Are Receptors Representations?

I. Introduction

By far the most dominant and successful picture of the mind during the history of cognitive science is what has come to be called the "Representational Theory". Of course, it might be somewhat misleading to refer to "the" representational theory, since there are many different theories in the sub-disciplines of cognitive science, and many of these invoke radically different notions of mental representation. In this paper, I want to look closely at one of these notions -- or more accurately, a family of notions -- that is becoming a popular way of thinking about mental representation in such fields as cognitive neuroscience, connectionist cognitive modeling, cognitive ethology and the philosophy of mind. The family of representational notions I'll explore is one I will simply call the "receptor notion"1. After explaining what I take this notion to involve, my aim will be to argue that it is not a useful theoretical posit of cognitive science. In effect, what I'll argue is that receptor representations are not really representations at all.

To show all this, my discussion will have the following organization. First, I'll try to spell out the basic idea of receptor representation as it appears in different areas of cognitive science. Of course, I can't provide an exhaustive survey of the many different theories in which this notion appears; however, I hope to provide enough of a sketch for you to get a rough sense of what this notion involves. Next, I'll raise what I take to be the single biggest problem for receptor representation – a problem I'll call the "functional specification challenge". As I hope to show, the problem suggests that the basic receptor notion requires some serious revision if it is to play any real explanatory role in cognitive science. To this end, I'll turn to philosophical work on mental representation – in particular, the work of Fred Dretske. I'll try to show that Dretske's own account of representation overlaps a great deal with the receptor notion, yet is significantly more sophisticated and robust. Thus, some have suggested that Dretske shows how the receptor notion can be enhanced in ways that could handle the functional specification worry. I'll explore two elements of Dretske's account that might be thought to do this – his theory of misrepresentation, and his discussion of how representations function. I'll then argue that neither one of these aspects of Dretske's account can save the receptor notion and that, in fact, Dretske's own account of representation is equally flawed. I'll conclude by further explaining just why, in light of these considerations, the receptor notion should be abandoned.

However, before we begin I want to make one preliminary comment about how I see all of this fitting in to the larger scheme of philosophical work on mental representation. In recent years, work on the nature of mental representation has become a major industry in the philosophy of mind. Almost all of this work has focused on a family of issues clustered around the core theme of explaining, in naturalistic terms, how mental states come to have intentional content. This topic is a perfectly natural one for philosophers of psychology to address. Questions about content are those which philosophers have special expertise, especially those trained in semantics and the philosophy of language.

But here my efforts will be aimed in a somewhat different direction. Rather than explore the various issues concerning mental representation content, I'm instead going to examine just what it is certain cognitive scientists seem to think representations do. While the two topics are closely related, I believe the latter topic has been severely neglected by the philosophic (and scientific) community. So while philosophers have hotly debated whether a frog's brain represents flys or small black dots, or how we should understand the content of a Twin- Earthling's beliefs, more fundamental questions regarding what, in naturalistic terms, a mental representation does, or exactly what it is about a structure that makes it a representation, have received very little attention.2 This is an oversight I hope to partly address in the course of this essay.

II. Receptors as Mental Representations

At the heart of the receptor notion of mental representation is the idea that because a given neural or computational structure is regularly and reliably activated by some distal condition, it should be regarded as having the role of representing (detecting, indicating, signaling, etc.) that condition. Such structures are viewed as representations because of the way they are triggered by environmental conditions. Unfortunately, in the scientific literature there is very little in the way of explicit exposition of this representational notion. Consequently, our understanding of the notion must be gleaned from the way it is employed in the descriptions and discussion of various theories.

One of the most frequently cited sources for the idea of receptor representations is Hubel and Weisel's (1962, 1968) important work on neurons in the visual system of cats, monkeys and rats. In a series of papers, Hubel and Weisel reported on the excitatory and inhibitory activity of neurons responding to visual stimuli, such as a slit of light presented at different angels of orientation. While they are commonly cited for discovering these so-called "edge detectors", in truth, Hubel and Weisel are extremely cautious about ascribing any sort of representational role to these neurons, and primarily restrict their discussion to their response profiles and receptive fields. By contrast, in their famous paper, "What the Frog's Eye Tells the Frog's Brain", Lettvin, Maturana, McCulloch and Pitts (1959), quite explicitly endorse the idea that certain neurons should be viewed as "detectors" precisely because they reliably respond to certain stimuli. Toward the end of their article, they note,

What, then, does a particular fiber in the optic nerve measure? We have considered it to be how much there is in a stimulus of that quality which excites the fiber maximally, naming that quality. . . We have been tempted, for example, to call the convexity detectors "bug detectors." Such a fiber responds best when a dark object, smaller than a receptive field, enters that field, stops, and moves about intermittently thereafter. (p. 1951)

This basic idea – that neural structures should be viewed as having the job of representing because they respond in a reliable manner to certain conditions – has become a common assumption in cognitive neuroscience. The following is typical of what one finds throughout the literature regarding the role of neurons involved in perceptual processing:

Neurophysiological studies . . . have identified individual neurons in the inferior temporal region of the macaque brain which fire in response to visual presentation of faces . . . Ensembles of such cells can thus be said to code for the presence of individual face stimuli or, less formally, to 'recognize' them. (Elliffe, 1999).

Indeed, researchers often skip the question of whether neural receptors function as representations, and instead ask about how the representational encoding is done. That is, researchers often begin with the assumption that neurons function as representations, and then explore, for example, whether the encoding is in single-cell "grandmother" representations, or instead distributed across a population of neurons. In more recent neuroscientific theories, this idea has been supplemented by introducing more formal theories of information, such as one suggested by Shannon and Weaver (1949). Since neurons often stand in some sort of nomic dependency relation to specific stimuli, they thereby qualify as "information carriers". Throughout many areas of the neurosciences today, especially in neuro-biological accounts of perception, representation and information carrying are often thought to be the fundamental tasks that neurons perform.

A similar notion of representation is increasingly becoming commonplace in certain forms of artifical intelligence. In particular, in connectionist modeling, the internal (so-called "hidden") units of networks are generally characterized as playing a representational role because of the way they respond to input to the network. A common task of multi-layer networks is to learn to discriminate between varying forms of stimuli. As a result of this learning, specific activation patterns of the internal units become correlated with specific types of input. If sonar echoes from undersea mines tend to generate patterns of one sort, whereas echoes from rocks generate patterns of a different sort, then those distinctive patterns are thought to function as internal "distributed representations" of mines and rocks (Gorman and Sejnowski, 1988). Connectionist modelers often note the similarities of these internal units to neural receptors, suggesting they provide a more biologically plausible form of representation than traditional computational symbols.

Another area where the receptor notion of representation appears quite frequently is cognitive ethology and studies of simple organisms. We already saw how some researchers have characterized neurons in the frog's brain as representations of bugs because of the way they respond to bug-type displays. However, not all cases of receptor representation involve neuronal states. In one famous example, researchers have suggested that iron traces employed in the propulsion mechanism of certain anaerobic bacteria serve as magnetesomes. Because these iron particles are reliably pulled in the direction of magnetic North and, as it happens, toward oxygen- free water, they are viewed as tiny compass-like representations that tell the bacteria which way to go (Blakemore and Frankel, 1981).

There are, of course, many more examples of the receptor notion employed throughout the cognitive sciences, but this should suffice to get across the basic idea. Some receptor representations exist deep within a given cognitive or neural system, whereas others are more on the periphery. Some are actual neurological structures, others are part of artificial intelligence systems, and still others are instantiated in different mediums. What we see with all of these is a common fundamental conviction that, roughly, an important type of cognitive representation results when some sort of internal state reliably responds to, is caused by, or in some way nomically depends upon, some external condition. When this occurs, such a state, whether it be called a "representation" or a "detector" or an "indicator" or some such, is viewed as having the functional role of representing that external condition.

To many philosophers working in naturalistic theories of meaning, none of this should come as much of a surprise. The central idea behind the receptor notion is a very old and familiar one in philosophy; namely, that there is a sort of "natural meaning"or "informational content" that results from a state's causal or nomic dependency upon some other state. It is this notion of representational content, made famous by Grice (1957), that comes into play when we use tree rings to inform us of the age of a tree, or that prompts us to say things like, "smoke means fire". Moreover, several philosophers have developed their own theories of content for mental representations by building upon this basic idea. In a minute, I want to say more about how this philosophical project links up with the receptor notion that is employed in cognitive science. But first, I want to raise an important worry for the receptor notion – a worry that I will suggest ultimately undermines its explanatory value.

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III. The Functional Specification Challenge.

A critical step in the analysis of any theoretical posit is the specification of the unique causal properties that give the posit its explanatory role and, indeed, help serve to define it. If we want to give a reductive analysis of genes, for example, then we need to first explicate the specific relations and causal roles that capture the sort of job we think genes perform (Kim, 1998). Then, with this functional specification in hand, we can go looking for the bio-chemical structures that fit the bill. Of course, along the way we may discover that our specification needs to be modified in some way. But we cannot make any progress in understanding how a given posit is actually realized unless we first have a fairly clear understanding of what it is the posit supposedly does. Among other things, the functional analysis should give us conditions that, when satisfied, uniquely instantiate the state in question. A functional specification would be inadequate if, say, it put forth properties possessed not only by the posit we want explicated, but by a wide array of other things as well.

Thus, in the case of cognitive representation, there needs to be some unique role or set of causal relations that structures possess that warrants our saying they serve a representational function. These roles and relations should enable us to distinguish the representational from the non- representational; hence they must go beyond the standard causal interactions commonly found throughout a biological system. For purposes of this paper, we can refer to this task as the "functional specification challenge". What we want is a functional specification that justifies our treating a certain state as playing a representational role.

Now given this, we can see that the receptor notion faces a prima facie difficulty. The problem is that the receptor notion does not appear to provide us with a model of representation that meets the functional specification challenge. That is, this notion of representation is one that fails to provide a demarcation between representational states and non-representational states. The property of reliably responding to some external condition is not, by itself, sufficient to justify treating some state or structure as a representation. After all, there are a wide range of non-
representational internal states that must, in their proper functioning, reliably respond to various states of the world. Our immune system, to take one example, functions in part by consistently reacting to infections and insults to our bodies. Yet no one suggests that any given immune system response (such as the production of antibodies) has the functional role of representing these infections in a cognitive system. While nomic dependency may be a necessary element of certain notions of representation, it clearly is not sufficient. Something more is needed if we want to invoke a notion of representation that is not explanatorily empty.

What could this "something more" amount to? Given the importance of intentional properties to any notion of representation, it is difficult to see how there could be a successful functional specification without an account of how the content of a representation is not only present, but in some way explanatorily relevant. It is hard (if not impossible) to see how there could be an account of something serving as a representation without a corresponding reason for treating the state's content as somehow germane to the job it performs. So it seems a minimal requirement for a successful functional specification of any notion of representation is that the content -- or, if you like, the fact that the representation has semantic content -- be an explanatorily relevant fact about that state. The information the state carries or transmits (whatever that may come to) must be in some way (though perhaps not causally) relevant to the sort of job it performs for the system. Otherwise, we would have no legitimate reason for treating a structure as some sort of representational device.

For example, the sort of task-decompositional explanations common to many classical computational models invoke sub-modules that must be understood as having input and output states representing things relevant to the sub-routine's function. If, say, we are to view a sub- module as serving as an adder, then we must regard its inputs as representations of numbers and its outputs as representations of sums. Consequently, our understanding of such models forces us to treat such states as serving as representations of certain items.

By contrast, the receptor notion, at least as it presented in many cognitive theories, carries no such motivation for treating a structure's causal or nomic regularities as serving to present information. There is nothing about the embedding architecture, or the explanatory strategy employed, that drives us to treat these structures as representations. Apart from some misleading language, the actual functional analysis provided for such systems suggests we should view their internal states as reliable causal mediators or perhaps relay switches, triggered by certain environmental conditions. These things have the function of causing something to happen in certain conditions. But then, many things function this way while properly performing their duties. This alone gives us no reason to treat them as representations.

It should be noted that other authors have, in different contexts, discussed concerns closely related the functional specification challenge (see, for example, Fodor, 1991). But despite their awareness of this issue, I do not believe philosophers have adequately addressed this worry, partly for reasons I hope to make clearer in a little bit. Briefly, I think there is a common assumption that by handling other problems associated with mental representation content, the functional specification problem becomes resolved in the process. Below, I'll try to show why this assumption is mistaken.

IV. Dretske to the Rescue

In the last section, I spelled out the core idea behind a popular notion of representation that appears in many cognitive theories; I also raised one central problem that at least initially seems to undermine the explanatory value of that notion. I would now like to turn to the philosophical support for this type of representation. After all, the scientific theories that employ the receptor notion were never intended to provide a robust philosophical defense of this posit. If we are going to give the receptor notion a fair hearing, we need look at philosophical accounts that are specifically tailored to handle concerns like the functional specification worry.

Unfortunately, finding a philosophical defense of a given notion of representation is not so simple. The problem is not that philosophers are unconcerned about mental representation; quite the contrary, it has been a central topic of philosophical discussion for the last 25 years. Rather, the difficulty lies with the types of issues philosophers have been concerned with. First, most philosophical writers do not see themselves as working to defend one or another particular notion of cognitive representation. Consequently, while there are aspects of the receptor notion that are shared by various philosophical accounts, it is far from obvious that many writers would describe their task as one of defending receptor representation. Secondly, not many philosophers of psychology have focused upon the functional specification problem per se. Hence, there has not been a wide-spread effort to spell-out causal relations or functional roles that are unique to representational states and could be used to help define and defend the receptor notion of representation. Much of this is unsurprising, when one looks at the standard way philosophers approach the topic of mental representation.

For the most part, the philosophical project has been one of providing a naturalistic theory of content for one specific family of representational notions; namely, propositional attitudes and, in particular, our ordinary notion of belief. Within the context of this project, the functional specification worry is much less of an issue. This is because the unique functional analysis that distinguishes beliefs from other cognitive states is already provided, at least in part, by our common sense or folk psychology. In other words, a rudimentary functional specification for our notion of belief is already given (for free) by the set of causal relations, that, in accordance with a functionalist theory of mental states, defines this mental state. To be a belief, as opposed to some other cognitive state, is to be something that interacts with inputs, other psychological states, and contributes to behavior in the manner prescribed by folk psychology.

Given this backdrop, it is natural to think that a complete theory of mental representation is a two-part endeavor. One part is what I have been calling the functional specification component, and, in the case of beliefs and other propositional attitudes, is already provided by our folk psychology. The other part is a story about how such a state acquires the content it has. It is in connection with this latter project where most of the philosophical effort has been devoted. So, many philosophers have adopted the project of providing a naturalistic theory of content for a notion of representation that comes with a functional specification more or less built in. Since the notion of belief tends to dominate philosophical work on mental representation, it is perhaps not surprising that philosophers have neglected other notions of representation along with their unique problems.

However, having said all that, I think there are some philosophical projects that come very close to providing what we are after. That is, some philosophical projects ostensibly devoted to providing a naturalistic account of belief content, in fact, provide what looks a great deal like a philosophical defense of the receptor notion of representation. Of these, perhaps the most notable and promising is the work of Fred Dretske (1988). Dretske offers an ambitious account of mental representation that is designed not only to provide a naturalistic account of content, but also to show how content is relevant to the production of behavior. While Dretske's stated focus is upon beliefs (or as he puts it, "proto-beliefs') I believe his project can be easily viewed as an account of low-level representational states that are part of the family I have been referring to as receptor representations. Indeed, his theory seems clearly motivated by examples of the very notion of representation we are trying to explicate, and many have appealed to Dretske as a way of defending receptor-style representations. What's more, because Dretske's account of content is so closely intertwined with an account of what it is for something to function as a representation, we see that he is, indeed, worried about providing a solution to what I have been calling the functional specification challenge. Thus, if anybody has given a carefully worked-out philosophical explication and defense of the receptor notion -- one that tackles the functional specification problem head-on -- it is Drestke. Consequently, showing what is wrong with Dretske's account will help us see the fundamental problem with receptor representation in general.

At the heart of Dretske's analysis of mental representation is the notion of "indication", a relation between two nomically connected conditions or states. For condition C to indicate another condition F, C must stand in a relation to F typically characterized by subjunctives like the following: If F had not occurred, C would not have occurred. This is roughly the same notion of informational content introduced earlier and, as we saw, shared by the receptor notion. 3

Thus, Dretske's account is built upon the same sort of content-grounding relation that is at the heart of receptor representations.

Yet as we saw in the last section, mere nomic dependency is insufficient to bestow full-blown representational status on cognitive structures. Dretske recognizes this, but for him the inadequacy is not couched in terms of our functional specification worry. Instead, Dretske focuses on a problem that has been the dominant concern of philosophers writing about mental representation for the last 15 years. The difficulty -- often called the problem of error or the "disjunction problem"-- concerns the nature of misrepresentation in causal accounts of content. If we assume that the critical content-bearing link between a representation and its intentional object is a dependency based on some form of causal connection, then it is difficult to see how there could ever be such a thing as misrepresentation. A frog's inner fly representation can also be triggered by a flying BB; yet if the inner state depends on both flies and flying BBs, it seems we really shouldn't say that it represents the presence of flies and misrepresents when triggered by BB's, but that it accurately represents the presence of flies or BB's. Hence, the purely causal or nomic dependency story does not allow for falsehood and, thus, fails to provide an adequate account of representational content.

To handle this difficulty, Dretske joins others -- such as Millikan (1984) and Papineau (1984) -- and introduces a teleological component to his account, thereby placing a tighter restriction on the sort of causal relations that matter. According to Dretske, internal indicators are elevated to full-blown mental representations when they are recruited -- either through natural selection or some sort of learning -- as a cause of certain motor output because of their relevant nomic dependencies. In other words, indicators become representations when they are incorporated into the processing so that they have a function of responding to certain conditions. Their job is to ensure that certain bodily movements are produced whenever the indicated condition obtains. So, in the example discussed earlier, the magnetesomes in anaerobic bacteria indicate the direction of magnetic North, which also happens to correlate with deeper, anaerobic water. Through a process of natural selection, these magnetesomes come to be wired to the bacteria's navigational system because of their nomic link to anaerobic water. They are thus given the functional role of indicating the direction of anaerobic water and, according to Dretske, thereby become anaerobic water representations.

This appeal to function does two things for Dretske. First, it enables him to handle the disjunction problem in an intuitively appealing way. Error is now possible because we can say that whenever an indicator is triggered by something other than what it is supposed to indicate-- something other than what it was, in fact, selected to indicate -- it is a case of misrepresentation. If the magnetesomes react to something other than magnetic north (and thus anaerobic water), we can say they are misrepresenting since they are responding to something other than what they were recruited to indicate. Second, it provides Dretske with a way of showing how informational content can be explanatorily relevant. Structures are recruited as causes of motor output because they indicate certain conditions. Thus, being an indicator is a causally relevant feature of a structure, and since Dretske regards the indication relation as a type of natural meaning, he suggests a type of meaning is causally relevant. Informational content serves as what Dretske calls a "structuring cause" – a feature that helps explain why a structure becomes incorporated into a system's functional architecture. It is in this way that Dretske believes meaning is explanatorily relevant and justifies our viewing a structure as having a representational function.

So on Dretske's account, to serve as a representation something must a) stand in some sort of nomic dependency relation to some distal state of affairs, and b) become incorporated into the processing because of this dependency, thereby acquiring the function of indicating those other states of affairs. While there are a number of further details and elements to Dretske's theory, for our purposes, the relevant question is whether there is anything here that might serve as the "something more" that could be used to beef-up the receptor notion of representation and resolve the functional specification problem. Many philosophers and cognitive scientists familiar with Dretske's work have assumed the answer to this question, or a least a similar one, is yes. Indeed, there appear to be at least two components of Dretske's account that could be thought to revitalize the receptor notion. The first is Dretske's teleological strategy for enhancing natural meaning in order to solve the disjunction problem. The second is Dretske's account of how content comes to play an essential role in the explanation of behavior. What I would like to do next is take a very close look at each of these elements. I plan to show that despite their initial appeal, neither of these factors can serve as an adequate answer to the functional specification problem for receptor representations. Consequently, what Dretske has to offer over and above nomic dependency does not help elevate the receptor notion and make it a legitimate notion of mental representation.

1. Does Dretske's Solution to the Disjunction Problem Help Solve the Functional Specification Problem?

Because the capacity for misrepresentation is considered a hallmark of intentionality, it is often assumed that the key to naturalizing content (and providing a theory of representation) is to explain in causal or physical terms how a state can succumb to error. As Millikan puts it "the core of any theory of representation must contain an explanation of how misrepresentation can occur" (1995, p. 186). And Dretske himself notes, "it is the power to misrepresent, the capacity to get things wrong, to say things that are not true, that helps define the relation of interest. That is why it is important to stress a system's capacity for misrepresentation. For only if a system has this capacity does it have, in its power to get things right, something approximating meaning" (1987, p. 65). So for many philosophers, explaining how something serves as a full-blown representation has largely been a matter of addressing the disjunction/error problem. It is in this way that I believe some have supposed that a solution the disjunction/error problem could serve as a solution to something like what I have been calling the functional specification challenge.

However, we need to bear in mind the specific way in which Dretske and others appeal to teleology to solve the disjunction/error problem. The relevant notion that does all the work is the notion of something malfunctioning -- of some device or element not playing the functional role it was designed or selected to perform. Of course, by itself the notion of malfunction (like the notion of function) has no intentional aspect or implication. Errors of this sort are ubiquitous throughout biological systems -- all sorts of states with non-representational functions are capable of malfunctioning. Indeed, all sorts of states with non-representational functions are capable of malfunctioning in virtue of being triggered by inappropriate or aberrant causes. So we don't get semantic notions of error, like falsehood or misrepresentation, through an appeal to teleology alone. One must first assume that the state in question is serving some sort of representational function, and then we can appeal to teleology to tell a story about how error arises through malfunction.

Consequently, the teleological solution to the disjunction/error problem cannot count as a solution to the functional specification problem because the former presupposes that the latter has already been given. That is, to solve the disjunction/error problem in the manner suggested by Dretske and other teleo-semanticists, we must begin with the assumption that the state in question has a representational function. The functional specification challenge raises the following question: "By virtue of what, exactly, is a state serving as a representation as opposed to something else?" However, the disjunction/error problem raises a different question; namely, "How does a representational state come to have a specific, appropriate sort of content as opposed to some sort of aberrant, disjunctive content; how do we show it represents flies and not flies or BBs?" Dretske's answer to the disjunction/error challenge involves showing what a state represents by appealing to what it ought to represent. But this leaves untouched the prior question about whether the state in question represents at all.

2. Does Dretske's Account of Representation Function Help?

If Dretske's account of misrepresentation cannot provide the "something more" needed to shore up receptor representation, then what about his account of how representations acquire their functional role? After all, a story about how a state comes to function as a representation seems to be the very thing we are after. Moreover, it is through this process that, according to Dretske, the structure's informational content comes to play an important explanatory role. So perhaps it is here that Dretske can give us something that can supplement the receptor notion and provide a solution to the functional specification challenge.

Recall again that for Dretske something becomes a full-blown representation when it acquires the function to indicate, and the key to acquiring that function is being recruited as a cause of some sort of physiological response because of the indication relation. In other words, a structure becomes a full-blown representation when it is recruited as a cause of motor output because it reliably responds to some sort of condition. As Dretske puts it,

These internal indicators are assigned a job to do in the production of bodily movement . . . in virtue of what they "say" (indicate or mean) about the conditions in which these movements have beneficial or desirable consequences . . .Learning of this sort mobilizes information-carrying structures for control duties in virtue of the information they carry (1988, pp. 98-99).

Dretske clearly assumes that if the relevant nomic dependencies are relevant to the proper functioning of the structure, then whatever information is carried by those dependencies must be relevant as well. The triggering mechanism of the Venus fly-trap "signals" the presence of insects because the mechanism reliably responds to certain kinds of movements. As he puts it, "there is every reason to think that this internal trigger was selected for its job . . . because it "told" the plant what it needed to know . . ." (p. 90). So the strategy for showing that information is causally relevant (and that something is thereby serving as a representation) is to show that the nomic dependency relation is causally relevant.

This assumption – that the explanatory relevance of information or natural meaning is apparently entailed by the explanatory relevance of some sort of nomic dependency – is a critical element of the receptor notion of representation. It is perhaps the main motivation behind the common tendency to treat things that respond as things that represent. If neural or computational structures are recruited as causes of bug-catching movements because of their tendency to react to bugs, then it certainly seems tempting to assume that they are serving as bug representations. But is this assumption warranted?

To answer this question, it will first help to get a clearer understanding of the relation between the purely causal/physical dependencies that are thought to "underlie" the indication relation, on the one hand, and the quasi-semantic, informational relation often said to be "carried by" these dependencies on the other hand. Although a great deal has been written about statistical notions of information and many writers – including Dretske – appear to reify information with expressions like "information flow" and "information carrying", we can here understand informational content in a fairly straightforward (and I hope relatively uncontroversial) way. Talk about information carrying can be understood as simply a way of saying that nomic relations between states of affairs allow us to use these states of affairs to discover things. When we say that A carries information about B, we do so because certain states of A depend upon states of B, and thus we (or someone or something) can use A to learn about B. Hence, there is, strictly speaking, nothing that is "carried by" A or "flows from" B to A. There is simply the natural dependency relation produced by a situation where certain facts regarding A entail certain facts about B. The (objective) fact that the bi-metallic strip carries information about ambient temperature, or is an indicator of temperature, comes down to this: states of the strip nomically depend upon the temperature in such a way that it can be used to learn facts about ambient temperature. Talk about the amount of information the strip carries is nothing more than a reference to the range of facts about temperature we can learn from it.

It is well worth noting that this situation is not restricted to the causal dependencies or co- variation relations that take center stage in theories of representation. Other sorts of nomic relations carry information as well. For example, if A is always larger than B, then A carries information about the size of B – the size of A can tell us something about the size of B. If A is heavier than B, or if A is always within a certain distance of B, then the weight or position of A can tell us something about the weight or position of B. Nomic relations carry information by virtue of entailment relations between facts expressing certain conditions of the world; being an information carrier is nothing more than being a thing that stands in some sort of relation to something else, such that the former can be exploited to gain knowledge about the latter.

Note that this analysis suggests that there is a significant difference between being an information carrier and being an informer. As Dretske himself notes, it is possible for something to be an information carrier (i.e., possess states that could inform about other state of affairs) without the relevant information being exploited (without anything actually using those states to learn about those states of affairs). To actually function as an informer, a structure needs to be plugged into the right sort of system in the right sort of way, such that the relevant entailment relations are put to a very specific sort of use.

Given this analysis, we can now formulate an answer to our earlier question regarding the explanatory connection – or lack thereof – between nomic dependency and informational content. Recall that our question was this: can the nomic dependency be explanatorily relevant without informational content being explanatorily relevant? Can something function as reliable respondent without functioning as an informer? And now the answer pretty clearly appears to be "yes". There are indeed a variety of ways in which something can be recruited because it stands in certain law-like relations with other things, without it being the case that it is recruited for the information that stems from those relations. Consequently, we can't establish that something functions as an information conveyer – as some sort of representation -- by showing only that the nomic relation that makes it a potential information conveyer is relevant to its function.

To help see this, it might help to first consider the sorts of informational relations just mentioned that do not, strictly speaking, involve co-variation . If it is a law that A is always heavier than B, then this relational property of A (along with A's weight) could be used to gain information about the weight of B. Thus, this "nomically heavier than" relation makes A an information carrier with regard to B. But this same "heavier than" relational property of A could be exploited in an entirely different way. It could be exploited instead to make A function as a counter- weight for B. Or, it could be exploited so that A is recruited as an anchor for B. Thus, the same "heavier than" relation could be explanatorily or causally relevant to a variety of different functional roles of A, not all (or even many) of which would involve the transmission of information. In other words, all of the following may well be true:

1) A could be recruited for a task because of a certain relation it stands to B.

2) The relevant relation happens to be one that makes A an information carrier with respect to B.

3) The task A actually performs is not representational, it is not one that involves the relaying of information.

Returning now to Dretske and receptor representation, this very same point applies to the various nomic dependency relations. The "reliably caused by" or "regularly co-varies with" relations can bestow a number of different functions upon a structure other than serving as some type of representation. In other words, the nomic dependency relation can be important for something's function, without the associated information being important. Suppose I decide that I want shade on my back porch a certain time in the afternoon. Paying close attention to the angles of the sun, I decide to plant a tall tree in a new location, knowing that its shadow will fall exactly where I want, when I want. Here, I would be exploiting a certain nomic regularity -- between the length of the tree's shadow and the position of the sun – to keep me cool. Of course, this very same nomic regularity is one that carries a considerable amount of information. The tree's shadow can be used to learn all sorts of things about the position of the sun and, with enough background information, I could use this arrangement as a crude sundial to tell me the time of day. However, given the way I am in fact using the tree's shadow, the information it carries is entirely irrelevant to its functional role. While the nomic relation that exists between the position of the sun and length of the shadow is relevant to explaining why the tree was planted and how the tree is employed, the fact that this nomic relation carries information is not.

Bearing these considerations in mind, we can now see a fundamental problem with Dretske's own cases of so-called representational systems. Consider the well-known example of a bi-metallic strip in a thermostat. If we look closely at the functional architecture of a thermostat, treated by Dretske as an example of an artificial representational system, it is far from clear why we should say that information carrying is the functional role of the strip, as opposed to reliably responding to certain conditions. Inside the thermostat, the strip is rigged up so that it switches on the furnace whenever the ambient temperature drops to a specified level. Thus, it serves as a reliable causal mediator between low temperatures and furnace activation. The functionality of the strip is to cause something to happen in very specific conditions. But in this respect, it is no different than many other devices that we ordinarily treat as having no representational function. The firing pin in a gun similarly bridges a causal gap between the pulling of the trigger and the discharge of the round. It also serves to reliably mediate between two distinct states of affairs -- to reliably go into a specific state when and only when a certain condition obtains. However, no one thinks the firing pin functions as some sort of representational device. We can say that both the bi-metallic strip and the firing pin carry information. But the information carried by the strip (like the information carried by the firing pin) is not explanatorily relevant to the job it performs inside a thermostat. Of course, the strip could play such a role. If we used the position of the strip to tell us about the ambient temperature, then its informational content would be vital to that role. But as it functions in a thermostat, as a condition-dependent trigger for furnace activation, there is no more reason to think that the strip serves to inform the furnace of the temperature than there is to think the firing pin serves to inform the shell about the status of the trigger.

Indeed, it appears the same thing can be said about nearly all of Dretske's cases of alleged representational systems. The triggering devices of carnivorous plants, the magnetesomes in anaerobic bacteria, the so called "bug detectors" of a frog's brain – none of these receptor-type examples provide any principled motivation for claiming that the structure in question is functioning to represent or inform. For instance, the iron deposits that serve as magnetesomes in anaerobic bacteria are wired to the bacteria's propulsion's devices because of the way they reliably respond to anaerobic conditions. We need some further reason, however, for thinking they are recruited into service because of the information that results from this relation. There is really no sense in which the bacteria's flagellum (their propellers) exploit the informational content carried by the magnetesomes -- no sense in which they use the magnetsomes to discover something about anaerobic conditions. It is one thing to serve as a causal mediator between A (anerobic conditions) and B (directional propulsion), it is an entirely different thing to serve as an informer about A for B.

It is important to be very clear on what is not being claimed here. I am not claiming that Dretske-
style indicators or receptors representations function in such a way that the relevant nomic dependency relations are irrelevant to their role. Instead, I'm claiming that these structures function so that the nomic relations that enable them to carry information are explanatorily relevant, though the information resulting from those nomic dependencies is not. In other words, these structure are employed qua-nomic-dependent or qua-reliable-respondent without being employed qua-information-carrier or, more to the point, qua-representation. It appears that what Dretske and proponents of receptor representation have failed to notice is that a structure can be employed because it reliably responds to some condition, without whatever information that arises from that relation being relevant to the structure's recruited task.

One obvious question worth asking at this stage is how we do we tell the difference between the two sorts of cases? How do we demarcate between: a) cases where only the nomic regularity is relevant to explaining the structure's function and b) cases where it makes sense to say that information is being exploited as well? What makes Dretske's examples of bi-metallic strips and magnetesomes cases of the former and not the latter?

While this is a huge question, we can begin to formulate an answer by contrasting uncontroversial examples of mere causal mediation with uncontroversial cases of actual representation. Consider the difference between the firing pin in a gun and the mercury in a thermometer. In the case of the former, there is no sense in which the information carried by the firing pin is exploited in its normal functioning. While an extended pin is a sign of a pulled trigger (along with a host of other things), nothing in any sense interprets or exploits this sign. By contrast, the mercury in a thermometer functions as a representation because there is a clear sense in which the information carried by the mercury is utilized – in this case, by people who want to learn about the temperature. This suggests that the way to distinguish structures that are serving as relay switches, causal mediators, and such, on the one hand, and structures that are serving as genuine representations, on the other, is that for the latter, but not the former, there results a sort of process that is properly or naturally viewed as a type of interpreting or learning about some other state of affairs. In other words, for representations there are explanatorily sound reasons that motivate construing their effects as involving interpretation. Perhaps one reason we mistakenly regard the bi-metallic strip in a thermostat or the magnetesomes in bacteria as representations (and not firing pins in guns or immune responses) is because they resemble actual representational devices – like thermometers and compasses – that, in their proper functioning, have this added dimension. But since this added element does not exist in thermostats and the bacteria -- since there is no explanatory motivation for thinking the furnace or the flagellum interpret -- we have no reason to view these and other receptor-like structures as representations.

This requirement might be taken to imply that I think representations are the sorts of things that simply cannot be elements of minds because they require a fully functioning mind in their employment. It seems given what I've said that to function as a representation is to function as something that is used by a cognitive system, not something cognitive systems are made out of. Dennett puts the point this way:

" . . . something is a representation only for or to someone; any representation or system of representation thus requires at least one user or interpreter of the representation who is external to it. Any such interpreter must have a variety of psychological or intentional traits . . . Such an interpreter is then a sort of homunculus. Therefore, psychology without homunculi is impossible. But psychology with homunculi is doomed to circularity or infinite regress, so psychology is impossible" (1978, p. 122).

However, as Dennett himself notes, representational psychology is indeed possible if we can get by without a full-blown mind as an interpreter. As we saw earlier, many psychological theories that invoke computational processes force us to regard certain processes as computing sub- routines relevant to the capacity being explained. But we can adopt such an analysis only if we view the input and output to these sub-routines as representing the inputs and outputs of the lower function being computed. Consequently, in the case of computational systems, there exist higher theoretical considerations, stemming from a specific explanatory strategy, that drive us to characterize certain states as playing a representational function – to regard them as information carriers that are exploited as such by the system. There is very real sense in which the output of an addition sub-module is consumed (to use Millikan's terminology) as representations of sums by subsequent computational modules. By contrast, there is nothing about the surrounding architecture of most receptor representations – nothing about the system in which they operate – that encourages this sort of computational, and, hence, representational analysis.

VI. Conclusion: Why Does It Matter?

I hope to have shown what is wrong with the receptor notion of representation and why it is mistake to view these sorts of internal states as representations. However, in response to my argument, people often complain that I am simply being stingy with the word "representation". After all, what is the harm in treating receptor states as some type of low-level representation? Although there may be no deep functional difference between firing pins or immune responses on the one hand, and states we call receptor representations on the other, what is wrong with simply stipulating the latter are working as primitive representations?

In response to this move, I am initially unsure of what to say. Certainly, we can choose to words however we like, and so we can call structures that have no representational function "representations", just as we can choose to call dogs "cats". But I suspect that this complaint stems from a more serious consideration. The more serious consideration is not so much about our how we use our words, but instead stems from a lingering pull toward what Dennett calls the "intentional stance" with regard to receptor-type systems. If we are so inclined, and nothing is really at stake by adopting this stance, then why not go ahead and do so? In truth, however, I actually think there is quite a bit at stake here. Let me close by offering three reasons for thinking that it is a bad idea to treat receptors as representations.

1) It Makes For Bad Science

When we treat various elements of cognitive systems as representations, we commit ourselves to a certain model or picture of the system's functional architecture. If we regard certain things as playing a role that they in fact are not playing, then that picture is inevitably going to be mistaken. The problem with receptor representation in cognitive explanation is similar to the following scenario: you are ignorant of how car engines work, so you investigate the operations of its various components. You notice that the firing rate of one component (which in fact is a spark-plug) nomically depends upon the depression of the car's accelerator pedal. You thereby conclude that the function of the spark-plug is to represent the position of the accelerator pedal, and subsequently base your understanding of the engine on this belief. The moral is clear: If you always assume that something functions as a representation simply because it reliably responds to something else, then your functional analysis of the system may prove to be well off the mark.

While many cognitive scientists routinely employ the receptor notion without much hesitation, at least some have come to recognize its potential for damage. In a short but important article, neuroscientists Walter Freeman and Christine Skarda (1990) describe how their research on the olfactory system was de-railed for several years because they mistakenly assumed that the electrical activity of certain neural arrays functioned to represent olfactory stimuli because of the way they responded to that stimuli. This interpretation led them away from a more accurate understanding of the roles of these neural states in brain dynamics. As they put it,

We have found that thinking of brain function in terms of representation seriously impedes progress toward genuine understanding . . . The EEGs of the olfactory bulb and cortex show a brief oscillatory burst of potential that accompanies each inhalation . . . For more than 10 years we tried to say . . . that each burst served to represent the odorant with which we correlated it . . . This was a mistake. After years of sifting through our data, we identified the problem: it was the concept of representation (1990, p 376).

Freeman and Skarda go on to conclude "the idea of representation is seductive and ennervating, promising good deals but delivering nothing new. When researchers refrain from using the term, knowledge of brain function is not significantly affected. We conclude that use of the term is unnecessary to describe brain dynamics" (1990, 375-376).

While I'm not prepared to go so far as Freeman and Skarda and banish the notion of representation altogether from cognitive neurosciences, I do believe their testimonial provides clear case of how the receptor notion can undermine scientific progress.

2) It Makes For Bad History

Our tendency to treat cognitive transducers or relay structures as representations has led to a mistaken conception of the history of psychology and cognitive science. The so-called "cognitive revolution" that occurred in the 50's and early 60's, whereby behaviorism was replaced by cognitive science, is thought to have involved a paradigm shift of Kuhnian proportions. It is generally thought that a major aspect of this transition involved the introduction of internal representations in several cognitive theories. However, many behaviorist models posited complex internal states that mediated transitions between stimuli and conditioned responses. Often these internal states were viewed as facilitators or relay switches, but behaviorists deliberately avoided mentalistic talk about representation. Given the dubious nature of receptor representations, it looks like they were right to do so. One of the few differences between some of these behaviorist theories and many contemporary models in the cognitive neurosciences is simply that the latter characterize these mediator states as representations. If this characterization is completely artificial and unwarranted, as I've suggested here, the cognitive revolution may be a lot less dramatic and far-reaching than it is currently assumed to be.

3) It Makes For Bad Philosophy

Finally, insofar as the receptor notion does not hold up to philosophical scrutiny, then, of course, any philosophical theory based upon the receptor notion is going to be on shaky ground. While philosophical accounts such as Dretske's can be used to support the receptor notion, the relationship between philosophy and science in this area is quite reciprocal. There is no doubt that various philosophical accounts of representation and content are inspired, at least in part, by the notions of representation employed by cognitive scientists and it is clear that a good deal of what philosophers say about content and representational function is inspired from the receptor notion discussed here. If this notion is fundamentally ill-conceived, then any philosophical project designed to incorporate and defend it will be misguided.

For example, the receptor notion encourages an unfortunate tendency to suppose that being a representation is, by and large, a function of a state's causal link to stimuli. Philosophical work on mental representation has largely adopted this attitude, where most of the work is concerned with the different head-world connections that might bestow content on a state. How much of this work will actually enhance our understanding of mental representation has yet to be seen. A more fruitful approach might be to focus upon the sorts of functional roles different types of representation play in cognitive models. A shift away from questions about how representations are produced and more toward questions about how they are used would not only help us understand more about representation function, but there is good reason to think it would also provide a more helpful perspective on representation content.4

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