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Chapter 3: Movement and the Image or Centrally Excited Sensation
Margaret Floy Washburn
IN the last chapter we found reason for thinking that a stimulus produces an effect on consciousness when it initiates a motor response which is partly checked in its execution by a process of inhibition, through the influence of an antagonistic motor response. The conscious process that is thus directly occasioned by the action of an outside stimulus is called a 'sensation,' and may be further distinguished as a 'peripherally excited sensation,' to denote the difference between it and a centrally excited sensation or image. For we can get, as conscious experiences, sensations not only from outside stimuli, but by the processes which are commonly known as 'memory' and 'imagination.' Not only can I see red when red light is acting on my eyes, but I can call up a mental image of red, and even, with fair accuracy, images of a whole series of different shades and tones of red. I can not only hear the tones of a violin playing the "Prize Song" from the Meistersinger when the violinist is actually before me (or the phonograph is actually running), but I can sit here in my study, with no actual sound stimuli acting on my ears save the voices of the children across the street, and hear the tones of the violin through the entire air. The very important question now confronts us as to whether these images or centrally excited sensations are each one of them dependent on an incipient motor process, as the corresponding sensations would be if outside stimuli were acting.
A fact clear to any observation is that there often intervenes, between the giving of a stimulus and the making of a movement that an outsider can see, a long interval. A man is sitting in his business office. To him there enters an acquaintance and
( 28) asks him to write a check for one hundred dollars. The man of business says nothing, and makes no visible movement for a considerable interval of time. His friend knows very well, however, that the request has been heard and is being pondered, and waits patiently. At the end of a certain period, the business man draws his check-book and his pen to him and carries out the request. He responds to the original stimulus by making the same movements which he might appropriately have made to it at once: but during the interval between stimulus and response. he will report from introspection, a train of processes has passed through his consciousness which had no outside stimulus: which belonged to the class of centrally rather than peripherally initiated conscious processes. He may have heard in memory the words of another friend urging the claims of the cause to which lie is asked to give; he may have had a mental picture of some scene from his past.
Now, it would be quite possible to hold (d) that while these conscious processes are, taken all together, the whole series of them, caused by the delay in responding to the original outside stimulus, and thus conditioned by the initiation of the final motor response, the several and individual centrally excited processes, images, or thoughts, that filled up the interval were not, each of them, dependent on an initiated motor response of its own. On the other hand, I think a very good case can be made out for the hypothesis (b) that each of these centrally excited processes, thoughts, or images, is dependent on its own special motor response. If the first view (a) is maintained, we should suppose that the energy of the stimulus S, not finding full discharge into the motor pathways of the response. passes directly through a series of sensory centres and finally, by this indirect route, finds its way back into the motor outlet which by the direct route was not fully open. As the nervous process traverses each of the series of sensory centres, there occurs, it would be held, a centrally excited conscious process in quality like the sensation which the centre in question would mediate if it were excited by an external stimulus. To take a simple ex-
(29) -ample: the words, 'I promised my wife to give this money,' may pass through the mind of the man who sits silently pondering between the request for the money and the writing of the check. According to hypothesis (a), the energy of the stimulus (the request for the money) passes directly through a series of auditory sensory centres, and the accompaniment in consciousness is the mental hearing of the words in question. The implication of this view is that every sensory centre may have its functioning accompanied by consciousness under two wholly unlike physiological conditions. The first condition is when the energy of an outside stimulus reaches the centre. As we have seen, it appears probable that in such a case consciousness results only when the motor response is partly but not fully produced; only when excitation is partly balanced by inhibition. The second condition is when energy travels to the sensory centre directly from another sensory centre. But if the mere passage of the nervous process from one sensory centre to another is sufficient to call up a conscious process; if, that is, the traversing of a sensory centre by a nervous process coming from another sensory centre and on its way to a third is sufficient to bring an 'image' into consciousness, why is not the passage of the nervous process through a sensory centre on its way to a free motor outlet sufficient to cause consciousness in the case of the peripherally excited process' Yet we have noted the probability that the traversing of a sensory pathway by the nervous process is unaccompanied by consciousness when the motor pathway is free. On hypothesis a, then, the conditions for consciousness produced by outside stimulation, on the one hand, and the conscious processes, 'centrally excited,' involved in memory and imagination, on the other hand, would be quite unlike: the former would demand not merely the traversing of a sensory centre by a nervous current, but the partial inhibition of a motor discharge: the latter would demand merely the passage of the nervous current through the sensory centre from another centre.
Other arguments against hypothesis a will present them-
( 30)
-selves later on. Hypothesis b, that each of the centrally excited processes which make up a train of images or thoughts has its own special motor response upon whose initiation it depends, may now be further developed.
Suppose that a certain motor pathway, M, has at various times in the past been excited by energy reaching it from two different sensory pathways, S and S'. And suppose that in a given case energy from S reaches it, the effect of which is partly compensated by an inhibition from some antagonistic centre. M will then be in the kind of incomplete excitation which according to the conclusion of our last chapter is accompanied by consciousness of the sensation S. But whatever reason there may he for thinking that a process, accompanied by consciousness, is set up in S by the incipient excitation of M, would appear to hold also for the setting up of a process in S', a centre which is not now receiving any excitation from outside, but which has formerly, under the influence of outside excitation, discharged into M. The very same process which, on our hypothesis, when added to the effect of an outside stimulus makes that effect conscious, will, when it occurs in a sensory centre that is not being externally excited, be accompanied by the type of consciousness that we call 'centrally excited,' the consciousness that occurs in mental images and thoughts. When ever a motor pathway is at the same time excited by a sensory pathway and partially inhibited by an antagonistic motor excitation, a process occurs in all sensory pathways connected with the motor pathway by low synaptic resistances, including the sensory pathway that is exciting the motor pathway in question. This process is accompanied by consciousness. When it occurs in a sensory pathway that is being excited by an outside stimulus, it gives rise to the type of consciousness that we call a peripherally excited sensation. When it occurs in a sensory pathway that has no out-
(31) -side excitation, it gives rise to the type of consciousness that we call a centrally excited sensation or a mental image.[1]
How, under this hypothesis, can we explain the occurrence of a train of images or centrally excited processes, each one calling up the next? During the period between the getting of a stimulus to action and the carrying out of the action, a man's thoughts drift from one idea to another: he may in thirty seconds or so have a long train of conscious processes without any external stimulus. In the psychological laboratory, we ask students to take a certain word as a starting-point, and to introspect, without controlling, the images which follow one another through their minds.
On the theory which we are developing, such trains of centrally excited processes depend on the type of movement associations which we have called 'successive movement systems.' Take as an example the case of learning a series of nonsense syllables which are visually presented, printed or written, before one. For the sake of simplicity let us neglect the part played by auditory sensations, peripherally or centrally excited, and consider the process of pronouncing the syllables, as one looks at them, simply as a motor process whose stimulus is visual. In reading the series through we have a succession of visual stimuli, each followed by its proper motor response. Now, as the series is repeatedly read, each motor process comes to have two stimuli: the original visual one, the sight of the printed or written syllable, and also the kinesthetic 'feel' of pronouncing the syllable just preceding. The more thoroughly the series is learned, the more the kinesthetic stimulus becomes sufficient by itself, without the visual one, to set off the movement. When the series is thoroughly learned, one can say it 'without book,' that is, without any visual stimuli at all. If, however, in reciting a series of syllables without the copy before
(32) one, there is a delay; if some inhibition arises, then one may recall the look of the missing syllable, more or less accurately; that is, delay in the discharge of a motor centre produces, in the sensory centre which formerly discharged into it, that process upon which there is based consciousness of the sensation, centrally excited, which would be peripherally excited if an actual stimulus were acting on the sensory centre and its motor response were delayed.
This is what happens, we may suppose, when the series of syllables is actually being recited; that is, when the movements of articulation are being fully performed. If a hitch comes in the recitation, a mental image of the look of the syllables may be recalled. What, now, takes place when the series is not recited aloud, or even whispered, but merely 'run through' mentally% The thoughts or images which we have fancied to occur in the mind of the man deliberating whether he shall write a check do not take place, each of them, in the course of a series of actual movements that are visible externally. But nevertheless, — and this is a crucial assumption for our whole hypothesis, — there probably are, going on in his muscles, slight actual contractions. So when we mentally run over a series of nonsense syllables that we have learned by heart, it must be supposed that slight actual movements of the articulatory muscles do occur. Introspection furnishes some evidence of the fact. Try, for instance, the following test: pronounce aloud or in a whisper the letter ' b' successively twenty-five times, and as you pronounce it try, absolutely simultaneously, to think of each of the other letters of the alphabet. You will find that, whether you think of them in auditory or visual terms, whether you mentally bear them or mentally see them, you have to slip them in between your pronunciations of 'b': the sequence of events in your consciousness has to be 'h a,' 'b c,' 'b d,' and soon. Or try James's old experiment of holding the mouth open and thinking of the word 'bubble.' In this case, to my introspection, the auditory image is impossible, but I can form pretty well a visual image of the word. In general,
(33) visual images of words are not so closely dependent on articulatory movements, whether strong or weak, as auditory images are, for all visual images are related also to other movements, as for example movements of the eyes.
Slight but actual movements occur when we are thinking. These movements it is difficult to christen by a suitable name, but for convenience of reference, we shall call them 'tentative movements.' A later chapter will be devoted to their consideration.
A train of associated ideas, each one suggesting the next by a process which travels directly from one sensory centre to another, rests according to this hypothesis on a train of movements each of which, by the kinesthetic excitations its performance produces, sets the next one off, while delays in the process bring about, in sensory centres that have formerly discharged into the motor centres now partially excited, the processes on which the images are based. The case is fundamentally the same whether the movement system consists of movements that are being fully performed, though hesitatingly, or of tentative movements; whether the series of syllables is recited or only thought of.
Two points in favor of some such hypothesis may he noted. The older view demanded that there should exist nervous pathways directly connecting one sensory centre with another. Now, in those lower regions of the nervous system of vertebrates which have been the subject of exact physiological investigation and experiment, there is no evidence that sensory centres ever have pathways of direct connection with one another. 'Association pathways' connect an afferent or sensory pathway with an efferent or motor pathway. Several sensory pathways converge on a single motor pathway, their 'final common path,' or upon 'internuncial pathways' leading to a final common path, but nowhere does any path directly bridge across from one afferent path to another. Yet apparently such connections would be as useful in the lower regions of the nervous system as they are supposed to be in the upper, cortical
(34) regions. Simplicity would argue for the supposition that all parts of the nervous system are built on —the same general plan, and that this plan involves the association of movements, but not the direct association of sensory processes.
A second argument for the kind of hypothesis we are trying to form is this: The older view supposed not only that direct pathways connected one sensory centre with another, but that when the two sensory centres were simultaneously excited, the resistances along the connecting pathway were lowered, so that at a later occasion, when a nervous process occurred in one of the centres, it travelled across to the other centre and 'centrally excited' it. Thus was explained the fact of experience that in order for one thing to recall another to our minds, by central excitation, we must have encountered the two things together. In order that the sight of a word printed in a foreign language shall recall its correct pronunciation, we must have heard the word pronounced on some former occasion while we were looking at it. But the older view neglected the fact that merely experiencing two stimuli together will not suffice to form a central excitation tendency. Stimuli may easily act simultaneously, each on its own sensory pathway, and the resistances between these pathways not be lowered at all. For example, one needs merely to suppose that when looking at the foreign word for the first time and hearing it pronounced, one hears it only inattentively. If the attention is not directed towards the sound, if both the look of the word and the sound of it are not attentively experienced, the look will not later recall the sound, nor vice versa. Not the simultaneous experiencing of two objects, but simultaneous attention to them, forms a tendency to recall.
It is clear on our theory why the formation of an 'association' demands attention. All association is association between movements. Now, when the association between movements is in process of establishment, the performance of the movements is necessarily subject to many inhibitions and delays, but of course it must be an actual performance. We must
(35) suppose then that the actual performance of movements — that is, something more than the mere innervation of the muscles —is what is involved in attention; the excitation being weakened and slowed by inhibitions, but nevertheless resulting in actual though slight movements. We have seen that there are two types of association between movements. The first type is characterized by the dropping out of movements, so that a stimulus takes on a new motor response and loses its old one. The second type preserves all the movements, and links them so that the performance of one furnishes the stimulus for the performance of another. Clearly, it is this second type of association that is the basis of trains of ideas, each one distinguished from the rest, for only where different motor responses are initiated are the sensory processes leading to them accompanied by distinguishable sensations.
One of many other points in favor of a motor theory of attention and the image is that it explains readily what we may call the transitory character of both. "The object of attention constantly fluctuates." We cannot hold an unchanging thing in the focus of attention: it seems to change of its own accord. Some years ago in the Vassar laboratory (124) an attempt was made to investigate experimentally a phenomenon that nearly every one has noticed: the fact that if you look steadily for some time at a printed or written word it takes on a strange and unfamiliar appearance. As our observers gazed steadily at a single printed word for three minutes, they noted that the word was constantly changing; constantly suggesting new pronunciations, new syllable divisions, and so on. 'Acre' would become 'ac-er,' 'a-cree,' and so on. This shifting of the object of attention has been referred to the fact that primitively a stimulus must be a change in the environment of an organism, and that an unchanging object cannot be a stimulus. But in fluctuations of attention it is not really the stimulus, not the external object, that changes. What changes is our reaction to it, and the reason for this constant change of response to a stimulus in attention is that the essence of attention is movement, and
(36) that the great majority of motor processes are of the phasic rather than the static or attitude type.
On our general theory, then, consciousness will depend on a certain ratio between excitation and inhibition of the motor response to a stimulus; while recall through association will depend on the motor response's being sufficiently innervated to be actually performed, though only in the weakened form of a tentative movement. We may tabulate the different ratios of inhibition and innervation of response which according to this theory correspond to different degrees of consciousness.
I. Motor response very slightly excited, not enough to produce consciousness. The effects of this excitation appear, however, in the phenomenon of 'readiness,' which will be discussed later. (gee page 82.)
II. Motor response more strongly initiated. but no actual performance of a movement, either tentative or full. This is accompanied by consciousness of the stimulus, but no associative activity can take place, because associative activity requires the actual performance of a motor response, to produce the kinaesthetic excitations necessary to set other motor pathways into action.
III. Motor response still more strongly initiated. Tentative movements occur slowly and with delays. Consciousness and associative activity both present: this is attention.
IV. Motor response still more strongly initiated. Tentative movements occur smoothly and without delay. Associative activity occurs, with comparatively little 'conscious accompaniment: these are the conditions which we may think of as underlying very rapid thinking, which is nearly unconscious.
V. Motor response fully initiated and entirely unopposed. Performance of full motor response without delay and without consciousness: the situation in secondarily automatic or habitual actions.
Thus, by supposing five different ratios of excitation to inhibition, we can make a bridge between the apparently con-
( 37) -tradictory but equally true statements that unconsciousness accompanies total inhibition of movement and that it accompanies perfect freedom of movement.
Can we form any notion of what physiological process underlies the difference which we consciously experience between a peripherally excited sensation and an image? We have assumed that the degree of consciousness, what M nsterberg would call the vividness, of both is due to the same underlying cause, the relation between the excitation and the inhibition of the motor pathways with which their sensory pathways are connected. Now, just what makes the difference between the color red as I see it before me and the color red as I remember or imagine it? Certainly it is not a difference in consciousness degree. The degree of consciousness depends on attention: but I may be wholly inattentive to the colors I am actually looking at, and fully absorbed in the color imagery that my imagination conjures up. If you ask me afterward what I thought of a certain red cushion, I may have no recollection that any such object was near me, although I spent some minutes apparently looking intently at it.
All authorities are agreed that centrally excited sensations are much less steady and enduring than peripherally excited sensations. I cannot hold in consciousness the image of red nearly so long as I can be conscious of the red that I actually see. On our theory, this short duration of the image is due to the fact that the stimulus which initiates the motor response on which consciousness of the image depends is produced by another movement; while the stimulus which initiates the motor response on which the peripherally excited sensation depends is an outside physical force. It is obvious that a stimulus supplied by a bodily movement must be of brief duration, unless indeed the movement is not a true movement, but a static motor process or attitude. Much discussion has centred about the problem as to whether a difference in intensity is characteristic of the difference between a sensation and an image. To immediate observation it seems clear that the color or tone or
( 38) smell one imagines is never so intense as an actual sensation. Now, it is not so easy as one might suppose to decide exactly what is meant by intensity.
Chapter 3: Movement and the Image or Centrally Excited Sensation
Margaret Floy Washburn
IN the last chapter we found reason for thinking that a stimulus produces an effect on consciousness when it initiates a motor response which is partly checked in its execution by a process of inhibition, through the influence of an antagonistic motor response. The conscious process that is thus directly occasioned by the action of an outside stimulus is called a 'sensation,' and may be further distinguished as a 'peripherally excited sensation,' to denote the difference between it and a centrally excited sensation or image. For we can get, as conscious experiences, sensations not only from outside stimuli, but by the processes which are commonly known as 'memory' and 'imagination.' Not only can I see red when red light is acting on my eyes, but I can call up a mental image of red, and even, with fair accuracy, images of a whole series of different shades and tones of red. I can not only hear the tones of a violin playing the "Prize Song" from the Meistersinger when the violinist is actually before me (or the phonograph is actually running), but I can sit here in my study, with no actual sound stimuli acting on my ears save the voices of the children across the street, and hear the tones of the violin through the entire air. The very important question now confronts us as to whether these images or centrally excited sensations are each one of them dependent on an incipient motor process, as the corresponding sensations would be if outside stimuli were acting.
A fact clear to any observation is that there often intervenes, between the giving of a stimulus and the making of a movement that an outsider can see, a long interval. A man is sitting in his business office. To him there enters an acquaintance and
( 28) asks him to write a check for one hundred dollars. The man of business says nothing, and makes no visible movement for a considerable interval of time. His friend knows very well, however, that the request has been heard and is being pondered, and waits patiently. At the end of a certain period, the business man draws his check-book and his pen to him and carries out the request. He responds to the original stimulus by making the same movements which he might appropriately have made to it at once: but during the interval between stimulus and response. he will report from introspection, a train of processes has passed through his consciousness which had no outside stimulus: which belonged to the class of centrally rather than peripherally initiated conscious processes. He may have heard in memory the words of another friend urging the claims of the cause to which lie is asked to give; he may have had a mental picture of some scene from his past.
Now, it would be quite possible to hold (d) that while these conscious processes are, taken all together, the whole series of them, caused by the delay in responding to the original outside stimulus, and thus conditioned by the initiation of the final motor response, the several and individual centrally excited processes, images, or thoughts, that filled up the interval were not, each of them, dependent on an initiated motor response of its own. On the other hand, I think a very good case can be made out for the hypothesis (b) that each of these centrally excited processes, thoughts, or images, is dependent on its own special motor response. If the first view (a) is maintained, we should suppose that the energy of the stimulus S, not finding full discharge into the motor pathways of the response. passes directly through a series of sensory centres and finally, by this indirect route, finds its way back into the motor outlet which by the direct route was not fully open. As the nervous process traverses each of the series of sensory centres, there occurs, it would be held, a centrally excited conscious process in quality like the sensation which the centre in question would mediate if it were excited by an external stimulus. To take a simple ex-
(29) -ample: the words, 'I promised my wife to give this money,' may pass through the mind of the man who sits silently pondering between the request for the money and the writing of the check. According to hypothesis (a), the energy of the stimulus (the request for the money) passes directly through a series of auditory sensory centres, and the accompaniment in consciousness is the mental hearing of the words in question. The implication of this view is that every sensory centre may have its functioning accompanied by consciousness under two wholly unlike physiological conditions. The first condition is when the energy of an outside stimulus reaches the centre. As we have seen, it appears probable that in such a case consciousness results only when the motor response is partly but not fully produced; only when excitation is partly balanced by inhibition. The second condition is when energy travels to the sensory centre directly from another sensory centre. But if the mere passage of the nervous process from one sensory centre to another is sufficient to call up a conscious process; if, that is, the traversing of a sensory centre by a nervous process coming from another sensory centre and on its way to a third is sufficient to bring an 'image' into consciousness, why is not the passage of the nervous process through a sensory centre on its way to a free motor outlet sufficient to cause consciousness in the case of the peripherally excited process' Yet we have noted the probability that the traversing of a sensory pathway by the nervous process is unaccompanied by consciousness when the motor pathway is free. On hypothesis a, then, the conditions for consciousness produced by outside stimulation, on the one hand, and the conscious processes, 'centrally excited,' involved in memory and imagination, on the other hand, would be quite unlike: the former would demand not merely the traversing of a sensory centre by a nervous current, but the partial inhibition of a motor discharge: the latter would demand merely the passage of the nervous current through the sensory centre from another centre.
Other arguments against hypothesis a will present them-
( 30)
-selves later on. Hypothesis b, that each of the centrally excited processes which make up a train of images or thoughts has its own special motor response upon whose initiation it depends, may now be further developed.
Suppose that a certain motor pathway, M, has at various times in the past been excited by energy reaching it from two different sensory pathways, S and S'. And suppose that in a given case energy from S reaches it, the effect of which is partly compensated by an inhibition from some antagonistic centre. M will then be in the kind of incomplete excitation which according to the conclusion of our last chapter is accompanied by consciousness of the sensation S. But whatever reason there may he for thinking that a process, accompanied by consciousness, is set up in S by the incipient excitation of M, would appear to hold also for the setting up of a process in S', a centre which is not now receiving any excitation from outside, but which has formerly, under the influence of outside excitation, discharged into M. The very same process which, on our hypothesis, when added to the effect of an outside stimulus makes that effect conscious, will, when it occurs in a sensory centre that is not being externally excited, be accompanied by the type of consciousness that we call 'centrally excited,' the consciousness that occurs in mental images and thoughts. When ever a motor pathway is at the same time excited by a sensory pathway and partially inhibited by an antagonistic motor excitation, a process occurs in all sensory pathways connected with the motor pathway by low synaptic resistances, including the sensory pathway that is exciting the motor pathway in question. This process is accompanied by consciousness. When it occurs in a sensory pathway that is being excited by an outside stimulus, it gives rise to the type of consciousness that we call a peripherally excited sensation. When it occurs in a sensory pathway that has no out-
(31) -side excitation, it gives rise to the type of consciousness that we call a centrally excited sensation or a mental image.[1]
How, under this hypothesis, can we explain the occurrence of a train of images or centrally excited processes, each one calling up the next? During the period between the getting of a stimulus to action and the carrying out of the action, a man's thoughts drift from one idea to another: he may in thirty seconds or so have a long train of conscious processes without any external stimulus. In the psychological laboratory, we ask students to take a certain word as a starting-point, and to introspect, without controlling, the images which follow one another through their minds.
On the theory which we are developing, such trains of centrally excited processes depend on the type of movement associations which we have called 'successive movement systems.' Take as an example the case of learning a series of nonsense syllables which are visually presented, printed or written, before one. For the sake of simplicity let us neglect the part played by auditory sensations, peripherally or centrally excited, and consider the process of pronouncing the syllables, as one looks at them, simply as a motor process whose stimulus is visual. In reading the series through we have a succession of visual stimuli, each followed by its proper motor response. Now, as the series is repeatedly read, each motor process comes to have two stimuli: the original visual one, the sight of the printed or written syllable, and also the kinesthetic 'feel' of pronouncing the syllable just preceding. The more thoroughly the series is learned, the more the kinesthetic stimulus becomes sufficient by itself, without the visual one, to set off the movement. When the series is thoroughly learned, one can say it 'without book,' that is, without any visual stimuli at all. If, however, in reciting a series of syllables without the copy before
(32) one, there is a delay; if some inhibition arises, then one may recall the look of the missing syllable, more or less accurately; that is, delay in the discharge of a motor centre produces, in the sensory centre which formerly discharged into it, that process upon which there is based consciousness of the sensation, centrally excited, which would be peripherally excited if an actual stimulus were acting on the sensory centre and its motor response were delayed.
This is what happens, we may suppose, when the series of syllables is actually being recited; that is, when the movements of articulation are being fully performed. If a hitch comes in the recitation, a mental image of the look of the syllables may be recalled. What, now, takes place when the series is not recited aloud, or even whispered, but merely 'run through' mentally% The thoughts or images which we have fancied to occur in the mind of the man deliberating whether he shall write a check do not take place, each of them, in the course of a series of actual movements that are visible externally. But nevertheless, — and this is a crucial assumption for our whole hypothesis, — there probably are, going on in his muscles, slight actual contractions. So when we mentally run over a series of nonsense syllables that we have learned by heart, it must be supposed that slight actual movements of the articulatory muscles do occur. Introspection furnishes some evidence of the fact. Try, for instance, the following test: pronounce aloud or in a whisper the letter ' b' successively twenty-five times, and as you pronounce it try, absolutely simultaneously, to think of each of the other letters of the alphabet. You will find that, whether you think of them in auditory or visual terms, whether you mentally bear them or mentally see them, you have to slip them in between your pronunciations of 'b': the sequence of events in your consciousness has to be 'h a,' 'b c,' 'b d,' and soon. Or try James's old experiment of holding the mouth open and thinking of the word 'bubble.' In this case, to my introspection, the auditory image is impossible, but I can form pretty well a visual image of the word. In general,
(33) visual images of words are not so closely dependent on articulatory movements, whether strong or weak, as auditory images are, for all visual images are related also to other movements, as for example movements of the eyes.
Slight but actual movements occur when we are thinking. These movements it is difficult to christen by a suitable name, but for convenience of reference, we shall call them 'tentative movements.' A later chapter will be devoted to their consideration.
A train of associated ideas, each one suggesting the next by a process which travels directly from one sensory centre to another, rests according to this hypothesis on a train of movements each of which, by the kinesthetic excitations its performance produces, sets the next one off, while delays in the process bring about, in sensory centres that have formerly discharged into the motor centres now partially excited, the processes on which the images are based. The case is fundamentally the same whether the movement system consists of movements that are being fully performed, though hesitatingly, or of tentative movements; whether the series of syllables is recited or only thought of.
Two points in favor of some such hypothesis may he noted. The older view demanded that there should exist nervous pathways directly connecting one sensory centre with another. Now, in those lower regions of the nervous system of vertebrates which have been the subject of exact physiological investigation and experiment, there is no evidence that sensory centres ever have pathways of direct connection with one another. 'Association pathways' connect an afferent or sensory pathway with an efferent or motor pathway. Several sensory pathways converge on a single motor pathway, their 'final common path,' or upon 'internuncial pathways' leading to a final common path, but nowhere does any path directly bridge across from one afferent path to another. Yet apparently such connections would be as useful in the lower regions of the nervous system as they are supposed to be in the upper, cortical
(34) regions. Simplicity would argue for the supposition that all parts of the nervous system are built on —the same general plan, and that this plan involves the association of movements, but not the direct association of sensory processes.
A second argument for the kind of hypothesis we are trying to form is this: The older view supposed not only that direct pathways connected one sensory centre with another, but that when the two sensory centres were simultaneously excited, the resistances along the connecting pathway were lowered, so that at a later occasion, when a nervous process occurred in one of the centres, it travelled across to the other centre and 'centrally excited' it. Thus was explained the fact of experience that in order for one thing to recall another to our minds, by central excitation, we must have encountered the two things together. In order that the sight of a word printed in a foreign language shall recall its correct pronunciation, we must have heard the word pronounced on some former occasion while we were looking at it. But the older view neglected the fact that merely experiencing two stimuli together will not suffice to form a central excitation tendency. Stimuli may easily act simultaneously, each on its own sensory pathway, and the resistances between these pathways not be lowered at all. For example, one needs merely to suppose that when looking at the foreign word for the first time and hearing it pronounced, one hears it only inattentively. If the attention is not directed towards the sound, if both the look of the word and the sound of it are not attentively experienced, the look will not later recall the sound, nor vice versa. Not the simultaneous experiencing of two objects, but simultaneous attention to them, forms a tendency to recall.
It is clear on our theory why the formation of an 'association' demands attention. All association is association between movements. Now, when the association between movements is in process of establishment, the performance of the movements is necessarily subject to many inhibitions and delays, but of course it must be an actual performance. We must
(35) suppose then that the actual performance of movements — that is, something more than the mere innervation of the muscles —is what is involved in attention; the excitation being weakened and slowed by inhibitions, but nevertheless resulting in actual though slight movements. We have seen that there are two types of association between movements. The first type is characterized by the dropping out of movements, so that a stimulus takes on a new motor response and loses its old one. The second type preserves all the movements, and links them so that the performance of one furnishes the stimulus for the performance of another. Clearly, it is this second type of association that is the basis of trains of ideas, each one distinguished from the rest, for only where different motor responses are initiated are the sensory processes leading to them accompanied by distinguishable sensations.
One of many other points in favor of a motor theory of attention and the image is that it explains readily what we may call the transitory character of both. "The object of attention constantly fluctuates." We cannot hold an unchanging thing in the focus of attention: it seems to change of its own accord. Some years ago in the Vassar laboratory (124) an attempt was made to investigate experimentally a phenomenon that nearly every one has noticed: the fact that if you look steadily for some time at a printed or written word it takes on a strange and unfamiliar appearance. As our observers gazed steadily at a single printed word for three minutes, they noted that the word was constantly changing; constantly suggesting new pronunciations, new syllable divisions, and so on. 'Acre' would become 'ac-er,' 'a-cree,' and so on. This shifting of the object of attention has been referred to the fact that primitively a stimulus must be a change in the environment of an organism, and that an unchanging object cannot be a stimulus. But in fluctuations of attention it is not really the stimulus, not the external object, that changes. What changes is our reaction to it, and the reason for this constant change of response to a stimulus in attention is that the essence of attention is movement, and
(36) that the great majority of motor processes are of the phasic rather than the static or attitude type.
On our general theory, then, consciousness will depend on a certain ratio between excitation and inhibition of the motor response to a stimulus; while recall through association will depend on the motor response's being sufficiently innervated to be actually performed, though only in the weakened form of a tentative movement. We may tabulate the different ratios of inhibition and innervation of response which according to this theory correspond to different degrees of consciousness.
I. Motor response very slightly excited, not enough to produce consciousness. The effects of this excitation appear, however, in the phenomenon of 'readiness,' which will be discussed later. (gee page 82.)
II. Motor response more strongly initiated. but no actual performance of a movement, either tentative or full. This is accompanied by consciousness of the stimulus, but no associative activity can take place, because associative activity requires the actual performance of a motor response, to produce the kinaesthetic excitations necessary to set other motor pathways into action.
III. Motor response still more strongly initiated. Tentative movements occur slowly and with delays. Consciousness and associative activity both present: this is attention.
IV. Motor response still more strongly initiated. Tentative movements occur smoothly and without delay. Associative activity occurs, with comparatively little 'conscious accompaniment: these are the conditions which we may think of as underlying very rapid thinking, which is nearly unconscious.
V. Motor response fully initiated and entirely unopposed. Performance of full motor response without delay and without consciousness: the situation in secondarily automatic or habitual actions.
Thus, by supposing five different ratios of excitation to inhibition, we can make a bridge between the apparently con-
( 37) -tradictory but equally true statements that unconsciousness accompanies total inhibition of movement and that it accompanies perfect freedom of movement.
Can we form any notion of what physiological process underlies the difference which we consciously experience between a peripherally excited sensation and an image? We have assumed that the degree of consciousness, what M nsterberg would call the vividness, of both is due to the same underlying cause, the relation between the excitation and the inhibition of the motor pathways with which their sensory pathways are connected. Now, just what makes the difference between the color red as I see it before me and the color red as I remember or imagine it? Certainly it is not a difference in consciousness degree. The degree of consciousness depends on attention: but I may be wholly inattentive to the colors I am actually looking at, and fully absorbed in the color imagery that my imagination conjures up. If you ask me afterward what I thought of a certain red cushion, I may have no recollection that any such object was near me, although I spent some minutes apparently looking intently at it.
All authorities are agreed that centrally excited sensations are much less steady and enduring than peripherally excited sensations. I cannot hold in consciousness the image of red nearly so long as I can be conscious of the red that I actually see. On our theory, this short duration of the image is due to the fact that the stimulus which initiates the motor response on which consciousness of the image depends is produced by another movement; while the stimulus which initiates the motor response on which the peripherally excited sensation depends is an outside physical force. It is obvious that a stimulus supplied by a bodily movement must be of brief duration, unless indeed the movement is not a true movement, but a static motor process or attitude. Much discussion has centred about the problem as to whether a difference in intensity is characteristic of the difference between a sensation and an image. To immediate observation it seems clear that the color or tone or
( 38) smell one imagines is never so intense as an actual sensation. Now, it is not so easy as one might suppose to decide exactly what is meant by intensity.