Karl Lashley Poses a Research Agenda The Lesion Technique
-Lashley made extensive use of the technique of ablation, where specific areas of the nervous system are destroyed by means of a surgical lesion. The basic goal of the ablation technique is to determine which behavior is impaired or destroyed following a punctate lesion, and thereby to infer which functions are typically served by that region of the brain.
-He strongly questioned the significance of specific neural zones and connections: it is very doubtful that the same neurons or synapses are involved even in two similar reactions for the same stimulus. The mechanisms of integration are to be sought in the dynamic relations among the parts of the nervous system rather than in details of structural differentiation.
-Lashley was calling into question localization, the belief that specific behavior resides in specific neural locations. At the same time, if less explicitly, he was also posing difficulties for reductionism, the scientific program that seeks to explain behavior entirely in terms of neural (or other lower-order) principles.
Equipotentiality and Engrams
-Lashley was attracted to the principal ideas of Gestalt psychology: perhaps the brain works as an integrated unit, responding as an organized totality to complex patterns of stimulation.
-He spoke of equipotentiality, the capacity of any part of a functional area to carry out a particular behavior. Impairment in performance is due not to the site of the injury, but rather to the amount of tissue destroyed. All of the cells of the brain are constantly active and are participating, by a sort of algebraic summation, in every activity. There are no special cells reserved for special memories. He pondered also the property of plasticity, the potential for remaining areas of the nervous system to take over when a specific region has been damaged.
-Lashley concluded that we would never find the engram, the discrete representation in the nervous system of specific ideas, concepts, or behaviors. During learning, information comes to be represented widely within large regions of the brain, if not throughout the brain as a whole. Whether the cells can be mobilized to carry out an impaired function depends upon the percentage of them still remaining after brain injury, the degree to which the pattern of behavior has been mastered beforehand, and the strength of motivation of the animal.
-Lashley he helped to cast doubt on the reflect arc, the bond whereby each response is triggered by a specific stimulus, which had been the principal neural model of behavior in higher (as well as lower) organisms.
Lashley's Iconoclasm
-Lashley demonstrated that many sequences of behavior exhibited long planning units which unfold too quickly for them to be altered or corrected “live”. In his view, it was necessary to reconceptualize current associationist models of the nervous system to allow for effects that can be manifest for a significant period after initial stimulation. No simple stimulus-response bonds can explain this behavior: one needs a model of the nervous system which is hierarchically arranged and features feedback and feed-forward mechanisms.
-Lashley has criticized the comparison between the brain ant the computer. The neuron, like a switch or valve, either does or does not complete a circuit. But at that point the similarity ends. The switch in the digital computer is constant in its effect, and its effect is large in proportion to the total output of the machine. The effect produced by the neuron varies with its recovery from [the] refractory phase and with its metabolic state. The number of neurons involved in any action runs into millions so that the influence of any one is negligible. Any cell in the system can be dispensed with. The brain is an analogical machine, not digital.
-Lashley described behavior that eluded current mechanistic models and strongly implied more abstract and hierarchically organized forms of representation. He did not call for explanations on the mentalistic level, but his work, and his talk of "plans" and "structures", cleared the way for Simon's belief in a symbol system, Piaget's call for mental operations, Miller's TOTE system, and Chomsky's resorting to rules and representations.
-The nervous system turns out to be far more specific, far less equipotential than Lashley had contended. His belief that the brain works in a Gestalt-like fashion would find few adherents today.
-It has been held by many scientists, especially neuroscientists, that the optimal way to account for human behavior and thoughts is in terms of the structure and functioning of the human nervous system. To some investigators, this neuroanatomical account can complement accounts proffered in psychological or behavioral language; but for others, neuroanatomical accounts may eventually render unnecessary accounts in terms of representations, or symbols, or other psychological argot. In the view of this latter reductionist group, cognitive science emerges as, at best, a temporary account of mental activity destined to vanish once an account in terms of synapses can be attained. The debate about the possibility and the desirability of reductionism lurks in the background in any account of neuroscientific work.
How Specific Is Neural Functioning?
Evidence for Localization
-Toward the end of the nineteenth century, the work of Fritsch, Hitzig, Ferrier and Broca made the pendulum began to swing from Flourens’s holism toward Gall’s localizationism.
It was a heyday for scholars of a localizationist persuasion. With increasingly sophisticated methods for testing animals, claims were made for specificity in each region of the cortex.
As additional case studies of brain-injured patients accumulated, claims about the astonishing specificity of certain cognitive deficits were forthcoming.
The Resurgence of Holism
-Against Broca, Pierre Marie showed that the third frontal convolution plays no special role in the function of language, and that each of Broca's patients had far more extensive lesions than Broca had reported, and the range of accompanying deficits had not been documented with sufficient precision. Within a few years, a variety of neurologists had endorsed his claims that cognitive functions are not highly localized in the nervous system. They adduced evidence that the same kinds of deficits could be obtained from individuals with lesions in a wide variety of areas; and conversely, patients with similar anatomical lesions often exhibited contrasting sets of deficits or even at times no deficits at all. They spoke of the plasticity of the nervous system, the capacity of uninjured areas to take over from injured areas, and the loss of abstract thinking and other functions as a consequence of the size, rather than the site, of lesion.
-Holists were far more sympathetic to the notion that behavior could not be explained satisfactorily in terms of neural circuitry. As they saw it, there was a continued need for explanation on the psychological level. There was a correlation between skepticism about localization and skepticism about reductionism –not a logically necessary association, to be sure, but a meeting of two ideas in the minds of many scientists.
-Localization of symptoms did not signify localization of function. A specific section of the brain may be necessary for a specific function, but there may be other sections that are also necessary for that function.
Evaluating the Evidence
-By the end of the 1940s, many investigators were seeking some rapprochement between the rabid holists and the extreme localizers. Claims about highly specific syndromes following highly specific lesions could not be maintained; the variation across patterns, and across clinics, was simply too great. Besides, any number of lines of investigation undercut the extreme holist position.
Donald Hebb's Bold Synthesis
-Hebb argued that behavioral patterns, such as visual perceptions, are built up gradually over long periods of time through the connection of particular sets of cells, which he called cell assemblies. To this extent, behaviors or percepts can indeed be localized in specific regions. However, with time, more complex behaviors come to be formed out of sets of cell assemblies, which he called phase sequences. These phase sequences are less localized, and involve much larger sets of cells drawn from disparate sections of the nervous system. A phase sequence inevitably involves some equipotentiality. Finally, by the time the organism has reached maturity and is capable of performing the most complex forms of behavior, it is difficult to attribute any behavior to a discrete set of neurons in a delineated region of the brain. It would be an oversimplification to see the developmental course as proceeding from localizing to holism; for, in other respects, the sequence is exactly the reverse. A beneficial effect of Hebb's work was to point up these various complexities and competing tendencies, making it less plausible for anyone to adopt a rigid localizing or an inflexible holist position.
The Hixon Symposium Revisited
-During the Hixon Symposium, the major neurologists, neurophysiologists, and neuropsychologists debated with one another the tenability of the localization position.
Hubel and Wiesel's Decisive Demonstrations
-In the late 1950s, David Hubel and Torsten Wiesel began to record with microelectrodes impulses from single cells in the cortex of cats and other animals. They demonstrated beyond any doubt that specific cells in the visual cortex respond to specific forms of information in the environment. They also demonstrated the critical role played by certain early experiences in the development of the nervous system. For those in sympathy with specificity and localization of function, the last few decades have yielded much confirming evidence.
The Molar Perspective Sperry on Split Brains
-Because they were suffering from intractable epilepsy, a small group of patients were subjected to a surgical intervention where the two halves of the brain were disconnected from one another. Sperry devised methods for testing separately the two halves of the brain.
He documented important differences in the functioning of two hemispheres. Sperry reinforced the impression that the left hemisphere is dominant for language and other conceptual and classificatory functions, while the right hemisphere assumes a dominant role for spatial functions and for other fine-grained forms of discrimination. However, the Sperry team was able to show some holistic conclusions: the right hemisphere of right-handed persons was capable of far more linguistic functioning than had hitherto been
thought. Besides, the younger they had been at the time of their operation, the more likely it was that patients would reveal well-developed capacities in both hemispheres.
Gradients of Plasticity and Hierarchy of Functions
-Sperry's results suggest that the degree of plasticity depends of the earliness of the injury.
Other factors also influence the degree of plasticity: for example, younger individuals who are left-handed or who have sustained some brain injury early in life also exhibit more plasticity than those who exhibit contrasting traits.
-With development, different nervous centers gain dominance, and the hierarchy among behavioral functions alters. For instance, in young children, sensory regions are dominant;
but in older individuals, the association cortexes and the "planning regions" of the frontal lobes become ascendant. According to Luria's studies, no function is carried out fully by a specific region, but nor is it the case, as Lashley implied, that all regions figure equally in a specific function. Rather, several anatomical regions may figure in the performance of a particular behavior, but each of them makes a characteristic and irreplaceable contribution.
-While, in general, it is preferable to sustain an injury to the brain early rather than late in life, and to exploit the plasticity of that developmental stage, early is not always better.
There are at least three caveats. First of all, sometimes an early injury manifests no apparent deficits at the time but produces severe deficits later in life. Second, when another area of the brain takes over, the "rescuer" may well sacrifice the potential for carrying out its own preordained functions. Finally, even when another area of the brain assumes a function, it may not do so in an optimal way.
-Other considerations militate against a purely "plastic" perspective. Work in experimental psychology documents that organisms are "prepared" to master certain behaviors and
"counterprepared" to learn other ones.
The Neural Base of Cognition: Studies of Two Systems
-It is now conceded that, at least at the level of sensory processing, the nervous system is specifically constructed to respond to certain kinds of information in certain kinds of ways.
There is also evidence for "neural commitment" at much more molar levels of representation, even extending to the two cerebral hemispheres. On the other hand, impressive evidence continues to accumulate documenting the resilience and plasticity in the nervous system, particularly during the early phases of development. As a tentative conclusion, then, it seems that some localization is accepted by all, but that important islands of plasticity remain within this general framework.
-Studies have reverted to a more circumscribed terrain. Neuroscientists are devoting the bulk of their time to the careful study of specific systems in specific organisms.
Eric Kandel Bridges a Gap
Recently Eric Kandel has succeeded in bridging the gap between the functioning of the individual nerve cell and the behavior of organisms. He focused on a snail, whose nervous system can be readily described and which is also capable of simple forms of learning.
Kandel has shown that these elementary aspects of learning are not diffusely distributed in the brain but rather can be localized in the activity of specific parts of neuronal networks.
Learning results from an alteration in the synaptic connections between cells; rather than necessarily entailing new synaptic connections, learning and memory customarily come about as a consequence of alteration in the relative strength of already existing contacts.
The potentialities for many behaviors of which an organism is capable are built into the basic scaffolding of the brain and are to that extent under genetic and developmental control. Environmental factors and learning bring out these latent capabilities by altering the effectiveness of the pre-existing pathways thereby leading to the expression of new patterns of behavior.
The Song of Birds
Fernando Nottebohm has studied the songs of birds. He showed that various deprivations exert predictable influences on the course of song development. Canaries, for example, require auditory feedback for normal development. They can, however, go on to produce a well-structured song even in the absence of hearing the vocalizations of other members of their species: their own songs suffice. In the chaffinch, however, both auditory feedback of one's own song and exposure to the songs of other birds are needed if the chaffinch is to produce a full normal song. Bird song is one of the few instances of brain lateralization among infrahuman animals. The aphasic canary can recover its prior songs because the homologous pathways of the right hemisphere have the potential of being exploited.
-The work of investigators like Nottebohm and Kandel is based on the premise that much can be learned at this point through the careful study of a single system in a single organism.
-The two research efforts proceed on somewhat different assumptions. Kandel hopes that by studying habituation and conditioning, he will eventually illuminate processes known to occur in a wide range of organisms, including humans. The Nottebohm line of research, on the other hand, studies bird song as a behavior that clearly exists only in birds, and any generalizations that may be validly extended from bird song to other systems in other organisms will only emerge after careful study of these systems on their own terms. There is the same tension between a modular and a general point of view.
-Hubel and Wiesel hold the belief that systems may work in their own way. Each region of the central nervous system has its own special problems that require different solutions. For the major aspects of the brain's operation no master solution is likely.
-The work of Kandel (and to a lesser extent of Nottebohm) raises afresh the issue of reductionism. It seems to some observers that an account of the classical psychological phenomenon of habituation in terms of neurochemical reactions is an important step on the road to the absorption of cognition by the neurosciences. Yet most scientists of a cognitive persuasion feel that such accounts, while informative, will still prove tangential to their ultimate interests.
Pribram's Holographic Hypothesis
Pribram argued that the brain is better analogized to a holographic process. Holography is a system of photography in which a three-dimensional image of an object can be reproduced (with the appearance of the third dimension preserved) by means of light-wave patterns recorded on a photographic plate or film. A hologram is the plate or film with the recorded pattern: information about any point in the original image is distributed throughout the hologram, thus making it resistant to damage. According to Pribram's holographic view, all parts of the brain are capable of participating in all forms of representation, though admittedly certain regions play a more important role in some functions, and other regions are more dominant for other functions. Just as many holograms can be superimposed upon
one another, so can infinite images be stacked inside our brains. Since the hologram records the same wave front over its surface, repeating it over and over, if only some of a shattered hologram is left, it will still suffice to reconstruct the entire image. What interests brain theorists about the hologram is this quality of a distributed memory: every piece of the hologram says a little bit about every part of the scene, but no piece is essential.
Three Historical Moments
We might single out three moments in the age-old debate about the degree of localization of representation in the human nervous system.
-The first moment involved scientific insights. When Descartes located the soul in the pineal gland, when Gall spoke about the representation of amativeness and of criminality in different lobes of the brain, each was announcing claims without benefit of experimental evidence.
-A significant step forward took place when it was possible to examine the effects of injuries to discrete areas of the nervous system. When Fritsch and Hitzig lesioned specific sites in the nervous system of dogs, when Broca and Wernicke looked at the effects of strokes in the human cerebral cortex, they were able to substantiate correlations between regions of the brain and forms of behavior.
-Finally, when Hubel and Wiesel recorded from discrete cells in the visual cortex of the cat, it became possible to ascertain with great specificity the function of particular units and the circumstances under which they would function (or fail to function) normally.
-The thrust of the new research lines has had two effects: first, to render localization as a more plausible general orientation; second, to direct the attention of active scientists to the operation of specific systems, rather than to continued debate on broad conceptual issues.
-The thrust of the new research lines has had two effects: first, to render localization as a more plausible general orientation; second, to direct the attention of active scientists to the operation of specific systems, rather than to continued debate on broad conceptual issues.