Scand. J. Psychol., 1976,17, 133-141
Muller’s doctrine of specific nerve energies A reevaluation in view of perceptual development ARILD LIAN
Abstract.-Some principles underlying Miiller’s doctrine are presented. This doctrine is briefly evaluated in view of modem research in sensory and perceptual psychology, whereupon its relationship to problems of perceptual development is examined. Two positions concerning developmental changes of intersensory relationships are presented. According to the first position, development starts with a well integrated supra-modal system and proceeds by a gradual differentiation of the separate modalities. According to the second, the senses constitute relatively independent functions by the infant, while integration of these functions takes place when the child grows. It is shown that studies supporting either of the two positions have been based on different behaviour systems, and that these studies, therefore, need not contradict each other. Consequently it is suggested that sensory functions should be defined in relation to particular forms of behaviour, and that forms of organism-environment interactions should serve as frames of reference for classifications of the senses.
A rationale for a systematic classification of the senses was first announced in the influential works of Charles Bell (1811, 1869) and Johannes Miiller (1838). These men proposed that each sense has its own specific energy and can respond only with its own peculiar quality, a proposition which has later become known as the doctrine of the specific energies of nerves. It is, however, Miiller’s name which is ordinarily associated with this doctrine. Therefore, this article will be based on some formulations in his Handbuch der Physiologie der Menschen which appeared in 1838 (see also Rand, 19.12; Dennis, 1948). The doctrine of the specific energies of nerves has been abundantly criticized in the literature (v. Hornbostel, 1925; Hartshorne, 1934; J . J. Gibson, 1966) and Pfaffman (1960) has suggested alternative ways of dealing with sensory functions. However, it has not been systematically examined in view o f studies on perceptual development. In the present article such examination will be made, but at first
University of Oslo, Norway
the basic principles underlying the doctrine will be presented together with a brief evaluation of its present status.
LAWS OF THE SPECIFIC ENERGIES OF NERVES
Miiller formulated ten laws about the specific energies of nerves. In this article only a few of them will be mentioned. Quotations of the laws are all taken from Dennis (1948). The first law stated that “external agencies can give rise to no kind of sensation which cannot also be produced by internal causes, . . .” (p. 157). The second law stated that “the same internal cause excites in the different senses different sensations; . . . in each sense the sensations peculiar to it” (p. 159), and the third law stated that “the same external cause also gives rise to different sensations in each sense, according to the special endowments of its nerve” (p. 159). Muller’s text was replete of instances which gave support to these laws. For example, he carefully described how the stimulus of electricity may give rise to a bright flash of light when applied to an eye and to a sensation of sound when applied to an ear. The first three laws seemed to imply t h at the senses, by virtue of their peculiar properties, can only inform the organism of their own states, not of external objects-a principle which was enunciated by way of a fifth law. To explain the perception of external objects, therefore, Miiller took the position of a representational theorist. In addition he maintained that “the nerves of the senses have assuredly a specific irritability for certain influences; for many stimuli, which exert a violent action upon one organ of sense, have little or no effect upon another” (p. 162). Miiller thus announced a separate principle which has been called the principle of Scand. J . Psvchol. 17
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specixc irritability, and which Sherrington later rephrased by the concept of adequate stimulus. According to Miiller, the principle of specific irritability shows why the perception is reliable. It should be emphasized that specific irritability is not the same as specific energy. Specific irritability implies a functional relation between the sense organs and the environmental flux of physical energy. Specific energy, on the other hand, implies a content of the nerves. What is the content of the nerves of a sense? Miiller said it was an energy. That term, however, caused much bewilderment among later research workers, for they tended to think that in a physical sense all sensory nerves are equal: The impulses transmitted along the auditory nerve do not differ considerably from the impulses transmitted along the optic nerve. Boring remarked that “. . . when he [Miiller] called the nervous essential an energy, he must have been influenced by such doctrines as the vis viva and the vis nervosa in his choice” (1942, p. 71). But it is quite clear from Miiller’s text that specific energy was supposed to be a psychological term. Thus he pointed out one place that quality would do as an alternative term for energy. He even said that “. . . the sensation of sound-is the peculiar ‘energy’ or ‘quality’ of the auditory nerve; the sensation of light and colours that of the optic nerve; and so of the other nerves of sense” (p. 163). Yet Miiller needed to say more of the specific energies of nerves, otherwise he could not explain how the senses convey information about qualitative differences. He proposed that “either the nerves themselves may communicate impressions different in quality to the sensorium, which in every instance remains the same; or the vibrations of the nervous principle may in every nerve be the same and yet give rise to the perception of different sensations in the sensorium, owing to the parts of the latter with which the nerves are connected having different properties” (p. 166). He added that “the proof of either of these propositions 1 regard as at present impossible” (p. 167). Today we are likely to think that the last proposition is the adequate one, and to bring the doctrine in correspondence with modern works in sensory physiology, it would have to be reformulated as the doctrine of the specific energies of cortical zones. In the part of the nervous system which is engaged in the receiving and processing of sensory information, the so-called primary projection areas are genFrnnri
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erally considered to possess high modal specificity. This means, for example, that in the primary occipital cortex there are virtually no cells which respond only to auditory stimulation, and in the primary temporal cortex there are no cells which respond only to visual stimulation. Hubel and Wiesel (1965) have shown that among the cells of afferent layer IV in the parastriate area immediately anterior and lateral to the primary visual area there are some cells which indicate highly complex receptive field organizations and which respond only to specialized properties of the visual stimulus. In particular, these cells respond best to appropriately oriented lines, bars, edges or contours. Thus we may say that the works of Hubel and Wiesel have shown a high degree of intra-modal specificity of cells in the occipital cortex. In the same area, however, there are also cells of a multimodal character. Morrell (1972) has shown that some of these cells change their respons patterns to light after concomitant presentation of visual and auditory stimuli. Yet these studies cannot directly confirm or discredit the doctrine of specific energies of nerves. The reason is that these studies were based on observations of neural activity patterns, they were not psychophysical studies. A neural activity pattern cannot safely be assumed to be analogous to a specific energy or a certain content of perception. On the other hand the studies of Hubel and Wiesel have made possible an extension of the principle of specific irritability to cortical zones. Another aspect of Miiller’s doctrine has apparently been more vulnerable to the criticism of modern research workers. This aspect is expressed in his sixth law: “The nerve of each sense seems capable of one determinate kind of sensation only, and not those proper to the other organs of sense; hence one nerve cannot take the place and perform the function of the nerve of another sense” (p. 165). According to a literal interpretation of this law the various senses work independently of each other, and they obey only their own specific laws of stimulation (the modality axiom). This axiom has been clearly contradicted by Kleint (1940), Klix (1962), Moustgaard (1969), and others who have shown that stimulation of one sensory system influences the functions of another sensory system. In the writer’s opinion, however, the modality axiom represents a debatable interpretation of Miiller’s text, in particular of his sixth law. A more reasonable interpretation of this law would be that
Doctrine of specific nerve energies and perceptual development
regardless the conditions under which stimulation takes place, the quality of the sensation is never converted into the sensory quality which is characteristic of another sense. Thus although vestibular stimulation may interfere with the perception of the main coordinates of visual space, the visual experiences themselves never lose their modality specific character. It should also be mentioned that some criticism has been raised against Miiller’s doctrine on the basis of electrophysiological recordings from afferent nerves. Pfaffmann (1962) who made such recordings from the gustatory nerves of a rat, maintained that there are no specific receptor mechanisms underlying the taste qualities of salt, sour, sweet and bitter. The stimulation of a taste bud by different chemicals, he said, leads to different afferent input patterns. “It is upon such patterns that discrimination or, more subjectively, the perception of different sensory qualities depends” (p. 391). Moreover, electrophysiological recordings from the nerves of other senses have shown that single nerve fibers cannot be assigned specific roles in the discrimination of sensory qualities. However, objections based on these findings are not quite pertinent to Miiller’s doctrine, since he did not suggest a specific mechanism underlying qualitative discriminations. Such mechanisms were postulated later by Helmholtz who may be said to have extended Miiller’s doctrine to the principle of fiber specificity. Pfaffmann’s criticism hits the latter principle, not the doctrine of specific energies of nerves. To be pertinent to the subject under discussion the researcher should start by an examination of the concept of a sense or a sensory modalitv. Miiller apparently defined a sense by reference to a class of sensations, hence psychophysical data have become basic ones in the study of the senses. But from the time of Miiller the concept of a sense has also implied certain discriminatory abilities, or one may say, certain functions relating the organism to his environment. These functions may depend on the age of the individual. Miiller’s doctrine should, therefore, be examined from a developmental point of view. PROBLEMS OF SENSORY SPECIFICITY AND DEVELOPMENT It is possible to argue for two positions regarding developmental changes in intersensory relation-
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ships. According to the first position, development starts by a relatively unselective processing system and proceeds by a gradual differentiation of the separate modalities. Bower (1974), who is an exponent of this position, called the early processing system a supra-modal perceptual system. According to the other position, the senses constitute relatively independent functions in infancy, while development involves an integration of these functions into more comprehensive systems. Birch (1962) has argued that the senses can be ordered in a kind of dominance hierarchy with respect to their role in learning and survival, and this dominance hierarchy, he proposed, depends on the age of the individual. Yet he was obviously an exponent of the second position, since he considered integration of the senses to be an equally important aspect of perceptual development. Differentiation and integration are supposed to have sensory specificity as a common frame of reference. In psychophysic studies of sensory specificity have required an intentional learning or instruction of particular forms of behaviour. Also studies of intersensory relationships require the use of some forms of behaviour, and depending on how these forms are developmentally related to the forms of behaviour used in psychophysics, such studies will either support a differentiation or an integration hypothesis. Thus studies of certain forms of spontaneous behaviour of infants and studies of cross-modal transfer and cross-modal matching performance have apparently given contradictory results. The former type of studies may be said to support a differentiation hypothesis, and the latter type of studies an integration hypothesis. Intersensory relationships in spontaneous hehuviour of infants Turning of the head, smiling. sucking, grasping and crying are examples of forms of behaviour which occur spontaneously by infants or which may be elicited by particular stimuli independent on previous learning. Wertheimer (1961) showed that auditory clicks which are randomly presented to the right and left of an infant a few minutes after birth release consistent movements of head and eyes in the direction of the source of sound. Apparently, the infant expected to see something in that direction. Bower,
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Broughton, and Moore (1970), using a technique with oppositely polarized beams of light in a shadow caster on infants (in their second week of life) wearing polarizing goggles, produced a virtual object in front of a projection screen. The situation apparently caused an expectation of something which can be touched, since the infants reached out for the intangible, binocularly present object. Also, as referred by Bower (1974),Bower and Wishart (1973)have shown that infants will try to grasp a source of sound which has been presented in darkness. Aronson and Rosenbloom (l%9) studied infants, 20 to 58 days old, with respect to an ability to coordinate visual and auditory stimulation. The infant subject was seated in a chair which was specially designed to provide maximum support for head and torso while permitting free movements of arms and legs. He was facing a window through which he could see his mother in an adjoining room. The mother stood directly in front of the infant at a distance of approximately 60 cm, and her voice was transmitted to the infant’s room via a stereo amplifier system. The two loudspeakers were set 101 cm apart, 90” to each side of the infant. In stage I of the experiment the mother spoke to the infant with the loudspeakers in balance, and hence her voice appeared to come from her seen position. In stage 11, one of the loudspeakers was made dominant and the mother’s voice would thus appear to emanate from either the left or the right side of the infant. As long as the mother’s voice seemed to come from her seen position, the infants were generally calm and attentive, but once her voice seemed to come from a different position, the infants showed a number of distress reactions. To ascertain that these reactions were caused by an auditory-visual discrepancy, and not merely by a shift in voice locus per se, an additional experiment was performed with the mother standing behind a curtain backdrop and hence invisible to the subject. The two infants who were tested remained calm throughout the procedure, i.e. they showed‘ no distress reactions to the lateral displacement of the mother’s voice. In another control experiment, the mother was replaced by a female laboratory assistant. The three infants who served as subjects reacted with distress, indicating that the effect was not dependent on the familiarity of the speaker. Aronson and Rosenbloom interpreted their results as “indicating that infants as young as Prod
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30 days are perceiving auditory and visual information within a common space” (p. 1163). Bower (1974)mentioned a later experiment by Aronson and Dunkeld which demonstrated thal older infants also reacted with distress to an auditory-visual discrepancy. However, they showed a double orientation in the situation, looking at the mother while turning their head towards the locus of the voice. The youngest infants oriented eyes and head towards a position midway between voice location and mouth location, i.e. their reactions were less differentiated. According to Bower these studies have shown that the tactual, auditory, and visual systems act in a unitary fashion, as if sensory signals are processed by a supramodal perceptual system. Modality specific analysers, he maintained, develop gradually by the growing child. However, the empirical support to Bower’s position has been weakened by a recent work of McGurk and Lewis (1974).As part of a more extended study of audio-visual integration in infancy, they replicated Aronson and Rosenbloom’s experiment while controlling for the sequence of presentations of normal and displaced face-voice conditions. They studied infants at three age levels: 1 month (25 to 31 days), 4 months (1 10 to 126 days), and 7 months (182 to 217 days). Five response indices were recorded: Frequency of smiling to the mother, frequency of vocalization, frequency of frowning, frequency of fretting and crying, and frequency of tongue protrusions. In addition, three measures of head turning and looking behaviour were recorded during each of the face-voice conditions. Dislocation of the voice to the right or left side significantly influenced the frequency of turning of the head and looking in the direction of the active loudspeaker only among the 4 and 7 months old infants. Yet, according to McGurk and Lewis, the youngest subjects may still have discriminated between the normal and displaced face-voice conditions, but “any such discrimination was not detected by this particular response index” (p. 650). Measures of distress such as fretting and crying were not significantly influenced by the spatial dislocation of the mother’s voice. Neither did the frequency of frowning and number of tongue protrusions discriminate between the experimental episodes at any of the age levels studied. Hence this study gave no support to the proposition that in-
Doctrine of speci3c nerve energies and perceptual development fants perceive information in a common audiovisual space. Evidently, Aronson and Rosenbloom assumed that dislocation of the voice-to the extent that it caused a disintegration of an audio-visual space-would always be followed by certain distress reactions. McGurk and Lewis’ experiment suggested that this assumption is a dubious one. It did not contraindicate the possibility that the particular form of behaviour shown by the infant in the normal face-voice condition is controlled by an integrated audio-visual stimulation. Dislocation of the voice may have served to terminate this form of behaviour and to activate another. The transition needed not be accompanied by distress reactions.
SUPRA- OR UNI-MODAL OBJECT CONCEPTS?
J. J. Gibson (1966) maintained that the primary function of the senses is to detect solid objects in the environment. Bower (1974) pointed out that even lower organisms respond to dimensions of objects which can be specified via any sensory modality, and hence that there are supra-modal mechanisms present in perception at an early stage in phylogenetic and ontogenetic development. Thus he reasoned that “. . . in the course of evolution, it is differentiation-rather than integration-that has been the sign of advance” (p. 119). Underlying this position is an assumption that “the object” is supra-modal, and that the perception of objects may be contrasted with the perception of sensory qualities. A child learns the concept of a ball by seeing, touching and manipulating a ball, and by listening to the sound it produces when dropped to the floor. But is there any specific perceptual experience which constitutes “the ball” for the child? Certainly he learns the name ball which might be considered as a supra-modal symbol for this’type of objects. It is possible to postulate a single neural mechanism which is involved in the designation of the object by its name irrespective of the sensory channel through which it is perceived. But is recognition of the object also mediated by such a mechanism independent of the modality of the sensory inflow? This problem was raised by Ettlinger (1967). He referred to cases of so-called modality-specific agnosia in clinical neurology. According to reports of such cases patients with visual
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agnosia may be able to recognize an object by touch but not by vision, indicating loss of the visual but not of the tactile object concept. On this ground Ettlinger asked whether object concepts are supraor uni-modal. Monkeys are generally considered capable of acquiring object concepts. At least they can be trained to discriminate forms. Ettlinger proposed the following procedure: A monkey is trained by vision to discriminate between a triangle and a disc. If configuration, size and colour are gradually changed, the animal may continue to treat the triangle as if it were possessed of the original properties. Under these circumstances the animal may be said to have acquired the concept of a triangle. At this point Ettlinger suggested two ways of proceeding in order to cast light on the problem of supra- vs uni-modal object concepts: The first requires that “we train monkeys, say, to discriminate between the same two shapes by vision and also by touch. After this we can remove the inferior temporal cortex, and we find impaired performance on the visual (but not tactile) version of the discrimination task. Similarly, we can remove posterior parietal cortex in other monkeys, and we find impaired performance on the tactile (but not visual) version of the task” (p. 54). As mentioned by Ettlinger, such impairments were actually found in a previous experiment performed by Bates and Ettlinger (1960). Following the other way of proceeding, Ettlinger performed an experiment in which unoperated monkeys were first trained to distinguish between a triangle and a disc by vision. Next, the transfer from vision to touch was studied by letting the same monkeys discriminate the same forms in the dark. 1n.case of no transfer, the animals were given a final test of retention of the first modality task. The results were consistently negative. Ettlinger found no evidence of positive cross-modal transfer effects, neither from vision to touch nor from touch to vision. Therefore, it is difficult to postulate supra-modal object concepts by animals, and neurological data have shown that they hardly exist by humans. Rather it seems likely that object concepts are uni-modal with respect to sensory basis. Ettlinger’s experiments have also shown that one cannot strictly distinguish the perception of objects from the perception of sensory qualities. Thus although ?he behaviour of lower organisms and human infants is directed towards external objects, one cannot safely assume that they respond to propScand. J . Psvchol. 17
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erties of stimulation which are common to different modalities.
Development of cross-modal transfer abilities and intersensory equivalences
Ettlinger referred to a number of other studies which have been performed to establish crossmodal transfer of a specific discrimination habit in animals. The results of these studies have proved consistently negative. Also, in a review of crossmodal transfer and sensory equivalence, von Wright (1970) concludes that “experiments show that the capacity to recognize and utilize specific stimulus similarities across modalities is very hard to demonstrate in infrahuman subjects” (p. 22). On the other hand, animals trained to respond to the onset of stimulation in one modality transfer this response to the onset of stimulation in another modality. To a certain extent it also appears that cross-modal transfer of a principle or a learning set has been possible among animals (Wilson, 1964). However, the effects have been weak, and in some studies they have been shown to work only in one direction (for example from audition to vision). Later Ward, Yehle, and Doerflein (1970) demonstrated cross-modal transfer of a specific discrimination habit in a prosimian (Galugo senegalensis) when using a so-called go/no-go training procedure. It thus seemed possible that the use of an aversive stimulus in avoidance conditioning is a prerequisite for cross-modal transfer in animals. But Frampton, Milner and Ettlinger (1973), who used the same training procedure with their monkeys, found an effect only on the initial error scores, not on training scores. Thus the literature is still scarce of evidence for cross-modal transfer among animals. In studies with human subjects it has been shown that cross-modal transfer occurs under conditions when verbalization is possible (Blank & Bridger, 1964). It is hard to say whether such transfer has ever been demonstrated independently of the linguistic skills of human subjects. To study the development of intersensory equivalences, cross-modal matching experiments have been performed. Such experiments require that the subject is made aware of an equivalence between two discrimination tasks. For example, he may be told to touch an object and afterwards be shown a sample of objects visually with the instruction to tell which is the same as the one felt. By way Scand. . I Psychol. . 17
of training procedures, animals may similarly be told to match objects presented to different modalities (Davenport & Rogers, 1970). The results of such experiments with animals have given evidence of significant but not perfect cross-modal matching (Davenport, Rogers & Russell, 1973). In children, the ability to perform cross-modal matching tasks show clear age trends (Birch & Lefford, 1963; 1967; Birch & Belmont, 1965). Hence it has been commonly assumed that language is involved in the mediation of cross-modal effects. On the other hand, these effects have also been said to depend on a general ability to explore objects efficiently. Milner and Bryant (1970), for instance, have shown that inter-modal and intra-modal matching abilities make corresponding improvements with age. According to E. J. Gibson (1969), object perception develops as “the organism comes to detect properties of stimulation not previously detected” (p. 76). This development, she maintained, will also bring about an improvement of intermodal perception, since “there is information in stimulation which is not tied to the specific sensations but is rather invariant over them” (p. 219). She said that “many distinctive features of objects are of this kind (corners, motions, temporal patterns, and transitions)” (p. 219). Parallel development of intermodal and intramodal matching abilities may also be explained from the point of view taken by Zaporozhets (1965). He emphasized the role of motor processes in perception: When we explore a n object, either visually or tactually, we make characteristic movements which are common to vision and touch, and which, therefore, were assumed to be responsible for the cross-modal effects. Neither E. J. Gibson nor Zaporozhets seemed to deny that language may be involved in the mediation of cross-modal effects. However, they suggested that these effects generally have other sources as well. Studies of crossmodal perception among animals seem to have confirmed their points of view, but it will soon be shown that their specifications of the cross-modal sources are entirely arbitrary ones.
DISCUSSION The epistomological aspect of Muller’s doctrine is the one most vigorously attacked by modern researchers in the psychology of perception. Thus E.
Doctrine of specific nerve energies and perceptual development
J . Gibson, J . J . Gibson, Bower, and others have stressed that the senses give direct information of external objects, not merely of the states of their nerves. Furthermore, these researchers all assumed that similar information of objects is mediated by different sensory systems. Yet they seemed to disagree on questions concerning developmental changes in intersensory relationships. While Bower apparently considered the sources of a supra-modal object perception available at birth, E. J. Gibson assumed they must be detected by learning and perceptual exploration. For Bower “the sign of advance” in perceptual development was the differentiation of modality specific analysers. For E. J . Gibson it was the evolvement of intermodal perception. Since the two researchers argued from different types of studies, both may be right. Bower argued from studies of certain forms of spontaneous behaviour by infants, i.e. behaviour which may involve orientations or attitudes of great survival value. In general, orienting reflexes, arousal and behaviour subserving certain mother-child relationships may constitute such orientations. As pointed out by Haffman (1962), most sensory nerves have also a nonspecific pathway leading through the reticular activating system with diffuse projections to the cortex and other neural structures. Hence it is possible to argue that the type of orientations mentioned can be elicited and maintained equally by the stimulation of different senses. Anyhow, the senses do not only serve discriminatory functions, they also serve the arousal and reinforcement of behaviour. The point is that studies of the latter type of functions may require that certain forms of spontaneous behaviour are taken as points of departure. On the other hand, studies of discriminatory functions almost always require that one makes use of learned behaviour. E. J. Gibson argued from studies of certain forms of learned behaviour which involved attention to particular aspects or parts of the environment. During such behaviour perception may still be “grossly selective”, but it cannot form an indiscriminate response to all aspects of the situation. Therefore, studies which are based on learned behaviour will necessarily show some specificity in perception. But it may be difficult to decide whether perception in a matching-to-sample performance, for instance, has been specific with respect to one sense or to features common to dif-
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ferent modalities. The problem is that one is still unable to define a modality or a specific energy independently of common language terms such as colour, sound and taste. A proposition which says that corners, transitions and movements are intermodal rather than intra-modal properties, introduces an arbitrary distinction which cannot be accounted for independently of some knowledge about the acquisition of language. Although we use the same word about movements detected via different sensory channels, one cannot say that movements produced by auditory stimulation are psychologically equivalent to movements produced by visual stimulation. Particularly, one cannot guarantee that movements produced by the stimulation of different senses by infants have equal effects on all forms of behaviour. Likewise, one cannot know whether self-produced movements during visual and haptic explorations are psychologically equivalent movements for the infant. Whether the researcher wants to study the perception of properties supposed to be modality specific ones or the perception of properties supposed to be intermodal ones, he must provide certain means of communication between subject and experimenter. Either verbal instructions or training procedures have to be used. It follows that modality specific and intermodal forms of perception can only be studied in organisms who have a certain capacity for learning. Perceptual properties may require different means of communication, and probably different learning capacities. In this way cross-modal matching abilities will tend to increase with age. But so will also the capacity for what is supposed to be a modality specific form of perception. Therefore, studies which show an increasing tendency towards integration of the modalities need not contradict studies which show an increasing tendency towards a modality specific form of perception. A final evaluation of Miiller’s doctrine awaits another definition of the concept of a sense. This concept tends to be used circularly as long as it is merely defined by reference to common language terms for sensations. However, it has also been used with reference to certain discriminatory abilities which relates the organism to the environment, and the only chance of avoiding a circularity of usage will be to analyse independently forms of organism-environment interactions. Such interactions should serve as frames of reference for clasS m n d I Prvrhol l i
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sifications of the senses. This means that, for instance, taste and smell may constitute one sense for certain forms of organism-environment interactions, whereas they may constitute different senses for other forms of interactions. Spontaneous behaviour such as turning of the head, smiling and sucking may serve a type of interactions which differ from the type of interactions served by matching behaviour in studies of form and colour perception, and possibly therefore, the nerve of a sense can “take the place and perform the function of another sense” only in the former type of organism4nvironment interactions, not in the other.
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v . Wright, J. M. (1970). Cross-modal transfer and sensory equivalence. A review. Scand. J . Psychol. II, 21-30. Zaporozhets, A. V. (1%5). The development of perception in the preschool child. In P. H. Mussen (Ed.), European research in cognitive development. Monogr. SOC.Res. ChildDev. 30, No. 2. Postal address: A. Lian Psykologisk Institutt Postboks 1094, Blindern Oslo 3, Norway
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Muller’s doctrine of specific nerve energies A reevaluation in view of perceptual development ARILD LIAN
Abstract.-Some principles underlying Miiller’s doctrine are presented. This doctrine is briefly evaluated in view of modem research in sensory and perceptual psychology, whereupon its relationship to problems of perceptual development is examined. Two positions concerning developmental changes of intersensory relationships are presented. According to the first position, development starts with a well integrated supra-modal system and proceeds by a gradual differentiation of the separate modalities. According to the second, the senses constitute relatively independent functions by the infant, while integration of these functions takes place when the child grows. It is shown that studies supporting either of the two positions have been based on different behaviour systems, and that these studies, therefore, need not contradict each other. Consequently it is suggested that sensory functions should be defined in relation to particular forms of behaviour, and that forms of organism-environment interactions should serve as frames of reference for classifications of the senses.
A rationale for a systematic classification of the senses was first announced in the influential works of Charles Bell (1811, 1869) and Johannes Miiller (1838). These men proposed that each sense has its own specific energy and can respond only with its own peculiar quality, a proposition which has later become known as the doctrine of the specific energies of nerves. It is, however, Miiller’s name which is ordinarily associated with this doctrine. Therefore, this article will be based on some formulations in his Handbuch der Physiologie der Menschen which appeared in 1838 (see also Rand, 19.12; Dennis, 1948). The doctrine of the specific energies of nerves has been abundantly criticized in the literature (v. Hornbostel, 1925; Hartshorne, 1934; J . J. Gibson, 1966) and Pfaffman (1960) has suggested alternative ways of dealing with sensory functions. However, it has not been systematically examined in view o f studies on perceptual development. In the present article such examination will be made, but at first
University of Oslo, Norway
the basic principles underlying the doctrine will be presented together with a brief evaluation of its present status.
LAWS OF THE SPECIFIC ENERGIES OF NERVES
Miiller formulated ten laws about the specific energies of nerves. In this article only a few of them will be mentioned. Quotations of the laws are all taken from Dennis (1948). The first law stated that “external agencies can give rise to no kind of sensation which cannot also be produced by internal causes, . . .” (p. 157). The second law stated that “the same internal cause excites in the different senses different sensations; . . . in each sense the sensations peculiar to it” (p. 159), and the third law stated that “the same external cause also gives rise to different sensations in each sense, according to the special endowments of its nerve” (p. 159). Muller’s text was replete of instances which gave support to these laws. For example, he carefully described how the stimulus of electricity may give rise to a bright flash of light when applied to an eye and to a sensation of sound when applied to an ear. The first three laws seemed to imply t h at the senses, by virtue of their peculiar properties, can only inform the organism of their own states, not of external objects-a principle which was enunciated by way of a fifth law. To explain the perception of external objects, therefore, Miiller took the position of a representational theorist. In addition he maintained that “the nerves of the senses have assuredly a specific irritability for certain influences; for many stimuli, which exert a violent action upon one organ of sense, have little or no effect upon another” (p. 162). Miiller thus announced a separate principle which has been called the principle of Scand. J . Psvchol. 17
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specixc irritability, and which Sherrington later rephrased by the concept of adequate stimulus. According to Miiller, the principle of specific irritability shows why the perception is reliable. It should be emphasized that specific irritability is not the same as specific energy. Specific irritability implies a functional relation between the sense organs and the environmental flux of physical energy. Specific energy, on the other hand, implies a content of the nerves. What is the content of the nerves of a sense? Miiller said it was an energy. That term, however, caused much bewilderment among later research workers, for they tended to think that in a physical sense all sensory nerves are equal: The impulses transmitted along the auditory nerve do not differ considerably from the impulses transmitted along the optic nerve. Boring remarked that “. . . when he [Miiller] called the nervous essential an energy, he must have been influenced by such doctrines as the vis viva and the vis nervosa in his choice” (1942, p. 71). But it is quite clear from Miiller’s text that specific energy was supposed to be a psychological term. Thus he pointed out one place that quality would do as an alternative term for energy. He even said that “. . . the sensation of sound-is the peculiar ‘energy’ or ‘quality’ of the auditory nerve; the sensation of light and colours that of the optic nerve; and so of the other nerves of sense” (p. 163). Yet Miiller needed to say more of the specific energies of nerves, otherwise he could not explain how the senses convey information about qualitative differences. He proposed that “either the nerves themselves may communicate impressions different in quality to the sensorium, which in every instance remains the same; or the vibrations of the nervous principle may in every nerve be the same and yet give rise to the perception of different sensations in the sensorium, owing to the parts of the latter with which the nerves are connected having different properties” (p. 166). He added that “the proof of either of these propositions 1 regard as at present impossible” (p. 167). Today we are likely to think that the last proposition is the adequate one, and to bring the doctrine in correspondence with modern works in sensory physiology, it would have to be reformulated as the doctrine of the specific energies of cortical zones. In the part of the nervous system which is engaged in the receiving and processing of sensory information, the so-called primary projection areas are genFrnnri
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erally considered to possess high modal specificity. This means, for example, that in the primary occipital cortex there are virtually no cells which respond only to auditory stimulation, and in the primary temporal cortex there are no cells which respond only to visual stimulation. Hubel and Wiesel (1965) have shown that among the cells of afferent layer IV in the parastriate area immediately anterior and lateral to the primary visual area there are some cells which indicate highly complex receptive field organizations and which respond only to specialized properties of the visual stimulus. In particular, these cells respond best to appropriately oriented lines, bars, edges or contours. Thus we may say that the works of Hubel and Wiesel have shown a high degree of intra-modal specificity of cells in the occipital cortex. In the same area, however, there are also cells of a multimodal character. Morrell (1972) has shown that some of these cells change their respons patterns to light after concomitant presentation of visual and auditory stimuli. Yet these studies cannot directly confirm or discredit the doctrine of specific energies of nerves. The reason is that these studies were based on observations of neural activity patterns, they were not psychophysical studies. A neural activity pattern cannot safely be assumed to be analogous to a specific energy or a certain content of perception. On the other hand the studies of Hubel and Wiesel have made possible an extension of the principle of specific irritability to cortical zones. Another aspect of Miiller’s doctrine has apparently been more vulnerable to the criticism of modern research workers. This aspect is expressed in his sixth law: “The nerve of each sense seems capable of one determinate kind of sensation only, and not those proper to the other organs of sense; hence one nerve cannot take the place and perform the function of the nerve of another sense” (p. 165). According to a literal interpretation of this law the various senses work independently of each other, and they obey only their own specific laws of stimulation (the modality axiom). This axiom has been clearly contradicted by Kleint (1940), Klix (1962), Moustgaard (1969), and others who have shown that stimulation of one sensory system influences the functions of another sensory system. In the writer’s opinion, however, the modality axiom represents a debatable interpretation of Miiller’s text, in particular of his sixth law. A more reasonable interpretation of this law would be that
Doctrine of specific nerve energies and perceptual development
regardless the conditions under which stimulation takes place, the quality of the sensation is never converted into the sensory quality which is characteristic of another sense. Thus although vestibular stimulation may interfere with the perception of the main coordinates of visual space, the visual experiences themselves never lose their modality specific character. It should also be mentioned that some criticism has been raised against Miiller’s doctrine on the basis of electrophysiological recordings from afferent nerves. Pfaffmann (1962) who made such recordings from the gustatory nerves of a rat, maintained that there are no specific receptor mechanisms underlying the taste qualities of salt, sour, sweet and bitter. The stimulation of a taste bud by different chemicals, he said, leads to different afferent input patterns. “It is upon such patterns that discrimination or, more subjectively, the perception of different sensory qualities depends” (p. 391). Moreover, electrophysiological recordings from the nerves of other senses have shown that single nerve fibers cannot be assigned specific roles in the discrimination of sensory qualities. However, objections based on these findings are not quite pertinent to Miiller’s doctrine, since he did not suggest a specific mechanism underlying qualitative discriminations. Such mechanisms were postulated later by Helmholtz who may be said to have extended Miiller’s doctrine to the principle of fiber specificity. Pfaffmann’s criticism hits the latter principle, not the doctrine of specific energies of nerves. To be pertinent to the subject under discussion the researcher should start by an examination of the concept of a sense or a sensory modalitv. Miiller apparently defined a sense by reference to a class of sensations, hence psychophysical data have become basic ones in the study of the senses. But from the time of Miiller the concept of a sense has also implied certain discriminatory abilities, or one may say, certain functions relating the organism to his environment. These functions may depend on the age of the individual. Miiller’s doctrine should, therefore, be examined from a developmental point of view. PROBLEMS OF SENSORY SPECIFICITY AND DEVELOPMENT It is possible to argue for two positions regarding developmental changes in intersensory relation-
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ships. According to the first position, development starts by a relatively unselective processing system and proceeds by a gradual differentiation of the separate modalities. Bower (1974), who is an exponent of this position, called the early processing system a supra-modal perceptual system. According to the other position, the senses constitute relatively independent functions in infancy, while development involves an integration of these functions into more comprehensive systems. Birch (1962) has argued that the senses can be ordered in a kind of dominance hierarchy with respect to their role in learning and survival, and this dominance hierarchy, he proposed, depends on the age of the individual. Yet he was obviously an exponent of the second position, since he considered integration of the senses to be an equally important aspect of perceptual development. Differentiation and integration are supposed to have sensory specificity as a common frame of reference. In psychophysic studies of sensory specificity have required an intentional learning or instruction of particular forms of behaviour. Also studies of intersensory relationships require the use of some forms of behaviour, and depending on how these forms are developmentally related to the forms of behaviour used in psychophysics, such studies will either support a differentiation or an integration hypothesis. Thus studies of certain forms of spontaneous behaviour of infants and studies of cross-modal transfer and cross-modal matching performance have apparently given contradictory results. The former type of studies may be said to support a differentiation hypothesis, and the latter type of studies an integration hypothesis. Intersensory relationships in spontaneous hehuviour of infants Turning of the head, smiling. sucking, grasping and crying are examples of forms of behaviour which occur spontaneously by infants or which may be elicited by particular stimuli independent on previous learning. Wertheimer (1961) showed that auditory clicks which are randomly presented to the right and left of an infant a few minutes after birth release consistent movements of head and eyes in the direction of the source of sound. Apparently, the infant expected to see something in that direction. Bower,
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Broughton, and Moore (1970), using a technique with oppositely polarized beams of light in a shadow caster on infants (in their second week of life) wearing polarizing goggles, produced a virtual object in front of a projection screen. The situation apparently caused an expectation of something which can be touched, since the infants reached out for the intangible, binocularly present object. Also, as referred by Bower (1974),Bower and Wishart (1973)have shown that infants will try to grasp a source of sound which has been presented in darkness. Aronson and Rosenbloom (l%9) studied infants, 20 to 58 days old, with respect to an ability to coordinate visual and auditory stimulation. The infant subject was seated in a chair which was specially designed to provide maximum support for head and torso while permitting free movements of arms and legs. He was facing a window through which he could see his mother in an adjoining room. The mother stood directly in front of the infant at a distance of approximately 60 cm, and her voice was transmitted to the infant’s room via a stereo amplifier system. The two loudspeakers were set 101 cm apart, 90” to each side of the infant. In stage I of the experiment the mother spoke to the infant with the loudspeakers in balance, and hence her voice appeared to come from her seen position. In stage 11, one of the loudspeakers was made dominant and the mother’s voice would thus appear to emanate from either the left or the right side of the infant. As long as the mother’s voice seemed to come from her seen position, the infants were generally calm and attentive, but once her voice seemed to come from a different position, the infants showed a number of distress reactions. To ascertain that these reactions were caused by an auditory-visual discrepancy, and not merely by a shift in voice locus per se, an additional experiment was performed with the mother standing behind a curtain backdrop and hence invisible to the subject. The two infants who were tested remained calm throughout the procedure, i.e. they showed‘ no distress reactions to the lateral displacement of the mother’s voice. In another control experiment, the mother was replaced by a female laboratory assistant. The three infants who served as subjects reacted with distress, indicating that the effect was not dependent on the familiarity of the speaker. Aronson and Rosenbloom interpreted their results as “indicating that infants as young as Prod
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30 days are perceiving auditory and visual information within a common space” (p. 1163). Bower (1974)mentioned a later experiment by Aronson and Dunkeld which demonstrated thal older infants also reacted with distress to an auditory-visual discrepancy. However, they showed a double orientation in the situation, looking at the mother while turning their head towards the locus of the voice. The youngest infants oriented eyes and head towards a position midway between voice location and mouth location, i.e. their reactions were less differentiated. According to Bower these studies have shown that the tactual, auditory, and visual systems act in a unitary fashion, as if sensory signals are processed by a supramodal perceptual system. Modality specific analysers, he maintained, develop gradually by the growing child. However, the empirical support to Bower’s position has been weakened by a recent work of McGurk and Lewis (1974).As part of a more extended study of audio-visual integration in infancy, they replicated Aronson and Rosenbloom’s experiment while controlling for the sequence of presentations of normal and displaced face-voice conditions. They studied infants at three age levels: 1 month (25 to 31 days), 4 months (1 10 to 126 days), and 7 months (182 to 217 days). Five response indices were recorded: Frequency of smiling to the mother, frequency of vocalization, frequency of frowning, frequency of fretting and crying, and frequency of tongue protrusions. In addition, three measures of head turning and looking behaviour were recorded during each of the face-voice conditions. Dislocation of the voice to the right or left side significantly influenced the frequency of turning of the head and looking in the direction of the active loudspeaker only among the 4 and 7 months old infants. Yet, according to McGurk and Lewis, the youngest subjects may still have discriminated between the normal and displaced face-voice conditions, but “any such discrimination was not detected by this particular response index” (p. 650). Measures of distress such as fretting and crying were not significantly influenced by the spatial dislocation of the mother’s voice. Neither did the frequency of frowning and number of tongue protrusions discriminate between the experimental episodes at any of the age levels studied. Hence this study gave no support to the proposition that in-
Doctrine of speci3c nerve energies and perceptual development fants perceive information in a common audiovisual space. Evidently, Aronson and Rosenbloom assumed that dislocation of the voice-to the extent that it caused a disintegration of an audio-visual space-would always be followed by certain distress reactions. McGurk and Lewis’ experiment suggested that this assumption is a dubious one. It did not contraindicate the possibility that the particular form of behaviour shown by the infant in the normal face-voice condition is controlled by an integrated audio-visual stimulation. Dislocation of the voice may have served to terminate this form of behaviour and to activate another. The transition needed not be accompanied by distress reactions.
SUPRA- OR UNI-MODAL OBJECT CONCEPTS?
J. J. Gibson (1966) maintained that the primary function of the senses is to detect solid objects in the environment. Bower (1974) pointed out that even lower organisms respond to dimensions of objects which can be specified via any sensory modality, and hence that there are supra-modal mechanisms present in perception at an early stage in phylogenetic and ontogenetic development. Thus he reasoned that “. . . in the course of evolution, it is differentiation-rather than integration-that has been the sign of advance” (p. 119). Underlying this position is an assumption that “the object” is supra-modal, and that the perception of objects may be contrasted with the perception of sensory qualities. A child learns the concept of a ball by seeing, touching and manipulating a ball, and by listening to the sound it produces when dropped to the floor. But is there any specific perceptual experience which constitutes “the ball” for the child? Certainly he learns the name ball which might be considered as a supra-modal symbol for this’type of objects. It is possible to postulate a single neural mechanism which is involved in the designation of the object by its name irrespective of the sensory channel through which it is perceived. But is recognition of the object also mediated by such a mechanism independent of the modality of the sensory inflow? This problem was raised by Ettlinger (1967). He referred to cases of so-called modality-specific agnosia in clinical neurology. According to reports of such cases patients with visual
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agnosia may be able to recognize an object by touch but not by vision, indicating loss of the visual but not of the tactile object concept. On this ground Ettlinger asked whether object concepts are supraor uni-modal. Monkeys are generally considered capable of acquiring object concepts. At least they can be trained to discriminate forms. Ettlinger proposed the following procedure: A monkey is trained by vision to discriminate between a triangle and a disc. If configuration, size and colour are gradually changed, the animal may continue to treat the triangle as if it were possessed of the original properties. Under these circumstances the animal may be said to have acquired the concept of a triangle. At this point Ettlinger suggested two ways of proceeding in order to cast light on the problem of supra- vs uni-modal object concepts: The first requires that “we train monkeys, say, to discriminate between the same two shapes by vision and also by touch. After this we can remove the inferior temporal cortex, and we find impaired performance on the visual (but not tactile) version of the discrimination task. Similarly, we can remove posterior parietal cortex in other monkeys, and we find impaired performance on the tactile (but not visual) version of the task” (p. 54). As mentioned by Ettlinger, such impairments were actually found in a previous experiment performed by Bates and Ettlinger (1960). Following the other way of proceeding, Ettlinger performed an experiment in which unoperated monkeys were first trained to distinguish between a triangle and a disc by vision. Next, the transfer from vision to touch was studied by letting the same monkeys discriminate the same forms in the dark. 1n.case of no transfer, the animals were given a final test of retention of the first modality task. The results were consistently negative. Ettlinger found no evidence of positive cross-modal transfer effects, neither from vision to touch nor from touch to vision. Therefore, it is difficult to postulate supra-modal object concepts by animals, and neurological data have shown that they hardly exist by humans. Rather it seems likely that object concepts are uni-modal with respect to sensory basis. Ettlinger’s experiments have also shown that one cannot strictly distinguish the perception of objects from the perception of sensory qualities. Thus although ?he behaviour of lower organisms and human infants is directed towards external objects, one cannot safely assume that they respond to propScand. J . Psvchol. 17
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erties of stimulation which are common to different modalities.
Development of cross-modal transfer abilities and intersensory equivalences
Ettlinger referred to a number of other studies which have been performed to establish crossmodal transfer of a specific discrimination habit in animals. The results of these studies have proved consistently negative. Also, in a review of crossmodal transfer and sensory equivalence, von Wright (1970) concludes that “experiments show that the capacity to recognize and utilize specific stimulus similarities across modalities is very hard to demonstrate in infrahuman subjects” (p. 22). On the other hand, animals trained to respond to the onset of stimulation in one modality transfer this response to the onset of stimulation in another modality. To a certain extent it also appears that cross-modal transfer of a principle or a learning set has been possible among animals (Wilson, 1964). However, the effects have been weak, and in some studies they have been shown to work only in one direction (for example from audition to vision). Later Ward, Yehle, and Doerflein (1970) demonstrated cross-modal transfer of a specific discrimination habit in a prosimian (Galugo senegalensis) when using a so-called go/no-go training procedure. It thus seemed possible that the use of an aversive stimulus in avoidance conditioning is a prerequisite for cross-modal transfer in animals. But Frampton, Milner and Ettlinger (1973), who used the same training procedure with their monkeys, found an effect only on the initial error scores, not on training scores. Thus the literature is still scarce of evidence for cross-modal transfer among animals. In studies with human subjects it has been shown that cross-modal transfer occurs under conditions when verbalization is possible (Blank & Bridger, 1964). It is hard to say whether such transfer has ever been demonstrated independently of the linguistic skills of human subjects. To study the development of intersensory equivalences, cross-modal matching experiments have been performed. Such experiments require that the subject is made aware of an equivalence between two discrimination tasks. For example, he may be told to touch an object and afterwards be shown a sample of objects visually with the instruction to tell which is the same as the one felt. By way Scand. . I Psychol. . 17
of training procedures, animals may similarly be told to match objects presented to different modalities (Davenport & Rogers, 1970). The results of such experiments with animals have given evidence of significant but not perfect cross-modal matching (Davenport, Rogers & Russell, 1973). In children, the ability to perform cross-modal matching tasks show clear age trends (Birch & Lefford, 1963; 1967; Birch & Belmont, 1965). Hence it has been commonly assumed that language is involved in the mediation of cross-modal effects. On the other hand, these effects have also been said to depend on a general ability to explore objects efficiently. Milner and Bryant (1970), for instance, have shown that inter-modal and intra-modal matching abilities make corresponding improvements with age. According to E. J. Gibson (1969), object perception develops as “the organism comes to detect properties of stimulation not previously detected” (p. 76). This development, she maintained, will also bring about an improvement of intermodal perception, since “there is information in stimulation which is not tied to the specific sensations but is rather invariant over them” (p. 219). She said that “many distinctive features of objects are of this kind (corners, motions, temporal patterns, and transitions)” (p. 219). Parallel development of intermodal and intramodal matching abilities may also be explained from the point of view taken by Zaporozhets (1965). He emphasized the role of motor processes in perception: When we explore a n object, either visually or tactually, we make characteristic movements which are common to vision and touch, and which, therefore, were assumed to be responsible for the cross-modal effects. Neither E. J. Gibson nor Zaporozhets seemed to deny that language may be involved in the mediation of cross-modal effects. However, they suggested that these effects generally have other sources as well. Studies of crossmodal perception among animals seem to have confirmed their points of view, but it will soon be shown that their specifications of the cross-modal sources are entirely arbitrary ones.
DISCUSSION The epistomological aspect of Muller’s doctrine is the one most vigorously attacked by modern researchers in the psychology of perception. Thus E.
Doctrine of specific nerve energies and perceptual development
J . Gibson, J . J . Gibson, Bower, and others have stressed that the senses give direct information of external objects, not merely of the states of their nerves. Furthermore, these researchers all assumed that similar information of objects is mediated by different sensory systems. Yet they seemed to disagree on questions concerning developmental changes in intersensory relationships. While Bower apparently considered the sources of a supra-modal object perception available at birth, E. J. Gibson assumed they must be detected by learning and perceptual exploration. For Bower “the sign of advance” in perceptual development was the differentiation of modality specific analysers. For E. J . Gibson it was the evolvement of intermodal perception. Since the two researchers argued from different types of studies, both may be right. Bower argued from studies of certain forms of spontaneous behaviour by infants, i.e. behaviour which may involve orientations or attitudes of great survival value. In general, orienting reflexes, arousal and behaviour subserving certain mother-child relationships may constitute such orientations. As pointed out by Haffman (1962), most sensory nerves have also a nonspecific pathway leading through the reticular activating system with diffuse projections to the cortex and other neural structures. Hence it is possible to argue that the type of orientations mentioned can be elicited and maintained equally by the stimulation of different senses. Anyhow, the senses do not only serve discriminatory functions, they also serve the arousal and reinforcement of behaviour. The point is that studies of the latter type of functions may require that certain forms of spontaneous behaviour are taken as points of departure. On the other hand, studies of discriminatory functions almost always require that one makes use of learned behaviour. E. J. Gibson argued from studies of certain forms of learned behaviour which involved attention to particular aspects or parts of the environment. During such behaviour perception may still be “grossly selective”, but it cannot form an indiscriminate response to all aspects of the situation. Therefore, studies which are based on learned behaviour will necessarily show some specificity in perception. But it may be difficult to decide whether perception in a matching-to-sample performance, for instance, has been specific with respect to one sense or to features common to dif-
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ferent modalities. The problem is that one is still unable to define a modality or a specific energy independently of common language terms such as colour, sound and taste. A proposition which says that corners, transitions and movements are intermodal rather than intra-modal properties, introduces an arbitrary distinction which cannot be accounted for independently of some knowledge about the acquisition of language. Although we use the same word about movements detected via different sensory channels, one cannot say that movements produced by auditory stimulation are psychologically equivalent to movements produced by visual stimulation. Particularly, one cannot guarantee that movements produced by the stimulation of different senses by infants have equal effects on all forms of behaviour. Likewise, one cannot know whether self-produced movements during visual and haptic explorations are psychologically equivalent movements for the infant. Whether the researcher wants to study the perception of properties supposed to be modality specific ones or the perception of properties supposed to be intermodal ones, he must provide certain means of communication between subject and experimenter. Either verbal instructions or training procedures have to be used. It follows that modality specific and intermodal forms of perception can only be studied in organisms who have a certain capacity for learning. Perceptual properties may require different means of communication, and probably different learning capacities. In this way cross-modal matching abilities will tend to increase with age. But so will also the capacity for what is supposed to be a modality specific form of perception. Therefore, studies which show an increasing tendency towards integration of the modalities need not contradict studies which show an increasing tendency towards a modality specific form of perception. A final evaluation of Miiller’s doctrine awaits another definition of the concept of a sense. This concept tends to be used circularly as long as it is merely defined by reference to common language terms for sensations. However, it has also been used with reference to certain discriminatory abilities which relates the organism to the environment, and the only chance of avoiding a circularity of usage will be to analyse independently forms of organism-environment interactions. Such interactions should serve as frames of reference for clasS m n d I Prvrhol l i
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sifications of the senses. This means that, for instance, taste and smell may constitute one sense for certain forms of organism-environment interactions, whereas they may constitute different senses for other forms of interactions. Spontaneous behaviour such as turning of the head, smiling and sucking may serve a type of interactions which differ from the type of interactions served by matching behaviour in studies of form and colour perception, and possibly therefore, the nerve of a sense can “take the place and perform the function of another sense” only in the former type of organism4nvironment interactions, not in the other.
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