Hurley's target article is pitched at a high level of generality. It speaks broadly of shared circuits, control, mirroring, simulation, mindreading, and so forth, giving the impression that its major theses apply equally across all applicable types of cognition. But there is good reason to doubt that this is accurate, and it is not entirely clear whether Hurley really intends it. Important sub-themes of the target article seem principally aimed at the relation between action and perception – for example, the falsity of the “classical sandwich architecture” (sect. 3, para. 1). Is everything she says about action, perception, and feedback supposed to apply equally to other domains in which shared circuits, mirroring, and mindreading are found? The article's level of abstraction leaves the distinct impression that the theses advanced at the various layers of analysis cut across all the domains, but that is dubious.

My chief worry centers on the relation between shared circuits (or mirroring) and control theory. Hurley is not alone in emphasizing such a connection (Gallese Reference Gallese2003; Wolpert et al. Reference Wolpert, Doya, Kawato, Frith and Wolpert2003). However, the case for tying the control-theoretic perspective to shared circuits, mirroring, and simulation is based mainly on the action-perception domain, where there is specific physiological, theoretical, and experimental evidence for efferent copy and reafferent input. Nothing of this sort exists, however, for a number of other domains where shared circuits and simulation are found.

To be specific, mirroring phenomena exist in several areas of cognition in addition to the motoric: in sensation, including pain (Jackson et al. Reference Jackson, Meltzoff and Decety2004; Singer et al. Reference Singer, Seymour, O'Doherty, Kaube, Dolan and Frith2004) and touch (Keysers et al. Reference Keysers, Wicker, Gazzola, Anton, Fogassi and Gallese2004), and in emotion (most clearly, disgust; see Wicker et al. Reference Wicker, Keysers, Plailly, Royet, Gallese and Rizzolatti2003). But in these domains, there are no established feedback or control-theoretic phenomena of comparable importance – or any sort at all. Here is a brief review of the shared circuits (or mirroring) findings across multiple domains. The shared areas or circuits for action are the premotor cortex and inferior parietal lobule interconnected with the superior temporal sulcus (STS)/middle temporal gyrus (MTG); for disgust, the insula; for fear, (possibly) the amygdala; for pain, the anterior cingulate cortex (ACC) and anterior insula; and for touch, the somatosensory cortices. In all cases, observing what other people do or feel is transformed into an inner representation of what we would do or feel in a similar, endogenously produced, situation. In many of these cases, moreover, evidence drawn from lesion studies and imaging studies indicates that mirroring produces mindreading of others' mental states (Goldman Reference Goldman2006; in press; Goldman & Sripada Reference Goldman and Sripada2005). However, only in the case of action is there clear evidence of feedback loops that fit the control-theoretic framework. So the notion that systematic relationships between shared circuits, simulation, and mindreading crucially depend on control-theoretic mechanisms is unsupported. Yet that is what Hurley suggests, since her architecture of social cognition is erected on a control-theoretic foundation.

Hurley writes that “the shared circuits model (SCM) shows how subpersonal resources for control, mirroring, and simulation can enable the distinctively human sociocognitive skills of imitation, deliberation, and mindreading” (sect. 3, para. 1). Her two bottom layers of analysis highlight adaptive feedback control and prediction of effects for improved control, and the three higher layers are explained in terms of these lower-level mechanisms. She makes no attempt, however, to explain how feedback and control account for simulational, empathic, or mindreading properties related to sensation and emotion. Indeed, the latter are barely mentioned. The explananda listed at her top level, the personal-animal level, all involve action and behavior; yet the social-cognitional phenomena featuring shared circuits include feelings and emotions rather than just action. If a single unifying framework underpinning all shared circuits phenomena is feasible, a different framework seems to be called for.

An alternative approach to many of the same phenomena is the Hebbian learning approach developed by Keysers and collaborators (Keysers & Gazzola Reference Keysers and Gazzola2006; Keysers & Perrett Reference Keysers and Perrett2004), closely aligned with Heyes's (Reference Heyes, Hurley and Chater2005) associative learning approach. When they are young, monkeys and humans spend a lot of time watching themselves. Neurons in the premotor cortex responsible for the execution of a hand-grasping movement will be active at the same time as the visual neurons in the STS respond to the sight of grasping. Given that the STS and area F5 are connected through area PF, ideal Hebbian learning conditions are met: what fires together wires together. Hence, the synapses going from STS grasping neurons to PF and then F5 will be strengthened as the grasping neurons at all three levels are repeatedly coactive. Given that many neurons in the STS show viewpoint-invariant responses, the sight of someone else grasping in similar ways suffices to activate F5 mirror neurons. The same Hebbian argument can be applied to sensations and emotions. While seeing oneself being touched, somatosensory activations overlap in time with visual descriptions of an object moving towards and touching our body. The Hebbian learning approach also has the virtue of not assuming that a particular modality is crucial to shared circuits. Damasio (Reference Damasio2003) emphasizes the importance of somatosensory representations, but somatosensory representations do not seem to be important for the representation of action or emotion. Analogously, Hurley does not make a strong case for a single control-theoretic explanation of all types of shared circuitry or the phenomena arising from them (e.g., empathy, mindreading). Indeed, she hardly mentions the existence of emotion and feeling cases. On the surface, these are just as arresting a set of cognitive phenomena as the motor-theoretic ones, and equally in need of explanation.

Can it be argued that the Hebbian associationist perspective is just another version of the control-theoretic one? After all, control theory also postulates association-based learning. True, but Hebbian learning does not posit comparable use of neural comparator systems, forward and inverse models, or other apparatus characteristic of control theory. Also, newly discovered properties of mirroring, such as modulation of mirroring by social relations between individuals (Singer et al. Reference Singer, Seymour, O'Doherty, Klaas, Dolan and Frith2006), aren't obviously explainable in control-theoretic terms. But this goes beyond the present purview.