I am very grateful to Vittorio Gallese for pointing me in the direction of current neuroscientific research showing that my account of primitive self-consciousness is in several significant respects compatible with what is currently known about the neuroscience of perception and action. In this reply I make explicit how the research to which he has drawn attention meshes with my account of primitive self-consciousness.
2. SELF-SPECIFYING INFORMATION IN VISION. In Chapter 5 of PARADOX I followed J. J. Gibson's (1979) ecological approach to vision in stressing the proprioceptive dimensions of visual experience. The optic array contains information about the self as well as information about the environment. I characterised this as one of the two most primitive form of self-consciousness -- both ontogenetically and phylogenetically. Visual proprioception is present at birth and appears early on in evolution.
3. One of the planks of Gibson's theory of self-perception is the notion of an affordance -- the idea that objects and surfaces in the environment have properties allowing different animals to act and react in different ways and that those properties are directly and immediately perceived in the environment as higher-order invariants. The perception of affordances is a form of self-perception. It is the pick-up of environmental information about one's own possibilities for action and reaction. The central claim here is that affordances are themselves part of the content of what is perceived. The animal does not work out his own possibilities for action by analysing what he sees. He just sees what he can and cannot do in a particular environment.
4. How might the Gibsonian view be confirmed or disconfirmed at the neural level? We can compare two different pictures of the relation between perception and action. On the first, more traditional picture, perception and action are insulated from each other by the mechanisms of "central processing". Put crudely, sensory input feeds into the central decision-making processes that integrate sensory information with other sources of information and determine a plan of action. This planned action is then implemented by the various motor systems. An influential version of this conception of the relation between perception and action is to be found in Fodor's "The Modularity of Mind" (Fodor 1983), where sensory input is fed into the central belief system by encapsulated modules that are domain-specific, fast and have shallow outputs. On this view, perhaps the dominant view within the cognitive sciences, bare sensory input is only transformed into meaningful perception when the central belief system is brought to bear. The environment is perceived from a point of view or perspective only in the minimal sense that the perceived environment is represented on a subject-centred frame of reference. The self features in the content of visual perception only as a geometrical point -- as the origin of the field of view.
5. On the second picture of the relation between perception and action, in contrast, the environment is not merely represented on a subject-centred frame of reference. It is represented as an environment within which the perceiver can act. The perceived environment is meaningful to the perceiver long before the central processes of belief fixation become involved -- if indeed they do become involved. The content of perception is such that it can feed directly into behaviour. Gibson's theory of ecological perception adopts this second picture of the relation between perception and action. Versions of this second picture have also been canvassed by philosophers from very different traditions (Dreyfus 1992, Hurley 1998).
6. The conception of primitive self-consciousness that I put forward in PARADOX will meet the Implementation Constraint only if something like the second picture of the relation between perception and action receives corroboration at the neural level -- if it turned out that the processing correlative with the environment becoming meaningful to the perceiver qua agent takes place at a relatively "early" stage in the perceptuo-motor system. It is interesting that much recent research carried out by various people (including Gallese and his co-workers) seems strongly to suggest that this is in fact the case (for reviews see Jeannerod 1997, Rizzolati, Fogassi and Gallese 2000). I will comment here on two particular sets of results.
7. PERI-PERSONAL SPACE. It is well known that there exist many different representations of space at the neural level (Wilson 2000), although from a phenomenological point of view the spatiality of the environment is of course perceived as a single and unified manifold. Space is represented egocentrically in many different ways, with a range of frames of reference centred on different body-parts. There are also many different allocentric coordinate systems nested within each other. The mere fact that perceptual experience is coded on egocentric frames of reference is not itself evidence for the second picture of the relation between perception and action. It is hard to see how the spatiality of perception could be anything other than egocentric (except perhaps in the case of God). But there is a particular type of egocentric frame of reference whose existence does point strongly in favour of the second picture. This is what has come to be known as peri-personal space (Rizzolati, Fadiga, Fogassi and Gallese 1997).
8. Experiments on the monkey ventral premotor cortex (area F4) have revealed the existence of neurons responsive to both visual and tactile stimuli (particularly those associated with movements of the head or arm). The visual and tactile receptive fields of these neurons (that is, the areas in objective space within which they detect stimuli in the relevant modality) are very closely linked. Visual stimuli can detected only in the area immediately around the skin surface (e.g. where tactile stimuli are detected). Moreover, the visual receptive fields move only when the body moves. They are not sensitive to changes in the direction of gaze that are unaccompanied by changes in posture. As Gallese (2000) suggests in his review, it seems likely that peri-personal space is a motor space, circumscribed by the range of efficacy of the various body-parts. The coding of objects' location in peri-personal space in the pre-motor cortex, and consequently at a relatively early stage in perceptuo-motor processing, seems strongly to support the second picture of the relation between perception and action.
9. CANONICAL F5 NEURONS. Further support for the second conception of perception and action comes from single neuron studies of area F5 in the monkey premotor cortex. The firing of neurons in F5 is correlated with distinct types of motor act. Studies have show that groups of F5 neurons exist that are sensitive to the following types of motor act:
grasping with the hand and mouth,
grasping solely with the hand,
holding,
tearing,
poking,
manipulating.
The grasping neurons are themselves subdivided into three classes according to the grip type that they involve. Small objects are grasped with the precision grip, in which the thumb is opposed to the index finger. Medium-sized objects are grasped with the thumb opposed to the other fingers (finger prehension) and large objects using whole hand prehension in which the fingers are opposed to the palm.
10. Among these F5 neurons correlated with particular types of action, some respond only when the action in question is performed (the F5 motor neurons). Others (the F5 visuo-motor neurons) also respond to visual stimuli. These visuo-motor neurons discharge when an object is perceived, according to the type of grasp that would be most appropriate for that object. Experiments by Murata et al. (1997) show that this neuronal activity occurs even if no movement is performed, and they strongly suggest that it occurs even when the monkey has no intention of grasping the object (by showing that the neurons fire even when the monkey is performing tasks in which grasping is not involved). The conclusion that seems to be imposed by the existence of these neurons in the pre-motor cortex is again that objects seem to be perceived in terms of the potential for action that they offer the perceiver. Again, self-specifying information (in the form of information about hand shape and size relative to the presented object) appears to be coded at an early stage in the process of perceptuo-motor transformation.
11. ACTIVE AND PASSIVE MOVEMENT: THE ROLE OF PROPRIOCEPTION AND SPATIAL REPRESENTATION. In his review of my book Gallese expresses concern with my emphasis on Gibsonian accounts of perception. He notes that Gibson's ecological theory of perception does not lay enough stress on the distinction between active and passive movement, suggesting that both types of movement function for Gibson as "just a tool to capture the higher-order invariant features which are prespecified in the afferent stimulation". In contrast, the position that emerges from the neuroscientific work he discusses and that he correctly identifies as being at the core of my account of primitive self-consciousness is one in which agency is a key component.
12. This point is well taken (although it is unclear how the notion of an affordance can be completely understood in terms of passive movement). Gibson's theory of visual perception is rather one-sided and it would be unwise either to have it as the sole element in an account of self-consciousness or to follow Gibson in some of his more extreme pronouncements. In PARADOX I tried to bring out how the self-specifying information in vision is complemented by self-specifying information in somatic proprioception, as well as by the awareness of spatial location implicated in navigation (what I called a point of view). These two further dimensions of primitive self-consciousness are closely linked with agential motor activity. The mechanisms of proprioceptive information are designed to operate in conjunction with the mechanisms of efference copy (or corollary discharge) to provide feedback on active movement and to distinguish active movement from passive movement. In so doing they underwrite a form of self-awareness qua agent.
13. The point is even clearer for the form of self-awareness implicated in the possession of what I termed a nonconceptual point of view. The basic idea here is that, since representing a particular space requires representing one's own location relative to it, the representation of space is a form of self-awareness. Since self-consciousness is essentially a contrastive notion, the richness of this form of self-awareness is a function of the complexity and richness of the representation of the environment within which the subject is located. I discussed the interplay between spatial representation and self-consciousness in terms of three central cognitive/navigational capacities:
the capacity to think about different routes to the same place;
the capacity to keep track of changes in spatial relations between
objects caused by its own movements relative to those objects;
the capacity to think about places independently of the objects or
features located at those places.
Of these, only the first strictly implies agency and active movement, but it is clear that the second and third will only emerge in the context of active movement within the environment.
14. PSYCHOLOGICAL SELF-AWARENESS AS AN AGENT. The contrastive nature of self-consciousness comes to the fore in psychological self-awareness, that is, awareness of oneself as a bearer of psychological as well as physical properties. In considering psychological self-awareness I proposed what I termed the Symmetry Thesis:
Symmetry Thesis A subject's psychological self-awareness is
constitutively linked to his awareness of other minds
The rationale for the Symmetry Thesis is that a subject's recognition that he is distinct from the environment in virtue of being a psychological subject is dependent upon his ability to identify himself as a psychological subject within a contrast space of other psychological subjects. This self-identification as a psychological subject takes place relative to a set of categories which collectively define the core of the concept of a psychological subject. Consequently, there are some psychological categories which a subject cannot apply to himself without also being able to apply them to other psychological subjects. These psychological categories are the categories which form the core of the notion of a psychological subject.
15. I analysed the notion of a psychological subject in terms of three components -- being an agent, being a perceiver and being a subject of reactive attitudes. The Symmetry Thesis entails that self-awareness relative to each of these three categories will be relative to awareness of other subjects falling under each of those categories. In PARADOX I explored the relation between psychological self-awareness and awareness of other subjects in infancy. The neuroscientific research to which Gallese draws attention is also highly relevant. It suggests that this personal-level interdependence has subpersonal correlates.
16. MIRROR NEURONS IN F5 AND THE SYMMETRY THESIS. In addition to the canonical neurons in F5 that have already been discussed above there is a second group of neurons sensitive to visual stimuli in F5. These are the so-called mirror neurons. Whereas the canonical neurons discharge in response to visually presented stimuli as a function of the type of action appropriate to the perceived object, the mirror neurons discharge in response to perceived actions. The surprising feature is that they discharge not only in response to actions performed by the subject, but also to actions performed by other individuals. Like the canonical neurons, the mirror neurons classify actions by type.
17. The existence of mirror neurons clearly shows that in the premotor cortex there is a level of coding actions that does not differentiate between those that are self-initiated and those that are other-initiated. It provides corroboration for the Symmetry Thesis in the following sense. The Symmetry Thesis implies that it is not possible to be aware of oneself as an agent without being aware of other individuals as agents. The functioning of the mirror neurons shows that the mechanisms for coding one's own actions serve equally as mechanism for coding the actions of others.
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