Brain mechanisms and learning

ELECTROENCEEHALOGRAPHY AND CLINICAL NEHROPHYSIOLOGY

BOOK

591

REVIEWS

Edited by CHARLES E . HENRY

Institute of Living, Hartford, Conn. (U.S.A.)

Brain mechanisms and learning. - - A. Fessard, R. W. Gerard and J. Konorski, Consulting Editors (C. C. Thomas, Springfield, Ill., 1961, 702 pp. $15.00). Everyone is suffering from a surfeit of symposia, but this volume has less chaff and more solid food for thought than most synthetic aggregates can provide; it is a worthy successor to the Laurentian Symposium on Brain Mechanisms and Consciousness held six years earlier. Inevitably, with thirty one fluent and original participants from thirteen countries the range of subjects and the diversity of approach are bewildering, and in the latent period between discussion and publication some confident assertions have already been relegated to the waste basket. On the other hand, very little of significant novelty has been gained in that time, so this volume must represent a fair approximation to current human knowledge about the brain as the organ of dynamic adaptation. The 64,000 word question is - - whose brain? Are we justified in assuming that learning, however defined, is mediated by similar mechanisms in all organisms? Or must we first identify and classify the basic processes in each individual or species or genus or phylum and then sort out what is common to all and peculiar to each? As a corollary, does ontogeny recapitulate phylogeny in nervous function as we used to say it does in structure? This would be too ingenuous - - yet we must frame some general hypothesis if the whole fabric of higher nervous function is not to crumble into academic rubble before our eyes. Gerard, with his habitual eloquence and clarity, poses the key questions in his opening contribution: "How does a s y s t e m . . , with an enduring architecture interact with the environment to give a new enduring a r c h i t e c t u r e ? . . . When does a reversible c h a n g e . . . become irreversible? What is the mechanism of the o p e r a t i o n . . , how is the change given an adaptive (or a non-adaptive) direction? How is the adapted individual s e l e c t e d ? . . . What is the mechanism of cumulative racial change?" Clearly nothing human is alien to this enquiry - yet this collection of papers includes only two of directly human relevance. Hebb reports on his studies of the nature of the trace in short-term memory for sets of digits when the same series was inserted at every third trial; he infers that the slate is not wiped clean between presentations, and that there-

fore some cumulative structural modification may develop in the brain even on a short-term basis, and without re-inforcement. His argument is not very convincing, however, for as far as we know a dynamic trace might also persist through subsequent stimuli - - both the dynamic and structural mechanisms are so conjectural that we can assign them almost any properties we please on a verbal level. As Gerard points out in his comment on Hebb, there are too many jumps - - from total behaviour to organ to cell to molecule, and these conceptual jumps disturb the course of appreciation almost as much as the transitions from worms to tits to rats to dogs to cats and primates as one turns from one contribution to another. Garcfa-Austt and his colleagues describe how the significance of the photic stimulus determines the characters of the evoked responses in human subjects - - but provide no details of the number or diversity of the subjects themselves. There is still too little mutual aid between physiological and psychological teams. Perhaps the least confusing way to approach this volume is a posteriori, by the General Discussion at the end, where four main topics are considered by free association: Inborn and reflex behaviour, Spontaneous behaviour patterns, Location of plastic change and Nature of the change. Here, of course, enlightenment is by aphorism rather than by axiom but an impression of personal standpoints and disciplinary bias is an essential preliminary to perusal of the main sections, where authorship is often veiled in the habitual scientese. There is really a great deal to be ~aid for a reversion to frankly first-personal accounts of observations in this strange borderland of science and philosophy. Let me follow my own advice: I find the personal statements in the Discussion sections more helpful than the carefully impersonal descriptions in the main contributions. I think this is because here at least some of the remarks and notions are accounted for quickly and clearly in a simple context, whereas in the prepared papers the experimental conditions (for example) are often described as if they were the outcome of natural law rather than the whim or hunch of the experimenter. This seems to me one of the greatest specific difficulties in following and contributing to research on Learning - - apart from those I have referred to already, the mysteries

Electroenceph. clin. Neurophysiol., 1962, 14:591-596

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of phylogenesis and ontogenesis, which are general problems in biology. However Learning is defined, I think it must include some sort of development or change with respect to time, and must also involve notions of association or contingency. The association may be a simple logical one - - as between different stimuli or environmental events or between single stimuli and single responses, or - - alas more often and nearly always in natural life - - complex relations between sets of "signals" of some sort and sets of "actions" which may or may not be such as to influence the subsequent signals. This may seem a childishly naif view or perhaps merely trite, but the implications are quite alarming when one considers how to co-ordinate one's thinking about observations and hypotheses such as arc described in this volume. Disregarding the diversity of recording and analysing techniques, the range of animal species and the individual variations, considering only the experimental situations set up by the experimenters, no two have operated in comparable conditions. The sensory stimuli most favoured were visual, auditory and tactile and the subjects or preparations were able to respond either not at all or in an arbitrary fashion or by operant control (that is, so as to obtain a reward or avoid a penalty connected with a stimulus). With three stimulus modalities, any two of which may be associated in sequence, and three possibilities of response (including zero) there seem to be fiftyfour distinct situations, varying from presentation of a single isolated stimulus with no possible effector response, to all the permutations and combinations of paired stimuli to one of which the organism can "learn" to perform an appropriate operant action. The first, single, situation is the elementary, classical physiological one in which the stimulus is intended merely as a means of studying some elementary intrinsic property of the nervous system, and the range of combinations includes all the classical conditioning arrangements. With these fifty-four basic situations (all feasible experimentally and possible as everyday experiences, too) there are quite innumerable sequences of situations of which the familiar conditioning procedure is one (deprivation - - motivation, novelty - - orientation, habituation, association, conditioning, extinction, etc.). So it is not surprising that, with such a rich variety of situations to choose from, no two experimenters have yet chosen exactly the same arrangement. This would matter little if there were general agreement about the basic phenomena, but the only point of complete accord is that conditioning depends on the conditions, so there are likely to be as many descriptions of conditioning as there are sets of conditions. In this case the search for contingent invariants, that is, the Natural Laws of Learning, is likely to remain a snark-hunt - - at least until we have reached a working agreement about what experimental situations are most realistic. The importance of specifying the external situation in detail is often overlooked even when the internal

conditions are particularly carefully controlled, as in mutilation and ablation experiments. Several contributors mention that apparently quite trivial changes in methods of testing for response may profoundly modify the results, even in animals with quite extensive cerebral lesions. Of course, all these warnings are much more easily uttered than heeded. It is si nple to remember to squeeze the bottom of the to, 4h-paste tube but very hard not to squeeze the middle. This sort of difficulty may be quite close to the heart of the problem, like the dog that didn't i~ark in the nighttime. Experimental biologists are nured to paradox - - non-linear complex systems are bound to be surprising if one is looking for logic m d proportion - but the traditional attitude to t'~oblems of brain function may have to turn a fex, somersaults yet. From the papers in this volume, and the more recent works of the contributors, ~ looks as though the learning brain is busy rubbing o~Lt inborn gibberish rather than inscribing the wisdom of experience. The nervous system may start whet~ Prometheus ended, with d~v~lpl01aov Ku~t&'rc0v y~Xo=~aa - - the countless chuckles of the waves, r a t h ~ than the tabula rasa of less sophisticated speculatc .~. The printing and figure repro,~uction are excellent, marred only by several literal ~:rrors, particularly in the discussions (which are alw~¢s hard work for the editor), some massive transpositions in the Bibliography under the letter P, and ~ quite inadequate index. W. GaEY WALTER Burden Ne~rologieal Institute

Die elektroeneephaiograpisehe Lichtreizantwort der mensliehen Illrnrlnde (The eleetroeneephalographie response to light of the human cerebral cortex). Leodegar Cig'finek (Verlag der Slo~aklsehen Akademie der Wissensehaften, Bratislava, P)61, 152 pp., Kes 21.30). This monograph contains a careh~l study of cortical responses recorded from the scalp in normal humans following brief intense light flashes from a stroboscope. The author has used a slightly modified superimposition technique according to Dawson (1947). In the occipital region a con,plex response was recorded, consisting mainly of three components. The first, with a latency of about 28 msec, was identified as the primary specific visual response from area 17. A second response, which followed the primary one, could also be elicited by other modalities of stimulation and was therefore identified as the unspecific one. It decreased with increasing 1requency of stimulation. While the primary responst was only affected to a minor degree by sleep and b~ administration of barbiturates, the secondary response increased in amplitude and in duration. Following the secondary response a repetitive after-discharge was often recorded which was assumed to be caused by activity in corticothalamic pathways. Electroenceph. clin. Neurophysiol. 1962, 14:591-596