Implicit Body Representations 1 RUNNING HEAD : Implicit Body Representations Implicit and Explicit Body Representations

ion at which these implicit maps are organised, using the same logic discussed above for tactile localisation and tactile size perception. If the hand is represented as two distinct 2D skin surfaces, there may be different distortions on each. If, in contrast, the two sides of the hand are integrated into a fully 3-D representation of the hand as volumetric object in the world, then consistent distortions should appear on both sides of the hand, and should be correlated across people. Longo and Haggard (2012a) found that distortions on the dorsal and palmar hand surfaces were qualitatively similar, and strongly correlated across participants, suggesting that the representations of the two skin surfaces are bound into a common representation, suggesting something more abstract than a purely 2-D representation. However, the distortions were of different magnitude on the two surfaces, being substantially reduced on the palmar surface. This is a clear violation of the geometry of 3-D space, suggesting something less abstract than a fully 3-D volumetric representation. Thus, Longo and Haggard (2012a) suggested that position sense may rely on something intermediate between a 2-D representation of distinct skin surfaces and a fully 3-D representation of the hand as a volumetric object, which they called a 2.5-D representation, in analogy to Marr’s (1982) ‘2.5-D sketch’ in vision. Relations Between Implicit and Explicit Body Representations Implicit Body Representations 16 In the first part of this paper, I have described several large and highly stereotyped distortions of body representation underlying perceptual processing. Intuitively, this seems quite surprising, since for most of us it seems like we have quite an accurate sense of what our body is like. Surely, if there’s anything we ‘know like the back of our hand’ it would be the actual back of our hand. Do the distortions I have described also characterise our conscious experience of our body, our body image? To address this question, Longo and Haggard (2010) adapted the ‘template matching’ procedure of Gandevia and Phegan (1999) to measure participant’s conscious experience of their hand. The same participants who produced the distorted hand maps in Figure 2 were shown arrays of hand images which had been stretched in various ways, resulting in a range of hand shapes, from very long and slender to very squat and wide. In contrast to their highly distorted hand maps in the localisation task, participants on average selected hands quite similar to their actual hand shape. Thus, the explicit image of the hand is approximately veridical, even as somatoperception relies on a set of implicit, and highly distorted, representations. Implicit Body Representations and the Cognitive Unconscious The dissociation between implicit and explicit body representations fits within a larger trend in psychology and the cognitive sciences over the past few decades emphasizing that much of cognitive processing remains inaccessible to conscious awareness as part of the so-called ‘cognitive unconscious’ (Kihlstrom, 1987). While we are clearly able to introspect on much of our psychological life, we are also unaware of much of the cognitive machinery underlying our thoughts, beliefs, and actions (e.g., Nisbett & Wilson, 1977; Tranel & Damasio, 1985; Tulving & Schacter, 1991). In the domain of perception, there are numerous clinical reports of preserved ability to use perceptual information which appears entirely inaccessible to conscious awareness, Implicit Body Representations 17 including blindsight (Weiskrantz, 1986), visual object agnosia (Milner & Goodale, 2006), and numb-sense (Paillard, Michel, & Stelmach, 1983). The research reviewed above is similar in showing dissociations between the cognitive machinery of perception and conscious awareness, but also strikingly different in showing that these implicit processes are highly inaccurate, in contrast to more veridical explicit representations. This parallels findings of implicit processes producing highly biased results in multiple domains, including reasoning and decision making (Tversky & Kahneman, 1981; Kahneman, 2011) and attitudes (Greenwald & Banaji, 1995; Nosek, Hawkins, & Frazier, 2011), even while more deliberate reflection may produce more rational decisions and more egalitarian attitudes. A Hierarchy of Body Representations What, then, is the relationship between our explicit, conscious body image, and implicit body representations? One possibility is that they reflect entirely distinct representations emerging from different sensory modalities, the body image arising through vision and distorted implicit representations through somatosensation. However, there is strong evidence for bidirectional interactions between the visual body image and somatosensory processing. For example, cutting off inputs from the peripheral nerves with cutaneous anaesthesia produces the subjective experience that that body part has gotten larger, both on the hand (Gandevia & Phegan, 1999) and the mouth (Türker, Yeo, & Gandevia, 2005). This experience may be familiar to anyone unfortunate enough to have had dental anaesthesia, in which the gums and teeth begin to feel enormous. Conversely, visual illusions producing the experience of the body being larger than it actually is produce corresponding changes in the perceived size of touch, as described above (Bruno & Bertamini, 2010; Taylor-Clarke et al., 2004). Thus, somatosensory and visual body representations do not appear to be entirely independent. Implicit Body Representations 18 Another possibility, which I will defend here, is that implicit and explicit body representations lie at opposite ends of a continuum of body representations. This continuum can be thought of in terms of the different spatial scales at which the body is represented, which I discussed in the first part of the paper. At one end are primary somatosensory maps, representing the body surface as a mosaic of individual receptive fields, each constituting a single ‘pixel’. At the other end is our conscious experience of our body as a volumetric object in the world. In between these extremes may be 2-D maps of individual skin surfaces (such as I have argued may underlie tactile localisation and tactile size perception), and 2.5-D representations (such as I have argued underlies position sense). Intriguingly, there is some evidence that different measures of the conscious body image may index different points along this continuum. For example, Longo and Haggard (2012b) compared three different measures of hand representation: (1) the localisation task measuring implicit body representations underlying position sense, (2) the template matching task described above, and (3) a ‘line length’ task in which participants judged whether a line presented on the monitor was shorter or longer than different parts of their hands. As in their previous study described above, Longo and Haggard (2012b) found that the hand representation revealed by the localisation task was massively distorted, while that revealed through template matching was approximately veridical. The line length task, however, appeared intermediate between the two. Participants in the line length task showed distortions of perceived hand size and shape qualitatively similar to those found in the localisation task, but smaller in magnitude. The template matching task, as a purely visual recognition task, may be a purer measure of the ‘visual’ end of this continuum of body representations, while the line length task may involve a larger contribution of the ‘somatosensory’ side of body representation. Implicit Body Representations 19 On this view, body representations emerge from the operation and mutual interactions of complementary bottom-up and top-down processes. First, from the bottom-up, somatosensation represents the body surface as a mosaic of discrete receptive fields, which become progressively agglomerated into larger and larger units of organisation, a process I call fusion. Second, from the top-down, vision starts out depicting the body as an undifferentiated whole, which is progressively broken into smaller parts, a process I call segmentation. Thus, body representation operates from the bottom-up as a process of fusion of primitive elements into larger complexes, as well as from the top-down as a process of segmentation of an initially undifferentiated whole into more basic parts. Implications for Clinical Disorders of Body Representation While most of the studies I have described have been conducted with healthy individuals, this research also has potential implications for understanding clinical disorders involving disrupted body representation. While this connection remains speculative, in this final part of the paper I will discuss some ways in which the distinction between implicit and explicit body representations may relate to conditions such as eating disorders. It has been widely accepted since the classic work of Hilde Bruch (1978) that anorexia nervosa involves a distorted body image. Indeed, such distortions are strong predictors of poor prognosis for recovery (Casper et al., 1979) and of relapse following remission of symptoms (Fairburn et al., 1993; Keel et al., 2005). Could the distortions of the conscious body image seen in such cases reflect normal distortions of somatosensory body representations which have risen into conscious awareness, implicit representations which have become explicit? While the majority of the results I have described have investigated representation of the hand, it is nevertheless intriguing that the distortions of implicit Implicit Body Representations 20 representations are for the hand to be wider and squatter than it actually is, mirroring the body image distortions of individuals with eating disorders who experience their body as fat. Two sets of considerations may seem to make this hypothesis unlikely. 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