659

Endnotes, page 670

660 The typical associationist method was found radically faulty. i The first English psychologist fully to recognize this was James Ward, and the brilliance and originality of his psychological writings were mainly due to this recognition. He made a devastatingly triumphant attack upon associationism.1 Sir Frederic Bartlett, 1932

What is conspicuous in its absence is any discussion of convincing victories for Hebbian learning, though a few interesting works are listed. It is fair to suggest, however, that Hebbian successes have been modest.2 Jeffrey Foss, 1997

Scientists dream up phantasies and then pursue a highly selective hunt for new facts which fit these phantasies. This process may be described as “science creating its own universe” (as long as one remembers that “creating” here is used in a provocative, idiosyncratic sense). A brilliant school of scholars (backed by a rich society to finance a few well-planned tests) might succeed in pushing any fantastic programme ahead.3 Imre Lakatos, 1978

B. Refutatio: Associationism and the Hebb Rule

When an axon of cell A is near enough to excite a cell B The crux of Hebb’s insight, slightly reconstructed, is this: and repeatedly and persistently takes part in firing it, coactivation of connected cells should result in a some growth process or metabolic change takes place in modification of weights so that the probability of the one or both cells such that A’s efficiency, as one of the postsynaptic cell firing given the presynaptic cell fires, is cells firing B, is increased.4 increased. This makes eminently good sense, because it I have made it an essential condition of learning that allows for associated world events to be represented by cell two central events occur together.5 interaction.8 Donald Hebb, 1949 Patricia Churchland & Terrence Sejnowski, 1992 HailiHebb! The idea that information is stored in neural circuits by When two neurons fire at the same time, the synapses changing synaptic links between neurons was first between them get strengthened. (It is easily summarized in proposed in 1949 (Hebb, 1949).9 the phrase “Fire together, wire together.”) We now know Hebb was basically right. Of course, nothing in nature is Robert Plomin, John DeFries, Gerald McClearn, ever quite so simple, and in real brains the details are & Peter McGuffin, 2008 more complicated. Our nervous systems run many variations of Hebb’s learning rule; for instance, some Certain findings can be interpreted as an example of an synapses change their strength in response to small idea first proposed by Donald Hebb in the 1940s: variations in the timing of neural signals, some synaptic Synapses are strengthened when the pre- and post-synaptic changes are short-lived, and some changes are long-lived. elements are synchronously active.10 But Hebb was erecting only a framework for the study of learning, not a final theory, and that framework has been Eric Kandel, Thomas Jessell, & Joshua Sanes, 2000 incredibly useful.6 Jeffrey Hawkins, 2004 Hebb was the first to propose explicitly the conditions under which the change in efficacy would occur.11 The Hebb rule forms the basis of much of the research done on the role of synaptic plasticity in learning and Christof Koch, 1999 memory.7 D.O. Hebb first proposed the mechanism.12 Paul Churchland, 1989 i In Chapters 1 to 13 “associated” was often replaced by “connected.”

661 Hebb was NOT the First to Propose “the Hebb Rule” Sensations A, when impressed alone, shall be able to excite in the Mind, b, c the Ideas of the rest.18 Now there is a basic law of association by stimulation, David Hartley, 1749 which operates in the case of neuronal activity, of reproductive remembering, and which is the foundation of all links between the memory neurons. We find that a Hartley’s notion that experiences consistently occurring drIve passes over to another, B, if neurons A and B have at together are recorded in the brain as an interrelated some time been simultaneously activated by some other package will make one conscious of the entire package is neurons. Thus a synapse has been facilitated through the remarkably modern. Donald Hebb reached essentially 13 simultaneous activation of A-B. the same conclusion 200 years later.19 Sigmund Freud, 1895

Our perceptions are linked with one another in our Although this rule strongly influenced new memory - first and foremost according to simultaneity of connectionism, it was not original with Hebb. It is based occurrence. We speak of this fact as association.14 on the associative laws of contiguity and frequency that go back at least to Aristotle; and David Hartley Sigmund Freud, 1900 anticipated Hebb in applying these associative principles to neural activity by 200 years. William James also Aristotle (384-322 B.C.) postulated his now famous laws anticipated Hebb’s rule, and Pavlov’s neurophysiological of association. The most basic law of association is the explanation of the development of conditioned reflexes law of contiguity, which states that when we think of followed Hartley and James very closely.20 something, we also tend to think of things that were experienced along with it. Aristotle’s laws of association The general idea is an old one, that any two cells or were to become the basis of learning theory for more systems of cells that are repeatedly active at the same than 2000 years. In fact, the concept of mental time will tend to become associated, so that activity in association is still at the heart of most theories of one facilitates activity in the other.21 learning. The belief that one or more laws of association can be used to explain the origins of ideas, the phenomena of memory, or how complex ideas are A Source Amnestic Reentrance of the Hebb Rule formed from simple ones came to be called “associationism.”15 Two maps of neuronal groups receive independent inputs (1 and 2). Map 1 responds to local features (for example, Since Aristotle, Western philosophers have traditionally visually detected angles) that are different from those to thought that learning is achieved through the which map 2 responds (for example, an object’s overall association of ideas. This concept was systematically movement). The two maps are connected by nerve fibers that carry reentrant signals between them. These fibers are developed by John Locke and the British empiricist numerous and dense and serve to “map the maps” to each school of philosophy, important forerunners of modern other. If within some time period the groups in map 1 are psychology.16 reentrantly connected to the groups in map 2, these connections may be strengthened. As a result of reentrant David Hartley (1705-1757) believed that sense signaling, and by means of synaptic change, patterns of impressions produced vibrations in the nerves, which responses in map 1 are associated with patterns of traveled to the brain causing similar vibrations in the responses in map 2 in a “classification couple”. Because of “medullary substance” of the brain. The brain vibrations synaptic change, responses to present inputs are also 22 caused by sense impressions give rise to sensations. After linked to previous patterns of responses. sense impressions cease, there remain in the brain As groups of neurons are selected in a map, other groups in reentrantly connected but different maps may diminutive vibrations that Hartley called “vibratiuncles.” also be selected at the same time. Correlation and 17 It is the vibratiuncles that correspond to ideas. coordination of such selection events are achieved by reentrant signaling and by the strengthening of Any Sensation A, B, C by being associated with one interconnections between the maps another a sufficient Number of Times, get such a Power within a segment of time.23 over the corresponding Ideas a, b, c that any one of the Gerald Edelman, 1992

662 Most Modelers are Politely Loyal to the Royal Rule Hebb’s idea has been incorporated into many models of neuronal competition and cooperation. It is also the basis Most modeling of neural learning networks has been of several mathematical models of activity-dependent based on synapses of a general type described by Hebb competition between afferent fibers from the two eyes. 24 These models simulate with remarkable accuracy the (1949) and Eccles (1953). At a very specific level, segregation of ocular dominance columns during Hebbian learning, as conveyed by the Hebb rule development.29 continues to be applied even in the most recent subsystems.25 Eric Kandel, Thomas Jessell, & Joshua Sanes, 2000

The O’Reilly and Johnson (1994) model provides a good Certain experimentally observed forms of plasticity can example of how connectionism may help us to understand be described at a more abstract level in terms of synaptic architectural biases in neural networks. Once the algorithms, in particular by temporally asymmetrical activation state of the network had reached equilibrium Hebbian learning rules. Such formulations are useful for each position, the weights between all units in the because a great deal is known from the literature on system were adjusted according to an associative Hebbian artificial neural networks about the computational 30 learning rule.26 possibilities of Hebbian synapses. Jeffrey Elman, Elizabeth Bates, Christof Koch, 1999 Mark Johnson, Anette Karmiloff-Smith, Domenica Parisi, & Kim Plunkett, 1996 In this chapter we largely focus on activity-dependent synaptic plasticity of the Hebbian type, meaning plasticity An associative net is an abstract wiring diagram that has based on correlations of pre- and postsynaptic firing. To been studied by theoreticians such as Marr, Christopher ensure stability and to obtain interesting results, we often Longuet-Higgins, Leon Cooper and others. The strength must augment Hebbian plasticity with more global forms of the connection is adjusted “by experience” on the basis of synaptic modification that, for example, scale the 31 of certain well-defined rules, usually so that pathways that strengths of all the synapses onto a given neuron.

are often activated together are strengthened in some way. Peter Dayan & L.F. Abbott, 2005 Such nets can serve to fine-tune a system that has been partly precision-wired or to assist in the recall of a complex output when an input (or better still a partial input) of Without appropriate adjustments of the synaptic something often associated with it arrives. Seeing a plasticity rules or the imposition of constraints, Hebbian person’s face, you remember that person’s name modification tends to produce uncontrolled growth of (although, alas, not in all cases). You may still be able to do synaptic strengths.32 it even though you have seen only part of the person’s face. The higher nervous system appears to be an exceedingly cunning combination of precision wiring The simplest implementation of saturation is to set any and associative nets.27 weight that would cross a saturation bound due to Francis Crick, 1979 application of a plasticity rule to the limiting value. Uncontrolled growth is not the only problem associated Hebbian learning principles can explain most of the with Hebbian plasticity. Synapses are modified cortical behavior I have mentioned in this chapter. independently under a Hebbian rule, which can have Remember, it was shown back in the 1970s that auto- deleterious consequences. For example, all of the synaptic associative memories using the classical Hebbian learning weights may be driven to their maximum allowed values, algorithm can learn spatial patterns and sequences of causing the postsynaptic neuron to lose selectivity to patterns. The main problem was that memories couldn’t different patterns of input. The development of input handle variation very well. According to the theory selectivity typically requires competition between different proposed in this book, the cortex has gotten around this synapses, so that some are forced to weaken when others limitation partly by stacking auto-associative memories in a become strong. We discuss a variety of synaptic plasticity hierarchy and partly by using a sophisticated columnar rules that introduce competition between synapses. In architecture.28 some cases, the same mechanism that leads to competition Jeffrey Hawkins, 2004 also stabilizes growth of the synaptic weights. In other cases, it does not, and saturation constraints must also be 33 imposed. Peter Dayan & L.F. Abbott, 2005

663 Dayan and Abbott (2005) then go on to discuss several Although Hebbian synaptic plasticity is a seductive decades’ of attempts to rescue the Hebb Rule. But concept, it suffers from a number of problems. First, nowhere do they discuss whether the Hebb Rule is synapses are modified whenever correlated pre- and essentially right or downright wrong.34 postsynaptic activity occurs. Such correlated activity can occur purely by chance, rather than reflecting a causal Within those areas to which the paradigm directs the relationship that should be learned. A second problem attention of the group, normal science leads to a detail of purely Hebbian modification; it is not competitive, so of information and to a precision of the observation- constraints must be added to obtain interesting results.39 theory match that could be achieved in no other way. Furthermore, that detail and precision-of-match have a value that transcends their not always very high intrinsic Quartz & Sejnowski’s (1997) main accomplishment is the presentation of increasing complexity in the developing 35 interest. brain. Although this cuts a colorful swath through current theories of learning, it leaves the central question untouched: How does the environment direct neural Insightful Modelers Dismiss the Hebb Rule structure? In answer: Q&S offer us only Hebb’s half- century-old suggestion once again. What is conspicuous The standard selection of synaptic dynamic equation is in its absence is any discussion of convincing victories for attributed to the “correlation learning” hypothesis of Hebbian learning, though a few interesting works are Hebb (1949), which is simply that concurrent activation listed. It is fair to suggest, however, that Hebbian of nodes increase the “synaptic efficacy” or strength of successes have been modest.40 the connection between them. In a nutshell, Jeffrey Foss, 1997

é = ci cj (1) We agree with nearly everything Foss discussed in his 41 insightful commentary. Apart from referencing Hebb’s (nonmathematical) conjecture, it seems the operative argument for using (1) Steven Quartz & Terrence Sejnowski, 1997 the Hebb “correlation learning” rule is simply that everyone uses it. For surely the problems with it warrants investigating alternatives. To begin, (1) promotes Experimentalists See What They Expect to See spurious causal connections. If any two processors or nodes are active in a network, no matter how big the What a man sees depends both upon what he looks at network, how far apart the nodes, or how independent and also upon what his previous visual-conceptual their patterns of activation, the Hebbian law (1) grows a experience has taught him to see.42 causal connection between them. Concomitant activation replaces concomitant variation. Worse, the Hebb’s hypotheses were an attempt to understand the spurious causal attributions tend to grow exponentially development and the organization of behavior based on fast (as can be seen from the exponential form of the the anatomical and physiological data available to him, exact solution of (1)). In practice this necessitates though they did not constitute a model for learning or “hardclipping” of interconnects both during training memory in a formal sense. Some 40 years later we now and classification sessions.36 have solid physiological evidence, verified in several laboratories, that long-term potentiation (LTP) in some The Hebb rule in its original form is unstable, i.e. their parts of the mammalian hippocampus follows the Hebb synaptic weights will all be driven to their maximal rule.43 Hebb’s learning rule has led to a fruitful line of 44 value.37 experimental research and a rich set of network models.

Terrence Sejnowski & Gerald Tesauro, 1989 See the instability of the Hebb Rule: http://www.principia-psychologica.edu/TheHebbError.mov But we are now critically aware that such “evidence” can There is a serious limitation to Hebbian learning, which and should be interpreted in alternative ways (Chapters is that all is can learn is pair-wise correlations.38 3, 5, 7, 10, 11). In particular, induced LTP can potentially have at least four different causes:

664 1. simultaneous pre- and postsynaptic firing levels If we may ask how the brain develops its manifold maps of Hebb’s Rule) OR various abstract feature domains, developmental 2. simultaneous changes in pre- and postsynaptic firing levels, neuroscience already holds out the sketch of an answer. OR Hebbian learning is a mindless, subconceptual process that 3. lagged (non-synchronous) pre- and postsynaptic firing continually adjusts the strengths or “weights” of the levels, OR trillions of synaptic connections that intervene between 4. lagged changes in pre- and postsynaptic firing levels.45 one neuronal population and another. The Hebbian process of weight adjustment is systematically sensitive to First critical question: How were the “Hebb-proving” temporal coincidences among the many axonal messages LTP-experiments designed in order to conclude that arriving, from an upstream population, to a given neuron the cause of LTP was the Hebb condition (1), and not in the receiving population. Specifically, if a cadre of the condition of (2), or not the condition of (3), or not connections, a subset among the great many connections 46 to a given neuron, repeatedly bring their individual the condition of (4)? messages to the neuron at the same time, then the weight of each connection in that united cadre is made Second critical question: Regardless its causes, did this progressively stronger. As neuroscience undergraduates are LTP effect “some growth process or metabolic change taught, “Neurons that fire together, slowly wire together.”53 take place in one or both cells”? We must ask, because when it comes to learning and memory such structural Paul Churchland, 2007 changes (memory) must never be confused with mere postsynaptic potentiation (e.g., LTP), as some do 47 Recent evidence suggests that afferent connections to neurons in the somatic sensory cortex are formed on the basis of correlated firing. It is thought that cells that fire Memories are made of long-term synaptic changes, called together wire together! 54 Long-Term Potentiation.48 Eric Kandel, 2000

The finding that LTP in the Schaffer collateral pathways requires simultaneous firing in both the postsynaptic and If I understand them correctly, these thinkers believe presynaptic neurons provides direct evidence that synaptic associationism describes most or all of the for Hebb’s rule, proposed in 1949 by the psychologist learning going on at the neuronal level, and that Donald Hebb. A similar principle is involved in fine- observed differences in the emergent forms of learning - tuning synaptic connections during the late stages of of gratification (Chapter 5), of anticipation (Chapter 7), of development.49 conceptualization (Chapter 10), and of causation (Chapter 11) Eric Kandel, 2000 - are due to innate differences in “neural subsystems.” While I can understand how a thinker may paint Science does not deal in all possible laboratory himself into such a corner, I have no idea on how to get 55 manipulations. Instead, it selects those relevant to the him out of there, if this book does not do the trick. juxtaposition of a paradigm with the immediate experience that that paradigm has partially determined. A brain can be victimized by a highly biased “training As a result, scientists with different paradigms engage in set.” Suppose the brain has reached a heIght different concrete laboratory manipulations.50 configuration that allows it to respond successfully to all of the examples in the (narrow and biased) set it has so NMDA receptors operate as a kind of molecular far encountered. Subsequent exposure to the larger coincidence detector as they are required for a biochemical domain of more diverse examples will not necessarily implementation of Hebb’s learning rule.51 result in the system’s moving any significant distance away from its earlier configuration, unless the relative Wulfram Gerstner & Werner Kistler, 2002 frequency with which it encounters those new and anomalous examples is quite high. For if the encounter The discovery of the NMDA-receptor, which seemed to frequency is low, the impact of those examples will be have the right kinetics to implement the Hebbian rule, insufficient to overcome the gravity of the false also generated a great deal of excitement and offered the minimum that captured the initial training set. The possibility that this economical rule could underlie system may require “blitzing” by new examples if their important facets of neural development.52 56 collective lesson is ever to “sink in.” Steven Quartz & Terrence Sejnowski, 1997

665 Refutatio: Aristotle’s Associationism & Hebb’s Rule Do all our experiences outside the laboratory associate themselves spontaneously? This again is not the case. We If we try to use these broad associative categories as may hear a telephone number dozens of times together explanatory principles, there naturally arises a belief that with a name, and may still remain unable to recall it the essential course of which mental structure is when the name occurs again. Between the name and the gradually built up is by the adding together of number there are no specific relations; they do not tend innumerable details which originally had no connexion to form a group spontaneously.61 whatever one with another. But the typical associationist method was found radically faulty. The first English When Sylvie’s teacher says, “No, no, six times eight is psychologist fully to recognize this was James Ward, and forty-eight, not forty nine,” Sylvie may in the twinkling the brilliance and originality of his psychological of an eye shed her error and learning the right answer writings were mainly due to this recognition. He made a for good. Neither she, nor her teacher, not anyone else, devastatingly triumphant attack upon associationism. has any idea which synaptic connections to weaken or Ward would have nothing to do with similarity as a which to strengthen - so how do the neurons know? principle of association, and he replaced contiguity by Neither Hebbian nor neo-quasi-Hebbian learning have succession.57 much to tell us in answer to this question.62

The fact is, great as are the advances that psychology In short, Hebb’s First Rule - his most famous - is in owes to the doctrine of association, the time has come to error.63 question its finality and to circumscribe its range. The restriction here contended for is one which the earlier writers on association fully allowed: association was Hebb’s Second Rule: No Brain, No Pain wholly confined to ideas that to begin with are distinct and that to the end are separable.i The process by which Lobotomy, a surgical operation that severs most of the ideas arise from impressions cannot then be explained connections between the frontal pole and the rest of the ii by association. And for long no such explanation was brain, and electroshock treatment, in which an electric attempted, but the practice of regarding ideas as merely current passed through the skull induces a brief the residues of sensations prepared the way for such convulsion, both appear to work best with patients suffering from emotional depression,iii and both attempt and the confusion to which it has led.58 frequently have the effect of banishing the depression.64

This mistaken identification by the Associationist Donald Hebb, 1949 psychology of later processes with simpler and earlier ones, which fail to explain them, has not only obscured the science with inappropriate concepts but has prevented the question which we have treated - that concerning the genesis and development of the psyche - from being ever effectually raised. The discussion of this question in Chapters 1 to 13 will incidentally yield the ii Carlson (2001, p. 348): The surgeons introduced various kinds of cutting 59 devices into the frontal lobes and severed white matter (bundles of axons). best refutation of such views. One rather gruesome procedure did not even require an operating room; it could be performed in a physician’s office. A transorbital leucotome, shaped From the point of view of science the law of association like an ice pick, was introduced into the brain by passing it beneath the upper eyelid until the point reached the orbital bone above the eye. the instrument by contiguity appears to me as a strange argument. Two was hit with a mallet, driving it through the bone into the brain. The end was processes A and B happen to occur together and, then swept back an forth so that it cut through the white matter. The patient whatever the nature of A and B may be, a bond is often left the office within an hour. ... In Norway lobotomy was used more than in any other country in the world. (Johannesen, 2006, p. 20) formed between them! I do not know a single law in iii Carlson (2001, pp. 348-349): Only after many years were careful studies physics or chemistry which could in this respect be performed on the side effects of the procedure. These studies showed that compared with the law of contiguity.60 although patients did perform well on standard tests of intellectual ability, they showed serious changes in personality, becoming irresponsible and childish. They also lost the ability to carry out plans, and most were i Ward (1919, p. 83): If the term “association” is to be correctly used, it must unemployable. And although pathological emotional reactions were imply that the presentations associated were from the first distinct, were eliminated, so were normal ones. What we know today about the effects of attended to as distinct, became associated solely in consequence of such prefrontal lobotomy - whether done transorbitally or by more conventional attention, and remain to the last distinguishable. means - tells us that such radical surgery should never have been performed.

666

667

668

The Spirit of Refutatio: Associationism and the Hebb Rule

To what extremes have we come in psychology, if it is necessary seriously to discuss such a thesis.

Wolfgang Köhler, A.D. 1947

669 Endnotes Appendix B 41 Quartz & Sejnowski (1997) p. 583; reply to 1 Foss’s comment Bartlett (1932) p. 206 42 2 Kuhn (1970) p. 113 Foss (1997) p. 565; comment to Quartz & 43 Sejnowski (1997) Sejnowski & Tesauro (1989) p. 94; my 3 underline Lakatos (1978) pp. 99-100 44 4 Hebb (1949) p. 62 Sejnowski & Tesauro (1989) p. 101; my 5 underline Hebb (1949) p. 127; my underline 45 6 JH Hawkins (2004) p. 164; my underline 46 7 JH Dayan & Abbott (2005) p. 281 47 8 JH, quoting Hebb (1949) p. 62 Churchland & Sejnowski (1992) p. 251; my 48 underline Plomin, DeFries, McClearn, & McGuffin 9 Plomin & al. (2008) p. 301; my underline and (2008) p. 301; my underline 49 Kandel (2000) p. 1260; my underline boldface 50 10 Kandel, Jessell, & Sanes (2000) p. 1122; my Kuhn (1970) p. 126 51 Gerstner & Kistler (2002) p. 53; my underline underline and boldface; ‘certain’ replaces ‘these’ 52 11 Koch (1999) p. 322; my underline and boldface Quartz & Sejnowski (1997) p. 543; mye 12 underline Churchland (1989) p. 186 53 13 Churchland (2007) p. 235; my underline Freud (1895, p. 319), the pioneering brain- 54 psychologist, had to invent his terms: ‘neuronal’ Kandel (2000) p. 388; my underline 55 JH replaces ‘pure ψ’; ‘memory’ replaces ‘ψ’ since the 56 ψ neurons “are the vehicles of memory and so Churchland (1989, p. 192; ‘brain’ replaces probably of psychical processes in general” (p. ‘system’ ; ‘heIght’ replaces ‘weight’). The 300); ‘drIve’ replaces ‘Q[uantity]’; ‘activated’ Five-Decade Society of Synaptic Associationism is obviously such a highly biased training set. replaces ‘cathected’; ‘synapse’ replaces ‘contact- 57 barrier’; ‘activation of’ replaces ‘cathexis’; my Bartlett (1932) p. 206 58 Ward (1919) p. 191 underline 59 14 Freud (1900) p. 539; my underline Ward (1919) p. 179; ‘which we have treated’ 15 Hergenhahn (1997) pp. 44-45; my underline replaces ‘on which we are entering’; ‘the psyche’ 16 replaces ‘ideas‘; ‘in Chapters 1 to 13’ inserted Kandel, Kupfermann, & Iversen (2000) p. 1240 60 17 Köhler (1947) p. 258 Hergenhahn (1997) p. 129 61 18 Köhler (1947) p. 264 Hartley (1749/1834) p. 41, quoted in 62 Hergenhahn (1997) p. 129; my underline Foss (1997) p. 566; comment to Quartz & 19 Sejnowski (1997) Hergenhahn (1997) p. 129 63 20 Hergenhahn (1997) p. 566 Hokland (2005, p. 49; ‘first’ inserted; ; ‘his 21 Hebb (1949) p. 70; my underline most famous’ inserted). Professor Nestor 22 Schmajuk at Duke University was asked by my Edelman (1992) p. 90; my underline 23 Edelman (1992) p. 85 university’s Psychology Department to grade my 24 Hopfield (1982) p. 462 thesis, but on reading it he refused to pass it. 25 Quinlan (1991) p. 6 Then professor Leigh McCullough at Harvard 26 Elman & al. (1996) pp. 327-330; my underline University and professor Thomas Hoff at 27 Crick (1979) pp. 187-188; my underline University of Oslo were asked to evaluate it. They 28 Hawkins (2004, p. 164, my underline; 174): In called it difficult to judge because of its form, and gave it an A. this chapter, I have introduced many speculative 64 ideas on how the neocortex works. I expect that Hebb (1949) pp. 271-272 several of these ideas will prove to be wrong, and probably all of these ideas will be revised. 29 Kandel, Jessell, & Sanes (2000) p. 1122; my underline: Except from making data more pleasing to look at - has data fitting ever contribute anything to science? “That detail and precision-of-match have a value that transcends their not always very high intrinsic interest.” (Kuhn, 1970, p. 65) 30 Koch (1999) p. 329; my underline 31 Dayan & Abbott (2005) p. 283; my underline 32 Dayan & Abbott (2005) p. 284 33 Dayan and Abbott (2005) p. 284; my underline 34 JH 35 Kuhn (1970) pp. 64-65 36 Kosko (1986) p. 278 37 Shouval & Perrone (1995) p. 745 38 Elman & al. (1996) p. 57 39 Song & al. (2000) p. 924; ‘seductive’ replaces ‘powerful’ 40 Foss (1997) p. 565; comment to Quartz & Sejnowski (1997)

670