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Creativity and the Corpus Callosum
Hemispheric Specialization
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Creativity and the Corpus Callosum ] oseph E. Bogen, M .D. *, and Glenda M. Bogen, R .N .t
Creativity provides a workable approach to an unsolved problem or a previously unrecognized opportunity.' Commonly, the essence or framework of the approach appears rather suddenly and further details are then filled in. Many people have pointed out that creativity occurs through several stages.
STAGES IN CREATIVITY Hermann Helmholtz, the great physicist, described the way in which his most important new thoughts had come to him. He said that after a period of lengthy investigation, in many directions, of some particular problem, " ... happy ideas come unexpectedly, without effort, like an inspiration. So far as I am concerned, they have never come to me when my mind was fatigued, or when I was at my working table ... . They came particularly readily during the slow ascent of wooded hills on a sunny day. "25 Henri Poincare, in his book Science and Method, 36 described in vivid detail the successive stages of two of his great mathematical discoveries. Both of them came to him after long preparation and a period of incubation during which no conscious mathematical thinking was done. Of his discovery of Theta-Fuchsian functions, he wrote: "I went away to spend a few days at the seaside and thought of entirely different things. One day as I was walking on the cliff the idea came to me." He also said: Often when a man is working at a difficult question, he accomplishes nothing the first time he sets to work. Then he takes more or less a rest, and sits down again at his table. During the first half-hour he still finds nothing, and then all at
*Clinical Professor, Department of Neurological Surgery, University of Southern California School of Medicine; Adjunct Professor, Department of Psychology, UCLA; The Hospital of the Good Samaritan, Los Angeles, California tRegistered Nurse, Pasadena, California Supported in part by NIH grant No. NS20187.
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once the decisive idea presents itself to his mind. We might say that the conscious work proved more fruitful because it was interrupted and [that the period ofl rest restored force and freshness to the mind. But it is more probable that th e rest was occupied with unconscious work, and that the result of this work was afterwards revealed to the geometrician exactly as in the cases I have quoted, except that the revelation, instead of coming to light during a walk or a journey, came during a period of conscious work, but independently of that work, which at most only performs the unlocking process, as if it were the spur that excited into conscious form the results already acquired during the rest, which till then remained unconscious.
He went on to say, It might be supposed that, by thinking overnight ... we might hope to find the solution to the product ready-made for us on waking, or [that] a verification could be made unconsciously. [But] all that we can hope for from these inspirations is to obtain points of departure. [There is] a second period of conscious work which follows the inspiration, and in which the results of the inspiration are verified and the consequences deduced. The rules are strict and complicated; they demand discipline, attention, will and consequently consciousness.
Such stories are not peculiar to mathematicians. Chief Justice Rehnquist wrote, "I began to realize that some of my best insights came not during my enforced thinking periods in my chambers, but while I was shaving in the morning, driving to work or just walking from one place to another. " 37 In order to result in a successful creation, an inspiration must next be verified to see that the idea or product really works. This verification phase is in many ways similar to the preparation phase in that the process is logical, often mathematical, and consciously deliberate. Graham Wallas suggested in 1926 that creativity proceeds in four distinct phases. 46 He named these four phases preparation, incubation, illumination, and verification. Although derived from his own introspection and his knowledge of observations such as those of Helmholtz and Poincare, rather than from empirical observation, Wallas' description has been accepted by most investigators of creativity. 38 In order to produce something both novel and meaningful one must have a period of preparation. This involves building a base of knowledge or a fund of information from which to draw. Following the preparation phase there comes a period of incubation during which time the information is sorted, compared, and rearranged, typically while one is unaware of the process. Then follows illumination-the ''I've got it!" stage. Almost everyone is familiar with the cartoonist's use of a light bulb to symbolize the instant illumination of an idea. There is then necessary a phase of deliberate reorganization and refinement, readily describable by the creator, in order to polish the final product. What can be the physiologic basis for this succession of stages? It seems clear that some very productive thinking goes on which is inaccessible to language (in that one cannot tell how it went on) and whose result becomes available in a sudden if simple insight, or sometimes in an already elaborate form. Where does this kind of thinking take place? To say that it comes from "the heart" describes the quality rather than the origin. To say that it
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comes from "intuition" is merely to rename it rather than to give it a physiologic source. It surely requires an elaborate neuronal system, of a size, complexity, and activity level comparable to that organ-namely, the left hemisphere-which produces the richness of human language . It is likely that at least some of the thinking that goes on during incubation takes place in the human right hemisphere. By contrast, the verification and preparation phases seem typically left hemispheric in origin. One sees readily the likelihood of a greater than usual hemispheric interaction during an individual's more creative moments, an interaction dependent upon the corpus callosum.
OUR PRESENT KNOWLEDGE OF THE CORPUS CALLOSUM The central location and the large size of the corpus callosum, especially in the human, certainly suggest an important role in mentation. As McCulloch and Garol3° put it in 1941, The corpus callosum and anterior commissure constitute a fibre system larger than the sum of all systems ascending to and descending from the cerebral hemispheres . .. little is known of its function. Apart from symptoms referable to lesions of adjacent structures, tumors and softenings and surgical sections have failed to produce any characteristic disorders except, possibly, impairment of coordination of the hemispheres in complicated symbolic activity.
This "complicated symbolic activity" had been inferred from the apraxia, agnosia, etc. occasionally observed by neurologists in the early years of this century. But as late as 1961, Glees 21 wrote (somewhat wistfully), The specific function of callosal fibres still eludes us, however, although a connection between the hemispheres would indeed appear highly desirable, and one of the size of the corpus callosum must serve some more than decorative purpose.
However, Glees took note of the emerging split-brain experiments of Myers and Sperry32 • 33 and observed that the corpus callosum "is the means whereby the two hemispheres can share and profit by each other's experience." During the past 30 years, an ever-increasing number and variety of experiments have made it quite clear that the commissures can transfer high level (that is, cognitive) information from one hemisphere to the other. 3 • 7 · 9 · 11 · 20 · 22 • 23 • 26 • 28 • 39 • 42 • 43 • 47 But the split-brain experiments do not yet tell us enough. The demonstration that each half of a split brain can have its own percepts, memories, and choices is usually dependent upon restriction of relevant input to only one hemisphere. However illuminating they may be, these experiments do not show us the full potential of the corpus callosum in the intact individual in natural surroundings. Moreover, the posterior one fourth of the corpus callosum, if left intact, can prevent the appearance of the entire disconnection syndrome as now conceived. 9 • 22 For the remainder (at least 150 million nerve fibers) 27 little more is known now than was known 30 years ago. Our new advantage in speculating on their function is our recognition that the hemispheres are not as much "ma-
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jor" and "minor" as they are complementary and that each hemisphere is capable of thinking on its own, in its own way. 5 • 10· 19· 28 • 41 · 47 It is increasingly recognized that hemispheric asymmetry had an adaptive value long before the emergence of reading and writing, perhaps before the emergence of organized language. 12 · 18· 35 This supposition gives a special, updated meaning to Hebb's opinion that "speech arises as the end product of a phylogenetic increase in the ability to entertain independent ideas or trains of thought at the same time. " 24 If learning can proceed simultaneously but independently and differently in each hemisphere, so may problem solving. This contributes to a less predictable, that is, a less stimulus-bound, behavior. In other words, specialization of the hemispheres for different trains of thought greatly increases the flexibility of the ensemble. Such differentiation necessarily produces a concomitant decrease in stability. The successful expansion of the human species (so far) suggests that the loss of stability is less important than the gain in flexibility. Creativity has not only made the human species dominant (and dangerous) on the earth; what may be more important for each one of us is that it gives value and purpose to human existence. Creativity requires more than the propositional skills and logical thought of the left hemisphere; it also needs the cultivation and collaboration of the other side of the brain. There is as yet no clear consensus on how best to describe the cognitive differences between the hemispheres. Twenty years ago we suggested the distinction "propositional versus appositional. " 4 Whatever terms are used (e.g., "analytic versus holistic"; see refs. 6 and 10 for many others), what is essential for the theory we offer here is the well-established fact that in most humans the two cerebral hemispheres function quite differently.34· 41 • 44 We believe that this involves, indeed requires, a significant degree of hemispheric independence- that the interhemispheric exchange be much of the time incomplete.
THE INCOMPLETENESS OF INTERHEMISPHERIC EXCHANGE Failure of information transfer from one hemisphere to the other is typical of the split brain, and this is usually contrasted with the function of the intact brain. When the information consists of discriminative sensory data, the contrast is greatest. Every right-handed patient with a complete cerebral commissurotomy fails to name an unseen object in the left hand, although recognition by the right hemisphere is obvious from the ability of the left hand to retrieve the test object when it is dropped into a bag containing a number of similar objects. Every such patient fails to replicate, with one hand, complicated postures imposed on the other. There is a wide variety of similar defects in interhemispheric transfer following commissural section. By contrast, such information transfers readily in the normal condition. For complex motor sequences, however, the effects of commissurotomy are less contrasting. This is because transfer is often incomplete even when the commissures are intact. In the best brains, a complicated task
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learned with one hand transfers only incompletely to the other. The inability of the corpus callosum to perfect interhemispheric transfer is obvious in acquired skills such as writing. Asymmetry of learning is not restricted to propositional skills; it also appears in the attempt to perform with the left hand other tasks learned originally in the right hand. We should not suppose that this is simply an expression of "cerebral dominance," because it is also true of tasks learned in the left hand and then attempted with the right, if the task is of sufficient complexity, such as the fingering of a musical instrument. The violin virtuoso, ltzhak Perlman, 29 has been quoted as saying: It's not hard remembering what it was like to start learning the fiddle. You just change hands and try to play that way, and the whole experience comes back to you.
A variety of studies, as Butler 16 put it, "lead to the conclusion that commissural transfer normally has a relatively weak influence on hemispheric activity." We have recently published an extensive review8 gathering together a wide variety of experimental evidence describing conditions in which interhemispheric transfer of high-level (ideational) information does not occur in the intact brain. That review did not include description of an experiment, done with cortical spreading depression (CSD), which is instructive with respect to creativity and is described next.
AN ANIMAL EXPERIMENT ON CREATIVITY In animals with smooth brains such as the rat and rabbit, the CSD involves (with minor exceptions) 45 the entire hemisphere. If a recurring stim ulus is used (such as a drop of concentrated potassium chloride), there will be wave after wave of CSD, and the cortex of the entire hemisphere can be kept in a nonfunctioning condition as long as several hours. If an animal is trained under certain circumstances while one hemisphere is depressed by CSD, the learning does not transfer. That is, if the animal is tested later, while the previously trained hemisphere is depressed, it must learn the problem from the beginning. In other words, if one hemisphere has certain learning experiences at a time when the other hemisphere does not, the acquired behavior may not transfer even though the commissures are
intact. Bures and his colleagues 15 used CSD in a long series of informative experiments; one is particularly enlightening. They trained rats in one method of escape (A) while the right hemisphere was rendered inactive by the method of cortical spreading depression. They then trained the rats in a different method (B) with the left hemisphere depressed. They then tested the rats under conditions requiring simultaneous use, in combination, of the two different methods (A, B). The rats rapidly solved the combination problem. Control rats were trained on A and then B without depression, that is, with both hemispheres working together in a normal way. These rats solved the combined problem little better than rats that had had no previous training at all. The lateralized acquisition of contrast-
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ing information apparently results in less interference between the first and second lessons, leaving the animal with each lesson in its respective hemisphere. Such a situation could easily lead to internal conflict, but it also seems more conducive to rapid learning in novel or complex situations. As Bures and Burefova14 say, .. . (the presence of) the independent and undated engrams creates thus, at least in some behavioral situations, better conditions for a "creative" combination of previous experiences than the usual sequential learning.
The differential lateralization of learning in the rat with commissures intact required for its demonstration an ingenious experimental method. However, differential lateralization in the human is an everyday, commonplace event that is less dependent on the circumstances of learning than on an intrinsic asymmetry of function arising out of a long-term trend during millions of years of evolution. Interhemispheric collaboration in the human need not be restricted to verbal-visuospatial interaction; since the right hemisphere has a special capacity for timbre and certain other aspects of music, interhemispheric communication could clearly contribute to musical creativity. Indeed, we can easily entertain the notion that creativity in general benefits from interhemispheric collaboration. However, no one supposes that all creativity involves the corpus callosum. As has often been noted (for review, see refs. 3 and 17) and as we ourselves know from personal experience, the left hemisphere alone, either following right hemispherectomy or right carotid amytal injection or isolated from the right by callosal section, is capable of a full range of verbal expression, including generation of an unlimited number of novel sentences. But when a solution requires combining of "dual memory codes, verbal and imaginal" (as Milner31 put it) one readily recognizes an important role for the corpus callosum.
THE LACK OF CREATIVITY
If some kinds of creativity are dependent on a transcallosal interhemispheric exchange, there are some obvious explanations for its absence. There may be first of all a deficiency in technical competence in a suitable medium; in the case ofliterary as well as mathematical creativity this is easily seen as a lack of propositional skill. Alajouanine2 averred, "To conceive is nothing, to express is all." He meant by this that pleasing musical themes and poetical images arise in the minds of many people who have not acquired the means for their expression. There are many persons possessing technical proficiency in music, drawing, or writing whose production is devoid of those innovative and informative values that distinguish an artist from a performer. This may be the result of a genetic deficiency or a deficiency in the environmental exposure necessary to the development of inherited potential. There is a large literature demonstrating a lack of structural development consequent to lack of requisite environmental stimulation. As Sperry4° put it,
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Many elements deeper in the brain centers must discharge only in very special activities, and, if these activities are not exercised-especially during maturational stages when the neurons seem to be particularly dependent on use-the neuron types involved may regress, leaving profound functional deficiencies in the integrative machinery."
We are accustomed to hear, these days, of the "culturally disadvantaged," those persons whose propositional potential has remained undeveloped for lack of proper schooling. There is likely a parallel lack of appositional development in persons whose education has narrowly emphasized reading, writing, and their concomitants. 6 Third, there must be the possibility of an unusually free communication between the two hemispheres, interrupting temporarily the ongoing state that has allowed independent processing. The conditions for a transient increase in callosal transmission have yet to be adequately explored. One problem is that inspiration occurs not only in repose but often in striving under pressure. In all likelihood the momentary suspension of hemispheric independence is contingent upon appropriate hemispheric affective states, perhaps the concurrence of one hemisphere's positive affect and active cognition with the other hemisphere's neutrality or receptivity.
SUMMARY A duality of mind is readily demonstrable in split-brain humans, and evidence is steadily accumulating that ongoing interhemispheric communication is incomplete in the intact brain. It is now certain that the corpus callosum can transfer high-level information from one hemisphere to another. When we take into account the well-established principle of hemispheric specialization, added to the impressive normality of split-brain humans in ordinary social situations, a physiologic explanation for at least some forms of creativity seems close at hand. What is required is a partial (and transiently reversible) hemispheric independence during which lateralized cognition can occur and is responsible for the dissociation of preparation from incubation. A momentary suspension of this partial independence could account for the illumination that precedes subsequent deliberate verification. From this point of view, we can understand better the opinion of Frederic Bremer, 13 who wrote years ago that the corpus callosum subserves "the highest and most elaborate activities of the brain" -in a word, creativity.
REFERENCES l. Adelson M: Creativity and the third culture . UCLA Educ 18:41-49, 1976 2. Alajouanine T: Aphasia and artistic realization. Brain 71:229- 241, 1948 3. Benson OF, Zaidel E (eds): The Dual Brain : Hemispheric Specialization in Humans . New York, Guilford Press, 1985
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4. Bogen JE: The other side of the brain. II: An appositional mind. Bull LA Neurol Soc 34:135-162, 1969 5. Bogen JE: Further discussion on split-brains and hemispheric capabilities. Br J Philosophy Sci 28:281-286, 1977 6. Bogen JE: Some educational implications of hemispheric specialization . In Wittrock MC (ed): The Human Brain. Englewood Cliffs, NJ, Prentice-H all, 1977, p 135 7. Bogen JE: Split-brain syndromes. In Frederiks JAM (ed): Handbook of Clinical Neurology, Vol 45. Amsterdam, Elsevier, 1985 8. Bogen JE: Mental duality in the intact brain. Bull Clin Neurosci 51:3-29, 1986 9. Bogen JE: Physiological consequences of complete or partial commissural section. In Apuzzo MLJ (ed): Surgery of the Third Ventricle. Baltimore, Williams and Wilkins, 1987 10. Bogen JE, Bogen GM: The other side of the brain. Ill: The corpus callosum and creativity. Bull LA Neurol Soc 34:191-220, 1969 11. Bogen JE, Gazzaniga MS: Cerebral commissurotomy in man: Minor hemisphere dominance for certain visuospatial functions. J Neurosurg 23:394-399, 1965 12. Bradshaw J, Nettleton N: Human Cerebral Asymmetry. Englewood Cliffs, NJ, PrenticeHall, 1983 13. Bremer F: Physiology of the corpus callosum. Res Pub! Assoc Res Nerv Ment Dis 36:424-428, 1956 14. Bures J, Burefova 0: The use of Leao's spreading cortical depression in the study of interhemispheric transfer of memory traces. J Comp Physiol Psych 53:558-563, 1960 15. Bures J, Burefova 0, Krivanek J: The mechanism and application of Leao's spreading depression of electroencephalographic activity. Prague, Academia, 1974 16. Butler S: lnterhemispheric relations in schizophrenia. In Gruzelier J, Flor-Henry P (eds): Hemisphere Asymmetries of Function in Psychopathology. Amsterdam, Elsevier/ North-Holland Biomedical Press, 1979 17. Code C: Language, Aphas ia, and the Right Hemisphere. Chichester, John Wiley & Sons, 1987 18. Corballis MC: Human Laterality. New York, Academic Press, 1983 19. Cronin-Golomb A: Comprehension of abstract concepts in right and left hemispheres of complete commissurotomy subjects. Neuropsychologia 24:881-887, 1986 20. Galin D, Johnstone J, Nakell L, et al: Development of the capacity for tactile information transfer between hemispheres in normal children. Science 204:1330-1332, 1979 21. Glees P: Experimental Neurology. London, Oxford University Press, 1961 , p 486 22. Gordon HW, Bogen JE, Sperry RW: Absence of deconnexion syndrome in two patients with partial section of the neocommissures. Brain 94:327- 336, 1971 23. Hamilton CR: Mechanisms of interocular equivalence. In Ingle D, Goodale M, Mansfield R (eds): Analysis of Visual Behavior. Cambridge, MA, MIT Press, 1982 24. Hebb DO: The problem of consciousness and introspection. In Delafresnaye JF (ed) : Brain Mechanisms and Consciousness. Springfield, IL, Charles C Thomas, 1954 25. Helmholtz H: Cited by Wallas G : Stages of control. In Rothenberg A, Hausman C (eds): The Creativity Question. Durham, NC, Duke University Press, 1976, pp 69-73 26. Hoppe KD: Split-brain and psychoanalysis. Psychoanal Q 46:220-244, 1977 27. Innocenti GM: What is so special about callosal connections? In Lepore F, Ptito M, Jasper HH (eds): Two Hemispheres, One Brain? New York, Alan Liss, 1986 28. Landis T, Graves R, Goodglas H: Dissociated awareness of manual performance on two different visual associative tasks: A "split-brain" phenomenon in normal subjects? Cortex 17:435-440, 1981 29. Mayer M: Recordings. Esquire, June 1968, p 12 30. McCulloch WS, Carol HW: Cortical origin and distribution of corpus callosum and anterior commissure in the monkey (Macaca mulatta). J Neurophysiol 4:555-563, 1941 31. Milner B: Clues to the cerebral organization of memory. In Buser PA, Rougeul-Buser A (eds) : Cerebral Correlates of Conscious Experience. Amsterdam, North Holland Publishing Company, 1978 32. Myers RE, Sperry RW: Interocular transfer of a visual form discrimination habit in cats after section of the optic chiasma and corpus callosum. Anat Record 115:351-352, 1953 33. Myers RE, Sperry RW: Interhemispheric communication through the corpus callosum. Mnemonic carry-over betwee n the hemispheres. Arch Neurol Psychiatry 80:298-303, 1958
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34. Nass RD, Gazzaniga MS: Cerebral lateralization and specialization in human central nervous system. In Mountcastle VB, Plum F, Geiger SR (eds): Handbook of Physiology, Sec 1: The nervous system, Vol V, Pt 2. Baltimore, Waverly Press, 1987 35. Papc;un G, Krashen S, Terbeek D, et al: Is the left hemisphere specialized for speech, language, and/or something else? J Acoust Soc Am 55:319-327, 1974 36. Poincare H: Science and Method, translated by F. Maitland. New York, Dover, 1952 37. Rehnquist WH: The Supreme Court: How It Was, How It ls. New York, Morrow, 1987 38. Rothenberg A, Hausman CR: The Creativity Question. Durham, Duke University Press, 1976, p 69 39. Sperry RW: Cerebral organization and behavior. Science 133:1749-1757, 1961 40. Sperry RW : Embryogenesis of behavioral nerve nets. In Dehaan RL, Ursprung H (eds): Organogenesis. New York, Holt, Rinehart and Winston, 1965 41. Sperry RW: Lateral specialization in the surgically separated hemispheres. In Schmitt FO, Worden FG (eds): Neurosciences : Third Study Program. Cambridge, MA, MIT Press, 1974 42. Sperry RW, Gazzaniga MS, Bogen JE: lnterhemispheric relationships: The neocortical commissures; syndromes of disconnection. In Vinken PJ, Bruyn CW (eds): Handbook of Clinical Neurology, Vol 4. Amsterdam, Elsevier, 1969, pp 273-290 43. TenHouten WD, Walter DO, Hoppe KD, et al: Alexithymia and the split brain. V: EEG alpha-band interhemispheric coherence analysis. Psychother Psychosom 47:1-10, 1987 44. Trevarthen CB: Hemispheric specialization. Handbook of physiology-The nervous system, III, Ch 25. Washington, DC, American Physiological Society, 1984 45. Van Harreveld A, Bogen JE: Regional differences in propagation of spreading cortical depression in the rabbit. Soc Exper Biol Med 91:297-302, 1956 46. Wallas G: Stages of control. In Rothenberg H, Hausman C (eds): The Creativity Question. Durham, NC, Duke University Press, 1976, pp 69-73 47. Zaidel E: Hemispheric monitoring. In Ottosson D (ed): Duality and Unity of the Brain. Hampshire, England, Macm illan, 1987 1645 Poppy Peak Drive Pasadena, California 91105
0193-953X/88 $0.00
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.20
Creativity and the Corpus Callosum ] oseph E. Bogen, M .D. *, and Glenda M. Bogen, R .N .t
Creativity provides a workable approach to an unsolved problem or a previously unrecognized opportunity.' Commonly, the essence or framework of the approach appears rather suddenly and further details are then filled in. Many people have pointed out that creativity occurs through several stages.
STAGES IN CREATIVITY Hermann Helmholtz, the great physicist, described the way in which his most important new thoughts had come to him. He said that after a period of lengthy investigation, in many directions, of some particular problem, " ... happy ideas come unexpectedly, without effort, like an inspiration. So far as I am concerned, they have never come to me when my mind was fatigued, or when I was at my working table ... . They came particularly readily during the slow ascent of wooded hills on a sunny day. "25 Henri Poincare, in his book Science and Method, 36 described in vivid detail the successive stages of two of his great mathematical discoveries. Both of them came to him after long preparation and a period of incubation during which no conscious mathematical thinking was done. Of his discovery of Theta-Fuchsian functions, he wrote: "I went away to spend a few days at the seaside and thought of entirely different things. One day as I was walking on the cliff the idea came to me." He also said: Often when a man is working at a difficult question, he accomplishes nothing the first time he sets to work. Then he takes more or less a rest, and sits down again at his table. During the first half-hour he still finds nothing, and then all at
*Clinical Professor, Department of Neurological Surgery, University of Southern California School of Medicine; Adjunct Professor, Department of Psychology, UCLA; The Hospital of the Good Samaritan, Los Angeles, California tRegistered Nurse, Pasadena, California Supported in part by NIH grant No. NS20187.
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once the decisive idea presents itself to his mind. We might say that the conscious work proved more fruitful because it was interrupted and [that the period ofl rest restored force and freshness to the mind. But it is more probable that th e rest was occupied with unconscious work, and that the result of this work was afterwards revealed to the geometrician exactly as in the cases I have quoted, except that the revelation, instead of coming to light during a walk or a journey, came during a period of conscious work, but independently of that work, which at most only performs the unlocking process, as if it were the spur that excited into conscious form the results already acquired during the rest, which till then remained unconscious.
He went on to say, It might be supposed that, by thinking overnight ... we might hope to find the solution to the product ready-made for us on waking, or [that] a verification could be made unconsciously. [But] all that we can hope for from these inspirations is to obtain points of departure. [There is] a second period of conscious work which follows the inspiration, and in which the results of the inspiration are verified and the consequences deduced. The rules are strict and complicated; they demand discipline, attention, will and consequently consciousness.
Such stories are not peculiar to mathematicians. Chief Justice Rehnquist wrote, "I began to realize that some of my best insights came not during my enforced thinking periods in my chambers, but while I was shaving in the morning, driving to work or just walking from one place to another. " 37 In order to result in a successful creation, an inspiration must next be verified to see that the idea or product really works. This verification phase is in many ways similar to the preparation phase in that the process is logical, often mathematical, and consciously deliberate. Graham Wallas suggested in 1926 that creativity proceeds in four distinct phases. 46 He named these four phases preparation, incubation, illumination, and verification. Although derived from his own introspection and his knowledge of observations such as those of Helmholtz and Poincare, rather than from empirical observation, Wallas' description has been accepted by most investigators of creativity. 38 In order to produce something both novel and meaningful one must have a period of preparation. This involves building a base of knowledge or a fund of information from which to draw. Following the preparation phase there comes a period of incubation during which time the information is sorted, compared, and rearranged, typically while one is unaware of the process. Then follows illumination-the ''I've got it!" stage. Almost everyone is familiar with the cartoonist's use of a light bulb to symbolize the instant illumination of an idea. There is then necessary a phase of deliberate reorganization and refinement, readily describable by the creator, in order to polish the final product. What can be the physiologic basis for this succession of stages? It seems clear that some very productive thinking goes on which is inaccessible to language (in that one cannot tell how it went on) and whose result becomes available in a sudden if simple insight, or sometimes in an already elaborate form. Where does this kind of thinking take place? To say that it comes from "the heart" describes the quality rather than the origin. To say that it
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comes from "intuition" is merely to rename it rather than to give it a physiologic source. It surely requires an elaborate neuronal system, of a size, complexity, and activity level comparable to that organ-namely, the left hemisphere-which produces the richness of human language . It is likely that at least some of the thinking that goes on during incubation takes place in the human right hemisphere. By contrast, the verification and preparation phases seem typically left hemispheric in origin. One sees readily the likelihood of a greater than usual hemispheric interaction during an individual's more creative moments, an interaction dependent upon the corpus callosum.
OUR PRESENT KNOWLEDGE OF THE CORPUS CALLOSUM The central location and the large size of the corpus callosum, especially in the human, certainly suggest an important role in mentation. As McCulloch and Garol3° put it in 1941, The corpus callosum and anterior commissure constitute a fibre system larger than the sum of all systems ascending to and descending from the cerebral hemispheres . .. little is known of its function. Apart from symptoms referable to lesions of adjacent structures, tumors and softenings and surgical sections have failed to produce any characteristic disorders except, possibly, impairment of coordination of the hemispheres in complicated symbolic activity.
This "complicated symbolic activity" had been inferred from the apraxia, agnosia, etc. occasionally observed by neurologists in the early years of this century. But as late as 1961, Glees 21 wrote (somewhat wistfully), The specific function of callosal fibres still eludes us, however, although a connection between the hemispheres would indeed appear highly desirable, and one of the size of the corpus callosum must serve some more than decorative purpose.
However, Glees took note of the emerging split-brain experiments of Myers and Sperry32 • 33 and observed that the corpus callosum "is the means whereby the two hemispheres can share and profit by each other's experience." During the past 30 years, an ever-increasing number and variety of experiments have made it quite clear that the commissures can transfer high level (that is, cognitive) information from one hemisphere to the other. 3 • 7 · 9 · 11 · 20 · 22 • 23 • 26 • 28 • 39 • 42 • 43 • 47 But the split-brain experiments do not yet tell us enough. The demonstration that each half of a split brain can have its own percepts, memories, and choices is usually dependent upon restriction of relevant input to only one hemisphere. However illuminating they may be, these experiments do not show us the full potential of the corpus callosum in the intact individual in natural surroundings. Moreover, the posterior one fourth of the corpus callosum, if left intact, can prevent the appearance of the entire disconnection syndrome as now conceived. 9 • 22 For the remainder (at least 150 million nerve fibers) 27 little more is known now than was known 30 years ago. Our new advantage in speculating on their function is our recognition that the hemispheres are not as much "ma-
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jor" and "minor" as they are complementary and that each hemisphere is capable of thinking on its own, in its own way. 5 • 10· 19· 28 • 41 · 47 It is increasingly recognized that hemispheric asymmetry had an adaptive value long before the emergence of reading and writing, perhaps before the emergence of organized language. 12 · 18· 35 This supposition gives a special, updated meaning to Hebb's opinion that "speech arises as the end product of a phylogenetic increase in the ability to entertain independent ideas or trains of thought at the same time. " 24 If learning can proceed simultaneously but independently and differently in each hemisphere, so may problem solving. This contributes to a less predictable, that is, a less stimulus-bound, behavior. In other words, specialization of the hemispheres for different trains of thought greatly increases the flexibility of the ensemble. Such differentiation necessarily produces a concomitant decrease in stability. The successful expansion of the human species (so far) suggests that the loss of stability is less important than the gain in flexibility. Creativity has not only made the human species dominant (and dangerous) on the earth; what may be more important for each one of us is that it gives value and purpose to human existence. Creativity requires more than the propositional skills and logical thought of the left hemisphere; it also needs the cultivation and collaboration of the other side of the brain. There is as yet no clear consensus on how best to describe the cognitive differences between the hemispheres. Twenty years ago we suggested the distinction "propositional versus appositional. " 4 Whatever terms are used (e.g., "analytic versus holistic"; see refs. 6 and 10 for many others), what is essential for the theory we offer here is the well-established fact that in most humans the two cerebral hemispheres function quite differently.34· 41 • 44 We believe that this involves, indeed requires, a significant degree of hemispheric independence- that the interhemispheric exchange be much of the time incomplete.
THE INCOMPLETENESS OF INTERHEMISPHERIC EXCHANGE Failure of information transfer from one hemisphere to the other is typical of the split brain, and this is usually contrasted with the function of the intact brain. When the information consists of discriminative sensory data, the contrast is greatest. Every right-handed patient with a complete cerebral commissurotomy fails to name an unseen object in the left hand, although recognition by the right hemisphere is obvious from the ability of the left hand to retrieve the test object when it is dropped into a bag containing a number of similar objects. Every such patient fails to replicate, with one hand, complicated postures imposed on the other. There is a wide variety of similar defects in interhemispheric transfer following commissural section. By contrast, such information transfers readily in the normal condition. For complex motor sequences, however, the effects of commissurotomy are less contrasting. This is because transfer is often incomplete even when the commissures are intact. In the best brains, a complicated task
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learned with one hand transfers only incompletely to the other. The inability of the corpus callosum to perfect interhemispheric transfer is obvious in acquired skills such as writing. Asymmetry of learning is not restricted to propositional skills; it also appears in the attempt to perform with the left hand other tasks learned originally in the right hand. We should not suppose that this is simply an expression of "cerebral dominance," because it is also true of tasks learned in the left hand and then attempted with the right, if the task is of sufficient complexity, such as the fingering of a musical instrument. The violin virtuoso, ltzhak Perlman, 29 has been quoted as saying: It's not hard remembering what it was like to start learning the fiddle. You just change hands and try to play that way, and the whole experience comes back to you.
A variety of studies, as Butler 16 put it, "lead to the conclusion that commissural transfer normally has a relatively weak influence on hemispheric activity." We have recently published an extensive review8 gathering together a wide variety of experimental evidence describing conditions in which interhemispheric transfer of high-level (ideational) information does not occur in the intact brain. That review did not include description of an experiment, done with cortical spreading depression (CSD), which is instructive with respect to creativity and is described next.
AN ANIMAL EXPERIMENT ON CREATIVITY In animals with smooth brains such as the rat and rabbit, the CSD involves (with minor exceptions) 45 the entire hemisphere. If a recurring stim ulus is used (such as a drop of concentrated potassium chloride), there will be wave after wave of CSD, and the cortex of the entire hemisphere can be kept in a nonfunctioning condition as long as several hours. If an animal is trained under certain circumstances while one hemisphere is depressed by CSD, the learning does not transfer. That is, if the animal is tested later, while the previously trained hemisphere is depressed, it must learn the problem from the beginning. In other words, if one hemisphere has certain learning experiences at a time when the other hemisphere does not, the acquired behavior may not transfer even though the commissures are
intact. Bures and his colleagues 15 used CSD in a long series of informative experiments; one is particularly enlightening. They trained rats in one method of escape (A) while the right hemisphere was rendered inactive by the method of cortical spreading depression. They then trained the rats in a different method (B) with the left hemisphere depressed. They then tested the rats under conditions requiring simultaneous use, in combination, of the two different methods (A, B). The rats rapidly solved the combination problem. Control rats were trained on A and then B without depression, that is, with both hemispheres working together in a normal way. These rats solved the combined problem little better than rats that had had no previous training at all. The lateralized acquisition of contrast-
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ing information apparently results in less interference between the first and second lessons, leaving the animal with each lesson in its respective hemisphere. Such a situation could easily lead to internal conflict, but it also seems more conducive to rapid learning in novel or complex situations. As Bures and Burefova14 say, .. . (the presence of) the independent and undated engrams creates thus, at least in some behavioral situations, better conditions for a "creative" combination of previous experiences than the usual sequential learning.
The differential lateralization of learning in the rat with commissures intact required for its demonstration an ingenious experimental method. However, differential lateralization in the human is an everyday, commonplace event that is less dependent on the circumstances of learning than on an intrinsic asymmetry of function arising out of a long-term trend during millions of years of evolution. Interhemispheric collaboration in the human need not be restricted to verbal-visuospatial interaction; since the right hemisphere has a special capacity for timbre and certain other aspects of music, interhemispheric communication could clearly contribute to musical creativity. Indeed, we can easily entertain the notion that creativity in general benefits from interhemispheric collaboration. However, no one supposes that all creativity involves the corpus callosum. As has often been noted (for review, see refs. 3 and 17) and as we ourselves know from personal experience, the left hemisphere alone, either following right hemispherectomy or right carotid amytal injection or isolated from the right by callosal section, is capable of a full range of verbal expression, including generation of an unlimited number of novel sentences. But when a solution requires combining of "dual memory codes, verbal and imaginal" (as Milner31 put it) one readily recognizes an important role for the corpus callosum.
THE LACK OF CREATIVITY
If some kinds of creativity are dependent on a transcallosal interhemispheric exchange, there are some obvious explanations for its absence. There may be first of all a deficiency in technical competence in a suitable medium; in the case ofliterary as well as mathematical creativity this is easily seen as a lack of propositional skill. Alajouanine2 averred, "To conceive is nothing, to express is all." He meant by this that pleasing musical themes and poetical images arise in the minds of many people who have not acquired the means for their expression. There are many persons possessing technical proficiency in music, drawing, or writing whose production is devoid of those innovative and informative values that distinguish an artist from a performer. This may be the result of a genetic deficiency or a deficiency in the environmental exposure necessary to the development of inherited potential. There is a large literature demonstrating a lack of structural development consequent to lack of requisite environmental stimulation. As Sperry4° put it,
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Many elements deeper in the brain centers must discharge only in very special activities, and, if these activities are not exercised-especially during maturational stages when the neurons seem to be particularly dependent on use-the neuron types involved may regress, leaving profound functional deficiencies in the integrative machinery."
We are accustomed to hear, these days, of the "culturally disadvantaged," those persons whose propositional potential has remained undeveloped for lack of proper schooling. There is likely a parallel lack of appositional development in persons whose education has narrowly emphasized reading, writing, and their concomitants. 6 Third, there must be the possibility of an unusually free communication between the two hemispheres, interrupting temporarily the ongoing state that has allowed independent processing. The conditions for a transient increase in callosal transmission have yet to be adequately explored. One problem is that inspiration occurs not only in repose but often in striving under pressure. In all likelihood the momentary suspension of hemispheric independence is contingent upon appropriate hemispheric affective states, perhaps the concurrence of one hemisphere's positive affect and active cognition with the other hemisphere's neutrality or receptivity.
SUMMARY A duality of mind is readily demonstrable in split-brain humans, and evidence is steadily accumulating that ongoing interhemispheric communication is incomplete in the intact brain. It is now certain that the corpus callosum can transfer high-level information from one hemisphere to another. When we take into account the well-established principle of hemispheric specialization, added to the impressive normality of split-brain humans in ordinary social situations, a physiologic explanation for at least some forms of creativity seems close at hand. What is required is a partial (and transiently reversible) hemispheric independence during which lateralized cognition can occur and is responsible for the dissociation of preparation from incubation. A momentary suspension of this partial independence could account for the illumination that precedes subsequent deliberate verification. From this point of view, we can understand better the opinion of Frederic Bremer, 13 who wrote years ago that the corpus callosum subserves "the highest and most elaborate activities of the brain" -in a word, creativity.
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