Orbach (1998, 1999) combines a memoir of his experiences as a graduate student of both Karl S. Lashley and Donald Hebb, with a critical review of the differences between Lashley's and Hebb's theoretical use of recurrent neural circuits. Orbach also reprints nine of Lashley's important papers and adds four previously unpublished lectures that Lashley delivered in 1952. Although flawed by stylistic difficulties and one major omission, the book provides a clear contrast between the early use of recurrent circuits by Lashley and the later approach taken by Hebb.
2. This explains what I liked best about Jack Orbach's (1998, 1999) book, "The Neuropsychological Theories of Lashley and Hebb." Orbach studied with both Lashley and Hebb, and the implicit subtitle of his book is: Memoirs of a student of Lashley and Hebb. Most of my coffee-break conversations during my first years at McGill were spent with Hebb and a small group of post-docs and graduate students. Orbach's memoir strikes a familiar note. He described his own masters' comprehensive exam, given on the spur of the moment in Hebb's office, when they both realised that Orbach hadn't completed the exam before leaving McGill to complete a doctorate at Princeton. After checking with two other senior members of the department, Hebb put his feet up, lit his pipe, and conducted the oral examination then and there, with questions about Lashley, Kohler, and Hull. This is so consistent with what I remember about Hebb that an image of Orbach sitting the exam could be constructed with absolutely no difficulty. I also remember Hebb's telling us about the reaction to "Organisation of Behaviour" (1949) when it first appeared, and then pulling out and showing us the same letter from a Jesuit priest that Orbach quotes in his book! Orbach has written a familiar and entertaining students-eye view of the man who supervised, by instruction and by example, my own professional apprenticeship.
3. Orbach writes in more detail about his time at the Yerkes Primate Laboratory, which was headed by Lashley, and which was also an important stage in Hebb's career. I cannot check his character sketch of Lashley and his description of life at Yerkes against my own experience, but they were lively and direct. Orbach brought the Yerkes lab to life, both by describing his own experiences and by quoting extensively from Hebb's reminiscences regarding work at Yerkes and his own relationship with Lashley.
4. But human interest is not why Orbach wrote this book. He tries to trace the evolution of neuropsychological theory based on the exploitation, by Lashley and Hebb, of the discovery by Ramon y Cajal and Lorente de No of recurrent neural circuits in the cortex. Orbach raises the question of priority between Lashley and Hebb in the use of the recurrent circuit finding. It is clear from Orbach's citations that Lashley first appreciated the significance of the recurrent circuit, but drew very different conclusions about it than did Hebb. Lashley used the recurrent circuit as a mechanism for sustaining waves of excitation through the neocortex. Postulating that there were recurrent circuits of varying lengths, and therefore, by analogy to tuned amplifier circuits of varying resonant frequencies, he proposed that the intermingled, multiple recurrent circuits would provide a means of generating complex, self-sustaining cortical waves of excitation of great complexity originating from many cortical and sub cortical locations. Lashley viewed the cortex as a complex receiver, capable of expressing activity from sense input and from lower centres as wave activity across the surface of the cortex, which could in turn influence motor output. It was on the wide but thin, recurrently interconnected structure of the cerebral cortex that Lashley postulated the sustained waves of neural activity that correspond to mind.
5. Publication dates alone show that Lashley is entitled to priority for developing the idea that recurrent circuits make it possible to sustain cortical activity in the absence of sensory input. But the citations and reprints also make clear that Lashley did not believe that experientially driven synaptic changes had anything to do with changes in the organisation of recurrent circuits. Lashley's papers, as cited or reprinted by Orbach, offer no explanation for how experience can permanently modify mental content carried by the waves of cortical activity that, according to Lashley, were sustained by recurrent circuits.
6. Orbach's book includes 130 pages of his own text, liberally enriched with quotes from Lashley, Hebb, and others, including many from Hebb's student and colleague Peter Milner. There are another 247 pages of reprinted papers by Lashley including 65 pages devoted to the four Vanuxem lectures delivered by Lashley at Princeton University in 1952, and published here for the first time. Unfortunately, the illustrations that accompanied the lectures did not survive, so they must be supplied by the imagination. Originally appearing three years after publication of "Organisation of Behaviour," these lectures present a comprehensive, post-Hebbian summary of Lashley's neuropsychological theory. Their publication is a valuable contribution to the history of neuropsychology.
7. As Orbach says in his introduction, many of Lashley's lectures read as if they could have been written by Hebb. Lashley discusses priming and set in what are almost cell-assembly terms. Orbach points out that Lashley's concept of the self-sustaining cortical nerve net is very much like the central autonomous process - the cell assembly - proposed by Hebb. Lashley's version contains no direct mechanism for altering synaptic connections through learning, which is a central principle of Hebb's theory. Lashley does propose that synaptic priming of neurons explains set, in words that paraphrase ideas expressed by Hebb in his own editions (1, 2 and 3) of the "Textbook of Psychology" (revised, Hebb & Donderi 1994). This clearly represents an independent convergence of views - but only so far. Lashley proposed a reduplication process in the cortex, whereby the spreading waves of excitation would reproduce themselves across the cortex due to repeated patterns of recurrent neural interaction. He did not propose a learning process to change the structure of the cortex, and therefore change the spatial and temporal characteristics of the interacting waves. Hebb did.
8. Since this book is supposed to define and relate the ultimate contributions of Lashley and Hebb, there is one very serious omission. Hebb published his "Essay on Mind" in 1980. He explained in this book the obstacles that stood in the way of developing his recurrent-circuit theory between 1945 and 1949, and how he overcame them. "Essay on Mind" makes clear what Hebb saw as the weaknesses of cell-assembly theory, and what refinements he thought were needed. This book is neither cited nor quoted by Orbach. At 75+, Hebb may not, as Orbach reported, have been willing to go on the lecture circuit as a superannuated celebrity like his respected colleague B. F. Skinner, but he was certainly capable of writing a realistic evaluation of his own contribution, a theoretical prospectus, and an interesting philosophical speculation on neuropsychology and psychology in general. Orbach's treatment would have been more complete if he had dealt with this late and under-appreciated book by Hebb.
9. The difference between Hebb's and Lashley's view of the cortical neural net emerges from a comparison of "Organisation of Behaviour" and "Essay on Mind," on the one hand, and the Vanuxem lectures, on the other. Hebb's original perceptual cell assemblies were created by the successive firing of neurons representing un-analysable perceptual units (Lashley's term) linked by eye movements, and Hebb thought that cortical neurons that triggered eye movements were involved as the assemblies were formed. Conceptually, Hebb's assemblies generated a small subset of self-sustaining cortical activity that was therefore more highly correlated over time than other concurrent cortical activity. The tool for creating assemblies was changing synaptic strength influenced by simultaneous firing in the pre- and post-synaptic excitatory neurons.
10. Lashley viewed the cortex as a continuous active surface, in which changing sensory input as well as input from previous cortical states was enough to determine the overall pattern of cortical activity. Lashley also provided for spatial and temporal reduplication of part or all of the complex cortical activity as a result of the cortical recurrent circuits. He attributed changes in cortical activity to the combined influence of changing sensory input and the previous state of cortical activity. Lashley's Vanuxem Lecture perspective is consistent with the approach he adopted and the phenomena he discussed in the other post-Hebbian papers reprinted in Orbach's volume. Neither theorist made use of cortical inhibitory interneurons; their existence was not proved at the time, although Hebb and his students recognised the importance of inhibitory neurons to cell-assembly theory as soon as the facts became well-established. Hebb discusses this in "Essay on Mind."
11. Computer-aided tomography, multiple-electrode EEG recording and magnetic resonance imaging have re-invigorated the study of cortical localisation, and have reduced the significance of some of Lashley's early experimentally based criticisms of localisation, which in 1952 still formed a major part of Lashley's Vanuxem lectures. Recent evidence demonstrates the almost immediate influence of sensory activity on synaptic growth, further strengthening the Hebbian postulate that neural circuits are directly modified by experience.
12. Nevertheless, Lashley formulated many interesting ideas about the relationship between brain and mind, and about the nature of mind, in the Vanuxem lectures. And, as Orbach suggested, if Hebb had read the lectures, he would probably have agreed with much of what Lashley said.
13. While presenting an interesting and on the whole scholarly discussion of the theoretical differences between Lashley and Hebb, reflected partly in what each man did or did not acknowledge in the work of the other, Orbach could not resist statements like "Hebb, in turn, must have been offended by the short shrift that he received from Lashley" (p. 62). Orbach tells us that he talked to Hebb and to many of Hebb's students, and that he read the memoirs and the literature; but none of this, including some verbatim quotations from Hebb, suggests that 'Hebb ... must have been offended ...'. Orbach thereby adds some highly dubious emotional spice to the intellectual pot, which is certainly rich enough not to require irrelevant spices of any kind. The tabloid asides are unverifiable, probably untrue, and they detract from the genuine differences in approach that Orbach should have stuck to.
14. When he comments on citations by Hebb or Lashley, Orbach spurns the scholarly footnote in favour of parenthetical italicised remarks that break the flow of the quotation and make it irritatingly difficult to concentrate on what either Hebb or Lashley was saying. The long quotations from Hebb and Lashley, as well as from Lashley's reprinted lectures and papers, are interesting in their own right. A footnote reference lets one know the author has something to say, but doesn't interrupt the narrative by shouting it in one's face. Orbach should have used footnotes.
15. Despite the annoying parenthetical style and the preoccupation with priority claims that both Hebb and Lashley were too busy to worry about, Orbach has made a useful contribution by publishing the Vanuxem lectures, by writing a memoir-cum-critical essay on life with Hebb and Lashley, and by giving the reader a chance to compare the developed neuropsychological concepts of Lashley with those of Hebb as expressed in "Organisation of Behaviour" and the first Textbooks. The book is weaker because Orbach ignored Hebb's "Essay on Mind." The final analysis of Hebb's contribution will not be complete without referring to it.
Hebb, D. O. (1949) The Organization of Behavior: a Neuropsychological Theory. New York: Wiley.
Hebb, D. O. (1980) Essay on Mind. Hillsdale, NJ: Erlbaum
Hebb, D. O. and Donderi, D.C. (1994) Textbook of Psychology, fourth edition, revised. Dubuque, Iowa: Kendall/Hunt Publishing Company.
Orbach, J. (1998) (Ed.) The Neuropsychological Theories of Lashley and Hebb. University Press of America
Orbach, J. (1999) Precis of: The Neuropsychological Theories of Lashley and Hebb. PSYCOLOQUY 10(23). ftp://ftp.princeton.edu/pub/harnad/Psycoloquy/1999.volume.10/ psyc.99.10.029.lashley-hebb.1.orbach http://www.cogsci.soton.ac.uk/cgi/psyc/newpsy?10.029