| SECTION 7 OF 8 | [1] [2] [3.1] [3.2] [3.3] [4] [5] [6] [7] [Ap] | DOWNLOAD PDF |
Reproduced from Experimental Psychology Society Monograph No. 2 1963
7. Relevance to the Theory of Perception
The fact that we have attempted to make some sort of a study of a case of recovery from early blindness does not give us the right to attempt an authoritative statement as to the importance of such cases for the theory of perception. In thinking about this single case, however, the whole problem has been much in our minds and has inevitably led us to develop some opinions.
In 1949, D. O. Hebb made a great deal of the cases assembled by von Senden in his well-known book on The Organisation of Behaviour. Indeed he regarded them as providing powerful evidence for the slow development of perception in infancy. "We are not used", he writes, "to thinking of a single perception as slowly and painfully learned, . . . but it has already been seen, in the discussion of the congenitally blind after operation, that it actually is". (Hebb, opus. cit., pp. 77 - 8). In the light of our experience with S.B., this inference strikes us as distinctly questionable. Is it really certain that what applies to perception after recovery of vision in the adult applies in essentially the same way to its normal growth in infancy?
First, what are the reasons for the slow recovery of vision in the adult patient? Hebb evidently believes that the situation of such a patient is essentially that of the child, with its normal perceptual development arrested until the eyes are opened. Although he is evidently aware of the "emotional crisis" through which such patients commonly pass, he plainly does not regard reduced motivation as the principal cause of slowness in learning. Indeed he draws special attention to the fact that motivation is not disturbed in the first phase after operation, immediately upon beginning to use the eyes. He suggests, further, that the "crisis of motivation" is due directly to the difficulty in achieving pattern vision, and is thus a direct response to the disheartening slowness with which perceptual skill is acquired.
But is it? In the case of S.B., the patient made extremely rapid progress after operation and very soon found that his vision was useful, as in telling the time. It did in fact change his whole way of life within a few weeks of the first operation and some progress continued for the next two years, until his death. It is true that he did not learn to read, and for this and many other reasons found himself handicapped in the world of sighted people. It is this sense of overall inadequacy, we suggest, rather than deficiency of pattern vision per Se, which produced a crisis which lasted until the end.
Further, if emotional crisis is primarily a response to slowness in perceptual learning, why does it not occur in the normal child? It could of course be argued that the process is more difficult for the adult, but throughout Hebb argues that the position in the child is basically similar to that of the adult whose vision is restored at operation. Is this a tenable assumption? The adult, after all, has developed a "touch world" which has served him well for many years, and which has become accepted as the principal vehicle of his occupational and social adaptation. The child, on the other hand, is concerned to develop a "visual world" ab initio, and although tactile and motor activities contribute in an important way to its evolution, it is difficult to think of the two cases as in any real sense similar.
We find similar difficulties where Hebb tries to show that the perception of form is largely built upon patterns of eye movements. (Hebb, opus cit., pp. 84 - 91). Hebb takes as evidence here the reported fact that operated patients count the corners of polygons in order to name them, moving their eyes from corner to corner as a man might move his fingers in active touch. Yet can one really infer that the patient is reproducing the normal visual behaviour of the young child? First, there is no clear evidence that, in the acquisition of form perception, children really do scan the contours of objects or figures in the manner suggested by Hebb. And secondly, it would indeed be surprising if people who had lived for years without vision did not, at least at first, tend to "touch" the salient features with their eyes though one must bear in mind that ocular scanning is often severely disturbed by nystagmus. Although further study of the role of eye movements in the growth of form discrimination is evidently needed, it would appear prima fade unlikely that cases of the kind described in this study can tell us anything very significant about the normal development of perception in infancy. [ Footnote 12.]
It is of course true that in nearly (though not quite) all these cases of recovery of vision after long-standing blindness the development of perceptual skill is very slow, even when considerable sight was present before operation. But this, we suggest, is due not to the fact that visual learning, whether in the child or the adult, is inevitably slow but to lack of practice in making appropriate use of the available input. It would seem that the difficulty, is not so much in learning per se as in changing perceptual habits and strategies from touching to seeing. A not dissimilar phenomenon occurs in many cases with gradual loss of a sense or a limb; the patient will, at a certain stage, do better to abandon using the sense or limb altogether; to change his way of life to avoid and ignore it even if it is partly functional. In the event of some recovery of function, great difficulty may be experienced by the patient (and by those concerned with his rehabilitation) in inducing him to revert to the use of the impaired sense or limb. Indeed this may be one of the principal obstacles in the re-education of patients with any type of higher neurological disability. [ Footnote 13.]
Another point of some interest in the present case is the relatively good intelligence and education of our patient, in which he resembles the patient reported by Latta (1904). Our patient, like Latta's, was much concerned while blind in trying to visualise the world and to know it as other men know it. One might even say that their attempt to see was made long before their eyes were opened to the light, and in this respect they differ not only from most other cases in the literature but also of course from infants.
The finding that S.B.'s visual space was not disturbed by the geometrical - optical illusions, and that apparent depth was not evoked by perspective drawings, show that his spatial organisation was far from normal. Since investigating the case, we have devoted considerable thought to the origin of these illusions and have arrived at the tentative view that they depend upon inappropriate scaling by the mechanism which produces size-constancy. If this should prove correct, it may be surmised that the figures failed to evoke constancy size-scaling in our patient, either through lack of early learning or maturational defect. It would seem of some importance, therefore, to devote attention to the study of these illusions in children with special reference to the development of size constancy.
Perhaps the most important outcome of our study is the evidence it provides for transfer from early touch experience to vision many years later. The fact that our patient was able, certainly with a minimum of training - and perhaps with none at all - to recognise by vision upper case letters which he had learned by touch, and that he was unable to recognise by vision lower case letters which he had not learned by touch, provides strong evidence for cross-modal transfer. It will be borne in mind, too, that it took him many months to learn to recognise by vision letters which he had not previously learned by touch. One may point out that the "control" provided by the lower-case letters is vital to the argument, since we can never wholly rule out the possibility of some residual vision (undoubtedly present in this case). The fact that S.B. could recognise the figures in the Ishihara Plates (by far the most surprising observation we made) gives evidence of transfer and, incidentally, renders it most unlikely that the modus operandi of transfer lies in identity of motor patterns (i.e. patterns of eye-movement corresponding with patterns of active touch). As has been said, gross observation of eye-movements, which consisted in large and apparently uncontrolled jerks resembling exaggerated saccades, made it impossible to believe that the patient could follow outlines of relatively small figures, such as the Ishihara digits, by controlled movements of the eyes.
We may conclude that this case does provide evidence of transfer of perceptual information from the tactual sphere to the visual modality. [ Footnote 14.] This seems somewhat at variance with the evidence from studies of cross-modal transfer in animals [ Footnote 15. ]and we can only speculate as to the reasons for the discrepancy. It may be that language is the decisive factor.
In our view, these cases tell us nothing about the classical philosophical problems of the nature of perceived space (as von Senden fondly hoped that they would) and they tell us little or nothing about the importance of early visual learning for perception in the adult. We cannot sustain the view of Hebb that these cases provide windows through which we may see the perceptual system of the infant - they are not living fossils in which the past is re-enacted. We studied an adult with a unique past; what we found was the effect of this past on a normal brain. We did not find the brain or the perceptual system of an infant and we learned little or nothing about the normal development of vision. At the same time, we believe that our case has demonstrated the impact of visual experience on a man to all intents and purposes long blind, and the gains and losses which this revelation brought in its wake. We have ascertained that vision, although it may prove genuinely useful to the man long blind, is at the same time a potential source of grievous hurt. We have further ascertained that, in such a case, direct transfer of information from patterns of touch to equivalent visual patterns is almost certainly possible.
continues to Appendices
