Making sense of mirror neurons - CiteSeerX

Synthese DOI 10.1007/s11229-008-9385-8

of

Author Proof

Making sense of mirror neurons

pro

Lawrence Shapiro

Received: 1 July 2008 / Accepted: 1 July 2008 © Springer Science+Business Media B.V. 2008

3 4 5 6 7

8 9

10

11 12 13 14 15

Abstract The discovery of mirror neurons has been hailed as one of the most exciting developments in neuroscience in the past few decades. These neurons discharge in response to the observation of others’ actions. But how are we to understand the function of these neurons? In this paper I defend the idea that mirror neurons are best conceived as components of a sensory system that has the function to perceive action. In short, mirror neurons are part of a hitherto unrecognized “sixth sense”. In this spirit, research should move toward developing a psychophysics of mirror neurons. Keywords Senses

orre cted

2

Grice · Mirror neurons · Perception · Psychophysics · Receptive fields ·

1 Introduction

Few discoveries in the past fifteen or so years have excited as much interest in neuroscience as that of mirror neurons. Casually described, these neurons appear to respond to the observation of goal-directed or intentional action. For instance, mirror neurons in a monkey’s brain will discharge when the monkey observes a human or other monkey grasping a piece of food, however they will not respond to identical grasping motions

unc

1

This paper has evolved quite a bit since I first presented it to a Metaphysics of Science Workshop in October 2007. Present to offer me very useful advice were Ken Aizawa, Carl Gillett, Tom Polger, Bob Richardson, and Jackie Sullivan. I’m also grateful to the audience that heard me deliver this paper at University of Cincinnati in November 2007, as well as an audience at the University of Wollongong in June 2008. L. Shapiro (B) Department of Philosophy, University of Wisconsin – Madison, 5185 H.C. White Hall, Madison, WI 53706, USA e-mail: [email protected]

123 Journal: 11229 MS: SYNTBI-2 CMS: 11229_2008_9385_Article

TYPESET

! DISK

LE

CP Disp.:2008/9/19 Pages: 18

Synthese

20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

of

19

pro

Author Proof

18

if no food is present. But how are we to understand the typical claim that mirror neurons discharge, or become activated, or respond in the presence of goal-directed or intentional action? One reading of this action. If I am right, the discovery of mirror neurons is nothing short of the discovery of a new sense. My case for conceiving of mirror neurons as parts of sensory systems requires some conceptual ground clearing and also some empirical substantiation. The ground clearing is necessary to forestall objections to the possibility of a “sixth sense”. Grice, almost 50 years ago, asked how we could decide whether a creature possesses “a faculty which should be counted as a sense, different from any of those with which we are familiar” (1962, p. 35). Toward answering this question, Grice proposed four apparently distinct criteria for distinguishing the senses from each other. But these criteria have often been recruited for another purpose (Heil 1983; Keeley 2002; Gray 2005). So, although Grice intended to answer the question “What makes sensory system A distinct from sensory system B?”, some have used his four suggestions to answer the question: “What makes sensory system A the kind of sensory system that it is?” It is not obvious that Grice’s criteria can serve this purpose. Just as one might be able to describe how a soufflé differs from a custard without being able to say what makes a soufflé a soufflé or a custard a custard, perhaps Grice’s criteria are not suited to the task of individuating the nature of a sensory system, i.e. for revealing which features of a sensory system matter for its definition. In short, Grice was asking a question about the difference makers; the second question is about the definers. The discovery of difference makers is easier than the discovery of definers. Definers, ordinarily conceived, are essential features, and the search for essences is fraught with familiar hazards. Indeed, it is jarring to find philosophers like Keeley struggling to derive from the science of neuroethology four criteria that are “individually necessary and jointly sufficient” (2002, p. 12) for individuating the senses; or Nelkin, who seeks “the real nature of the senses” (1990, p. 149, his italics). This search for real natures, if feasible at all, would seem to hold little value for those engaged in scientific research. Perhaps even less so for those working in the special sciences, where there is slight hope, and even slighter need, to discover the real natures of kinds like monetary transactions, genes, agonistic behavior, and so on. In contrast, the search for difference makers does not have to dally in suspect notions like necessity, modality, and other ideas associated with essences. I can easily and correctly describe differences between elephants and eels without trying to define, or even having to grant, elephant and eel essences. I suppose one might ask whether an eel would be an elephant if and only if it had a trunk, thereby pushing the idea that trunks are essential to elephanthood, but this would be a strange question to ask of one whose goal was simply to distinguish elephants from eels. I have no idea whether an eel would be an elephant if and only if it had a trunk, but I know how to tell elephants from eels. Keeping straight the distinction between the search for difference makers and the search for definers will lend a new perspective toward recent critical discussions of Grice’s four criteria. I will argue that these criteria may fail to provide definers for sensory systems, but they can nevertheless deliver difference makers. The payoff in the present context is that Grice’s criteria may suffice to distinguish the sensory system

orre cted

17

unc

16


TYPESET

! DISK

LE

CP Disp.:2008/9/19 Pages: 18

Synthese

82

2 The counting question

66 67 68 69 70 71 72 73 74 75 76 77 78 79 80

83 84 85 86 87 88 89 90 91 92 93 94 95

96 97 98

pro

65

orre cted

64

At least since Aristotle’s day, common wisdom puts the number of human senses at five. Aristotle’s reason for distinguishing five senses was, roughly, that human beings seem capable of distinguishing five different sorts of properties. Thus, Aristotle says, “colour is the special object of sight, sound of hearing, flavour of taste. Touch, indeed, discriminates more than one set of different qualities. Each sense has one kind of object which it discerns, and never errs in reporting that what is before it is colour or sound…”2 Despite the enduring legacy that Aristotle’s reckoning has among the folk, scientists who study perception have largely cast aside the number five in favor of larger numbers so as to include proprioception, vestibular perception, and distinctions between various kinds of touch. Philosophers, trying to make sense of these divisions between the senses, have made use of four criteria that Grice (1962) offered in his pursuit of a similar project. Grice thought that one might try to distinguish the senses on the basis of one or more of the following:

unc

Author Proof

63

of

81

of which mirror neurons are a part from other sensory systems, even if the criteria fail to define the real nature of this novel sensory system. But it is one thing to have made room for a new sensory system, distinct from those we already know about, and another to show that mirror neurons are part of a novel sensory system. In this context, the search for difference makers between sensory systems is of some use, because they identify properties that sensory systems tend to possess, but they cannot be the end of the story. The job of difference makers (in the present context) is to tell how to distinguish one sensory system from another. They quite explicitly do not purport to show what makes some given system a sensory system.1 The second part of the paper is devoted to gathering empirical support for my claim that mirror neurons are part of a sensory system. This support comes from an examination of the methods that psychologists use to reveal the properties of mirror neurons. My argument here will take the form of the classic “duck” argument: if mirror neurons behave like sensory organs, then they are sensory organs. Naturally, one might wonder why it should be of interest whether mirror neurons are part of sensory system. What, if any, payoff is there in construing mirror neurons as having a perceptual function? I will consider this question in the concluding section. Roughly, the idea I will defend is that once mirror neurons are recognized as serving a perceptual role, research methods and goals that are standard practice in studies of perceptual systems can be effectively applied in future investigations of mirror neurons.

61 62

1. The objects of perception: This was Aristotle’s idea: each sense has a class of special objects that cannot be sensed by the others. Thus, vision is distinct from audition because among its special objects is color, which cannot be heard. 1 I’m grateful to Ken Aizawa for discussion on this point. 2 On the Soul, Bk II, ch. 6. tr. J. A. Smith, http://classics.mit.edu/Arisotle/sould.2.ii.html. Aristotle recog-

nized that there are “common sensibles,” such as motion and number that are perceivable by more than one sense.


TYPESET

! DISK

LE

CP Disp.:2008/9/19 Pages: 18

Synthese

104 105 106 107 108 109

110 111 112 113 114 115 116 117 118 119 120 121 122 123 124 125 126 127 128 129 130 131 132 133 134 135 136 137 138 139 140 141 142 143

of

103

pro

102

Having made these suggestions, Grice proceeds to question whether the first two ideas are truly independent of each other. In charmingly Gricean fashion, he argues that they are not. Briefly, any collection of properties demarcated as in the province of vision (color, shape, etc.) appears to be arbitrary without the grounding that something like sensory experience provides. Why designate color a visual property but not hardness except for the fact that colors cause in us visual sensations and hardness causes tactile sensations? On the other hand, Grice objects, the identification of sensory experiences seems impossible without reference to the properties that cause the experience: “such experiences (if experiences they be) as seeing and feeling seem to be, as it were, diaphanous…and the attempt to describe the differences between seeing and feeling seems to dissolve into a description of what we see and feel” (1962, p. 45). Thus, determining the objects of perception requires appeal to sensory experiences, but sensory experiences acquire their identity in virtue of the properties that cause them. In my view, (2) is the wrong way to go about distinguishing senses, but we should not give up on (1). Let me first explain why (2) is problematic. After having criticized his second criterion, Grice ends up endorsing it. He argues that an identification of sensory systems by reference to the qualia they produce does not, after all, require that qualia be “tied” to the properties that cause them. He says, “[T]here is a generic resemblance signalized by the use of the word ‘look,’ which differentiates visual from nonvisual sense-experience. This resemblance can be noticed and labeled, but perhaps not further described” (1962, p. 53). Grice’s point is that, although sensory experiences may be diaphanous, and thus indescribable except by allusion to the properties that cause them, there is nevertheless a way things look and a way things feel, and looks and feels can be distinguished even if it is impossible or difficult to articulate how something looks or how something feels. But, even granting that we are able to distinguish looks from feels, sounds, and other sorts of sensory experiences, the suggestion that sensory systems be distinguished on this basis faces insuperable difficulties. These difficulties all arise from the apparent fact that perception is possible without sensory experience. Coady (1974) notes this in his response to Grice when he points out that “dumb” animals see, hear, and so on (1974, p. 111). By “dumb” animals, Coady must mean those that we doubt are capable of sensory experience but still capable of responding to the sights and sounds in their environments (e.g. ants?). Even if one suspects Coady of begging the question

orre cted

Author Proof

101

2. The experiences of objects: The sensations or qualia one experiences when seeing a maraschino cherry are nothing like those one experiences when hearing a sousaphone. To each sensory system belongs an identifying class of sensory experiences. 3. The physical stimuli to which the senses respond: The visual system responds to a spectrum of frequencies of electromagnetic radiation. The auditory system is sensitive to pressure waves. Differences in physical stimuli to which sensory systems respond suffice to mark differences between sensory systems. 4. The anatomy of sensory organs: The visual system consists of eyes, rods, cones, and various nuclei. The auditory system consists of ears, cilia, bones, and various other nuclei. These differences in the anatomy of sensory organs justify the distinctions between the senses.

unc

99 100


TYPESET

! DISK

LE

CP Disp.:2008/9/19 Pages: 18

Synthese

149 150 151 152 153 154 155 156 157 158 159 160 161 162 163 164 165 166 167 168 169 170 171 172 173 174 175 176 177 178 179 180 181 182 183

of

147 148

pro

Author Proof

146

(an advocate of Grice’s second condition might insist that ants’ obvious visual abilities are the best kind of evidence that they have visual experience), there remains the very damning point that perceptual psychologists routinely distinguish between sensory systems in a variety of organisms without a thought about whether the organisms they study undergo sensory experiences of any kind. Clearly true also is that our judgments that an organism can sense seem to depend more on the organism’s ability to respond appropriately to its environment than they do on whether the organism undergoes various sensory experiences. This is apparent in cases of blindsight, as Heil (1983) also notes, in which patients display rudimentary visual abilities while denying that they have corresponding visual experiences.3 Likewise, as Keeley (2002) observes, some sensory systems, e.g. the vomeronasal system that detects pheromones, seem not to create any qualia. Finally, patients who have been fitted with Bach-y-Rita’s tactile vision substitution system, which relies on the patients’ tactile sense to receive information in light, can obviously perceive their surroundings but, most frequently, report no experience of any sort (Bach-y-Rita 1996). Many who reject Grice’s first suggestion, that for each sensory system there is a unique class of objects or properties that the system can detect, do so because often different sensory systems can perceive the same object or property. Thus, it is possible both to see that the blade is sharp and to feel that the blade is sharp. Similarly, RoxbeeCox (1970) notes that a surface’s smoothness can be both seen and felt. Apparently moved by this objection, Roxbee Cox sought to identify a special class of objects— “key features”—that can be perceived by only one kind of sensory system. For instance, Roxbee Cox chose “having some colour property” as a key feature of vision and “having some loudness and timbre” as a key feature of hearing (1970, p. 538). In response to this approach to distinguishing the senses, Heil (1983) argues that there is nothing contradictory in the suggestion that key features for one sensory modality and for one kind of organism might be perceived via a distinct sensory modality for another kind of organism. Perhaps some organisms can see sounds or can identify surface colors by touch. On the other hand, Nudds dismisses Roxbee Cox’s idea, simply saying that “[t]here is little reason to think that… in general there are properties that play this special role in perception” (2004, p. 10). However, I think critics of Roxbee Cox’s strategy are putting too much weight on the search for definers, as I called them earlier, and not enough on the search for difference makers. I share Heil and Nudds’ doubts about whether senses can be defined in terms of unique classes of objects that might provide necessary and sufficient conditions for individuating the particular senses. Nevertheless, Roxbee Cox’s suggestion seems eminently reasonable as a means by which to tell apart the senses. As far as we know, most organisms that perceive color do so visually, and most that perceive pitch and timbre do so by hearing. Moreover, if there is reason to suspect that an organism

orre cted

145

unc

144

3 Caution is necessary here, because one might still attribute the blindness of blind sight victims to some failure to recognize the visual qualia that are in fact present. Gertler (2001) has defended this interpretation of blind sight phenomena. Insofar as unrecognized qualia are possible, there is reason to be suspicious of Keeley (2002) argument that some sensory systems, e.g. the vomeronasal system that detects pheromones, are unattended by qualia.


TYPESET

! DISK

LE

CP Disp.:2008/9/19 Pages: 18

Synthese

188 189 190 191 192 193 194 195 196 197 198 199 200 201 202 203 204 205 206 207 208 209 210 211 212 213 214 215 216 217 218 219 220 221 222 223 224 225

of

187

pro

Author Proof

186

can feel colors or see sounds, these suspicions should not lead us to abandon the idea that senses can be differentiated on the basis of what they detect, but should instead prompt further investigation of how they detect the properties that they do. If, for instance, an organism appears to distinguish color patches on the basis of touch, one should want to understand how mechanoreceptors and other mechanisms of touch can sense color properties. But more significantly, even if such mechanoreceptors were discovered, this would not diminish the usefulness of color perception as a means by which to distinguish vision from touch. For, there is clearly a difference in how the two sensory systems detect color. So, even if the ability to detect color, in the end, is not a difference maker, there are other difference makers that would come to light when seeking to explain how two distinct sensory systems might both detect color. Obviously, this line of response assumes that vision and touch have already been distinguished—that the distinction does not rest solely on an adumbration of key features. This is true as far as it goes, however the criticism disregards the distinction I have been urging between the projects of distinguishing the senses and defining the senses. The search for difference makers is a great deal more relaxed than the search for essences. If color is usually but not always sensed visually, then the detection of color is a good but not perfect way to distinguish vision from the other senses. But, furthermore, there might be other difference makers between the senses, such as the physiological ones I mentioned above. There is no reason that an examination of the objects of perception cannot help to isolate physiological structures that are distinctive of different sensory systems, and that investigation of these different physiological structures might not reveal that sensory systems tend to differ in the kinds of physical stimulation to which they respond, which in turn might lead to hypotheses about which objects and properties different sensory systems are capable of detecting. I see no reason that the senses cannot be distinguished on the basis of several properties, where the study of one such property might lead to the discovery of others, which in turn might lead to an expansion or modification of other features that prove or have proven useful in distinguishing the senses from each other. There is no need, as Keeley (2002) seems to believe, to limit one’s search for a list of individually necessary and jointly sufficient conditions. Grounds for distinguishing the senses are likely to evolve as we learn more about what the senses do and how they do what they do.4 In this respect, the task of classifying the senses seems on par with taxonomic practices in other sciences. In the spirit of this nod toward reflective equilibrium, let us examine Grice’s third and fourth criteria. The fourth might appear easy to dismiss. Given the goal of providing general criteria for distinguishing sensory systems, an appeal to the details of a species’ or an organism’s anatomy and neurophysiology presents a challenge. The visual systems of the nautilus, the dragon fly, the scallop, and the octopus are all anatomically and neurophysiologically quite distinct. Some of these eyes use lenses, others do not; some employ mirrors, others do not, and so on. The prospect of locating

orre cted

185

unc

184

4 In this light, the proposal of O’Regan and Noë (2001) and Noë (2004) that senses be distinguished by the

senorimotor contingencies they incorporate might be seen as a recent attempt to uncover further difference makers between the senses.


TYPESET

! DISK

LE

CP Disp.:2008/9/19 Pages: 18

Synthese

230 231 232 233 234 235 236 237 238 239 240 241 242 243 244 245 246 247 248 249 250 251 252 253 254 255 256 257 258 259 260 261 262 263 264 265 266 267 268 269 270 271

of

229

pro

Author Proof

228

a physical trait common to all and only sensory systems of a given type seems dim. Moreover, the very identification of a particular organism’s sensory systems would seem to require reference to other criteria, e.g. the objects they detect, or the sensory experiences they produce, or the nature of the physical stimuli that activate them. This is Roxbee Cox’s reason for rejecting a physiological basis for distinguishing the senses. He asks, “Why is the familiar classification not arbitrary, whereas a classification that grouped together the left eye, the left hand, and the left ear as the organs of a certain kind of perception would seem arbitrary and absurd?” (1970, p. 532). Roxbee Cox thinks that an answer to his question requires that one “fall back on such considerations as the character of the experience, or the properties perceived” (1970, p. 533). Roxbee Cox is surely right: one cannot carve out the physiological bits of an organism’s body that constitute a single sense without drawing on further considerations. But this is hardly a reason to reject physiological discoveries as potential difference makers. Having noted that an organism is capable of color discriminations, a logical step toward understanding how this capacity is realized will involve an investigation of the organism’s nervous system. Presumably, an investigation of this sort would result in a non-arbitrary collection of physiological components. Further confirmation that this collection is non-arbitrary might come from the observation that these same components work together in a coordinated manner whenever light shines into the organism’s eye. Of course there is no sensory system comprising the left eye, left hand, and left ear. The operations of these parts do not exhibit the tightly knit coordination we expect to see between components of a single system. The point about the diversity of structures that realize a single kind of sensory system raises a different sort of issue. It is fair to wonder why, say, the visual systems of a nautilus and of a human being are of a kind. Certainly the nautilus’s pinhole eye hardly resembles the human being’s camera eye. However, sensory systems consist of more than just the peripheral organs by which information from “outside” can be brought “inside.” For instance, visual systems seem all to depend on visual pigments in order to transduce light into electricity. But even if this were not so, justification for conceiving of the sensory systems of the nautilus and the human being as both visual might come from several sources. Perhaps both systems respond to the same physical stimuli (electromagnetic radiation) and detect similar properties. Perhaps the nautilus visual system shares physiological components with the visual system of some third species, which, in turn, is physiologically similar in many other respects to the human visual system. Recall that the search for difference makers need not assume that sensory systems of one sort differ essentially from sensory systems of another. Within a class, sensory systems will have more in common with each other than they will with sensory systems of another class, but these resemblances between members of a class may be more or less of a physiological nature; more or less with regard to the objects and properties they perceive; more or less with respect to the kind of physical stimulation that they transduce. Left to consider is Grice’s third suggestion: sensory systems are to be defined by the kind of physical stimulation to which they respond. Heil (1983) favors this suggestion to Grice’s others. Seeing involves detection of the information in light, hearing involves detection of information in pressure waves, smell is a response to particular

orre cted

227

unc

226


TYPESET

! DISK

LE

CP Disp.:2008/9/19 Pages: 18

Synthese

277 278 279 280 281 282 283 284 285 286 287 288 289 290 291 292 293 294 295 296 297 298 299 300 301 302 303 304 305 306 307 308 309 310

of

275 276

pro

Author Proof

274

orre cted

273

chemicals, and so on. Gray (2005) finds two problems with the suggestion.5 First, scientists (as well, I think, as common sense) often distinguish between sensory systems that are responsive to the same kind of physical stimulation. Second, scientists (and perhaps common sense), will sometimes allow that a single sensory system can respond to two different kinds of physical stimuli. Gray illustrates the first point with his discussion of the pit viper. Pit vipers have two membrane-covered pits, one on each side of the head. The sensory cells that cover the membranes are sensitive to electromagnetic radiation in the infra-red range, thus providing the pit viper with the equivalent of night-seeing goggles. Pit vipers use the thermal imaging that their pits provide in order to distinguish warm prey from their cooler backgrounds. Among the reasons to conceive of the viper’s pits as distinct sensory organs, Gray argues, are these. First, although sensitive to the same kind of physical energy, the viper’s eyes and pits respond to non-overlapping ranges of this energy. Second, the organs evolved independently of each other, and in response to different kinds of selection pressures. Third, the organs can be used independently of each other, and indeed are so used once the sun has set. In illustration of his second point—that a single sense organ might respond to distinct categories of physical stimuli—Gray discusses the heat sensors in the noseleafs of vampire bats.6 Using their noseleafs, vampire bats appear able to detect heat by two distinct means. First, direct contact with a warm object allows the transfer of kinetic energy from the object to the bat’s noseleafs. Second, the bat’s noseleafs are sensitive to electromagnetic radiation. Because kinetic energy and radiant energy are different physical kinds, Gray claims that bats’ noseleafs constitute a single sensory system that is responsive to different kinds of physical stimulation. Among the reasons to claim that noseleafs are a single sense is that the cause of the kinetic energy that the noseleafs have been selected to detect—capillaries—is the same as the cause of the radiant energy that noseleafs have evolved to detect. Moreover, we might assume, the behavior the bat exhibits in response to either kinetic or radiant energy is invariant. There is thus no warrant for distinguishing between kinetic and radiant energy detection on behavioral grounds. Gray takes himself to have shown that the kind of physical stimulation to which a sensory system responds does not constitute a defining feature of a sensory system—different kinds of sensory systems may respond to the same kind of physical stimulation (the pit viper case), and the same kind of sensory system may respond to different kinds of physical stimulation (the noseleaf case). From the perspective I am taking toward classifying the senses, I think Gray’s points are important. They lay bear the risk one assumes when trying to define sensory systems in terms of essential features. Far better to avail oneself of a variety of considerations that might clarify differences and similarities between sensory systems.

unc

272

5 Gray’s actual target is Keeley (2002), who relies on Grice’s third suggestion as a necessary condition in a

set of four jointly sufficient conditions for the individuation of sensory systems. Heil needn’t be concerned with Gray’s criticisms because he is willing to concede some vagueness in the boundaries between the senses (1983, pp. 22–23). 6 Gray (2005) cautions that noseleaf science is still in its infancy and so the vampire bat example might

better be seen as a thought experiment.


TYPESET

! DISK

LE

CP Disp.:2008/9/19 Pages: 18

Synthese

342

3 Mirror neurons—a sixth sense?

315 316 317 318 319 320 321 322 323 324 325 326 327 328 329 330 331 332 333 334 335 336 337 338 339 340

343 344 345 346 347 348 349 350 351 352 353

pro

314

orre cted

Author Proof

313

The point of the preceding has been to triangulate on those features of sensory systems that serve to distinguish them from each other. And the point of doing this is to evaluate the possibility that mirror neurons are part of a novel sensory system. So far I have not said anything in defense of this possibility. However, I have, as promised, cleared some conceptual ground. The results are this. First, whether mirror neurons are parts of a distinctive sensory system does not depend on the satisfaction of some set of necessary and sufficient conditions. Second, while I have been at pains to discuss the difference makers between sensory systems, this discussion serves as well to motivate acceptance of some common features of sensory systems. That is, it is reasonable to distinguish sensory systems from each other on the basis of their objects, their physiology, and the physical stimuli to which they are sensitive because sensory systems have objects,

unc

312

of

341

In the preceding paragraphs I have argued that a classification of the senses should attend to the contents of perception as well as the physiological components involved in perception. Examination of the kinds of stimulation that affect sensory organs is doubtless another important source of data for classifying the senses, and Gray is perhaps right to emphasize as well the value of evolutionary considerations. The pluralism in criteria for distinguishing the senses that I advocate might appear to some as extravagant. However, I am far from endorsing an “anything goes” approach. I have already argued, in agreement with Keeley, that qualia appear to be of little significance in the classification of sensory systems. I am also happy to concede that for some purposes some criteria might be more important than others. When this is so, sensory systems usually classified as belonging to the same kind might find themselves classified as different kinds. Indeed, that researchers have not fixed on a determinate number of senses suggests that there is disagreement about how much weight to place on various difference makers. This is especially apparent in discussions of the sense of touch. Gardner and Martin (2000) clump touch, proprioception, temperature sense, pain, and itch together into a single somatosensory system despite differences in the receptors these senses use and in the stimulus energies to which the receptors respond. Thus, together with the vestibular sense, Gardner and Martin count six senses. Others distinguish the various senses within the somatosensory system for a total of ten. Still others add senses like hunger and the feelings associated with full bladders or stomachs. In my view, this indeterminacy in the number of human senses is further support for my claim that perceptual psychologists are not interested in isolating a set of necessary and sufficient conditions for defining the senses. Rather, the existence of different sensory taxomonies shows nothing more than that perceptual psychologists differ with respect to how much emphasis to place on the various criteria that bear on the individuation of the senses. These differences in emphasis, as far as I can tell, pose no threat to the study of the senses. Indeed, sensory systems are perhaps the best understood of all cognitive systems. Insofar as various schemes of classification agree, there is consensus about the importance of some criteria; differences in classification emerge from disagreement about less central features of the sensory systems.

311


TYPESET

! DISK

LE

CP Disp.:2008/9/19 Pages: 18

Synthese

358 359 360 361 362 363 364 365 366 367 368 369 370 371 372 373 374 375 376 377 378 379 380 381 382 383 384 385 386 387 388 389 390 391 392 393 394 395 396

of

357

pro

Author Proof

356

have a distinctive physiology, and are sensitive to kinds of physical stimulation. Thus, we should expect the mirror neuron system (henceforth MNS) to exhibit these characteristics as well. But more can and should be said in favor of construing mirror neurons as parts of a sensory system. Just as damage to sensory systems leads to particular kinds of impairments, we should expect that damage to mirror neurons will lead to a similar sort of impairment. We also know that sensory systems are sensitive to learning and to cognitive modulation. If the MNS displays similar sensitivities, this is all the more reason to regard the system as sensory in nature. Before turning to the details of the MNS, let me introduce mirror neurons more generally. Most research on mirror neurons has been performed on macaque monkeys. Mirror neurons are present in region F5 of the macaque’s premotor cortex as well as in area PF of the macaque’s parietal lobe and perhaps the superior temporal sulcus.7 Whether human beings possess mirror neurons remains controversial, but the growing consensus is that they do and that they are to be found in the superior temporal sulcus (Gallese 2006; Rizzolatti and Craighero 2004) and in the inferior frontal gyrus (Broca’s area), which is thought to be a homologue of the macaque’s F5 (Gallese et al. 1996). Like other neurons in monkey premotor cortex, mirror neurons discharge when the monkey takes a particular motor action. However, they also discharge when the monkey observes another monkey taking an action. Some features of the actions to which mirror neurons discharge are necessary for a response, but others are not. For instance, mirror neurons will not respond to intransitive actions, nor will they respond to object-directed actions if no object is present. Thus, mirror neurons will discharge if the monkey observes a hand moving towards and grasping a piece of food, but will not discharge if simply the hand moves, or if the hand moves and grasps at nothing. In other respects, mirror neurons show a fair degree of flexibility. Thus, a mirror neuron that responds to the observation of a monkey hand grasping will also activate in response to a human hand grasping. Proximity appears to make no difference to the mirror neuron’s behavior: the neuron will discharge regardless of whether the observed action is near or far, indicating that the size of the hand’s image on the monkey’s retina is not relevant to the mirror neuron’s behavior. There are different sorts of mirror neurons. Some discharge when the monkey observes grasping motions, others discharge when observing mouth motions. Ferrari et al. (2005) have identified a class of mirror neurons in F5 that they call tool-responding mirror neurons. These “discharge when the monkey observes actions performed by an experimenter with a tool (a stick or a pair of pliers)” (2005, p. 212). Tool-responding mirror neurons will discharge strongly as a monkey watches an experimenter obtain food by piercing it with a stick, but will fire not at all or only weakly if the experimenter mimes taking food with a stick, or uses his hand rather than the stick to procure the food. Mirror neurons in the cortex of the superior temporal sulcus (STS) show responses to actions such as “walking, turning the head, bending the torso, and moving the arms” (Rizzolatti and Craighero 2004, p. 171). Some mirror neurons are bi-modal. They will respond to actions that are seen, but they will also respond to the same actions when

orre cted

355

unc

354

7 Gallese et al. (1996) distinguish the mirror neurons in F5 from neurons in STS, but other researchers

do not.


TYPESET

! DISK

LE

CP Disp.:2008/9/19 Pages: 18

Synthese

402 403 404 405 406 407 408 409 410 411 412 413 414 415 416 417 418 419 420 421 422 423 424 425 426 427 428 429 430 431 432 433 434 435 436 437 438 439 440 441 442

of

401

pro

400

orre cted

Author Proof

399

heard (Kohler et al. 2002). Still other mirror neurons fire in response to actions, the final part of which is hidden from view (Umilta et al. 2001). Perhaps most incredibly, some mirror neurons seem to respond to intentions. If the experimenter who grasps the food does so with the intention of eating it, one class of mirror neurons will discharge rapidly; a different class of mirror neurons will discharge when the experimenter grasps the food with the intention to place it in a container (Fogassi et al. 2005). This surprising range of stimuli to which mirror neurons respond has led to several hypotheses regarding their function. According to one hypothesis, mirror neurons facilitate imitative behavior. One difficulty with this hypothesis is that monkeys do not imitate actions although chimpanzees, which lack mirror neurons, do (Lyons et al. in press). The second and perhaps more popular hypothesis is that mirror neurons enhance action understanding. The neurons that respond to actions are the same neurons that fire when the monkey itself takes an identical or similar action. Because the monkey understands the meaning, purpose, intent, of its own actions, the discharge of the appropriate mirror neurons permits it to understand the meaning, purpose, intent of others’ actions (Rizzolatti et al. 2001). With these basics in hand, let us first consider some common elements in the physiology of sensory systems and ask whether the MNS displays similar elements. Sensory systems are usefully divided into three components: sensory receptors, neural circuitry, and sensory cortex. Sensory receptors are the sensory system’s first contact with stimulation. The job of the receptor is to transduce stimulus energy of some kind—light, vibration, pressure wave, chemical—into electrical signals that the neural circuitry can then begin to process. Neural circuitry, comprising ganglion cells as well as nuclei in the thalamus and other regions, collects signals from receptors and organizes it in various ways, creating cells with receptive fields that respond selectively to patterns of receptor firings and that enhance the contrast between stimuli. Some neural circuitry is present in the cortex as well, and can be distinguished from other cortical cells because they perform neural processing similar to that which occurs within ganglion cells and in other nuclei. The outputs of the neural circuitry are then sent to areas in the cortex where the final stages of perception take place. Among the data psychologists collect in order to learn about the properties of cells involved in neural processing are those that display a cell’s receptive field. These data will be displayed in the form of a tuning curve. Thus, Fig. 1 below shows a tuning curve for a cell in a monkey’s visual cortex. This cell fires in response to a bar-shaped stimulus (the cell has a bar-shaped receptive field), and the tuning curve illustrates how the cell responds to bars of different orientations. This particular cell fires most rapidly when the bar is oriented at 90 degrees. The plotting of tuning curves in order to understand the properties of a sensory cell is completely routine in investigations of sensory systems. Tuning curves map the sensitivity of auditory nerve cells as well as mechanoreceptors and thermoreceptors. The fact that mirror neuron function can be measured with a tuning curve thus provides some reason to regard mirror neurons as playing a sensory role in the perception of action. Figure 2 below illustrates the response of three mirror neurons in a monkey’s inferior parietal lobule as the monkey observes either an experimenter grasping an object and moving it toward his mouth or grasping an object and placing it in a container.

unc

397 398


TYPESET

! DISK

LE

CP Disp.:2008/9/19 Pages: 18

Synthese

of

orre cted

pro

Author Proof

Fig. 1 Adapted from Gur et al. (2005, p. 1209). The tuning curve indicates the activity of the neuron in terms of spike counts in response to lines of different orientations

Fig. 2 From Fogassi et al. (2005, p. 664). A comparison of the tuning curves for three mirror neurons shows that some (87) are most active when exposed to grasping to eat, others (39) are most active when exposed to grasping to place, and others do not discriminate between these two actions

444 445 446 447 448 449 450 451 452 453 454 455 456 457 458 459 460

Mirror neuron 87 is more sensitive to grasping-to-eat than it is to grasping-to-place, MN 39 shows the opposite sensitivities, and MN 80 did not discriminate between the two actions. The motivation for the experiment in which these data were collected is exactly parallel to that which drives a researcher to develop tuning curves for the more familiar sensory cells of vision and hearing. Mirror neurons appear to sense goal-directed behavior, and manipulation of this sort of behavior in an experimental setting uncovers the kinds of actions toward which individual neurons are most highly tuned. If mirror neurons are sensory cells, then we ought to expect their tuning curves to behave like the tuning curves of other sensory cells. Sensory cells typically show the effects of habituation when exposed to repetition. Thus, a tuning curve measuring a cell’s sensitivity to bar orientation will flatten as the cell is exposed repeatedly to bars with similar orientations (Dragoi et al. 2000). Researchers have found the same effect in mirror neurons (e.g. Hamilton and Grafton 2006). Figure 3 below shows the effects of habituation on two clusters of mirror neurons in human beings. In contrast to monkey studies, studies of mirror neuron activity in human beings often employ fMRI because single cell recordings are prohibited (for good reason). The tuning curves derived from the activity of the two clusters of

unc

443


TYPESET

! DISK

LE

CP Disp.:2008/9/19 Pages: 18

Synthese

of pro

Author Proof

Fig. 3 From Hamilton and Grafton (2006, p. 1135). Clusters of mirror neurons (a and b) show greater habituation to repeated goals than they do to novel goals even when the novel goal requires duplication of previous arm trajectory

463 464 465 466 467 468 469 470 471 472 473 474 475 476 477 478 479 480 481 482 483 484 485 486 487 488 489

orre cted

462

cells show flattening with increased exposure to the stimulus, which in this case is a movie showing a human hand grasping an object. Interestingly, this study also lends support to the hypothesis that the objects of perception of the MNS are actions rather than simply movements. Subjects in the experiment were exposed to four different kinds of stimuli (hence the four curves in each graph). When subjects saw repetitions of the same arm trajectory toward the same object, their mirror neurons show signs of habituation. However, when shown repetitions of the same trajectory but each time toward a different object, subjects’ mirror neurons do not habituate. Subjects’ MNs habituated only when they saw repeated arm trajectories toward the same object or novel trajectories toward the same object. Thus, MNs seem to respond to the goal of grasping—the acquisition of some object. This explains why habituation occurs when the goal of the action remains constant through trials even when the trajectories toward the goal might change. The points I have made above are intended as support for treating mirror neurons as a kind of sensory cell. Like sensory cells in other modalities, mirror neurons display a preferential sensitivity to stimuli of a particular kind. They also react to repetition in the same way that other sensory cells do. There are further similarities between MNs and sensory cells that I could describe. For instance, the behavior of MNs, like that of other sensory cells, is modulated by states of arousal such as hunger (Cheng et al. 2006). MNs change their sensitivities in response to learning (Calvo-Merino et al. 2005). However, rather than belaboring the comparison, I wish now to consider another reason that mirror neurons should be conceived as sensory organs. Because sensory systems have the job of telling their possessors about the world, damage to sensory systems leads to distinctive impairments. A victim of kidney or liver failure remains able to form beliefs about his surroundings. He will not bump into objects, leave his hand on a hot stove, eat noxious food, and so on. This is evidence that kidneys and livers are not parts of sensory systems. If damage to mirror neurons produces the sort of behavioral deficits that damage to other sensory systems induces, this is further support for the claim that mirror neurons are sensory cells.

unc

461


TYPESET

! DISK

LE

CP Disp.:2008/9/19 Pages: 18

Synthese

511

4 Concluding remarks

495 496 497 498 499 500 501 502 503 504 505 506 507 508 509

512 513 514 515

516 517 518 519 520 521 522 523

524 525 526 527 528 529

pro

494

orre cted

493

In the first part of this paper I argued that there are several criteria to which a classification of sensory systems should attend. These criteria, although neither necessary nor sufficient conditions, serve well as difference makers—as reasons to distinguish one sensory modality from another. Among these difference makers are: (i) the objects of perception—different sensory systems often deliver information about different objects or properties; (ii) the anatomy of sensory systems—the physiology of sensory systems typically differ, as one would expect if sensory systems often deliver different kinds of information on the basis of different kinds of physical stimulation; (iii) the physical stimulation to which sensory systems respond—the world is replete with a variety of physical stimulation that is capable of carrying information about different properties in the world.

unc

Author Proof

492

of

510

Using transcranial magnetic stimulation (TMS), researchers have produced “virtual lesions” in the inferior frontal gyrus of human subjects.8 In one experiment, a group of lesioned subjects was asked to imitate patterns of finger movements on a keypad and then to produce finger movements on the keypad that followed the movement of a red dot skipping across the keys. The motor output in both tasks was identical, however performance suffered in the imitation task. Iacoboni (2005) concludes that mirror neurons are “essential” to imitation. But what role do mirror neurons play in imitation tasks? A prominent answer in mirror neuron literature is that mirror neurons “code” for action—they sense action. Autism spectrum disorder (ASD) provides yet another piece of evidence suggestive of the sensory function of mirror neurons. A number of studies have shown abnormalities in the mirror neuron systems of ASD patients (Iacoboni and Dapretto 2006). Patients with ASD also typically exhibit deficits in social cognition. Current thinking on these issues seeks to link the deficits of ASD patients to an inability to imitate the behavior of others around them, or to understand the behavior of others around them. This description of ASD fits easily into the framework for thinking about mirror neurons that I have proposed. Patients with ASD, like those who are blind or deaf, fail to recognize an important feature of their environment. They do not recognize that the behavior of others around them is goal-directed and guided by intentions (Gallese 2006). Lacking this recognitional capacity, ASD patients are unable to navigate successfully through social environments.

490 491

As the description of these criteria makes evident, they are interrelated. This fact alone makes plain why past efforts to isolate a single condition on which to base a distinction between sensory systems have met with failure. Careful attention to these difference makers, along with perhaps others (for instance, Gray’s 2005 suggestions regarding evolutionary function; O’Regan and Noë’s 2001 focus on sensorimotor contingencies), can guide a taxonomy of sensory systems that pays for itself in terms 8 TMS temporarily knocks out neural activity by inducing a precisely localized electric current.


TYPESET

! DISK

LE

CP Disp.:2008/9/19 Pages: 18

Synthese

535 536 537 538 539 540 541 542 543 544 545 546 547 548 549 550 551 552 553 554 555 556 557 558 559 560 561 562 563 564 565 566 567 568 569 570 571 572 573 574 575

of

534

pro

533

orre cted

Author Proof

532

of the deeper understanding of particular sensory systems and their relations to each other that it renders. In the second part of this paper I have tried to accomplish two things. First, I have tried to make the case for conceiving of mirror neurons as part of a sensory system. Admittedly, the case I have made is far from air tight. Having given up on the idea of essential properties, I do not know how to demonstrate that mirror neurons are part of a sensory system. Rather, the case is abductive. Mirror neurons share with other sensory systems a variety of properties that make their identification as sensory neurons quite natural: their behavior can be measured with a tuning curve, they show the effects of habituation, their behavior is modulated by states of arousal and the effects of learning. Like other sensory systems, damage to the mirror neuron system leads to characteristic deficits of a sort that show a kind of blindness to particular features in the environment. The best, or at least a very good, explanation for the various properties we observe mirror neurons to exhibit is that they do indeed have a sensory function. Secondly, I have tried to show that mirror neurons, or the system of which they are a part, constitute a sensory system distinct from others on the standard lists because, significantly, they differ in their perceptual objects from these other systems. Mirror neurons, unlike other sensory systems, detect actions. A question that I have not adequately addressed concerns the physical stimulation to which mirror neurons respond. The receptors in the visual system respond to electromagnetic radiation, those in the auditory system to pressure waves, in the olfactory system to chemicals. But what physical stimuli carry information about actions? Here, I concede, is a difference between the mirror neuron system and the paradigm sensory systems. Positioned as they are in association cortex, mirror neurons have access to information from other sensory systems. Indeed, some mirror neurons, as I have mentioned, are bimodal and will respond to either the visual observation of an action or the sound of an action (Rizzolatti and Craighero 2004). Clearly mirror neurons are responding to stimuli that have already been processed by other sensory systems. However, this should not by itself disqualify mirror neurons from having a sensory function. There seems nothing unreasonable in supposing that the mirror neuron system lacks its own receptors because it can avail itself of receptors in other sensory systems. By piggy backing on other sensory systems, the mirror neuron system spares itself the trouble of collecting information that is already available. Nevertheless, the fact that some mirror neurons are bimodal suggests that mirror neurons can “read through” the visual and auditory stimulation so as to detect whatever invariants there are that specify action. In closing, I would like to address a very serious challenge to my claim that mirror neurons are part of a hitherto unrecognized sensory system. How radical is the thesis that mirror neurons are parts of sensory systems? Researchers ordinarily speak of mirror neurons “coding” for actions and having sensory properties. Indeed, mirror neurons are found in sensory areas of the cortex. Perhaps my efforts to defend the idea that mirror neurons are sensory cells have been for naught because it is something widely believed already. Stoking this worry is the fact that psychologists are, as I showed above, applying methods of perceptual psychology to the study of mirror neurons. Finally, even if psychologists haven’t yet fully pledged themselves to the idea that the mirror neuron system is a sensory system, one might wonder why such a commitment

unc

530 531


TYPESET

! DISK

LE

CP Disp.:2008/9/19 Pages: 18

Synthese

581 582 583 584 585 586 587 588 589 590 591 592 593 594 595 596 597 598 599 600 601 602 603 604 605 606 607 608 609 610 611 612 613 614 615 616 617 618 619 620 621

of

580

pro

579

orre cted

Author Proof

578

is necessary. If psychologists are content to treat mirror neurons as sensory cells, what more is to be gained in designating them as sensory cells? The answer to this challenge, I believe, must look for a reason to classify mirror neurons as sensory cells that is distinct from those already mentioned. Suppose that areas of research that promised to shed further light on mirror neuron function would remain unexplored unless investigators approached the mirror neuron system just as they would approach the study of other sensory systems. This idea is not far-fetched. It is not until one realizes that a solution is acidic that one bothers to measure its pH, observe its reaction with metals, and so on. Similarly, it is quite possible that failure to identify the mirror neuron system as a sensory system will create missed opportunities to learn more about them. Two examples illustrate this point. The first involves a question we have already encountered. Sensory systems rely on receptors to collect information in light, sound, and so on. Gibson (1966, 1979) showed that stimulation such as light contains an invariant structure that can specify features in one’s environment. But what is the invariant structure in light that indicates action? The experiment I discussed earlier in which mirror neurons dishabituate in response to identical arm trajectories toward distinct objects but not distinct arm trajectories toward the same object suggests that mirror neurons do not respond simply to motion. But what is present in the stimulus array that cues mirror neuron activity? Although this is an area of investigation that strikes one in the face once one thinks of mirror neurons as sensory cells, to my knowledge no one is working on this question. In essence, a psychophysical study of mirror neurons is what is needed. Another example to motivate classifying mirror neurons as a sensory cells derives from a recent approach to understanding how sensory neurons can make coarse discriminations in some tasks and fine-grained discriminations in others. In solving this mystery, it is crucial to realize that a tuning curve for a sensory cell can carry different kinds of information. Traditionally, tuning curves have been thought to reveal the stimulus to which a neuron is most sensitive. Hence, a neuron might be designated as “for” the detection of bars with a 45 degree orientation, because the neuron’s spiking frequency peaks when presented with such a stimulus. But the steepness of a tuning curve’s slopes is also an important feature of a neuron’s behavior. The steeper the slope, the more sensitive the bar-orientation neuron is to differences in stimuli. A sharp tuning curve indicates that the neuron’s activity changes dramatically in response to small differences in the orientation of the stimulus, whereas a broader tuning curve indicates that the neuron is not as sensitive to changes in the orientation of the stimulus. Butts and Goldman (2006), using a measure for quantifying the amount of information about the stimulus that a tuning curve can contain, developed a mathematical model showing that in a low-noise environment, i.e. one in which the neuron’s background activity is low, a tuning curve with steep slopes carries more information than it would in a noisy environment, and thus a neuron’s activation is more effective in discriminating between similar stimuli. In contrast, in a high-noise environment, where the neuron exhibits appreciable background activity and thus produces a shallower tuning curve, the tuning curve carries more information about a single stimulus—the one corresponding to the curve’s peak.

unc

576 577


TYPESET

! DISK

LE

CP Disp.:2008/9/19 Pages: 18

Synthese

636

References

626 627 628 629 630 631 632 633 634

637 638 639 640 641 642 643 644 645 646 647 648 649 650 651 652 653 654 655 656 657 658 659 660 661 662 663 664 665 666 667 668 669 670 671 672

pro

625

Bach-y-Rita, P. (1996). Sensory substitution and qualia. In J. Proust (Ed.), Perception et Intermadalité (pp. 81–100). Paris: Presses Universitaires de France (Reprinted from Vision and mind: Selected readings in the philosophy of perception, pp. 497–514 by A. Noë, & E. Thompson, Eds. Cambridge: MIT Press). Butts, D., & Goldman, M. (2006). Tuning curves, neuronal variability, and sensory coding. PLoS Biology, 4, e92. Calvo-Merino, B., Glaser, D. E., Grezes, J., Passingham, R. E., & Haggard, P. (2005). Action observation and acquired motor skills: An fMRI study with expert dancers. Cerebral Cortex, 15, 1243–1249. Cheng, Y., Meltzoff, A., & Decety, J. (2006). Motivation modulates the activity of the human mirror-neuron system? Cerebral Cortex, 17, 1979–1986. Coady, C. (1974). The senses of martians. The Philosophical Review, 83, 107–125. Dragoi, V., Sharma J., & Sur, M. (2000). Adaptation-induced plasticity of orientation tuning in adult visual cortex. Neuron, 28, 287–298. Ferrari, P., Rozzi, S., & Fogassi, L. (2005). Mirror neurons responding to observations of actions made with tools in monkey ventral premotor cortex. Journal of Cognitive Neuroscience, 17, 212–226. Fogassi, L., Ferrari, P., Gesierich, B., Rozzi, S., Chersi, F., & Rizzolatti, G. (2005). Parietal lobe: From action organization to intention understanding. Science, 308, 662–667. Gallese, V. (2006). Intentional attunement: A neurophysiological perspective on social cognition and its disruption in autism. Brain Research, 1079, 15–24. Gallese, V., Fadiga, L., Fogassi, L., & Rizzolatti, G. (1996). Action recognition in the premotor cortex. Brain, 11, 593–609. Gardner, E., & Martin, J. (2000). Coding of sensory information. In E. Kandel, J. Schwartz, & T. Jessell (Eds.), Principles of neural science (pp. 411–429). New York: McGraw-Hill. Gertler, B. (2001). Introspecting phenomenal states. Philosophy and Phenomenological Research, 63, 305–328. Gibson, J. J. (1966). The senses considered as perceptual systems. Prospect Heights: Waveland Press, Inc. Gibson, J. J. (1979). The ecological approach to visual perception. Boston: Houghton-Mifflin. Gray, R. (2005). On the concept of a sense. Synthese, 147, 461–475. Grice, H. (1962). Some remarks about the senses. In R. J. Butler (Ed.), Analytical philosophy, Series 1 (pp. 133–153). Oxford: Blackwell (Reprinted from Vision and mind: Selected readings in the philosophy of perception, pp. 35–54 by A. Noë & E. Thompson, Eds. Cambridge: MIT Press). Gur, M., Kagan, I., & Snodderly, D. (2005). Orientation and direction selectivity of neurons in V1 of alert monkeys: Functional relationships and laminar distributions. Cerebral Cortex, 15, 1207–1221. Hamilton, A., & Grafton, S. (2006). Goal representation in human anterior intraparietal sulcus. The Journal of Neuroscience, 26, 1133–1137. Heil, J. (1983). Perception and cognition. Berkeley: University of California Press.

orre cted

Author Proof

624

unc

623

of

635

Although the details of Butts and Goldman’s discovery are very interesting in their own right, in the present context I present their work as an example of research that begins with fundamental questions about the nature of sensory processing. The application of Butts and Goldman’s findings to the tuning curves of mirror neurons seems a natural extension of their research once one conceives of mirror neurons as sensory cells in their own right. Does noise affect mirror neuron sensitivity in the way it does other sensory cells? What are the fine differences in stimuli that are distinguished as the steepness of a mirror neuron’s tuning curve slope increases? Knowing that some animal looks like a duck, walks like a duck, and quacks like a duck can be useful if one has a well-developed science of ducks. Using principles of duck science, researchers are in a better position to understand the behavior of a duckish creature than they would be if they had to begin their investigations from scratch. Of course, principles of duck science may in the end turn out not to be what’s needed, but then that’s worth knowing too.

622


TYPESET

! DISK

LE

CP Disp.:2008/9/19 Pages: 18

Synthese

678 679 680 681 682 683 684 685 686 687 688 689 690 691 692 693

of

677

pro

Author Proof

676

orre cted

675

Iacoboni, M. (2005). Neural mechanisms of imitation. Current Opinion in Neurobiology, 15, 632–637. Iacoboni, M., & Dapretto, M. (2006). The mirror neuron system and the consequences of its dysfunction. Nature, 7, 942–951. Keeley, B. (2002). Making sense of the senses: Individuating modalities in humans and other animals. The Journal of Philosophy, 94, 5–28. Kohler, E., Keysers, C., Umilta, M., Fogassi, L., Gallese, V., & Rizzolatti, G. (2002). Hearing sounds, understanding actions: Action representation in mirror neurons. Science, 297, 846–848. Lyons, D., Santos, L., & Keil, F. (in press). Reflections of other minds: How primate social cognition can inform the function of mirror neurons. Current Opinion in Neurobiology. Nelkin, N. (1990). Categorising the senses. Mind and Language, 5, 149–65. Noë, A. (2004). Action in perception Cambridge: MIT Press. Nudds, M. (2004). The significance of the senses. Proceedings of the Aristotelian Society, 102, 31–51. O’Regan, J., & Noë, A. (2001). A sensorimotor account of vision and visual consciousness. Behavioral and Brain Sciences, 24, 939–1031. Rizzolatti, G., & Craighero, L. (2004). The mirror-neuron system. Annual Review of Neuroscience, 27, 169–192. Rizzolatti, G., Fogassi, L., & Gallese, V. (2001). Neurophysiological mechanisms underlying the understanding and imitation of action. Nature Neuroscience Reviews, 2, 661–670. Roxbee-Cox, J. (1970). Distinguishing the senses. Mind, 79, 530–550. Umilta, M., Kohler, E., Gallese, V., Fogassi, L., Fadiga, L., & Keysers, C. (2001). I know what you are doing: A neurophysiological study. Neuron, 31, 155–165.

unc

673 674


TYPESET

! DISK

LE

CP Disp.:2008/9/19 Pages: 18