METAPHILOSOPHY
Vol. 5, No. 2, April 1974
THE POSSIBILITY OF TYPE-MATERIALISM
F. MUCCIOLO
LAURENCE
The mind-body identity thesis (IT hereafter) is roughly the
claim that mental events are identical with physical events in the
brain. Or, more exactly but still only roughly:
(IT) (x)(Ey)(x is a psychological event+(y is a neural event
& x=y)).
Many identity theorists regard IT as a case of a theoretical
identity. In an early discussion of IT, Feigl wrote:
In order to decide whether the mental and the physical can in
some sense be identified, it is indispensable to cast at least a
brief glance at the logic of reduction and identification in the
sciences, especially in physics, biology, and psychology.’
If IT can plausibly be thought of as a case of a theoretical
identity, then its justification can be based on an appeal to the
reductive relations that obtain between two scientific theories.
Thus the terms flanking the identity sign, say ‘4’ and ‘a’, are
such that ‘4’ is a psychological term which occurs in a psychological theory T,, ‘@’ is a neurological term which occurs
in a neurological theory T,, and the justification for ‘$*=
‘@’ is essentially that T, is micro-reducible to T,. Likewise the
justification for the empirical claim that temperature is identical
with mean kinetic energy is based on an appeal to the fact that
thermodynamics is micro-reducible to mechanics. Psycho-neural
identity statements are analogous to thermodynamical-mechanical identity statements; IT is analogous to the claim that thermodynamical events are identical with mechanical events. The
former may be thought of as reduction-functions which map
elements of the domain of the reduced theory T, onto elements
of the domain of the reducing theory T,. Functions of this sort
have traditionally been called “bridge laws” or “correspondence
‘Herbert Feigl. “The ‘Mental’ and the ‘Physical’” in Feigl, et al. (eds.), Minnesota
Studies in the PhiIosophy of Science 11, p. 438.
133
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LAURENCE F. MUCCIOLO
rules”. While there is still considerable controversy in the literature on reduction, we shall assume that something like the
analysis provided by Schnafber is adequate.2
There are, of course, many philosophers who have advanced
arguments against the view that IT is a theoretical identity.
Kim has argued that the micro-reduction of psychology to
neurology only requires psycho-neural correlation statements.
If this is correct, then the reductive relations which may obtain
between psychological and neurological theories will not justify
the identification of psychological and neural events. However,
Kim’s argument depends on a general claim in the philosophy of
science that correlation reduction-functions suffice for the purposes of a micro-reduction.’ A different sort of argument has
been advanced by Fodor. He has argued that the micro-reduction
model required by the “unity of science hypothesis” is seriously
misleading. According to Fodor, the relation between psychological and neurological theories is not one of micro-reduction
since psychological events are not reducible into anything, least
of all neurons or states of neurons.’ Finally Kripke has argued
that if IT is a theoretical identity, then it is necessarily false. On
Kripke’s view of identity, theoretical identities are necessary if
they are true at all. Since it appears that one can imagine circumstances in which IT is false, it follows on Kripke’s view that
IT is necessarily false. Indeed identity theorists make it a point
to tell us that it is conceivable that IT is false.5 I have attempted
to meet each of these objections elsewhere.” The primary aim of
this paper is to consider the objection that there is empirical
evidence that IT is false. Fodor and Block have recently argued
that it is empirically likely that (many) psycho-neural identity
statements are false if they have the form of type-to-type
identity statements.‘
There are two versions of IT depending on whether one takes
2Cf. Kenneth F. Schaffner, “Approaches to Reduction”, Philosophy of Science
XXXIV, No. 2 (June, 1967), pp. 137-47.
3Jaegwon Kim, “On the Psycho-Physical Identity Theory”, American Philosophical
Quarte& 111 (1966), pp. 227-35.
4 Jerry A. Fodor. Psychological Explanation, pp. 11 1-12.
.;Saul Krioke. “Identitv and Necessitv” in Milton Munitz (ed.) Zdentitu and
Individuation, pp. 135-64.61 consider Kim’s argument in my “Scientific Reduction and the Mind-Bdy
Problem” (in preparation); Fodor’s views are discussed below and in my “Fodor on
the Unity of Science, The Philosoohical Forum Ill, No. 1, pp. 133-37;Kripke’s
argument is attacked in my “On Kripke’s Argument Against the Mind-Body Identity
Theory” (in press).
7N. J. Block and J. A. Fodor, “What Psychological States Are Not”, Philosophical
Review (April, 1972). pp. 159-81.
135
the identity theorist to be making a claim about types or tokens,
namely :
Type Materialism:
(x)(Ey)(x is a type of psychological event+(y is a type of neural
event & x=y))
THE POSSIBILITY OF TYPE-MATERIALISM
Token Materialism:
(x)(Ey)(x is a token psychological event+(y is a token neural
event & x=y))
Thus type materialism involves the claim that organisms are in
type-identical psychological states if and only if they are in typeidentical neural states. Type materialism entails token materialism,
but not conversely. If Fodor and Block are correct in believing that
type-to-type psycho-neural identities are false, then IT cannot be
regarded as a theoretical identity since reduction functions must
be type-to-type identities. Against Fodor and Block I wish to
argue that the empirical evidence they cite is not incompatible
with type materialism and that there is reason to believe that
many psycho-neural identities will be discovered. The instance
of IT which I shall focus on deals with the identification of
memory with a type of neural state. Fodor uses the engram as
an example when discussing theory construction in psychology.
Moreover, what prima facie empirical evidence there is against
type-to-type psycho-neural identities is mainly due to Lashley’s
work on memory and learning. In section I it will be argued that
psychological states are in principle reducible to neurological
states on Fodor’s account of the relationship between psychological and neurological theories. The point here is to show the
logical or conceptual possibility of micro-reducing psychology to
neurology. In section I1 Lashley’s doctrine of equipotentiality is
considered and it is argued that it does not (as Fodor and Block
believe) entail that type materialism is false. I shall argue that
there is now available a coherent and plausible hypothesis about
what a memory trace is, that is, a type of neural state, which is
not only compatible with but also explains Lashley’s experimental results. If this is correct, we will have reason to believe
in the empirical possibility of reducing psychology to neurology.
According to Fodor’, theory construction in psychology can
8Psychological Explanation, pp. 107-11. See also, “Functional Explanation in
Psychology” in May Brodbeck (ed.) Readings in the Philosophy of the Sociul Sciences,
pp. 223-38.
136
LAURENCE F. MUCCIOLO
be thought of as exhibiting two phases. Although these phases
may be simultaneous in actual scientific practice (for example,
in physiological psychology), they are logically distinguishable.
In phase I theory construction, psychologists attempt to arrive
at theories that provide functional characterizations of the
mechanisms responsible for the production of behavior. The
criteria employed for individuating psychological constructs are
based primarily upon hypotheses about the role they play in the
etiology of behavior. Such inferred entities must be sufficiently
complex to account for whatever behavioral capacities the organism can be demonstrated to possess and specific aspects of
the organism’s behavior must be explicable by reference to
specific features of these inferred entities and their interactions.
An example of such an inferred entity is the memory trace or
engram. A psychologist might explain failures of memory by
reference to the decay of a hypothetical memory trace, and
attempt to attribute to the trace properties that will account for
such observed features of memory as selectivity, stereotyping,
and so on. As more is discovered, for example, about the effects
of pathology upon memory, or about differences between short
and long-term memory, the properties attributed to the trace and
to whatever other psychological systems which are supposed to
interact with the trace must be correspondingly elaborated.
The hypothesized psychological constructs are individuated
primarily by reference to their alleged causal consequences. It
follows that phase I theories postulate functionally equivalent
mechanisms if and only if they postulate constructs of which the
behavioral consequences are in theoretically relevant respects
identical. Phase I theories are sometimes compared with descriptions of a machine table. Different psychological states are
identified with different types of machine table states. Neurological theories (phase I1 theories) are then compared to descrip
tions of the hardware, that is, the physical machinery into which
such tables are programmed. Since two physical realizations of
the same table may differ in their physical structure, functional
equivalence is independent of physical similarity.
In the second phase of theory construction one specifies the
neurological systems that do in fact exhibit the functional
characteristics enumerated by phase I theories. The physiologist
has the job of determining whether the nervous system has
sub-systems whose functional characteristics correspond with
THE POSSIBILITY OF TYPE-MATERIALISM
137
those required by psychological theories. For example, given the
postulation of a memory trace in a phase I theory, the neurologist must provide systems of the CNS (central nervous system)
which are capable of going from one steady state to another and
that are capable of remaining in the relevant states for periods
of time that are a t least comparable to known retention periods.
If no mechanism is able to account for such facts on the neural
level, then the trace is the wrong model for the functional
organization of memory.
It is a consequence of this account of psychology and neurology that phase I and I1 theories must be mutually adjusted to
each other. Phase I psychological theories require operations on
the part of the neurological mechanisms that underlie behavior.
In phase I1 theories the nervous system is articulated into systems that must be matched with those functional characterizations postulated in the first phase. Fodor emphasizes that this
problem of fitting and adjusting such theories to each other is
an extremely complex one and has often been abbreviated by
identity theorists. What Fodor has in mind here are philosophers like Brandt and Kim’ who argue for IT by appealing to
constant correlation and simplicity considerations. The evidence required to justify psycho-neural identities is something
considerably more complex than mere correlation plus simplicity,
it is rather a plausible adjustment of psychological characterizations of function to considerations of neurological plausibility,
and vice versa. I have argued elsewhere that identity statements
are not in general justified by known correlations plus simplicity.” A fortiori, correlation is often evidence against identity.
The justification involved with theoretical identities has to do
with the relations that obtain between the reduced and the reducing theory. For example, our identification of a memory trace
with a type of neural state will be justified provided that we are
then able to explain how an organism habituates. In many cases
of theoretical identities the evidence for the identity could not
be a correlation since we cannot test for one part of the alleged
correlation. For example, we cannot test or measure mean
kinetic energy without measuring temperature. Hence the justification for this identity could not be that we have discovered a
correlation between temperature and mean kinetic energy and
PRichard Brandt and Jaegwon Kim, “The Logic of the Identity Theory”, The
loumal of Philosophy LXIV (September, 1967), pp. 515-37.
10Cf. my “Scientific Reduction and the Mind-Body Problem”.
MPH
E
138
LAURENCE F. MUCCIOLO
Occam’s Razor. Granted that there is such a correlation, but it
is trivial. Identities logically imply correlations.
Given this account of the relationship between psychology
and neurology, Fodor argues that:
(F) Functionally defined states are not reducible to mechanistic states.
If functionally defined states cannot be identified with mechanistically defined states, then it follows that phase I theories are
not micro-reducible to phase I1 theories. I want to argue that (F)
is false, although I find Fodor’s account of psychological theory
construction both plausible and extremely illuminating.
In order to see what (F) involves we shall consider one of
Fodor’s examples. The concept of a valve lifter is functionally
defined; a valve lifter is defined by the role it plays, to wit, it
opens the valves permitting fuel to enter the cylinder in order to
drive the piston. The concept of a camshaft is mechanistically
defined. Fodor argues that a valve lifter is not a camshaft, although this token valve lifter may be nothing but a camshaft.
According to Fodor, a mechanistic account tries to explain how
valves get lifted, it does not try to reduce valve lifters to camshafts. It does not because a valve lifter may be realized in
various ways; different mechanisms or structures can satisfy the
functional requirements for being a valve lifter. Similarly neurological theories seek to explain what neurological transactions
are involved when drives are reduced, motives entertained, objects perceived, and so on. Different neural mechanisms may
well satisfy the functional requirements for drive reduction.
When someone identifies a token functional state with a token
mechanism one is not thereby committed to saying in mechanistic terms what all members of the functionally defined class
have in common. In other words, token identities do not entail
type identities. A token pain may be identical with a token
brain state. But we needn’t believe that the type pain (which
is functionally defined) is identical with a type of brain state.
Fodor believes that if IT is construed as requiring type-to-type
identities, then it is very likely false.
In sum, then, Fodor’s general argument against type materialism is this. The principles we employ for individuating psychological states in phase I theories and those for individuating
neural states in phase I1 theories are logically independent. Since
THE POSSIBILITY OF TYPE-MATERIALISM
139
what counts as one sensation, wish, drive, and so on, is not
specified by reference to the organism’s neurophysiology, it is
likely that an organism may persist in a given psychological
state while undergoing neurological change. There may be sets
of neurologically distinct brain states, whose members are
nevertheless psychologically indistinguishable. Alternatively,
Fodor accepts the following theses :
(I) Psychological theories are phase 1 theories; psychological
states or events are functionally defined states which are
inferred in order to explain behavior.
(11) Neurological theories are phase I1 theories; neural states
exhibit the functional characteristics enumerated in
phase I theories.
(111) Phase I theories are not reducible to Phase I1 theories;
psychological states are not identical with types of neural
states.
If this is correct, then the best the materialist can do in addition to defending token materialism is to assert the following
thesis :
Functional materialism: Psychological states are type identical with functionally equivalent
neural states.
But, then, the materialist’s thesis is at best no clearer than the
unclear notion of functional equivalence. The theoretical identity (as defined above) is thus lost.
There are various versions of (I) and (11) which, if true, render
(111) plausible. On one reading the first thesis may be construed
as asserting that psychological states are machine table states.
(11) would then be construed as asserting that neural states are
the physical states into which these machine table states are
programmed. From this it would follow that psychological states
are not identical with types of neural states:
(1’) Psychological theories are sets of descriptions of a
machine table.
(11’) Neurological theories are sets of descriptions of the hardware, that is, the physical machinery into which such
machine tables are programmed.
(III’) Psychological theories are not reducible to neurological
theories.
140
LAURENCE F. MUCCIOLO
The logical net model of McCulloch and Pitts” is an example
of a theory which satisfies (I’). According to this model, a
modular net is a collection of modules (or formal neurons)
whose behavior is governed in accordance with the laws of
Boolean algebra. Any modular net is a finite automaton. A
Turing machine is a finite automaton plus a potentially infinite
tape and a device for scanning one square of the tape a t a time,
printing a new symbol in the scanned square, and moving the
tape one square to the left or one square to the right. This
Turing machine may be realized by both the CNS and a computer. It does not delineate the physical mechanisms which can
satisfy it. Although a neurological theory can explain how the
CNS can realize such a logical net, it cannot explain the phase
I theory. On this interpretation, phase I theories are not reducible to phase I1 theories because the laws of the phase I theories
have wider application than those of the phase I1 theories.
It is important to point out that Fodor would no longer
accept this argument. He has argued quite decisively against
the view that psychological states are identical with types of
machine table states.’* Thus (I’) is false. Moreover the logical
net model of McColloch and Pitts is not a phase I psychological
theory since it does not explain any behavior until it is given
an interpretation. An uninterpreted formal theory is not a
scientific theory since it is neither true nor false. Any physical
theory may, from a logical point of view, be a Turing machine.
But from this it does not follow that no theories are capable of
being reduced to other theories. It was pointed out above in
what sense phase I psychological theories may be compared with
descriptions of a machine table. But such theories are not,
strictly speaking, sets of descriptions of a machine table. The
alleged irreducibility of psychological to neural states involves
the claim that the former are functionally defined whereas the
latter are not. It does not involve the hardwarelsoftware distinction, but only the function /mechanism distinction.
Consider again Fodor’s claim that a valve lifter is not reducible to a camshaft. He supposes that the reduction of such
a functionally defined entity requires that we be able to say in
mechanistic terms what all things of this type have in common.
In other words a functionally defined class F is reducible only
“Cf. W. S. McCulloch and Walter Pitts. “A Logical Calculus of the Ideas
Immanent in Nervous Activity”, in The Bulletin of Mathematical Biophysics V .
pp. 115-33.
1a“What Psychological States Are Not”.
THE POSSIBILITY OF TYPE-MATERIALISM
141
if all members of F have some nonfunctional property P. Since
there is no such shared property in the case of valve lifters,
a valve lifter is not reducible to a camshaft. Surely this is correct.
But the reason why such a reduction is impossible is that there
is no shared nonfunctional and theoretically relevant property,
not merely because the object is functionally defined. If we are
unable to reduce F, we can try to redistribute the members of
F into classes F’, F”, . . . , F n and say what each have in common.
Gendron points out that :
If it is objected that such entities have not been theoretically
reduced insofar as they belong to class (F), the answer is: so
much the worse for being a member of class (F).I3
The point here is that the class of valve lifters does not form a
natural kind. In general what counts as a natural kind is theoretically determined. Micro-reductions have determined in many
cases what is to count as a natural kind and what is not. Fodor
in fact says that “what makes a kind ‘natural’ is the power and
generality of the theories that we are enabled to formulate when
we taxomonize in that way”.” The crucial question is therefore
whether psychological states which are functionally defined can
be given nonfunctional correlates, or at least some redistribution
which will do so. The fact that valve lifters do not form a natural
kind and cannot be given nonfunctional correlates is entirely
beside the point.
There is another version of (I)-(111) which needs to be considered :
(I”) Psychological states are individuated in terms of the
role they play in the production of behavior.
(11”) Neural states are individuated in terms of their anatomical and topographical properties.
(111”) Psychological states are not identical with types of
neural states.
If the principles we employ for individuating psychological and
neural states are logically independent, and if neural states are
individuated in terms of their anatomical properties, then there
is no reason to think that psychological states will be identical
13Bernard Gendron, “On the Relation of Neurological and Psychological Theories :
A Critique of the Hardware Thesis” in Robert S. Cohen and Marx W. Wartofsky
(eds.) Boston Studies in the Philosophy of Science VIII. pp. 487-95.
14Psychological Explanation, p. 119.
142
LAURENCE F. MUCCIOLO
with types of neural states. There is no reason since the CNS
is able to undergo considerable substitutability among its parts.
Even if we discovered correlations between psychological states
and neural structures, it is unlikely that a theoretical identity
would be justified. That is, it is unlikely that such an identity
would be of any help in the reduction. Suppose we know that a
psychological state P is accounted for by a phase I law L.
Would the fact that P=N,where N is an anatomical construct,
explain L? It is unlikely that one will be able to account for
the fact that P acts in accordance with L by citing the putative
identity P=N.
The difficulty with this argument is that (II”) is false; there is
no reason for limiting neurological theories to anatomically
defined states. Neurological theories will include such theoretical
entities as slow graded potentials, nerve impulses, EEG states,
and as we shall see shortly, micro-structures of junctional slow
potentials. Although Fodor sometimes appears to limit neurological taxonomies to anatomical or morphological classifications,
one must be willing to include whatever classifications neurologists in fact use.
If these considerations are correct, then Fodor has not provided an acceptable a priori argument against the reduction of
psychology to neurology. We shall now turn to an examination
of the claim that psychological states are not type identical with
neural states unless these neural states are functionally defined.
Since this is an empirical claim the evidence for it needs to be
examined.
I1
Historically, two conflicting approaches to the neurophysiology of learning have been advanced and with certain modifications, this dichotomy continues to dominate much theory con~truction.’~
We will refer to these as the cellular-connection a p
proach and the aggregate-field approach. The former derives
from the neuron doctrine and the latter derives from Gestalt
theory. According to the cellular-connection hypothesis, cell
aggregates are thought to be specialized for a unique function.
These are either innate permanent connections or learned asso1 X f .
Eric R. Kandel and W. Alden Spencer, “Cellular Neurophysiological
Approaches in the Study of Learning”. Physiological Review 48 (1968), pp. 65-134.
THE POSSIBILITY OF TYPE-MATERIALISM
143
ciative connections between neurons. According to the aggregatefield hypothesis, certain interactions important to the organization of behavior cannot be specified in terms of permanent
connections between neurons. The most articulate exponent of
the “field” approach was Lashley. Lashley’s doctrine of neurological equipotentiality holds that any of a wide variety of
psychological functions can be served by a wide variety of brain
structures. For example, the learning process and the retention
of habit are not dependent upon any finely localized structural
changes within the cerebral cortex. This view is incompatible
with “connection” approaches to learning in which learning and
memory essentially involve changes in synaptic structure, or
definite anatomical paths specialized for unique psychological
functions. In the case of memory, Lashley concluded from his
experimental work that :
the memory trace is located in all parts of the functional area;
that various parts are equipotential for its maintenance and
activation. ‘
However, the generality of Lashley’s doctrine has been disputed
by several studies. For example, Kandel and Spencer marshall a
good deal of evidence for the cellular-connection hypothesis.’‘
Nevertheless there still appear to be cases of psychological functions which support the “field” approach, particularly in the case
of memory. Block and Fodor are aware of this and write as
follows :
While the generality of this doctrine may be disputed, it does
seem clear that the CNS is highly labile and that a given type
of psychological process is in fact often associated with a
variety of distinct neurological structures.”
They conclude that if IT requires that organisms are in typeidentical psychological states if and only if they are in typeidentical neural states, then if equipotentiality is true, then IT
is false.
It is, of course, possible that type-to-type psychoneural
identities should turn out to be true. Block and Fodor’s point is
16K. S. Lashley, “In Search of the Engram” in F. Beach et al. (eds.) The Neuropsychology of Lashley. p. 492.
‘“Cellular Neurophysiological Approaches in the Study of Learning”.
18“What Psychological States Are Not”, p. 160.
144
LAURENCE F. MUCCIOLO
simply that we have no current evidence that this is so and thus
we have no warrant for adopting a philosophical thesis which
requires that it be so. It is this last point which I wish to
contest. It seems to me that in the case of memory we do have
evidence for believing that psychological states are identical with
types of physical states. Moreover the theory which I shall a p
peal to in order to defend this represents an alternative to both
the cellular-connection and the aggregate-field approaches. Indeed if one supposes that these are the only hypotheses available,
then one will find it hardly surprising that Lashley wrote:
I sometimes feel, in reviewing the evidence on the localization
of the memory trace, that the necessary conclusion is that
learning just is not possible at all. It is difficult to conceive of
a mechanism which can satisfy the conditions set for it.
Nevertheless, in spite of such evidence against it, learning
does sometimes occur.1Q
These remarks illustrate quite well Fodor’s point about fitting
and adjusting phase I and I1 theories. The memory trace is
postulated in the first phase of theory construction. In the
second phase, the physiologist must provide us with a mechanism which exhibits the functional characteristics enumerated in
the first phase. Moreover this mechanism must be compatible
with our neurological theories and with our experimental findings. Lashley’s experimental work appears to be incompatible
with the notion of a localized trace. So it would follow,
if Lashley is correct, that the trace is the wrong model for
memory.
Neurophysiologists working with a cellular-connection hypothesis have mapped the brain, traced nerve pathways, and as a
result of such work hypothesized visual, auditory, someasthetic
and motor areas in the brain. They did, however, remain baffled
by the memory trace, although many believed that they could
describe the nerve pathways from a stimulus input to a muscular
response. But these presumed pathways could hardly be reconciled with Lashley’s experimental results.” In one study, Lashley
showed that rats could remember and perform certain complex
actions even though he had cut out nerve pathways. Indeed
they could remember after as much as 90 per cent of the
19%
Search of the Engram”, p.. 501.
ZoLashley’s experiments are reviewed in “In Search of the Engram”.
THE POSSIBILITY OF TYPE-MATERIALISM
145
primary visual cortex had been surgically removed. It is this sort
of evidence which supports Fodor’s contention that there is
considerable substitutability among various neural structures.
It is this sort of evidence which supports the “field” approach.
Given that each sensory system has considerable reserve capacity and that it makes little difference in terms of performance
which parts are destroyed, one infers that information is distributed throughout the brain region of the organism. Such information is duplicated in many locations, and these different parts
are said to be more or less “equipotent”. Presumably this explains why the rats in Lashley’s experiment could remember
after large parts of their brains had been altered or removed.
Apart from the theoretical issues connected with a general
“field” approach, these observational results do show that any
version of materialism which requires that a memory trace correspond to one of these parts (or even a disjunction) is likely to
be false. But the important point is that this does not rule out
the possibility that another type of neural structure may correspond with the trace. Indeed there is now available a hypothesis about the nature of this structure which is compatible
with Lashley’s experimental findings, type-materialism, and the
micro-reduction of psychology to neurology.
In order to formulate this hypothesis, several concepts of
neurology and physics need to be introduced. Two types of
neuroelectric activity need to be distinguished. One type of
neuroelectric activity is the nerve impulse. Nerve impulses are
unit discharges which are propagated in an “all or none”
fashion. In contrast, there are what are known as graded slow
potentials which wax and wane and are sensitive to chemical
changes. The point of this distinction is that the effective intraneural mechanism is characterized by the incrementing of graded
activity into a propagated nerve impulse; the effective junctional
(or synaptic) mechanism is characterized by the opposite, that
is, nerve impulses become decremented into local slow potentials.
An aggregate of such slow potentials present over an extended
location a t any time can be described as a state which has a
micro-structure. The arrival of impulses at junctions is never a
solitary event. These synaptic events, which are composed at
the junctions between neurons, form a pattern and these patterns
generate stationary wavefronts. They can be described in wave
mechanical terms. A given micro-structure of wave fronts can
interact with other micro-structures arising in overlapping junc-
146
LAURENCE F. MUCCIOLO
tional contacts. The hypothesis can now be formulated. This
process as described generates a neural hologram from which,
given the appropriate input, an image can be reconstructed. The
physical hologram is the mechanism which exhibits the functional characteristics of the memory trace. Similarly the psychological process of remembering can be thought of as consisting in the reconstruction of an image. The hologram provides us
with an understanding as to how this is accomplished. The
philosophical point here is that the materialist can identify the
memory trace with a type of neural state, to wit, a neural hologram. In order to clarify this we need to consider in some
detail the nature of a hologram.
A hologram is an encoded record of the wave patterns emitted by or reflected from an object. In neurology, encoding is the
process in which information is distributed in the brain. However, the concept of a hologram was arrived at outside of neurology. Gabor“ studied the propagation of light from the viewpoint of information theory and deduced from Huygen’s wave
theory of light that all the information in an image must exist
in every plane between the object and the photographic plate.
If this information could be extracted at any arbitrary plane,
one could recreate the image without using a lens. Gabor showed
how to extract this information (that is, the amplitude and phase
characteristics of the light) on film. Such a record is called a
hologram because it contains the “whole” information. When
the hologram is reilluminated, the light emerging from it forms
an image of the object.
This special type of “lensless” image-recording on a photographic plate is called holography.’z The object to be photographed is illuminated with the coherent light from a laser. A
photographic plate is placed in the path of the waves reflected
from the object. Another beam of coherent light (called the
reference beam) is simultaneously reflected from a mirror towards the photographic plate and forms an interference pattern
with the wavefront reaching the plate from the object. This
interference “halts” the waves from the object, and an interference pattern (called an interferogram) is recorded in the emulsion of the photographic plate. The plate when developed and
*‘Dennis Gabor, “A New Microscopic Principle” Nature 161 (May 15. 1948)
pp. 777-78. See also, George Stroke, “Holography” The Science Teacher (Oct., 1967)
pp. 73-88; E. N. Leith and J. Upatnieks, “Photography by Laser” Scientific American
212 11965) pp. 24-35.
* K f . “Holography”, p. 73.
147
fixed, will show only the lines and whorls of the interference
pattern. It does not show an image of the object. But when a
beam of light similar to the reference beam is directed onto
the back cf the plate, the waves which have reached the plate
from the object are reconstructed and an observer looking
through thc plate will see the original object just as it was. In
fact, if the observer shifts the angle of view, the observer will
see background objects that might have been hidden from view
in a straight-on-view. The observer may be the human eye, or it
may be a camera which can receive reactivated waves just as
though they were coming from the original object. The result is
a picture of the original object. And by varying the angle at
which the waves reach the camera, the picture will show different
views of the object. This provides a three-dimensional effect.
Holograms have several properties which are relevant to
problems concerning memory.23 First, as already noted, the
image seen by looking through the hologram is complete, and
three-dimensional. The reconstructed image has all the visual
properties of the original object and there is in fact no visual
test which can distinguish the two. Second, holograms have the
property that several images can be superimposed on a single
plate on successive exposures, and each image can be recovered
without being affected by the others. Third, it has been learned
that holograms can be constructed and reconstructed without
the use of lasers. Finally, each part of the hologram however
small can reproduce the entire image. The hologram can be
broken into small fragments each of which can be used
to construct a complete image. This is because each point on the
hologram receives light from all parts of the object and therefore contains, in an encoded form, the entire image.
Pribram has advanced the hypothesis that the hologram is the
model for encoding information in the brain. Optical systems
are not the only ones that can be subjected to the holographic
process. Holograms are not dependent on the physical presence
of “waves”, although they are most easily described by the equations of wave mechanics. Thus it is possible to apply the model
of the hologram to brain processes. Indeed the hologram has
many of the properties necessary for an adequate memory
mechanism.
THE POSSIBILITY OF TYPE-MATERIALISM
*JCf. Karl H. Pribram. “The Four Rs of Remembering” in Pribram (ed.) On the
Biology of Learning, pp. 198-99.
148
LAURENCE F. MUCCIOLO
According to P ~ i b r a minteractions
,~~
among patterns of excitation which fall on receptor surfaces become, after transmission
over pathways organized in a parallel fashion, encoded by
virtue of horizontally interacting processes in the slow potential
activities of neural aggregates to form a microstructure whose
design depends on the functional organization of neural junctions. The neural representation of input is holographic. The
essence of the holographic hypothesis is that images are reconstructed when representation in the form of distributed information systems are appropriately engaged. These representations
operate as filters or screens. Holography in this frame of reference is conceived as an instantaneous analogue cross-correlation
performed by matched filters. In the brain correlation can take
place at various levels. In peripheral stations correlation occurs
between successive configurations produced by receptor excitation. The residuals left by adaptation through decrementing form
a buffer memory register to be updated by current input. In
central stations correlation entails a more complex interaction
which includes patterns arriving from other stations. In contrast
to a field hypothesis, the holographic hypothesis asserts that the
mechanism of these correlations is not some disembodied
“floating field” or disembodied wave forms. The mechanism
consists in a microstructure of junctional slow potentials which
can be described either in statistical terms or in the wave
mechanical language of convolutional integrals and Fourier
transformations. This microstructure is generated by the arrival of impulses a t a set of junctions, When such arrival patterns converge from at least two sources their design would
produce interference patterns. Accordingly the information
representing the input is distributed over the entire extent of
the neural pattern just as it is over the entire extent of the
optical holographic pattern.
In sum, then, the patterns set up by arriving impulses form a
microstructure of waveforms that can interact with similar
microstructures arising in overlapping junctional contacts. These
other microstructures are derived from the spontaneous changes
in electrical potential that ceaselessly occur in nerve tissue, and
from other sources within the brain. Immediate cross-correlations result, and these can add in turn to produce new patterns
of nerve impulses. This process has a more or less lasting effect
on protein molecules and perhaps other macromolecules at the
24Karl H. Pribram, Languages of the Brain, ch. 8 .
THE POSSIBILITY OF TYPE-MATERIALISM
149
synaptic junctions and can serve as a neural hologram from
which, given the appropriate input, an image can be recon~tructed.*~
The holographic hypothesis fits many of the facts we know
about memory. Since the holographic image is three-dimensional
it can account for the three-dimensional imaging involved in
remembering. Second, the hologram has a great capacity to usefully store information since several images can be superimposed
on a single plate (or neural aggregate). For example, ten billion
bits of information have been usefully stored holographically in
a cubic centimeter.“ The important property of holograms for
our purposes is the fact that each part of the hologram can
reproduce the entire image. The hologram can be broken into
small fragments each of which can be used to construct a complete image; each fragment contains the “whole” information.
These properties of the hologram are just those demanded by
the facts of brain physiology as they pertain to perception and
memory. In particular, if memory corresponds with a neural
hologram, then Lashley’s experimental findings can be accounted
for. From this frame of reference it is not surprising that the
memory of the rats in Lashley’s experiment was not destroyed
by removing large parts of their brains. In another experiment Lashley showed that the removal of as much as 80 per cent
of the sensory input mechanism fails to impair pattern percep
tion. This result was especially puzzling to followers of the
cellular-connection approach, since peripheral sensory receptors
had been mapped point-to-point with areas in the cortex. Both
findings are no longer puzzling since holograms have the
property that any small part can be used for the reconstruction of the entire image.
Conclusion
The memory trace may be thought of as postulated in a phase
I theory. Various neural structures may realize a given engram.
However, we can say what these various structures have in
2”The hologram is not, however, the only mechanism involved in memory. Pribram
advances a full-blown theory of memory which considers such questions as how
information is extracted, how an organism is reinforced, and so on. In order to
explain the decoding of information the Spinelli unit is hypothesized. A detailed
discussion of this issue may be found in “The Neurophysiology of Remembering”
Scientific American (January, 1965), Languages of the Bruin. especially chapter 2
and 8, and in “The Four R’s of Remembering”.
*%5anguages of the Brain, p. 150.
150
LAURENCE F. MUCCIOLO
common: They are neural holograms. Holograms are not functionally characterized; they are describable either in statistical
or wave mechanical terms. If this is correct, a memory trace
may be identified with a type of neural state, to wit, a neural
hologram. This structure is, as already noted, micro-reducible.
I t consists in a microstructure of graded slow potentials.
Granted that there are many different microstructures which can
realize the same hologram, and thus the same memory trace.
But this is no argument against type-materialism. A given
temperature (macrostate) may correspond to many distinct
microstates. For example, if there are just 4 molecules in a
collection each of which can occupy any one of 8 positions, and
each of which may have a velocity of 1 or 2 feet/second, then
the total number of distinct microstates is 4”. If the mean
kinetic energy is 5/4, then this is compatible with 393,212
distinct microstates. Similarly a memory trace is compatible, on
the theory advanced by Pribram, with many different microstructures. The trace is identified with the neural hologram
just as temperature is identified with mean kinetic energy. And
these identities I submit are type-to-type identities. Indeed both
may be described in statistical terms. Moreover it is a consequence of these considerations that an organism could persist
in a given psychological state (for example, remembering) while
undergoing neurological change or be in the same psychological
states at different times although the corresponding neural
microstructures be different. Thus the memory trace is typeidentical with a neural hologram. And if the memory trace is
identical with a neural hologram then it has the location of the
hologram. Thus Lashley’s conclusion that “it is not possible to
demonstrate the isolated localization of a memory trace anywhere within the nervous system”*‘ is mistaken. And if remembering is the process of reconstructing an image, then it
may be located in the space occupied by the neural hologram.
Finally, since the laws necessary to account for the holographic
neural mechanism are the laws of chemistry and physics, there
needn’t be any unexplained psychological laws. As in all cases of
theoretical reduction, our body of knowledge has been unified
and thus our understanding of ourselves and the world has
been increased.
COLLEGE OF NEW ROCHELLE
2‘“In Search of the Engram”, p. 501.