P. M. S. Hacker - Philosophical Foundations of Neuroscience

Popper’s three-world doctrine impressed Eccles, and he formulated his dualism in terms of it. World 1, the material world of the cosmos, he declared, consists of mere material things and of beings that enjoy mental states. The latter, being a subset of the entities in World 1, he refers to collectively as ‘World 1 M’. This ‘world’ stands in reciprocal causal interaction with World 2 by means of what he terms ‘the liaison brain’ ( HM 211).

The impact on Eccles of Kornhuber’s research on readiness potential

Research done by Kornhuber and his colleagues (see §1.6.1) on changes in electrical potential antecedent to a voluntary movement had revealed that the so-called readiness potential began up to 800 milliseconds before the onset of the muscle action potential, and led to a sharper potential, the pre-motion positivity, beginning at 80–90 milliseconds prior to the movement. The patterns of neuronal discharges eventually project to the appropriate pyramidal cells of the motor cortex and synaptically excite them to discharge, so generating the motor potential (a localized negative wave) just preceding the motor pyramidal cell discharge that initiates the movement. The question on which Kornhuber’s research seemed to throw light was: ‘How can willing of a muscular movement set in train neuronal events that lead to the discharge of pyramidal cells of the motor cortex and so to the activation of the neuronal pathways that lead to the muscle contraction?’ ( HM 214). It is striking that Eccles took these discoveries to betoken empirical confirmation of mind–brain interaction of a kind (but in a different location) that had been envisaged by Descartes. He argued as follows:

What is happening in my brain at a time when the willed action is in the process of being carried out? It can be presumed that during the readiness potential there is a developing specificity of the patterned impulse discharges in neurons so that eventually there are activated the pyramidal cells in the correct motor cortical areas for bringing about the required movement. The readiness potential can be regarded as the neuronal counterpart of the voluntary intention. The surprising feature of the readiness potential is its very wide extent and gradual build up. Apparently, at the stage of willing a movement, there is a very wide influence of the self-conscious mind on the pattern of module operation. Eventually this immense neuronal activity is moulded and directed so that it concentrates onto the pyramidal cells in the proper zones of the motor cortex for carrying out the required movement. The duration of the readiness potential indicates that the sequential activity of the large numbers of modules is involved in the long incubation time required for the self-conscious mind to evoke discharges from the motor pyramidal cells … It is a sign that the action of the self-conscious mind on the brain is not of demanding strength. We may regard it as being more tentative and subtle, and as requiring time to build up patterns of activity that may be modified as they develop.( HM 217)

Cartesian problems recapitulated :

(1) Interaction

So, Eccles conceived of what he called ‘the dualist-interactionist hypothesis’ as helping to ‘resolve and redefine the problem of accounting for the long duration of the readiness potential that precedes a voluntary action’ ( HM 217). Descartes, as we have noted, conceived of the mind as operating upon the pineal gland to generate the minute fluctuations in the animal spirits (the role-equivalent of neural transmitters) in the ventricle in which he thought the pineal gland was suspended. This, he held, enabled the acts of will of the mind to affect the motions of the animal spirits, which are then transmitted to the muscles. But the question of how an immaterial substance could actually interact causally with a material object such as the pineal gland to produce the appropriate minute motions was left totally unanswered. In much the same way, Eccles thought that the ‘self-conscious mind’ interacts causally with the pyramidal cells of the motor cortex, gradually (rather than instantaneously) getting them to discharge. But the question of how an immaterial entity such as the mind can interact causally with neurons was left equally unanswered.

(2) Reifying the mind

Both thinkers erred in conceiving of the mind as an entity of some kind. Had they heeded Aristotle in thinking of the mind, or more accurately, of the psuche-, not as an entity but as an array of powers or potentialities, they would have been much closer to the truth, and would not have become enmeshed in insoluble problems of interaction. For it patently makes no sense to ask how one’s abilities to do the various things one can do interact with one’s brain.

(3) Misconceptions about the will

Both thinkers erred in imagining that voluntary movements are movements produced or caused by antecedent acts of will. 15For although there are such things as acts of will – namely, acts performed with great effort to overcome one’s reluctance, aversion or difficulties in acting in adverse circumstances – obviously the vast majority of our ordinary voluntary actions involve no ‘act of will’ in this sense at all. We shall examine this conception in chapter 9.

Eccles was further confused over the object of the alleged act of will, which is variously characterized as (i) a muscular movement, (ii) an action or (iii) a movement of a limb.

Confusions about the object of the alleged act of will

It is, of course, possible to intend to move a muscle – for example, to flex a muscle – but that is something we rather rarely intend to do, and although the movement of muscles is involved in all our positive, physical acts ( by contrast with acts of omission and mental acts), what we intend, and what we voluntarily perform, are actions (such as raising our arm, writing a letter, saying something, picking up a book, reading a book, and so on), and not the constitutive muscle movements of these actions, of which we are largely unaware. But it is easy to see why a neuroscientist who is attracted to dualism should confuse the objects of the will. For, according to the dualist conception, the mind has causally to affect the brain, and the causal powers of neural events in the brain causally affect muscle contraction.

Problems of volitional interaction between mind and brain

This raises yet a further insoluble problem for the dualist. The ‘self-conscious mind’ is supposed to influence the pattern of module operation, gradually moulding and directing it so that it concentrates on the pyramidal cells in the proper zones of the motor cortex for carrying out the intended movement. But how does the ‘self-conscious mind’ know which pyramidal cells to concentrate on, and how does it select the proper zones of the motor cortex? For it would need such knowledge in order to execute such actions. And it is certainly not knowledge of which the self-conscious mind is conscious. To these questions there can be no answers, any more than the nineteenth-century innervationist ideo-motor theories of voluntary movement ( chapter 9), favoured by such eminent scientists as Helmholtz and Mach (and psychologists such as Bain and Wundt), could answer the question of how the mind, in addition to having images of kinaesthetic sensations that allegedly accompany voluntary movements, directs the currents of energy going from the brain to the appropriate muscles. ( There must be appropriate feelings of innervation – of ‘impulse’ or ‘volitional energy’, they thought, otherwise the mind could never tell which particular current of energy, whether the current to this muscle or the current to that one, was the right one to use.)