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an inhibition. The chief exponent of this theory is Exner. He enters into the subject from a physiological standpoint, with an intimate knowledge of the nervous system and its processes. It is impossible to do justice here to Exner's construction of a physiological basis for his theory of attention. We have quoted in our introductory chapter the evidence he adduces in favor of the new concept of Bahnung. But his systematic application of this and of more familiar concepts cannot be properly valued without a careful review of the work as a whole. Obscurities in our summary of his theory will be removed, we think, by reference to the chapters preceding that on attention, in which Exner describes and schematizes the nerve processes correlative to the more elementary processes of conscious and unconscious movement. We, however, can attempt to present here only those portions which stand in immediate connection with the explanation of the peculiar and distinguishing characteristics of attention itself.
Of all the sections preceding the chapter on attention, that which deals with reaction times (pp. 156-162) seems to us to stand in closest connection to the subsequent definition of attention, and as a preliminary to the citation of that definition, we quote the author's explanation of the motor type of reaction.
"There can scarcely be a doubt that this form of a typically voluntary movement is to be referred to the fact that the intention (Willensintention) to execute a definite movement as quickly as possible upon the appearance of an expected sense stimulus, depends upon a change which the cortical processes bring about in the relations affecting the excitability of sub-cortical centres. The reader will at once recognize the similarity between the relations here described and those already mentioned in the account of Bahnung [quoted in our first chapter] and of the regulation of voluntary movement by ideas [pp. 151-154]. In fact it was through the investigation of reaction times that I was led to believe in the existence of the phenomena which I have described above under the name of phenomena of "facilitation," and for which I later found proof in experiments on animals.'
If we bear in mind, then, that voluntary action is to be thought of as "a change which cortical processes bring about in the relations affecting the excitability of sub-cortical centres," and that this change is referred to as either a facilitation or an inhibition, or both, we have the two most important ideas involved in the definition and explanation of attention. Exner states the definition in these terms:
1"Entwurf zu einer Phys. Erklärung der Psychischen Erscheinungen.' p. 158.
"We have shown that an act of will can bring about changes in our nervous system, the effect of which is that an excitation now follows a path A, and now, when a different change has been voluntarily set up in place of the former, takes the path B. The changes thus set up have the character of states. I see in this interaction of different parts of the nervous system the essence of what we ordinarily call attention."
Exner's schemata are so serviceable in the course of his explanations that we reproduce here the "schema for the demonstration of the effect produced by the direction of the attention to sense-impressions." And we quote from the context the passages that give the substance of Exner's explanation and application of the schema.
"If we apply the diagram to the case of the motor form of reaction when only the group of muscles represented by M, is to contract, then we must think of the cell m, as being charged through 1; and perhaps also the cell a, is charged through 1. Then when the stimulus enters through S it will pass through a, and m, to M1. In this instance the paths facilitated lie in the sub-cortical centres. Similar changes can take place in the cortex, as we shall see later.""" "But attention can also be applied to the stimulus entering through S without being connected with a tendency to movement, as when, for example, some particular part of an object interests us. The physiological state of attention then consists in a charging of the a-cells through the a-fibres. [These a-fibres will carry excitations both to and from the cortex: e. g., pp. 153, 155 and 164.] A feeble excitation by way of S will then bring these cells to the point of
1Op. cit., p. 163.
2 P. 163 (paraphrase).
discharge; and if the charging is still kept up through the a-fibres, the a-cells will remain in continuous excitation until they, or the central terminations of the a-fibres, or both, are exhausted, - in which case we speak of fatigued attention."
"But although attention may thus be exclusively sensory, 'it will probably be found, in accordance with the diagram, that there must be a tendency to movement in sensible attention, since the m-cells receive stronger impulses than when the a-cells are not charged; impulses which are stronger before as well as at the time of the entrance of the sensation. Introspection shows that
at least those muscles whose region of innervation is nearly related to the sensory region concerned can scarcely be kept at rest if the attention is directed to the corresponding sensations.' "I 2
Examining the physiological phenomena of attention more closely, we find that whether the attention is turned to senseimpressions or movements or memorial images, it always causes certain paths of the nervous system to become specially practicable (fahrbar), and to remain in this state a longer or shorter time; and, furthermore, reduces the conductive power of a great number of other paths. The more intensive the attention, the lower is the excitability of these other paths. .. I will designate this total state of the system by the term Attention [not Aufmerksamkeit], and accordingly speak of attentional inhibition and attentional facilitation. The dividing line between them may have to be drawn at very different points in different sense departments if we choose to venture on comparisons of department with department, and will certainly vary from case to case with the disposition of the whole nervous system."13
Exner's definition of attentional inhibition is rather perplexing. He says, "By the term attentional inhibition, I mean a state of the centres somewhat like that which prevails in a reflex organ or in a centre subserving instinctive movements, a centre which is stimulated to action by an adequate stimulus but prevented from acting by the will. I mean, i. e., an increased tonus of the cells, in spite of which discharge is obstructed. And this region of simultaneous facilitation and inhibition may be variable."4 Since Exner's concept of the will is a thoroughly psychophysical one, the statement quoted cannot mean what on the surface it seems to mean. The passage must be interpreted in the light of his general treatment. The last sentence of the quotation shows that here, as elsewhere, attentional inhibition and facilitation are being considered together as coöperative and complementary processes. To make the passage at all intelligible or consist ent, we have to paraphrase it in the following fashion. If we attend to an object there may be a rise in the tonus of a region a. This region may now be compared to that connected with a reflex organ in which there is a permanent "facilita
tion" of discharge in certain directions. In both cases there is a tendency to discharge, but actual discharge will be prevented if there is at the same time an attentional facilitation of some other region. Exner might also refer here to his account of the inhibition of motor phenomena in the passage already quoted from pp. 163 and 164. But apparently he thinks of that as so exceptional that after once stating its possibility, he makes no further use of it. And with this exception he always refers to attentional inhibition as attendant upon and simultaneous with attentional facilitation. 1
The most important point with regard to attentional facilitation is the relation of attention to the intensity of a sensation. As Exner says, "From the preceding explanation we might expect the intensity of a sensation to increase as the attention was more fully directed upon it. As a matter of fact, if the cella, (Fig. 48) is charged, and at the same time receives impulses through the fibre,, the retroactive excitations must be greater than if , had carried no impulses to a1. In other words, the intensity of the intercellular tetanus (see chapter II, p. 94) depends upon each of the coöperating factors."72 We recall at once the conclusive objections against the consideration of attention as nothing else than an increased intensity of sensation so concisely stated by Külpe. But Exner's further development of his theory shows that it is not touched by the objections stated by Külpe. For although he holds that attention does increase the intensity of a sensation, he also maintains that there is a recognizable difference in consciousness between the enhancement due to increase of stimulus and that which results from increase of attention. Another diagram will give his idea more exactly.
The accompanying statement is as follows:
"Nevertheless, the excitations which reach the organ of consciousness will be different according as the external stimulus, or the attention, is increased. And if they are different, the two cases will be distinguished in consciousness. Let a represent a cell through which impulses of attentional facilitation, following the path C1, flow from the organ of consciousness toward 8; and let there be, running from 8 to the cortex, the other path, C,, which has occurred so often in our previous figures. There will then be an intercellular tetanus established between a and 8. The impulses given off from a may be diffused along several paths; we are ourselves concerned with the further path C2, which may also give occasion to a conscious sensation.
'Notice that this statement holds only with regard to attentional inhibition. It does not refer, of course, to the purely physiological inhibitions described in Exner's second chapter, pp. 71-76.
2 P. 168.
3 "Outlines of Psychology," § 76, 1; tr., p. 441.
"Now when we consider the diffusion experienced by every excitation that penetrates the gray matter, due to the ramification of its paths, we cannot doubt that conscious processes may be aroused by the stimulation of S by way of C, as well as by way of C1, and probably also by way of C1. There is just as little doubt that in case of simultaneous facilitation by way of C1 the relation between the impulses flowing into the organ of consciousness by way of C2 and C, is altered. There are at least no reasons for assuming that the relation remains the same, while there are many for supposing that it undergoes a change (cf. p. 58).
"In other words, if C, conducts an excitation of a certain intensity to the cortex, and at the same time C, conducts one of much less intensity, we have one total impression. If C, conducts an excitation of the same intensity as in the first instance, and C2 another of greater intensity, then we have another total impression. These two impressions must therefore be distinguishable in consciousness, just as according to the modern theories of color-vision the impression of red-orange is distinguishable from the impression of yellow-orange owing to the relatively unequal excitation of the same kinds of fibres.""
Exner gives two illustrations to show how this theory explains the facts "in the domains of sensitivity and motility." The first instance is an application of the schema to the fact that we have a sensation of blackness when there is no excitation of the retinal elements. The second instance is an explanation of the apparent movements of objects when the motor apparatus is rendered incapable of functioning.2
Exner further limits the influence of attention to processes accompanied by consciousness. Sense impressions have,
1Pp. 168, 169. * Pp. 169, 170.