Beginning in the 1920s, K. S. Lashley startled psychologists with his theories of the memory trace within the cerebral cortex. Using terms such as terms mass action, equipotentiality, and sensory/motor equivalence, Lashley presented evidence that the engram is widely distributed in the brain, and that unactivated synapses, like activated ones, seem to show evidence of learning. His research and nativistic theories made him world famous by 1929, when he was just 39. He spent his professional career searching for a mechanism for the reduplication of the engram. While his contemporaries tried to specify the locus of the engram in the brain, Lashley found it everywhere. He liked to quip that the problem is not to find where the trace is located, but where it is not. Lashley's student, D. O. Hebb, published his empiricistic theories in 1949, in "The Organization of Behavior," and the monograph created a sensation. Hebb used Lorente de No's reverberatory circuit to provide a mechanism to maintain activity in the cerebral cortex after the stimulus terminated, the so-called central autonomous process. This led him to the cell assembly, a complex reverberatory circuit that could be assembled by experience. Changes in resistance at the synapse with learning came to be called the Hebb synapse. That monograph was highly praised for the breadth of its treatment. The present book documents how Lashley anticipated Hebb's introduction of the reverberatory circuit by some 12 years. Lashley's Vanuxem Lectures of 1952 are printed for the first time, together with nine of his previously published theoretical papers. Lashley's and Hebb's theories are reviewed and reevaluated fifty years after publication of Hebb's monograph, and a systematic effort is made to compare and contrast the views of teacher and student.
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AUTHOR'S RATIONALE FOR SOLICITING COMMENTARY
My rationale for seeking open peer commentary is primarily that the book says some things about both Lashley and Hebb that some peers might find controversial and startling if not downright outrageous. To get these views out in the open may be of pedagogical value not only to to me but to the neuropsychological community at large. Obviously, I don't believe that my arguments are wrong or weak. But the feedback I get might conceivably persuade me to rethink the matter.
1. Part 1 of the book opens with a summary of Lashley's last public lecture given at the University of Rochester in 1957, one year before his death and eight years after the publication of Hebb's monograph. In this lecture, Lashley was still consumed with the notion of irradiating waves of excitation in the cerebral cortex, a notion he developed in detail in 1942. In citing theories of stimulus equivalence, Lashley wrote 'That of Hebb is most in accord with conditioned reflex theory. He assumes that multiple paths are developed by learning. Such learning is ruled out by a mass of evidence for innate discriminations and equivalencies.' In this unpublished address, Lashley cited Hebb's empiricistic theory for the first and only time. He never cited the monograph itself in the literature.
2. An early chapter entitled 'Setting the Stage' offers another look at Lashley's early critique of the native Watsonian connectionism of his day. Lashley's early efforts to revise and revitalize neuropsychological theory are reviewed. The problem, Lashley suggested in the 1920s, was the omission of the brain from the Watsonian S-R formula. And when a model of cortical function was finally introduced, using the analogy of the telephone switchboard, it was based on the idea of linear reflex activity in the spinal cord, as suggested by Dewey, leaving no room for psychological categories that require sustained activity in the brain such as thought, memory, emotion, motivation, selective attention and the like. And then, along came Pavlov who undercut all contemporary speculations of psychologists with his physiological theories of conditioned reflexes and brain function. It was at this point that Lashley burst upon the scene.
3. Hebb must have experienced an epiphany when he was introduced to the reverberatory circuit of Lorente de N. He realized that this anatomical curiosity provided him with a mechanism for the autonomous central process that he developed so masterfully in the 1949 monograph. Hebb's revelation involving the reverberatory circuit was especially important for it gave neurological meaning to the earlier proposals of central motive state of Morgan and central excitatory mechanism of Beach as well as the putative reduplicated memory trace of Lashley. However, Lashley had already appropriated the reverberatory circuit for neuropsychological theory in 1937, some 12 years before Hebb's monograph was published and some three years before its presentation by Hilgard and Marquis in their Conditioning and Learning of 1940. This is documented with excerpts from Lashley's papers published in 1937, 1938, 1941, 1942 and 1949. The latter two papers are republished in their entirety in this volume.
4. The next chapter deals with the learning theory that synaptic resistance is reduced by the passage of the nerve impulse. Lashley's 1924 assault on this theory is reviewed in detail. (This 1924 paper is also reprinted in this volume.) In Lashley's own words, 'Among the many unsubstantiated beliefs concerning the physiology of the learning process, none is more widely prevalent than the doctrine that the passage of the nerve impulse through the synapse somehow reduces synaptic resistance and leads to the fixation of a new habit . . . but no direct evidence for synaptic resistance has ever been obtained. The hypothesis is not based upon neurological data but is merely a restatement of the observed fact that increased efficiency follows repeated performance . . . Familiar learning curves are obviously an expression of these successive integrations and we have no knowledge of the conditions prevailing in formation of a new simple neural integration. (On the other hand,) the instantaneous character of simpler associations in man . . . suggests that . . . a single performance serves to fix the habit. Even if this were the case for every simple reintegration within the nervous system, we should still get the appearance of gradual improvement through practice because of the formation of many simple associations . . . The fact of gradual improvement in complex functions cannot therefore be taken as evidence for a gradual wearing down of synaptic resistance by repeated passage of the nerve impulse' (Lashley, 1924). The reemergence of this theory in Hebb's monograph as a neuropsychological postulate is documented and evaluated. In the fourth edition of Hebb's Textbook (1994), Donderi refers to the postulate as Hebb's rule. Today, it is frequently referred to as the Hebb synapse.
5. Next, 'Lashley's Memory Mechanisms', considers:
i. Lashley's view that the memory trace is reduplicated in the cerebral cortex and the implications of that view on the interpretation of cerebral localization. In 1952, Lashley wrote 'I have never been able by any operation on the brain to destroy a specific memory' even when the memory is elicited by electrical stimulation of the part of the cerebral cortex that is subsequently removed.
ii. Lashley's early introduction of the reverberatory circuit in neuropsychological theory is documented. It is important to note that Lashley never abandoned the principle of synaptic transmission in favor of a cortical field theory, as had been alleged by Hebb and others. This claim is fully documented.
iii. Lashley assumed throughout his career that memory is a unitary function. He was of course aware of the distinction between long and short term memory but he never referred to the modern distinction between storage and retrieval. Nor did he ever consider associative and working memory as distinct forms of memory when he searched for the engram in the cerebral cortex.
iv. Lashley's position on the continuity-discontinuity debate is reviewed as well as his championing the concept of instinct at a time when the concept was falling into disfavor in America. In 1950, Lashley championed the European ethologists' views of fixed action patterns though he himself preferred the term instinct. His article on instinct in the Encyclopaedia Britannica of 1956 is especially noteworthy.
6. The next chapter, entitled 'Issues of Priority, Opinion and Criticism', includes:
i. a reconsideration of Lashley's obsession with theoretical criticism in his later years, as alleged by Hebb.
ii. an interpretation of the meaning of Lashley's refusal of Hebb's offer to coauthor the 1949 volume with him.
iii. the history of the reverberatory circuit in the psychological literature, and questions of priority as far as the integration of the reverberatory circuit into neuropsychological theory is concerned.
iv. the fact that Lashley failed to acknowledge data and theory that were unfavorable to his views, during his search for the engram. This is documented.
v. Lashley's opinion of Hebb's theories was never known because Lashley hardly ever spoke of them. But Lashley's 1947 review of Hebb's manuscript-in-progress is revealing in this regard. Revealing as well is Lashley's letter of congratulations to Hebb after publication of his 1949 monograph.
Finally, the personal relationship of Lashley, the teacher, and Hebb, the student, is delineated.
7. The next chapter, titled 'Hebb's The Organization of Behavior 50 Years After Publication', offers a contemporary view of Hebb's enduring contributions to neuropsychological theory. Hebb bolstered his theories with the following facts:
i. adults seem to be able to sustain brain injury with fewer permanent consequences than can infants and children;
ii. learning in children is much more difficult compared to similar learning in adults.
Hebb further argued that:
iii. distinctions should be made between primitive unity, non-sensory figure and identity in perception;
iv. stimulus equivalence and generalization are learned in early life;
v. the ratio of association cortex to sensory cortex should be considered in phylogenetic development;
vi. the evidence of post-tetanic potentiation supports the importance of the Hebb synapse in learning (this phenomenon was described after the 1949 monograph was published but it found its way into Hebb's later writings);
vii. there is a distinction between intelligence A (innate potential) and intelligence B (achievement);
viii. following Tolman, Hebb introduced a new way of thinking about neuropsychological problems in his 1960 presidential address to APA. That discourse is today named cognitive psychology;
ix. later research on the stabilized retinal image supported cell assembly theory.
8. The Left and Right Cerebral Hemispheres reviews the case of Alex, a nine year old boy whose left hemisphere was removed for the relief of intractable epileptic seizures. Though he never learned to speak before surgery, Alex began to show remarkable gains in speech and language and in cognitive skills in general. Alex's postoperative achievements challenge the widely held view, shared by Hebb, that early childhood is a particularly critical period for the acquisition of cognitive skills. It must be concluded that clearly articulated, well-structured, and appropriate language can be acquired for the first time as late as nine years of age with the right hemisphere alone. Hebb and Lashley did no live to see this case. My guess is that Hebb would have had great difficulty in explaining Alex's achievements, but Lashley would have chuckled and muttered in so many words, 'You see, not only do you have reduplication of the memory trace within a hemisphere but also between hemispheres.'
9. The next chapter is entitled, 'A Comparison of Lashley and Hebb on the Concepts of Attention and Stimulus Generalization'. On the concept of attention, Lashley took off from his observations of attempted solutions in rats during the learning of the maze. It was Spence's concession on this matter that persuaded Lashley that he had bested the neo-behaviorists on the continuity-discontinuity debate. In the 1942 paper (reprinted in this volume), Lashley argued that a pattern of excitation in the cortex in the form of a reverberatory circuit may form a relatively stable and permanent foundation, modifying the effects of later stimulation, as attention determines the selection of stimuli. These ideas precede Hebb's formulation of the central autonomous process by some seven years. And then in the Vanuxem Lectures of 1952, Lashley went way beyond Hebb when he introduced the ideas of a priming or pre-setting of circuits based upon the spacing of the end-feet on the post-synaptic cell. Hebb was by far the more accomplished writer and so, with the publication of his monograph in 1949, he captured the attention of the neuropsychological community with ideas that did not differ substantially from Lashley's.
10. However, on the matter of stimulus generalization their positions were radically different. Lashley's position is nativistic stimulus generalization, if it exists at all, is built into the organism. His conception derived from his critique of the neo-Pavlovian view of a gradient in stimulus similarity underlying stimulus generalization. His discontinuity position on learning led him to write in 1946 'Stimulus generalization is generalization only in the sense of failure to note distinguishing charateristics of the stimulus or to associate them with the conditioned reaction. A definite attribute of the stimulus is abstracted and forms the basis of reaction; other attributes are either not sensed or are disregarded. So long as the effective attribute is present, the reaction is elicited as an all-or-none function of that attribute. Other characteristics of the stimulus may be radically changed without affecting the reaction' (Lashley and Wade, 1946). The neo-Pavlovian gradient of similarity on a stimulus continuum is an artifact of inattention. Such a stimulus generalization is generalization by default.
11. Hebb's concept of stimulus generalization was developed in connection with his delineation of the formation of the cell assembly underlying the perception of a triangle. Hebb's contribution was to perceptual theory. He proposed the startling idea that a simple figure like an outline triangle is not perceived as a whole, innately, as alleged by the gestaltists. He went on to show how the elements of line and angle become integrated into a unified perception of a triangle. To persuade the skeptical reader, Hebb introduced the idea of perceptual identity, something that has to be learned. He then proposed a mechanism involving neural fractionation and recruitment. Fractionation eliminates the variable cells that are excited extramacularly. Macular excitation, which is due to ocular fixation, remains constant despite the variable angular size of the stimulus object.
12. In short, stimulus generalization emerges secondarily from the slow development of each complex concept. And yet, there is some doubt regarding the universality of stimulus generalization according to Hebb. His theory cannot always predict stimulus generalization from the learning of a simple discrimination. Take for example the learning to discriminate a vertical from a horizontal line. In this case, the stimuli belong to the category of primitive unity for which, unlike the triangle, no learning is required, according to Hebb, to build a unified percept. Nevertheless, our best guess is that, empirically, after the initial learning to discriminate the two lines, the organism would show stimulus generalization to a vertical rectangle vs. a horizontal rectangle and even to a vertical row of circles vs. a horizontal row of circles. Since there is no initial learning to build a unified perception of the vertical and horizontal lines, it is difficult to see how Hebb would derive the empirical data of stimulus generalization in this case.
13. A late chapter of commentary is entitled, 'Lashley's Enduring Legacy to Neuropsychological Theory'. A contemporary perspective is offered in reviewing the concepts of vicarious functioning, equipotentiality, reduplicated memory trace, the reverberatory circuit. Lashley's lesson that synapses inactive during learning can show the effects of learning is emphasized. Lashley's lesson was never acknowledged by Hebb or any of his students. Lashley derided the use of wiring diagrams in neuropsychological theory especially those derived from computer technology. Neurons are live metabolizing cells, he argued, not inert paths like copper wires. They interact at synapses, which are not solder joints like soldered copper wires. The synaptic contacts are variable. Furthermore, synaptic contacts may be excitatory and/or inhibitory. Soldered wires are always excitatory and fixed. Both are pathways to be sure but the differences between neurons and copper wires far outnumber their similarities. Thus brain organization cannot be modelled by circuit diagrams representing inert pathways.
14. An epilogue presents a number of personal vignettes of both Lashley and Hebb. Lashley's career was reviewed earlier in some detail in Orbach (1982). The most disturbing part of this story has to do with Lashley's racism, as alleged by Weidman (1996). I would not have raised this matter in a scholarly volume concerned with Lashley's contributions to neuropsychological theory were it not for Weidman's allegation that Lashley's racist attitudes influenced his theoretical views. But, did these odious attitudes of Lashley affect his science? I can attest to Lashley's anti-African-American attitudes, but I can find no evidence that Lashley's racism colored his theories. A lifelong student of genetics, Lashley had an abiding interest in the concept of instinct and in the genetics of behavior in general. These facts must have eluded Weidman. Both Hebb and Lashley were honored many times during their lifetimes. It is especially noteworthy that Hebb was appointed Chancellor of McGill University, and that he was nominated, in 1965, for the Nobel Prize.
15. During his freshman year, at the age of 16, Lashley studied general zoology and comparative anatomy with Albert M. Reese at the University of West Virginia. Reese appointed him departmental assistant, at a salary of $0.25 per hour. One of the new assistant's first tasks was to sort out various materials in the basement. The result of this assignment can best be expressed in Lashley's own words: 'Among them I found a beautiful Golgi series of the frog brain. I took these to Reese and proposed that I draw all of the connections between the cells. Then we would know how the frog worked (sic!). It was a shock to learn that the Golgi method does not stain all cells, but I think almost ever since I have been trying to trace those connections' (Beach in Orbach, 1982). Only later did Lashley realize that functional variables such as spatial and temporal summation, excitatory and inhibitory states, and micro-movements of elements influencing synaptic contact need not be represented microscopically. The lesson is that neurons are not inert and static, like soldered wires. They are live metabolizing cells with synaptic contacts that vary. If Lashley were alive today, there is no doubt that he would continue to scold modern neuroscientists who still have not become aware of the importance of this fact.
16. Part 2 of the book consists of nine of Lashley's major theoretical papers reprinted in their entirety. These are listed in the References below. Part 2 also includes Lashley's four Vanuxem Lectures given at Princeton University in 1952, and published here for the first time. In these lectures, Lashley referred to the anatomical observations of Lorente de N and emphasized the neural net as the active neural unit in the cerebral cortex. He introduced the idea of a neural priming or presetting, concepts all highly reminiscent of Hebb's theorizing on the central autonomous process and the cell assembly. The term neural lattice was coined by Lashley in 1949. This term was discarded by Hebb in his 1949 monograph in favor of cell assembly.
http://www.wabash.edu/depart/psych/Courses/Psych_81/LASHLEY.HTM http://www.archives.mcgill.ca/guide/volume2/gen01.htm#HEBB, DONALD OLDING http://www.princeton.edu/~harnad/hebb.html http://www.cogsci.soton.ac.uk/bbs/Archive/bbs.amit.html
Hebb, D. O. (1949) The Organization of Behavior: a Neuropsychological Theory. New York: Wiley.
Hebb, D. O. and Donderi, D.C. (1994) Textbook of Psychology, fourth edition, revised. Dubuque, Iowa: Kendall/Hunt Publishing Company.
Hilgard, E. R. and Marquis, D. G. (1940) Conditioning and Learning, NY: Appleton-Century.
Lashley, K. S. (1924) 'Studies of cerebral function in learning. VI. The theory that synaptic resistance is reduced by the passage of the nerve impulse.' Psychol. Rev., 31, 369-375.
Lashley, K. S. (1931) 'Mass action in cerebral function.' Science 73, 245-254.
Lashley, K. S. 'The problem of cerebral organization in vision.' Biol. Symp, 1942, 7, 301-322.
Lashley, K. S. (1949) 'Persistent problems in the evolution of mind.' Quart. Rev. Biol., 24, 28-42.
Lashley, K. S. (1950) 'In search of the engram.' In Symp. Soc. Exp. Biol. No. 4, Cambridge, Eng.,: Cambridge Univ. Press.
Lashley, K. S. (1951) 'The problem of serial order in behavior.' In Jeffress, L. A. (Ed.) Cerebral mechanisms in behavior, New York, Wiley.
Lashley, K. S. (1952) Vanuxem Lectures delivered at Princeton University in Feb. 1952. Untitled.
Lashley, K. S. (1954) 'Dynamic processes in perception.' In Adrian, E. D. Bremer, F. and Jasper, H. H. (Eds.) Brain Mechanisms and Consciousness. Illinois, Charles C. Thomas, 422-443.
Lashley, K. S. (1968) 'Cerebral organization and behavior.' In The Brain and Human Behavior, Proc. Ass. Res. Nerv. Ment. Dis., 36, 1-18.
Lashley, K. S. and Wade, M. (1946) 'The Pavlovian theory of generalization.' Psychol. Rev., 53, 72-87.
Lashley, K. S., Chow, K.-L, and Semmes, J., (1951) 'An examination of the electrical field theory of cerebral integration.' Psychol. Rev., 58, 123-136.
Orbach, J. (1982) Neuropsychology After Lashley: Fifty Years Since the Publication of Brain Mechanisms and Intelligence. Hillsdale, NJ.: Lawrence Erlbaum Associates.
Weidman, N. (1996) 'Psychobiology, progressivism, and the anti-progressive tradition.' J. Hist. Biol, 29, 267-308.