, Alberts, Wright, & Feinstein, 1967; Head & Holmes, 1911, for early studies; see Gallace & Spence, 2008). The present review aims to discuss the results of those studies that have addressed this topic. Given the close relationship that has been highlighted between the sense of touch and the representation of the body (e.g., Haggard, Taylor-Clarke, & Kennett, 2003; Knoblich, Thornton, Grosjean, & Shiffrar, 2006), we decided to give more visibility in the present review to those studies where Psyche, Volume 16, number 1 Touch and the Body 33 this important link can be stressed. By contrast, although a large body of research has dealt with topics related to awareness of pain and body representation (e.g., Moseley, 2004, 2005; Moseley, Parsons, & Spence, 2008), we will not discuss the extensive body of literature surrounding this topic here. In this review of the literature, we start by describing the anatomical organization of the somatosensory cortex and its connectivity with other brain areas. By doing so, we define some of the key aspects of the neural information processing of tactile information that will become useful later when trying to understand the results of both behavioral and psychophysiological studies relevant to the awareness of touch. Next, we report the results of those neuroimaging and neuropsychological studies that have investigated the role of the somatosensory cortex in mediating the awareness of tactile information. Taken together, these studies highlight the importance of the somatosensory cortex for our awareness of the body and the tactile sensations occurring across its surface. We will also