In the shared circuits model (SCM), Susan Hurley provides an impressive, overarching multi-level, multi-layered heuristic model to explain how we perceive intentional action, and to explain imitation and mindreading. Wide-ranging in its coverage of philosophy of mind, comparative and developmental psychology, and neuroscience, the SCM warrants a critical evaluation from within each of these domains.
The SCM proposes five layers of “functional subpersonal” description. The author explicitly resists stating any phylogenic or ontogenic relations amongst them. Our first question is to ask what, if any, developmental progression exists between these five layers. Since overt imitation is found in human newborns (Meltzoff & Moore Reference Meltzoff and Moore1983b; Reference Meltzoff and Moore1989), does this mean for example that layer 3 exists at birth? Mindreading abilities are typically observed by the age of 3, and appear in layer 5. Is Hurley's claim that one cannot pass from layers 3 to 5 without passing via layer 4? The developmental differences from one layer to the next are in need of considerable further specification.
Our second question is to ask whether, in humans, any double dissociations exist among these layers in certain neurodevelopmental conditions. These are not discussed in Hurley's account. For example, Autism Spectrum Conditions entail deficits in both mindreading (Baron-Cohen Reference Baron-Cohen1995a; Frith Reference Frith2001) and affective empathy (Baron-Cohen & Wheelwright Reference Baron-Cohen and Wheelwright2004). In contrast, psychopathy entails intact mindreading abilities (Richell et al. Reference Richell, Newman, Baron-Cohen, Wheelwright and Blair2003) but a deficit affective empathy, since they show little psychophysiological response to signals of distress when compared with matched controls (Blair et al. Reference Blair, Jones, Clark and Smith1997; Prinz Reference Prinz, Hurley and Chater2005). If these two conditions represent a double dissociation between mindreading and affective empathy (Blair & Perschardt Reference Blair and Perschardt2003; Chakrabarti & Baron-Cohen Reference Chakrabarti and Baron-Cohen2006), how does the SCM explain such fractionation? This dissociation would suggest that intact layers 3 and 4 are not essential for an intact layer 5.
Our third question is regarding where joint attention and joint action are situated in the model. These emerge by the age of 9–14 months in typically developing infants (Scaife & Bruner Reference Scaife and Bruner1975). Humans may be unique among primates in developing joint attention. They use their index finger to point to share interest, without any special teaching, in every culture where it has been studied (Tomasello et al. Reference Tomasello, Carpenter, Call, Behne and Moll2005). Joint action is also made possible by joint attention and intention detection. Imagine the classic scenario of needing to move a heavy log through a narrow exit, and achieving this with a conspecific but without using language. Person 1 has to attract the attention of person 2, and then succeed in drawing person 2's attention to the far end of the log rather than his own end of it. This would entail glances and gestures that convey (without words) “I'll pick up my end and you pick up your end.” Next, person 1 has to make clear to person 2 that he wants him to move his end of the log leftwards, while person 1 moves in the opposite direction, so as to rotate the log (role reversal). We guess this thought experiment involving joint action could be achieved effortlessly between two typical humans from age 18 months – and all without language – and some relevant data exist to support this prediction (Carpenter et al. Reference Carpenter, Tomasello and Striano2005; Phillips et al. Reference Phillips, Baron-Cohen and Rutter1995; Ross & Lollis Reference Ross and Lollis1987). Other models already exist to account for such coordinated action, such as the shared attention mechanism (SAM) within the human mindreading system (Baron-Cohen Reference Baron-Cohen, Moore and Dunham1995b). If the SCM cannot account for such a capacity, which arguably lies at the root of later social cognition, then what does the SCM add over earlier models? Notice that a mirror-neuron system might not be up to the task of explaining such joint action, since this might lead person 2 to move left when person 1 does (to mirror his or her movement), whereas person 1 may have been intending person 2 to recognise that the goal was to rotate the two ends of the log in opposite directions to get it out of the exit. Reading another's intention during coordinated action may involve goal representation that cannot be explained by mirroring alone.
Our fourth question is concerning how the SCM applies to the processing of facial expressions of discrete emotions. In a recent neuroimaging study in our lab, involving passive viewing of dynamic facial expressions of emotion, we found that trait empathy correlated maximally with different brain regions for different emotions (Chakrabarti et al. Reference Chakrabarti, Bullmore and Baron-Cohen2006). A similar result (Lee et al. Reference Lee, Josephs, Dolan and Critchley2006) showed that different brain regions were maximally active during explicit imitation of different emotion expressions. Interestingly, both of these studies found a common, emotion-independent role for the inferior frontal gyrus. This could possibly be accommodated by the SCM, with the assumption that the perception of emotion-independent facial muscular movement is mapped onto inferior frontal gyrus activity. However, the finding that both passive viewing and explicit imitation of different emotion expressions are associated with activity in different brain regions raises a problem for the SCM framework, which makes no provision for how perception of different emotions would recruit layers 4 and 5 to different extents.
Our final concern is the broad question of what testable predictions the SCM makes. In science, models are useful if they make novel, falsifiable predictions. While we reiterate how the SCM provides an impressive review integrating large literatures relevant to studies on action perception, we conclude that it leaves questions of central importance unaddressed and it is not clear how it is an improvement over earlier models.
