The reflex arc debate

The pathway followed by the stimulus (impulse) from beginning to end is the reflex arc. The general reflex arc (see figure in Purves) of the human nervous system has a minimum of five components:

• The receptor organ which receives the stimulus

• The afferent (sensory) neuron which carries the stimulus from the receptor organ to the CNS by way of the appropriate peripheral nerve. The cell body of this afferent neuron is located in the posterior root ganglion of a spinal nerve or the individual ganglion of a cranial nerve.

• The internuncial neuron (INTER = between; NUNCIA = messenger) within the spinal cord or brainstem or the internuncial neuron synapses.

• The efferent (motor) neurons - the axon of the motor neuron passes out of the CNS by way of the appropriate peripheral nerve.

• The information is then delivered by the motor neuron to the effector organ. Somatic motor neurons lead to striated muscle fibers, particularly in skeletal muscles. Autonomic (visceral) motor neurons lead to smooth muscle tissue, cardiac muscle tissue, or glands.

John Dewey (1859-1952) Important educational philosopher; functionalist, pragmatist and anti-reductionist (like William James).

John Dewey (1896) The Reflex Arc Concept in Psychology, Psychological Review, 3. pp. 357-370.

The full paper is available on the net. In it Dewey argued that the neural circuitry of the reflex arc was being misinterpreted by behaviourist psychology which was using stimulus and response as basic units. Physiology implied a unity of coordinated action, but psychology was treating ``stimulus'' and ``response'' as separable: one isolatable and independent event as ``cause'' and a subsequent and isolatable (albeit contingent) event as the ``effect''.

``It being admitted that the sensori-motor apparatus represents both the unit of nerve structure and the type of nerve function, the image of this relationship passed over into psychology, and became an organizing principle to hold together the multiplicity of fact.'' (Dewey, 1896)

In the behaviourist view, learning at all ages involved the gradual strengthening (due to experience) of S-R associations. Any complex behaviour (even language) could be built out of a series of simple connections - associated chains of neurons. Eg. Skinner's pigeons playing ping-pong.

In 1948 Karl Lashley, a neuropsychologist working in the behaviourist tradition came to pose a serious challenge to behaviourism. In his paper ``The Problem of Serial Order in Behavior'', presented at the Hixon Conference on ``Cerebral Mechanisms in Behavior'', CalTech, 1948, Lashley questioned the application of the reflex arc to complex human behaviours, eg. playing tennis, performing on a musical instrument and speaking. Reasons:

• These action sequences unfold too rapidly for every step in the chain to be based on the previous one (eg. one tone after another in a fast musical composition).

• Errors, eg. slips of the tongue, often include anticipation of words that are to occur only much later in a sequence.

So behaviour sequences must have to be planned and organised in advance. Organisation is hierarchical, beginning with broad overall plans. Eg. in speech:

• Overall intention prompting the utterance (highest nodes of the hierarchy)

• Syntax and vocabulary

• Production of sounds

Complex behaviour is not a result of environmental prompting; rather, central brain processes actually precede and dictate the ways in which an organism carries out these behaviours.

In the behaviourist view, the nervous system is static/ inactive most of the time. Lashley believed that the nervous system consists of active, hierarchically organised units, with control emanating from the centre rather than from peripheral stimulation, which includes feedback and feed-forward (used in neural net context) mechanisms.

It is now recognised that connections between neurons are not simple serial chains but complex parallel networks containing alternative pathways.

In the same conference Lashley presented his arguments against localisation of brain function (first in 1929 book).