Basic Neural Mechanisms in Behavior [jnl article] - K. Lashley (1930) WW.pdf - Bio Med 4 - hilakku

BASIC NEURAL MECHANISMS IN BEHAVIOR

Basic Neural Mechanisms in

Behavior

By K. S. Lashley (1930)

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BASIC NEURAL MECHANISMS IN BEHAVIOR

Basic Neural Mechanisms in Behavior [ 1 ]

K. S. Lashley (1930)

Behavior Research Fund, Chicago

First published in Psychological Review , 37 , 1-24

Among the systems and points of view which comprise our efforts to formulate a science of

psychology, the proposition upon which there seems to be most nearly a general agreement is

that the final explanation of behavior or of mental processes is to be sought in the physiological

activity of the body and, in particular, in the properties of the nervous system. The tendency to

seek all causal relations of behavior in brain processes is characteristic of the recent development

of psychology in America. Most of our text-books begin with an exposition of the structure of

the brain and imply that this lays a foundation for a later understanding of behavior. It is rare that

a discussion of any psychological problem avoids some reference to the neural substratum, and

the development of elaborate neurological theories to 'explain' the phenomena in every field of

psychology is becoming increasingly fashionable.

In reading this literature I have been impressed chiefly by its futility. The chapter on the nervous

system seems to provide an excuse for pictures in an otherwise dry and monotonous text. That it

has any other function is not clear; there may be cursory references to it in later chapters on

instinct and habit, but where the problems of psychology become complex and interesting, the

nervous system is [p. 2] dispensed with. In more technical treatises the neurological explanations

are made up mostly of assumptions concerning the properties of the nerve cell which have no

counterpart in physiological experiment. Thus we find the superiority of distributed over

concentrated practice seriously 'explained' by the 'fact' that successive passage of neural impulses

over a synapse reduces its resistance least when the impulses come in quick succession.

There is no direct evidence for any function of the anatomical synapse: there is no evidence that

synapses vary in resistance, or that, if they do, the resistance is altered by the passage of the

nerve impulse. If the explanation is to be given in terms of established facts, as it must be, then it

is limited to the following form: the superiority of distributed practice is due to the discontinuity

of the neurons, the polarity of conduction, the fact of learning, and the superiority of distributed

practice.

This is a typical case of the neurological explanations to be found in our psychological literature.

With such conditions prevailing, it seems time to examine critically the relations between

psychology and neurology and to attempt an evaluation of current notions concerning the

mechanisms of the brain.

INADEQUACY OF CURRENT THEORIES

The starting point for our attempts to account for behavior in terms of nervous processes has

been either the cerebral localization of functions or the theory that all nervous integration is

patterned after the spinal reflex. I need scarcely point out the difficulties encountered by the

older doctrine of cerebral localization. It expresses the fact that destruction of definite areas

results in definite symptoms and the probable inference that these different parts have diverse

functions, but it has given us no insight into the manner in which the areas or centers exercise

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BASIC NEURAL MECHANISMS IN BEHAVIOR

their functions or the way in which they influence one another. It is only, by applying

psychological conceptions like that of association, or by turning to the theory of reflexes that the

doctrine of localization is made to express the dynamic relations of behavior.

[p. 3] The extension of the theory of reflex conduction, first derived from studies of the spinal

cord, to problems of cerebral function provided a welcome addition to the psychophysical

doctrine of localization. It gave a clear interpretation of localized areas as relay points or centers

along the course of the reflex arc and seemed to explain the functional relations of the areas.

However, the theory has not worked well in application to the details of behavior. To understand

the difficulties we should have clearly in mind the form and limitations of the theory. It states

that the mechanism of cerebral function is essentially the same as that of the spinal reflexes,

involving the conduction of nerve impulses from the sense organs over definite, restricted paths

to the effectors. The performance of a habit, whether of speech or of manipulative movement, is

determined by the existence of definite connections between a limited number of nerve cells,

which are always functional in that habit. The model for the theory is a telephone system. Just as

two instruments can be connected only by certain wires, so the sense organs and muscles

concerned in any act are connected by nerve fibers specialized for that act.

Perhaps few neurologists would agree to such a bare statement. They point to the incalculable

number of nerve cells, the interplay of inhibition and facilitation, and suggest that in so complex

a system there are limitless possibilities. But the fact remains that the essential feature of the

reflex theory is the assumption that individual neurons are specialized for particular functions.

The explanatory value of the theory rests upon this point alone, and no amount of hypothetical

elaboration of connections alters the basic assumption.

Both the doctrines of localization and of conditioned reflexes imply the correspondence of

structural and functional units - the specialization of minute areas or of single cells for definite

limited functions. Recent experimental and clinical evidence seems to show that there is no such

correspondence, and thus to present fatal difficulties to both theories. I shall sketch the main

lines of this evidence, then turn to a consideration of other possible mechanisms.

[p. 4] ANALYSIS OF THE ADEQUATE STIMULUS

The notion of the reflex arc was developed in studies of spinal preparations in which protopathic

stimuli or muscle tensions are the chief sources of excitation. Under these simple conditions

something like a point for point correspondence between receptor cells and muscle groups could

be demonstrated, as in the case of the scratch reflex.

We first attempted the extension of this conception to instinctive behavior, on the assumption

that the adequate stimulus to nursing, to the recognition of the mate or young, to the recognition

of the nest site, to sexual excitement might be expressed in terms of the excitation of such and

such receptor cells. This proved to be a vain hope. The adequate stimulus in such cases may be

described in terms of a pattern having definite proportions but always, within wide limits, it is a

matter of indifference to what receptor cells this pattern is applied.

A survey of various types of behavior shows that this is an almost universal attribute of the

adequate stimuli.[ 2 ] It is most obvious in pattern vision and can be demonstrated in animals with

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BASIC NEURAL MECHANISMS IN BEHAVIOR

a rather primitive cortex. I have recently improved the technique for study of vision in the rat so

that habits of pattern vision may be established in 20 or 30 trials. It is thus easy to test the

equivalence of stimuli under conditions where previous associations are ruled out. Not only do

we find transposition as Köhler has described it for chimpanzees, but even more striking

equivalencies. An animal trained to discriminate patterns of solid white on a black ground is

undisturbed by reversal of the brightness relations, by substitution of outlines for the solid

figures, or even by partial outlines which retain some of the proportions of the original figures.

In many cases it is clear that the equivalent stimuli involve none of the retinal elements which

were activated during learning. Here we have a situation where a habit is formed by the

activation of one set of receptors and executed im- [p. 5] mediately upon stimulation of an

entirely different and unpracticed group. The equivalence of stimuli is not due to the excitation

of common nervous elements. The equivalent patterns have in common only ratios of intensity or

of proportion in the spacial distribution of excited points. I might multiply examples of this sort

indefinitely, but the studies of the Gestalt psychologists leave little doubt that such a condition is

the rule for all stimuli with which we deal in the study of behavior.

ANALYSIS OF REACTIONS

Turning to motor activity, we are confronted by an identical problem. If we train an animal in a

maze and observe carefully his subsequent errorless running, we find little identity of movement

in successive trials. He gallops through in one trial, in another shuffles along, sniffing at the

cover of the box. If we injure his cerebellum, he may roll through the maze. He follows the

correct path with every variety of twist and posture, so that we cannot identify a single

movement as characteristic of the habit.[ 3 ]

I have earlier reported cases of the direct adaptive use in the performance of motor habits of

limbs which were paralyzed throughout training and whose motor paths consequently could not

have been exercised during training.[ 4 ] It is not helpful to say that previously formed general

habits are utilized in such performances, for the preexisting habits have not been associated with

the new situation and the problem of the spontaneous association of the new patterns remains

unsolved.

The problem of equivalence of motor responses has been less studied than that of equivalence of

stimuli, but the phenomenon seems to be equally common. Activities ranging from the building

of characteristic nests by birds to the so-called purposive activities of man show the absence of

stereotyped movements in the attainment of a predetermined [p. 6] goal. The most familiar and

most striking example is that of grammatical form in speech. Once we learn a new word, we use

it in correct grammatical relations in limitless combinations with other words, without having to

form new associations for each new setting.

It is only in certain acts of skill that stereotyped movements are recognizable and the uniformity

of these is a result of long practice. We seem forced to conclude that the same motor elements

are not necessarily used in the learning and performance of motor habits and that motor elements

can be utilized directly when no specific associations have been formed with them.

PLASTICITY IN CENTRAL ORGANIZATION

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BASIC NEURAL MECHANISMS IN BEHAVIOR

Studies of the central nervous system give a similar picture. The functions are relatively

independent of the structural elements. I can only cite a few of the lines of evidence, but

sufficient, I believe, to establish the point.

First with respect to the specificity of conduction paths. The final motor neurons have been

studied by Weiss.[ 5 ] He grafted additional limbs on salamanders, cutting the nerve which

supplied the original limb so that the regenerating fibers came to innervate both the original and

the new limb. The two limbs innervated by the same nerve showed synchronization of

movements in corresponding muscle groups. Histological examination showed that the axons of

the original nerve had branched so that the muscles of the two limbs were supplied by fibers

from the same axons. There is no selective outgrowth of regenerating fibers and the branches of

the same axon do not necessarily go to corresponding muscles. It seems, then, that the

coördination of the two limbs is not a function of the particular fibers which innervate each

muscle, but is due to some property of the nerve impulse such that the same fiber can selectively

elicit either of two antagonistic movements. These experiments are still the subject of

controversy, but the objections raised against the results are not particularly impressive and,

though they may raise some [p. 7] doubt on this conclusion, they certainly do not establish the

specificity of the axon. The results of Weiss are in harmony with many facts revealed by the

study of the central nervous system.

In work with injuries to the spinal cord Miss Ball and I[ 6 ] have found that orientation of the rat

in the maze is undisturbed by interruption in the cervical cord of either the pyramidial,

rubrospinal, or any other of the long descending tracts. The impulses controlling turning and

threading the maze somehow get down the cord after the destruction of any half of the

descending fibers. I have more recently been working with double hemisections of the cord. In

these preparations one half of the cord is divided in the upper cervical region, the other half

below the nucleus of the phrenic nerve, so that all the long fibers are interrupted above the motor

centers for the limbs. After three months such preparations show coördinated movements in

walking and are able to control the limbs for orientation in response to stimuli applied to the

head. The control is established in spite of the permanent interruption of all the long spinal paths.

We have also been accumulating evidence upon the functions of the projection and association

tracts of the cerebrum in the rat. The data are not yet complete, but it seems fairly certain that the

interruption of the projection fibers to a part of a functional area produces far less pronounced

symptoms than destruction of the cortical area supplied by those fibers. We have now a large

number of cases in which linear lesions sever the connections between the different anatomical

areas of the cortex or divide the association fibers within single areas. It is rare that any

symptoms can be detected in such cases, unless there is involved a considerable destruction of

cortical tissue. The most capable animal that I have studied was one in which the cortex and

underlying association fibers had been divided throughout the length of each hemisphere. His

I.Q., based on ten tests, was 309.

In higher forms there is evidence for a somewhat greater [p. 8] specificity of long tracts in the

central nervous system, but even in man the evidence is unequivocal only for the pyramidal

system, which we have reason to believe is a part of the postural system and not especially

concerned in the higher integrative functions of the brain, and for sensory paths of the cord.

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