CHAPTER VIII

THE DIRECT CONNECTION BETWEEN THE

MOTOR AND SENSORY MEMORIES

THERE is a tendency for the nervous force from an impression to pass to all parts of the brain ; it only brings a past impression in the sensory memory before the consciousness, when the sum of the impressions reaches the necessary degree of intensity. As an impression is only increased in intensity by a conscious revival, and associations are only formed under similar circumstances, unless an impression be actually revived, this tendency to revival, if entirely unfelt, produces no increase of intensity. The transient character of dreams is due to the consciousness being at such a low ebb when they are received.

When a co-ordinate action has been learnt, the motor memory has reached a fair degree of intensity, and so the nerve-force passes directly from the impression in the sensory memory centre to that in the motor memory centre, and tends to bring about the movement reflexly, that is to say, the motor and sensory memories become directly connected beneath the plane of consciousness, the intervention and controlling influence of the mind becoming less and less required, and the movement becomes more and more truly reflex. When the movement is right-sided, only one set of ganglia is

employed. As this movement is repeated, so does this direct connection become more and more complete, a sensory stimulus at once exciting the required movement. In other cases the individual, wishing to write certain words, leaves it to the motor memory to do so. There is nearly always a certain modifying action of the cerebrum, until after many years of repeated movement this has become practically nil, as in the case of the reporter who could fall asleep and still go on reporting.

As both the sensory and motor memories are beneath the plane of consciousness, the mind has no cognizance of the direct combination or the movements produced by it, other than by the information afforded by fresh sensory impressions.

The faculties having performed their functions of establishing the sensory and motor memories of certain impressions, have, when these have reached a certain intensity, become superfluous, and may be usefully employed on other subjects: this the direct connection' allows them to be.

How contiguous impressions become associated has been discussed, and how the revival of impressions takes place has also been noticed.

I will first describe the direct association of the sensory and motor memories in ordinary shorthand writing, because this is a subject which is learnt at a much later period than ordinary writing, and, therefore, the reader will be more easily able to compare the following account with his own experience. The same applies to ordinary writing, which is described on page 164, where the direct connection is not mentioned, in order to avoid confusion, but will be described here.

The first necessary step in learning to write short

hand is to learn the alphabet; that is, that a straight line slanting downwards from the left to the right is P, a perpendicular stroke T, a horizontal stroke K, and so on. Then the shorthand character, the longhand character, and the corresponding sound become component parts of one impression, and are represented by two movements, one designating the longhand letter the other the shorthand letter, but they are recognised as distinct, though both representing the same sound.

After a short period of practice, the student is able to remember the forms he should make to represent the articulate sounds. Every repetition of the movement increases the facility with which it is made, and the intensity of the motor memory.

As this intensity grows, less and less attention need be paid to the movement, until a suggested letter is written at once.

There are two ways in which words may be written:

1. From dictation or copying from a book.

2. To express ideas with, or to write down some words previously in the mind.

In the first class of cases the part which the direct connection plays is obvious; thus, a man, who is accustomed to write from dictation, will write down the words and be thinking of something entirely different at the same time, and be, if asked, quite unable to say what it is that he has written down, only recognising that it is his work by the handwriting. What has taken place?

The sound of the speaker's voice has revived other impressions; these impressions are associated with, and have been followed by, the movements necessary for the

execution of certain forms, and so the pen being on the paper, these forms are written, the execution of the words being left entirely to the motor memory.

It is the same with copying from a printed book; the memory of similar printed words is associated with that of the corresponding spoken words, and these are associated with the motor memory necessary for the formation of written letters.

It will be seen from the above that dictation and copying can be carried out mechanically, and involve very little mental labour, as do all the processes of revival and performance of movements when executed by the basal ganglia; when the cerebral hemispheres are used there is much greater effort required; thus, let the reader compare the difference, when he is mentally tired, between simply copying some writing and reconstructing the sentences and putting them in his own words.

Notice the great difference there is between writing a word which is well known and has been written many times previously, and one which, however well known, we have not been accustomed to write. In the first case the word will be written easily and without any thought, whilst in the latter the word is written slowly and with considerable trouble, and very likely a mistake will be made in the spelling.

How far this machinery extends will depend upon the ability and the amount of practice the individual has had.

A boy learns that the form B represents the sound bee; in the same way he learns that the forms L, A, C, K, represent the sounds el, ay, see, kay, respectively.

Then he learns that the whole word, BLACK, repre

sents the sound blâk; that is, the two, the sound and the form, form component parts of one impression.

Now, if he wish to write the word, he must first have a consciousness of its constituent letters; that is, be able to spell correctly, then have a sensory memory as to know how those letters should be formed, and a motor memory so that the right muscles are employed.

The revival of a previous sensory impression is usually brought about directly; that is, the sight of the word brings the necessary constituent writing characters to the mind, at the time of the reception of the impression, or the revival may be indirect by the cerebrum, as when we wish to spell a word we have just heard.

The sensory memory remembers the attempts which have been previously made, and by the aid of them corrects the movements, until an accurate co-ordination of the muscles is obtained; then by degrees the motor memory acquires such an intensity and accuracy that the slightest impulse from the cerebrum brings about the requisite movements.

It will be seen that when the sensory and motor memories have attained this degree of perfection, the cerebrum which brought it about has become clearly unnecessary, and can usefully have some other employment, and this we find to be the case.

For instance, when an individual has learnt to write the letter B with facility, the sensory memory of the letter has become directly associated with the motor memory for the production of the written form; that is the sound bee,' or the sight or thought of the letter, is sufficient to bring about the execution of the necessary movements without any interference from the cerebrum.