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series of conditioned reflexes, can maintain a continuous circulation of neural impulse between the thalami and the cortex: we think that these multitudinous engrammic systems may be activated either (1) from the essential organs of the thalami, or (2) from the peripheral receptor organs, or (3) concurrently from both these sources, and that by some means at present beyond our knowledge the neural impulse whether from the thalami or the receptor organs or from both may be under a continual process of re-direction to different parts of the entire aggregate of engrammic systems.

"The sensory cortex is the storehouse of past impressions1," and these engrams which are the outcome of the ontogenetic life process come doubtless to incorporate in their systems the multitudinous neurones found in the different layers of the cortex; and in the unimaginable intricacy and complexity of the multitudinous paths formed by means of these cells and by the varying resistances offered by the synapses between them there is we think gradually built up and stabilized the neural counterpart of what we retain of these past experiences.

In the stratified arrangement of the multitudinous neurones in the cortex and the diverse complexities of the engrams embracing the neurones in these different strata we see the neural mechanism involved in 'Generalization,' in 'Classification' and in 'Abstraction': in the Universals' and 'Particulars' of the Philosophers, in the 'Genera' and 'Species' of the Logicians and Biologists and in logical constructions like the 'Tree of Porphyry.' "Are genera and species," said Huxley, "realities or abstractions?" Viewed from the standpoint we are now presenting we think we must tentatively answer that they are abstractions.

We have already expressed the opinion that in the activation of engrams by the essential organs of the thalami will be found the neural correlates of a large part of the mental processes concerned in what we call 'ratiocination'; and in the results of the reciprocal interaction of the internal stimuli from these organs and the external stimuli from the peripheral receptor organs will we think be found the neural correlates of what the logicians call 'judgments.' Of such judgments we may take as a simple illustration the case of someone pointing to a rose and saying, "This rose is red!" Here we think there comes into play that condition which psychologists call 'pre-perception': that this involves the partial activation by the essential thalamic organs of a large number of engrams

1 "Sensory Disturbances from Cerebral Lesions," by Henry Head and Gordon Holmes Brain, XXXIV, p. 189.

-of the whole series of engrams which in the aggregate form the neural correlates of the mental symbol or concept 'rose' and all its several differentiae which happen to be stored in the cognitive dispositions of the person expressing the judgment; and that the sensory impulses from the peripheral receptor organ (the eye), simultaneously re-enforcing the internal stimuli from the essential thalamic organs to the engram which forms the neural correlate of the concept 'red,' furnishes the combination of neural processes which forms the neural correlate of, and which issues in, the judgment. And in logical judgments dealing with a greater number and a greater complexity of phenomena-judgments which we call 'considered judgments': judgments which are not immediate but which are mediated by a discursive mental process-we consider that essentially the same neural processes will be found to be involved despite their greater complexity. Here we think is a suggestive theme to occupy the united attention of the logicians and the neurologists.

One of us (G. G. C.) has shown reasons from the psychological side for thinking that all the usual mental processes which we call observation, inference, imagination, reason, etc., can be resolved into the reactions and interactions of 'percepts' and 'concepts'1: percepts being in his arrangement of terms the psychological correlates of the interactions of fresh sensory impulses from the various kinds of receptor organs with the processes of the organized neural tissues which are the outcome of the ontogenetic life process. If throughout his argument we substitute for the psychological term 'concept' the neural term 'engram,' as denoting its neural correlate, the relation of the views expressed in this paper with his theory of epistemology will become at once apparent.

We have sketched now the more fundamental considerations upon which in our opinion depends a solution of that part of the psycho-neural problem which is concerned with reflective thought, and these considerations seem to bring us to the following conclusions.

CONCLUSIONS.

1. We adopt the view of the concept postulated by Bergson and elaborated by Santayana and Campion as the psychological or mental element or atom desiderated and sought by Wm. James as the first step towards the formulation of an elementary psycho-physic law. This provides an element-largely the abiding, organized and cumulative effects of sense-perception-of which all our cognitions are gradually built up during man's brief span from infancy to age.

1 Elements in Thought and Emotion.

2. We have shown that the growth of this mental element in its various manifestations is congruous with the neurological processes which establish in the course of the ontogenetic life history physiological paths for impulses between the multitudinous neurones in different lobes and areas of the brain. We adopt Semon's term 'engrams' to denote these physiological paths. We regard the 'engram' as the neural correlate of the 'concept' and the latent condition of the engram as the condition which usually subsists when its conceptual correlate is 'below the threshold of consciousness,' and the active or 'ecphorized' condition of the engram as the condition which usually subsists when its conceptual correlate is 'above the threshold of consciousness.' But we think that this is only restrictedly true and that it must be held to be merely one of a larger complex of factors. It is often we think possible for an engram to be in an active or 'ecphorized' condition without its conceptual correlate becoming above the threshold of consciousness. We consider that herein may be found the explanation of those psycho-pathic states which are characterized by what are usually known as 'suppressed' phases of past experience. In these cases we think that the engrams which form the correlates of the suppressed experience may have become functionally dissociated from larger engrammic systems and especially from direct communication with the essential thalamic centres, and that it is their dissociated functioning which gives rise to the symptoms which are known to be associated with such suppressed phases of past experience.

3. We regard the aggregation of the 'engrammic systems' of neural paths when in a latent condition, i.e. when no impulse is passing through them, as the neural correlate of what is called 'the subconscious mind.'

4. We consider that just as a concept itself is changing by a process of growth in the way described during the life process of the individual this is accompanied by and has its neural correlate in a corresponding growth in the 'engram,' and that just as in our processes of active thought that quality which we call our 'attention' is transferred rapidly from concept to concept in all their wide relations and through an unending series, so also is this associated with a corresponding change in the correlative pattern of neural impulse in the attendant 'engrams.'

5. We note and emphasize the fact which neurological science has brought to light within the last half-century that all afferent nerve stimuli in the cerebro-spinal nervous system pass into the optic thalami on their way both to the essential organs of the thalami and to the various cortical areas of the brain.

6. We adopt the view of Head and Holmes that the thalami form the great junctions where the afferent stimuli from all the receptor organs may be redistributed. This function of the thalami as a great junction for afferent stimuli provides we think the neural mechanism by which the perception of touch through one of the fingers may be the means of exciting, e.g. what are called 'visual images,' by means of the optic lobes.

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7. We adopt the view of Head and Holmes that the essential organs of the thalami perform in some way the function of the central seat of consciousness for the affective side of sensation and the cortex the seat of the discriminative perceptions of space, form, colour, sound, position, etc.

8. We regard the essential thalamic organs as affording by some form of conditioned reflexes one means of activating the various engrammic systems, and alternatively that such activation may be the result of stimuli from the peripheral receptor organs or concurrently from both of these sources.

9. We regard that part of an active engrammic system in which the neural potential is most intense and which is in most direct communication with the essential thalamic organs as the neural correlate of what the psychologists call the 'focus of attention' and those parts of the same active engrammic system in which the neural potential is less or which are in less direct communication with the essential thalamic organs as the neural correlate of what the psychologists call the 'fringe of consciousness.' But this in itself is in our opinion inadequate to explain all the associated phenomena which require for their full elucidation further research in the physiology of special sensation.

10. We regard the activation of the various engrammic systems by the essential thalamic organs as the neural process involved in what is usually called 'memory' and in what Semon called the 'mnemic excitation' (as distinguished from the original excitation) which 'ecphorizes' the multitudinous engrams, changing them from a latent to an active condition, so bringing above the threshold of consciousness' the multitudinous and ever-changing concepts of which the '‘engrams' form the neural correlates. We also regard this activation of engrams by the essential thalamic organs as furnishing the neural correlates of what in the chaotic vocabulary of our present discords, have been variously termed 'the datum' of consciousness, the 'a priori data' of the perceptual processes and 'pre-perception.'

11. We regard the fibres which descend from the cortex to the

thalami as conveying stimuli from the engrams in the various areas of localized perceptions or cognitive dispositions, and that these stimuli excite relay cells in the thalami which again send stimuli to the same cortical areas and that the continual action of this circulatory system of neural impulse from cortex to thalami and from thalami to cortex goes far to constitute the thalami as organs contributing in important ways to the continuity of reflective thought.

12. With the concept as an element of thought on the psychological side having the characteristics described and the engram as described by Semon furnishing its neural correlate; the first of these entities being infinitely mobile and ever in course of merging into others; and this mobility having as its neural correlate an ever-shifting range of activation in an otherwise latent aggregate of engrammic systems, we seem to possess a view of complementary processes, psychological and neural, congruous with one another and affording together the needful explanation of the continuity and fluidity of thought. As to the processes by which the requisite changes in the direction of the neural potential are being constantly made we are unable at present to offer any hypothesis, but we imagine that when these are discovered they will be found somewhat similar to the processes which direct, alternate and adjust with finely graded and continually changing intensity the stimuli to the nerves supplying the balanced groups of muscles which alternately flex and extend the limbs.

13. These views seem logically to bring us to the conclusion that it is necessary for us to regard what is usually called 'inhibition' as being, at any rate in some cases, more in the nature of a re-direction or intensive modification of neural potential than the effects of a positive neural discharge inhibiting other neural discharges. In this we find ourselves in agreement with the opinions expressed by Bianchi in his book on The Mechanism of the Brain and the Function of the Frontal Lobes, and by Morley Roberts in his book on Warfare in the Human Body1, but we realize that this conclusion questions a current view of the inter-relation of cortical and thalamic functionings. We think that the implications involved in the phrase 'cortical control' need to be further investigated and the attendant phenomena possibly re-interpreted.

14. We regard the formation and functional maintenance of the multitudinous engrammic systems as the neural mechanism involved in the storage of past experience.

15. We regard the formation of complex engrams embracing several 1 Op. cit. chap. iv, "Inhibition and the Cardiac Vagus."

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