Page:Encyclopædia Britannica, Ninth Edition, v. 20.djvu/71

PSYCHOLOGY 59 xion of Ipres- |>ns 1,1 iages. that often a state of distraction ensues, such as the train of ideas left to itself never occasions. The better to hear we listen ; the better to see we look ; to smell better we dilate the nostrils and sniff ; and so with all the special senses : each sensory impression sets up nascent movements for its better reception. 1 In like manner there is also an adjustment for images which can be distinguished from sensory adjustments almost as readily as these are distin- guished from each other. We become most aware of this, as, mutatis mutandis, we do of them, when we voluntarily concentrate attention upon particular ideas instead of remaining mere passive spectators, as it were, of the general procession. 'To this ideational adjustment may be referred most of the strain and "head-splitting" connected with recollecting, reflecting, and all that people call head- work ; and the "absent look" of one intently thinking or absorbed in reverie seems directly due to the absence of sensory adjustment that accompanies the concentration of attention upon ideas. But, distinct as they are, impressions and images are still closely connected. In the first place, there are two or three well-marked intermediate stages, so that, though we cannot observe it, we seem justified in assuming a steady transition from the one to the other. As the first of such intermediate stages, it is usual to reckon what are often, and so far as psychology goes inaccurately, styled after-images. They would be better described as after- sensations, except perhaps when the sense of sight is speci- ally in question, inasmuch as they are due either (1) to the persistence of the original peripheral excitation after the stimulus is withdrawn, or (2) to the effects of the exhaustion or the repair that immediately follows this excitation. In the former case they are qualitatively identical with the original sensation and are called " posi- tive," in the latter they are complementary to it and are called " negative " (see EYE, vol. viii. p. 823). These last, then, of which we have clear instances only in connexion with sight, are obviously in no sort re-presentations of the original impression, but a sequent presentation of dia- metrically opposite quality ; while positive after-sensations are, psychologically regarded, nothing but the original sensations in a state of evanescence. It is this continu- ance and gradual waning after the physical stimulus has completely ceased that give after -sensations their chief title to a place in the transition from impression to image. There is, however, another point of resemblance : after- sensations are less affected by movement. If we turn away our eyes we cease to see the flame at which we have been looking, but the after-image remains and is projected upon the wall, and continues still localized in the dark field of sight even if we close our eyes altogether. But the fact that movement affects their localization, though it does not exclude them, and the fact also that we are dis- tinctly aware of our sense-organs being .concerned in their presentation, both serve to mark them off as primary and not secondary presentations. The after- sensation is in reality more elementary than either the preceding percept or its image. In both these, in the case of sight, objects appear in space of three dimensions, i.e., with all the marks of solidity and perspective ; 2 but the so-called after-image 1 Organic sensations, though distinguishable from images by their definite though often anatomically inaccurate localization, furnish no clear evidence of such adaptations. But in another respect they are still more clearly marked off from images, viz., by the pleasure or pain they directly occasion. 2 The following scant quotation from Fechner, one of the best observers in this department, must suffice in illustration. " Lying awake in the early morning after daybreak, with my eyes motionless though open, there usually appears, when I chance to close them for a moment, the black after-image of the white bed immediately before me and the white after-image of the black stove-pipe some distance away against the opposite wall. . . . Both [after-images] appear as lacks all these. Still further removed from normal sensa- tions (i.e., sensations determined by the stimuli appropri- ate to the sense-organ) are the " recurrent sensations " often unnoticed but probably experienced more or less frequently by everybody cases, that is, in which sights or sounds, usually such as at the time were engrossing and impressive, suddenly reappear several hours or even days after the phy- sical stimuli, as well as their effects on the terminal sense- organ, seem entirely to have ceased. Thus workers with the microscope often see objects which they have examined during the day stand out clearly before them in the dark ; it was indeed precisely such an experience that led the anatomist Henle first to call attention to these facts. But he and others have wrongly referred them to what he called a " sense-memory " ; all that we know is against the supposition that the eye or the ear has any power to retain and reproduce percepts. "Recurrent sensations" have all the marks of percepts which after-images lack ; they only differ from what are more strictly called " hallu- cinations" in being, as regards form and quality, exact reproductions of the original impression and in being independent of all subjective suggestion determined by emotion or mental derangement. In what Fechner has called the " memory-after-image," or primary memory-image, as it is better termed, we have the ordinary image in its earliest form. As an instance of what is meant may be cited the familiar experience that a knock at the door, the hour struck on the clock, the face of a friend whom we have passed unnoticed, may sometimes be recognized a few moments later by means of the persist- ing image, although the actual impression was entirely disregarded. But the primary memory-image can always be obtained, and is obtained to most advantage, by looking intently at some object for an instant and then closing the eyes or turning them away. The object is then imaged for a momt very vividly and distinctly, and can be so recovered several times in succession by an effort of atten- tion. Such reinstatement is materially helped by rapidly opening and closing the eyes, or by suddenly moving them in any way. In this respect a primary memory-image re- sembles an after-sensation, which can be repeatedly revived in this manner when it would otherwise have disappeared. But in other respects the two are very different : the after- sensation is necessarily presented if the intensity and direction of the original excitation suffice for its production, and cannot be presented, however much we attend, if they do not. Moreover, the after-sensation is only for a moment positive, and then passes into the negative or complement- ary phase, when, so far from even contributing towards the continuance of the original percept, it directly hinders it. Primary memory-images, on the other hand, and indeed all images, depend mainly upon the attention given to the impression ; provided that was sufficient the faintest impression may be long retained, and without it very in- tense ones will soon leave no trace. The primary memory- image retains so much of its original definiteness and intensity as to make it possible with great accuracy to- compare two physical phenomena, one of which is in this way remembered while the other is really present ; for the most part this is indeed a more accurate procedure than that of dealing with both together. But this is only possible for a very short time. From "Weber's experiment* with weights and lines 3 it would appear that even after if they were in juxtaposition in the same plane ; and, though when my eyes are open I seem to see the white bed in its entire length, the after-image when my eyes are shut presents instead only a narrow black stripe owing to the fact that the bed is seen considerably fore- shortened. But the memory -image on the other hand completely reproduces the pictorial illusion as it appears when the eyes are open '* (Elemtnie der Psychophysik, ii. p. 473). 3 Die Lehre vom Tastsinne, &c., p. 86 sq.