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MIND-BRAIN SUPERVENIENCE & NEURAL RECRUITMENT

Abstract: The doctrine that at least some mental properties supervene on neural properties is nearly universally accepted among philosophers of mind. The spectrum of supervenience relations is broad, but all of them require that identity with respect to base properties in the actual world necessitates identity with respect to supervening properties in the actual world. Hence, it is nearly anodyne among philosophers of mind that identical neural properties necessitate identical mental properties for at least some mental properties. This paper points to a phenomenon, neural recruitment, that speaks against this popular thesis.

Jerry Fodor has claimed that “any scientifically useful notion of psychological state ought to respect [mind-brain] supervenience.”1 His reasoning seems to be as follows.

(i) In order to be scientifically useful, a state must have causal powers

(ii) In order to have causal powers, psychological states must respect mind-brain supervenience, for:a. Psychological states per se do not have causal powers,

butb. Brain states have causal powers, andc. Subvening states “transfer” their causal powers to

supervening states

Hence:(iii) In order to be scientifically useful, psychological states

must respect mind-brain supervenience

This conclusion is not peculiar to Fodor, though. Rather, psycho-neural supervenience is widely claimed and assumed by contemporary philosophers of mind. This paper asks whether some of the best-motivated theoretical equipment of present psychology respect mind-brain supervenience. In particular, we shall ask whether the

1 (1987: 30)

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computational mechanisms for visual processing—about which there is relative agreement among cognitive psychologists—supervene on the neural mechanisms for visual processing—about which there is much agreement among cognitive neuroscientists. The answer we receive is that this is not a case of psycho-neural supervenience, and, what’s more puzzling, they fail to do so thanks to a purportedly widespread phenomenon: neural recruitment. As such, this result does not seem to be limited to our present investigations; rather, if recruitment is a genuine phenomenon—which is otherwise indubitable—then the mechanisms posited by cognitive psychology do not supervene on the mechanisms posited by cognitive neuroscience. If it is true that the empirical studies of the mind and brain reveal the mental and cerebral “natural kinds”, respectively, then this suggests that psycho-neural supervenience, as traditionally conceived, is false for those natural kinds. It is not denied, however, that the mind supervenes on the brain. Indeed, mind-brain supervenience akin to “weak global supervenience” is briefly proposed.

We address psychological mechanisms because, like psychological states, they are supposed to have a causal influence on publicly observable intelligent behaviors. That is to say, if (i) is true, they must be causally potent. Thus, if the argument above is sound, psychological mechanisms ought to respect mind-brain supervenience. But, what is more important is how psychological mechanisms differ from psychological states: they are species-wide and their neural correlates are uncommonly well documented. Unlike the representational states that they operate over, psychological mechanisms vary very little from person to person, and so they and their neural substrates are far more easily studied and modeled. Indeed, concerning the visual mechanisms we’ll address, we shall see that there are vast amounts of data linking them to very specific regions of the brain. One might say, “the mechanisms for visual processing supervene on neural mechanisms if any psychological phenomena do”, without exaggerating more than philosophers of mind are wont.

We now proceed to clarify our terms. Supervenience is first, followed by psychological mechanisms, and then neural mechanisms. The first two sections should not be very provocative to philosophers, especially, in the second section, those who are sympathetic to the Computational Theory of Mind and/or the peripheral modularity of mind thesis. The third section, however, may raise some eyebrows. It is proposed there that the proper supervenience bases for psychological mechanisms are local neural mechanisms. Many philosophers have proposed, on the contrary, that psychological phenomena supervene on global brain states. Although I think this proposal is misguided, I have opted to simply evade this consideration by limiting my question to the entities posited by cognitive

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neuroscience, which posits are local neural mechanisms. My weak conclusion is thus limited to the relation between the properties of cognitive psychology and neuroscience, namely, that the former do not supervene on the latter. In some places, I also reference an apparently stronger conclusion, but it is conditional: if the empirical sciences of the mind and brain reveal the natural kinds apt to those domains of inquiry, then natural mental kinds do not supervene on natural cerebral kinds.2

SUPERVENIENCE

Supervenience is typically construed as a relation between two sets of properties, A and B, where:

(S) A supervenes on B just in case two things cannot differ in their A-properties without also differing in their B-properties

This formulation is taken to be roughly equivalent to:

(S`) if A supervenes on B, then, necessarily, two things identical with respect to their B-properties are identical with respect to their A-properties, or as it is sometimes put: necessarily, B-twins are A-twins.3

We shall focus on S, but we shall also avail ourselves of S` or the language of A- and B-twins when it is convenient. It is also proposed, however, as a relation between sets of event-types, facts, or processes. Distinct supervenience relations can thus be generated by tweaking the kinds of entities it relates. These distinctions are immaterial as regards our thesis, however, and so this paper shall employ the term “properties”, but, without building into it anything tendentious.

There are two axes along which distinct supervenience relations might be plotted, corresponding to the modal force of “cannot” and the specification of “things” in S (or the scope of necessity and the spatio-temporal boundaries of twins in S`). Let us briefly sketch the relations generated by toggling these elements.

The modal force of “cannot” in S might be logical, metaphysical, or nomological, among others. For each of these forces, there is a distinct supervenience relation.

(SL) A-properties supervene on B-properties logically just in case two things cannot differ in their A-properties without also differing in their B-properties, as a matter of logic.

2 I will have something to say, however, about purported psycho-neural supervenience relations that do not relate the properties that best capture the experimental regularities. 3 See McLaughlin (1995)

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(SM) A-properties supervene on B-properties metaphysically just in case two things cannot differ in their A-properties without also differing in their B-properties, as a matter of metaphysics.

(SN) A-properties supervene on B-properties nomologically just in case two things cannot differ in their A-properties without also differing in their B-properties, as a matter of law.4

The differences between these relations are immaterial for us. We’re interested in actual world cases, if there are any, in which two entities differ in their psychological properties but not their neural properties. Psycho-neural supervenience according to SL, SM, and SN all prohibit such an occurrence. As long as the modal force of a supervenience relation is strong enough to apply to all actual cases, psycho-neural supervenience of such a sort is inconsistent with the findings reported below.

Though the differences in the modalities of supervenience relation do not concern us, differences plotted along the other axis mentioned above are especially significant, and we shall discuss them at length here. On first blush, it may not seem to matter what are the “things” to which sub- and supervening properties accrue. It is the relations between the properties that interest us, after all, and as long as the two things are apt for comparison, it seems it should be inconsequential what they are, trucks, tomatoes, suicides, or starbursts. But this impression is mistaken.

First, there is an intuitive difference between a supervenience relation that holds only when the relevant entities share the same possible world and one that holds between entities across worlds. Suppose, for example, that Jones and Porter have the same eating habits. If health supervenes on eating habits, then Jones and Porter are equally healthy. But this doesn’t plausibly hold if Jones lives in a universe where doughnuts are healthy for humans and Porter lives in our world. Rather, if it holds at all, then it holds only if Jones and Porter occupy the same possible world. Compare this relation to a case of supervenience that plausibly spans possible worlds. Suppose that Jones and Smith are indiscernible as regards Aristotelian virtues. If moral goodness supervenes on the classical virtues, then they should be indiscernible with respect to goodness as well, no matter which worlds they live in. If moral goodness supervenes on Aristotelian virtues, then prima facie all persons with the same virtues are necessarily of equal moral goodness. The first sort of supervenience, which holds only intra-worlds, is often called weak

4 Each of these relations also admits of variations, of course, depending on which logic is referenced for SL or which law(s) are referenced for SN, or on the specifics of the metaphysics referenced in SM.

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supervenience (WS); inter-world supervenience is typically called strong supervenience (SS). We won’t be very concerned about this distinction because they both demand psychological identity from neural identity, and we don’t think this occurs. Still, it’s a useful distinction to have in hand.

(WS) A-properties weakly supervene on B-properties just in case for any possible world w and any two things in w, t1 and t2, t1 and t2

cannot differ in their A-properties without also differing in their B-properties

(SS) A-properties strongly supervene on B-properties just in case for any possible worlds w1 and w2 and any things t1 in w1 and t2 in w2, t1

and t2 cannot differ in their A-properties without also differing in their B-properties

Second, when the two entities we compare are individuals, the conceptual implications of being B-twins or A-twins are relatively straightforward: two individuals are B-twins just in case they share all the same B-properties. When the things we’re interested in include individuals among their constituents, however, determining identity with respect to B- or A-properties becomes tricky. This is especially salient when we’re comparing possible worlds. Suppose, for example, that we want to know whether the set of color properties supervenes on the set of shape properties, and we have before us worlds w1 and w2 for comparison. First we need to determine whether they are shape-twins. Does it suffice for them simply to have the same numbers of squares, circles, etc.; or is it that for each square to the left of a triangle and below a rectangle in w1, there must be a square to the left of a triangle and below a rectangle in w2? The dominant view is that there must be at least one isomorphism from individuals with a particular shape property in w1 to individuals with that same shape property in w2.5 Using for the set of properties being mapped, the needed isomorphism is typically called a -preserving isomorphism:

() A one-one isomorphism between the inhabitants of w1 and w2 is -preserving iff for every -property F, Fx in w1 iff F(x) in w2.6

If there is such an isomorphism in hand, and if worldwide color properties do supervene on worldwide shape properties, then there should also be an isomorphism for the color properties from w1 and w2. Now, depending on the relation between these two isomorphisms, we can define three strengths of supervenience for worldwide

5 McLaughlin (1995), Bennett (2004)6 This is borrowed directly from Bennett (2004), who attributes it to McLaughlin.

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properties, weak global supervenience (WGS), intermediate global supervenience (IGS), and strong global supervenience (SGS):

(WGS) A set of properties A weakly globally supervenes upon a set of poperties B iff for any worlds w1 and w2, if there is a B-preserving isomorphism between w1 and w2, then there is an A-preserving isomorphism between them.

(IGS) A set of properties A intermediately globally supervenes upon a set of properties B iff for any worlds w1 and w2, if there is a B-preserving isomorphism between w1 and w2, then at least one B-preserving isomorphism between them is also A-preserving.

(SGS) A set of properties A strongly globally supervenes upon a set of properties B iff for any worlds w1 and w2, every B-preserving isomorphism between w1 and w2 is an A-preserving isomorphism.

There is much discussion regarding the relations between each of these and the varieties of non-global supervenience. Given some plausible assumptions about worlds containing only a single individual, it seems that each of these is equivalent to some form of supervenience between individuals.7 Fortunately, though, we can skirt these issues for now; we shall return to them in discussing the objections. Our interest in global supervenience doesn’t concern possible worlds. Rather, we are interested in its specification of a very weak dependence between sets of properties, as in WGS, but we shall apply it to the sets of global cerebral properties and of global mental properties.8 Although the standard function of global supervenience is to relate the properties of individuals in worlds, it can be applied to any entities that count individuals amongst their constituents. Brains, as we shall see, count neural regions and mechanisms among their constituents, and so we can specify a notion of global supervenience for brains and the individuals that populate them. Call it weak global brain supervenience (WGBS)

(WGBS) A set of properties A weakly globally brain supervenes upon a set of properties B iff for any brains c1 and c2, if there is a B-preserving isomorphism between c1 and c2, then there is an A-preserving isomorphism between them.

We are interested in the case where the B-properties are neural properties and the A-properties are mental. If WGBS holds, then if there is an isomorphism between the distribution of your cerebral

7 See Bennett (2004)8 Compare Haugeland (1988)

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properties and mine, then there is an isomorphism between the distributions of our mental properties as well. Notice that since these need not be the same isomorphisms, WGBS is consistent with the failure of psycho-neural supervenience as traditionally conceived. That is, WGBS is consistent with the following situation. The mental isomorphism between brain one and brain two, being distinct from the cerebral isomorphism, maps the psychological property psy onto cerebral property nu in brain one but onto cerebral property nu* in brain two; neural identity does not necessitate psychological identity on this view.9

PSYCHOLOGICAL MECHANISMS

In this paper, we shall say that psychological mechanisms are computational processors that execute the cognitive tasks at which tend to humans excel. They operate over the representational states that carry information about our external world, but they themselves do not encode information about an agent’s local environment. The famous Language Acquisition Device (LAD) is a psychological mechanism, for example. It takes linguistic data as input and searches the class of language structure hypotheses for those compatible with it. It does so, moreover, irrespective of what the input is. The input has an influence on the result of the search and the language eventually acquired, to be sure, but the process of acquiring one’s first language, as long as it’s performed by the LAD, is universal. The input—mental representations—vary with the environment, but the mechanism that processes them remains the same. We’re interested in the mechanisms here.

As has become conventional, this reflects a tripartite division of cognitive capacities, most closely associated with David Marr’s 1982 work Vision. On the “top level” is a description of the operation or function subserved by the cognitive process to be explained. This level is often called the computational level, perhaps because it features a description of what the mind computes. Next is a description of how the relevant process executes the operation described at the “top level”, typically in terms of an algorithm. Accordingly, this is often called the algorithmic level. Psychological mechanisms, as we’re defining them, execute these algorithms. Finally, there is a concrete system that supports the mechanism executing the algorithm such that the algorithm is implemented in this system. For obvious reasons, this level is often called the level of implementation. In computers, the implementation level is electronic; in the case of humans, it is neural. Thus, the neural mechanisms we’ll be discussing are the units of the human implementation of psychological mechanisms. Note that this view doesn’t demand that the algorithm level supervene on the

9 See Bennett (2004) for elaboration of this result as regards WGS.

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implementation level. Rather, it is consistent with this view that neural mechanisms implement multiple psychological mechanisms.

As we’re defining them, then, psychological mechanisms are part of our native cognitive endowment as humans or, as it is also called, our cognitive architecture. For our purposes, their key feature is that they’re invariant across subjects. Like an advanced nervous system in general or opposable thumbs, the psychological mechanisms we’ll be discussing are proposed to be characteristic of our species. As compared to the belief that WATER IS WET or the representational state COW, psychological mechanisms do not come and go in the normal course of a life and are not at all subject to an agent’s rational control. When it comes to realization in the brain, there are distinct advantages to considering invariant psychological mechanisms.

First, invariant mechanisms have been easy—much easier than psychological representations—to investigate for both their unanticipated behavioral effects and their brain bases. And, as we shall see below, these studies have revealed much about their brain bases, (though they have not shown them to supervene on neural mechanisms.) Second, they are invulnerable to “content externalist” arguments to the effect that representational states do not supervene on brain states because the former are sensitive to various environmental factors.10 There is no convincing evidence effect that psychological mechanisms vary with environmental factors in an individual lifetime.11 This suggests a third point, which arises when it is supposed that either cerebral activities or environmental factors determine psychological phenomena. If environmental factors determine a class of phenomena, then experimental results from distinct cultures, languages, etc. should vary. As we have noted, though, no significant environmental variations have been found for the present class of psychological mechanisms. Hence, by modus tollens, they are not determined by environmental factors, and, by disjunctive syllogism, they should be determined by the brain’s activities. Thus, I propose that psychological mechanisms are very plausible candidates for psycho-neural supervenience, perhaps the most plausible presently available.

NEURAL MECHANISMS

The consensus in the neuroscience community is that the components of the brain that are relevant to psychological processes

10 See Putnam (1973), Burge (1979)11See Pylyshyn (2003) for a defense of invariance in the visual system’s psychological mechanisms against a litany of purported counter-examples. For example, claims that “experts” exhibit a refined, say, color-recognition mechanism are quite plausibly handled by the observation that refinement in the post-perceptual concept-sorting process explains the phenomena better.

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are localized neural mechanisms.12 As a working definition, we can say that neural mechanisms are those sets of entities and activities of the brain that contribute to the explanation of a mental phenomenon. Brain entities and activities include the electrophysiological properties of nerve cells, the structures and conformation changes of molecules, and the processing functions of brain systems; computational-representational states are not included.13 14

Churchland and Sejnowski (1992), for example, propose that an explanation of memory will come in the form of “orderly, local changes in individual cells”15. We shall not delve into the debate over how to define mechanisms16, but I submit that this counts as a neural mechanism because it refers to the properties and activities of neurons and not to computational processes or representational states. Notice that psychological relevance is built into definition such that whichever events in the brain are relevant to psychological phenomena, these are, for us, neural mechanisms. The phenomenon of neural recruitment plausibly applies to whichever brain events these are so long as they are local, and so our argument should apply to the entire class of local and psychologically-relevant brain events as well. This should suffice to differentiate psychological and neural mechanisms for our discussion.

Thus, there is a straightforward reason to take neural mechanisms to be the most plausible supervenience bases for psychological phenomena, namely, this is what the available evidence suggests. Every psycho-neural supervenience claim is a claim about

12 For a philosophical gloss on this claim, see Craver (2007); for sources with more authority in the neurosciences, see Morgan and Stellar (1950) or Shepherd (1994).13 The wording here was heavily influenced by Craver (2007: 6-9). Craver, however, thinks that neural mechanisms are “multi-level”, and so he includes “the representational and computational properties of brain regions” in his list of the brain’s entities and activities. Since we are interested in the relation between computational-representational processes an states and neural processes and states, defining neural mechanisms in such a way would surely undermine the thrust of our inquiry. Our definition thus belongs to Craver’s argumentative target, whom he calls Fundamentalists. Fundamentalists are epitomized by Churchland and Sejnowski (1992), Barlow (1972), Bickle (2003), and Barondes (1999). 14 Notice that we built into our definition that neural mechanisms contribute to the explanations of mental phenomena. We are not assuming that mental phenomena are explained fully by reference to neural mechanisms, we’re not assuming that all mental phenomena are neural phenomena, and we’re not assuming that mental phenomena supervene on neural mechanisms. These are the questions we aim to answer, or, at any rate, to address. This definition does, however, ensure that neural mechanisms are relevant to at least some psychological processes. That is, we are not advocating substance dualism, just the failure of supervenience for psychological and neural mechanisms.15 Churchland and Sejnowski (1992: 239), quotation cited in Craver (2007: 11)16 See, for example, Simon (1969), Haugeland (1998), Lycan (1987), or Salmon (1984).

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the cerebral properties that are relevant to psychological phenomena, and the plausibility of a supervenience claim is directly proportional to the empirical evidence that supports it. The people who are best empirically informed agree that the brain is composed of neural mechanisms and that neural mechanisms are the cerebral properties relevant to psychological phenomena. In the search for supervenience bases of psychological mechanisms, then, local neural mechanisms are a promising place to start. I propose we defer to the experts and start with neural mechanisms, but it is worthwhile to rehearse why locality has seemed promising.

Let’s call the view that regions of the cerebral cortex can be demarcated according to the distinct psychological phenomena they support localism. For a localist, mind-brain supervenience entails that at least some distinct psychological phenomena co-vary with activities in distinct regions of the cortex. As regards pathological evidence, localism predicts that brain damage results in insurmountable deficits in the psychological processes subserved by the damaged region(s). Phrenology, the theory that personality traits can be located in species-wide regions of the brain, is an early version of this view. And, let us call the view that the cerebral cortex is not thus divisible into regions according to their relevance to distinct psychological phenomena anti-localism. Anti-localism claims that psychological phenomena supervene on global brain states and that all cognitive deficits are surmountable in principle as long as one element from the class of brain states subvening a particular psychological phenomenon can be achieved. Pierre Flourens, an early champion of anti-localism, kept birds with brain legions, and pointed out that they exhibited no specific cognitive deficits.

Developments in the nineteenth and twentieth centuries, however, have made localism much more attractive. Most famously, Paul Broca noted the strong correlation between deficits in linguistic production and the region of the brain now called Broca’s area, while Carl Wernicke documented a similar correlation between a type of aphasia and what is now called Wernicke’s area. But what seems to have been most influential in turning the tide toward localism were discoveries involving the visual and motor cortices. Gustav Fritsch and Edvad Hitzig discovered in 1870 that stimulating a dog’s cerebral cortex incited it to move its limbs, suggesting what has since been confirmed: that the cerebral cortex is the seat of motor function. And a similar discovery in vision science was soon to follow. Gordon Holmes and William Tindall Lister studied wounded soldiers in World War I and developed a topographical map of the occipital lobe by correlating lesions and visual deficits.17 And, as was mentioned earlier, the visual sciences have since had many more successes in

17 Glickstein (1988)

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locating concrete brain bases for visual mechanisms. As a search through the literature attests, the localist outlook has been fecund, and it is not an exaggeration to say that it has won they day over the competing view. The result of this section and the last, then, is this: if the properties of scientific psychology supervene on the scientific properties of neuroscience, then psychological mechanisms supervene on neural mechanisms.

Neural RecruitmentNow let us come to the point: contrary to claims of psycho-

neural supervenience, neural mechanisms co-vary with multiple psychological mechanisms. Indeed, in some subjects, a numerically identical neural mechanism underwrites multiple psychological mechanisms. In other words, neural twins are not necessarily psychological twins. This occurs thanks to a phenomenon that is likely familiar when given a general description, but which has received little attention in the philosophy of mind literature, though it is at least prima facie relevant: neural recruitment. The general idea is that though a local brain region is normally the seat of a particular cognitive faculty, damage to this area can incite an adjacent area to “compensate” for the loss. That is, the adjacent area is “recruited” to subserve the tasks associated with the psychological mechanism enabled by the damaged area, though a difference in activity is not recorded in the “recruited” area.18 19 We shall illustrate neural recruitment by reference to prosopagnosic face recognition. This shall also demonstrate the tension between neural recruitment and psycho-neural supervenience. For we shall see that entities identical with respect to the property being the lateral occipital complex sometimes fail to be identical with respect to the property being the mechanism for the visual recognition of non-face objects. In these cases, rather, these entities bear the property being the mechanism for the visual recognition of faces.

First, it must be established that the visual recognition of faces draws on distinct psychological and neural mechanisms as compared to the visual recognition of non-face objects. There is substantial evidence for this from both experimental psychology and cognitive neuroscience.20 For starters, they are doubly disassociated. Object

18 Rossion et al (2008) are especially emphatic about the absence of a difference in activities in the relevant areas of the brain (the ventral lateral occipital complex) between their prosopagnosic patient, PS, and their “normal” comparison subjects.19 It should be noted that this use of “neural recruitment” is not the only use found in the relevant literature. In some cases, rather, it is used simply to indicate the neural region supporting a cognitive process. One might say, for example, that visual processes recruit the visual system for their operations.20 See Farah (1996) for a summary accessible to philosophers of mind and even drawing heavily on a text that’s likely familiar to them: Fodor (1984).

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agnosia is the inability to recognize non-face objects, even while one retains the ability to recognize faces. In one dramatic case, a subject was unable to determine that he was looking at fruit, but he knew that it was arranged so as to resemble a face. As far as we know, object agnosia results from damage to a family of areas including the ventral part of the lateral occipital complex, which is implicated in tasks requiring the recognition of non-face objects.21 Prosopagnosia is a selective impairment of the cognitive ability to recognize faces thanks to damage in the fusiform face area and the occipital face area, which are near to but distinct from the lateral occipital complex. In The Man Who Mistook His Wife for a Hat, Oliver Sacks tells the story of a man who became afflicted with severe prosopagnosia after a car accident; lying in his hospital bed, he couldn’t recognize his wife or daughter, but he could see that they had brought him flowers. In light of these discoveries, it is standard fare to claim that there are distinct psychological and neural properties corresponding to the visual recognition of faces as compared to the visual recognition of non-faces.

In some cases, however, these afflictions are not so tragic: face recognition can still be achieved even though the fusiform and occipital face areas have been damaged. How does this occur? As seems rather intuitive, though anthropomorphic, another region of the brain is “recruited” to compensate for the loss. Rossion et al (2008) investigated face recognition processes in prosopagnosics and concluded, “following prosopagnosia, areas that do not respond preferentially to faces such as the ventral part of the LOC [lateral occipital complex] may still be recruited for compensatory or residual individual face perception.”22 That is to say, the patient that Rossion et al studied, PS, could not draw on the neural mechanism underlying face recognition in the fusiform face area because it was damaged. Nonetheless, PS can recognize individual faces at a rate better than chance, and she does so thanks to the operations of the neural mechanism typically responsible for recognizing non-face objects.

Thus, it may be said that in prosopagnosics, activity in the LOC does not suffice to bring about object recognition processes; sometimes it brings about face recognition processes. This is because the LOC has been “recruited” to subserve two psychological mechanisms. When it comes to object recognition, then, neural duplicates—two instances of significantly increased activity in the LOC—are not necessarily psychological duplicates—we get object recognition in one instance and face recognition in another. In other words, two entities identical with respect to the property being the lateral occipital complex are not identical with respect to the property

21 Kanwisher et al (1996), (1997); Puce et al (1996); Bingel, et al (2007)22 Rossion, et al (2008: 318)

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being the mechanism for the visual recognition of non-face objects. The psychological mechanism for non-face object recognition does not supervene on its apparent neural substrate, the ventral LOC.

I submit that if psycho-neural supervenience is true, there should be some evidence to support it. As far as I know, however, there is none. Moreover, the areas we have discussed are plausibly the most promising for empirical support of the psycho-neural supervenience claim, but an investigation into their relation infirms the claim instead. It may not follow that the presumption should now be against psycho-neural supervenience, but if psycho-neural supervenience holds, then there should be an adequate explanation of the results reported here. Moreover, notice that “recruitment” may be a widespread phenomenon. If so, then it seems that any psycho-neural supervenience claim is vulnerable to counterexamples in which the neural property is “recruited”. In other words, evidence for the recruitment of a neural property nu is evidence that nu duplicates are not psychological duplicates. Indeed, we can give a general argument against psycho-neural supervenience claims from recruitment evidence:

For any psycho-neural supervenience claim between psy and nu:(i) nu can be recruited to underwrite ~psy(ii) recruited nu is nonetheless neurally indistinguishable

from nu(iii) yet, an entity with nu is psychologically distinguishable

from an entity with recruited nu: one has psy and the other has ~psy

hence:(iv) psy does not supervene on nu

If this is right, then advocates of psycho-neural supervenience owe us an explanation of neural recruitment, and not just of the particular supervenience failures mentioned here. We shall have more to say about this in responding to the objections.

Objections

1. The Wildcard Supervenience Objection

Objection: Notice, however, that there is a psycho-neural supervenience relation that does hold here. Instead of taking psychological properties to be those established by the empirical results alone, we can reclassify psychological and/or cerebral phenomena so as to satisfy the demand for supervenience. In this case, for example, we can say that the psychological property being

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the un-recruited object recognition mechanism supervenes on the neural property being the lateral occipital complex, while the property being the recruited object recognition mechanism supervenes on the neural property being not the lateral occipital complex. It is too hasty to say that psycho-neural supervenience has failed.

Reply: I have some sympathy for this response, but I think there are at least two reasons to be more circumspect here.

First, supervenience is cheap. As long as we are willing to fiddle with the relata, there is a supervenience relation between any two kinds of property. Let me illustrate. Suppose you want to know if colors supervene on shapes. Intuitively, there is no such supervenience: things of a single shape can have many colors. This failure of supervenience, however, is in fact just a failure of imagination in defining color and shape properties. We could require all color properties to be disjunctions of complement properties. The set of color properties would thus consist of being red or not-red, being blue or not-blue, being yellow or not-yellow, etc. and on others. Voila: these color properties supervene on shape properties. Two things cannot differ in their color properties without differing in their shape properties, for two things just cannot differ in their color properties: all things instantiate all of these color properties. The point is that many of the psycho-neural supervenience relations out there are trivial, but we are interested only in the ones that aren’t trivial, and, so far as I know, no one has come up with a good way to distinguish the two. Gerrymandering properties to secure supervenience requires caution, then, because the resulting relation may simply fail to be about the minds and brains we’re interested in.

Nonetheless, there are classifications of mental and cerebral phenomena that capture the well-established regularities in those domains; we can be reasonably sure that if there is a supervenience relation between these, then this is non-trivial supervenience. This brings us to the second reason to be cautious here. The aforementioned route to psycho-neural supervenience requires us to deviate from what are presumably our best theories about the “natural joints” of the mind and brain. It is presumably these joints that we are interested in when we’re interested in the relation between the mind and brain, rather than just any classifications of properties or mechanisms. As a strategic guideline, then, we should be reluctant to pursue the relations between properties other than these in our search for the true mind-brain relation. There may still be cases in which it is savory to recast mental and cerebral properties, but, as I say, we ought to be cautious. Before we jump to this option, we should re-evaluate our reasons for demanding psycho-neural supervenience in the first place. My guess is that we shall find them less convincing than many have supposed.

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Rejoinder: Not all “gerrymandered” psychological properties are equal, and not all supervenience relations with gerrymandered properties are trivial. It is true that disjoined complements trivially supervene on all properties, but there remain re-configurations of the psychological and/or neural properties here that are both a) sensitive to the experimental results and b) consistent with psycho-neural supervenience. The following suggestions for recasting sub- or supervening properties all exhibit both of these characteristics. [I shall reply to each immediately following its articulation.]

The “non-face object mechanism” is really the “object mechanism”23

What this experiment shows isn’t that the lateral occipital complex is unfit to be a supervenience base, but that the psychological mechanism it underlies, the “mechanism for the visual recognition of non-face objects”, is poorly named. It may be true that this mechanism is used primarily for non-face object recognition in the usual case; and, again in the usual case, it may be true there is another mechanism that performs better at face recognition tasks; but it doesn’t follow from either of these that the mechanism used primarily for object recognition doesn’t also execute the rudiments of a face recognition algorithm. Indeed, the evidence marshaled above is evidence to the contrary. It is evidence that the mechanism in question can also serve to recognize faces, even though this is not its usual job, and even though it doesn’t do this job as well as the mechanism specialized for face recognition does.

On this view, then, things are nearly exactly as we thought they were. The mechanism for face recognition supervenes on the fusiform face area, and “the mechanism for non-face object recognition”, which is really a mechanism for recognizing any object, supervenes on “the object areas” including the lateral occipital complex. The information isn’t about supervenience at all; it’s simply that the object recognition mechanism isn’t specialized to non-faces. Rather, it’s not specialized at all; it’s a general visual recognition mechanism. Indeed, once we put it this way, the previous view looks rather silly: Why, evolutionarily speaking, would there be a mental mechanism specialized for recognizing only non-faces? It’s much more likely that somewhere along the evolutionary line, we developed a specialized face processor to supplement the general object recognition mechanism common in many species. The evidence above isn’t a failure of supervenience, it’s just one more mark of our redundant evolutionary design.

23 This line of reasoning was suggested by Lizzie Schechter.

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Reply: This line of response admits of two substantial shortcomings, though I shall mention only one now. It shares the second with the next response, and so this flaw shall be discussed in the next section.

The problem is this. This response doesn’t work for all cases of recruitment. Notice that it is plausible that the object recognition mechanism also serves in recognizing faces because, intuitively, the computational process of recognizing faces seems not unlike the computational process of recognizing non-face objects. It is easy to see how a mechanism could be specialized for one and not the other, but a mechanism’s being unable to execute face recognition tasks doesn’t follow from its being specialized for non-face object recognition tasks. Nor vice-versa. On the contrary, it seems plausible that recognizing objects like noses, eyes, etc. would be a step in a face recognition algorithm (even though face recognition is widely held to be holistic rather than atomic24). Thus, it is quite plausible to say that the neural mechanism underwriting non-face object recognition isn’t subserving a distinct psychological process when it supports face recognition tasks. The plausibility of this line of response, then, turns on the intuitive similarity between the psychological processes in question. As such, it is implausible for cases of neural recruitment in which the neural mechanism underwriting a process psy1 is recruited to underwrite another psychological process psy2, even though psy1

isn’t a plausible replacement for psy2. There are multifarious contingencies that contribute to whether

one psychological process is a plausible replacement for another in supporting the execution of relevant tasks, of course, but let me try to put a point on this notion all the same. Recall the three cognitive levels we sketched above. Psychological mechanisms are specified by algorithms that underwrite the performance of cognitive tasks, and they are implemented in the brain by neural mechanisms. Psychological mechanisms and their neural implementations, then, are confined by their algorithms. An algorithm randomly matching input items with output items, for example, is capable of supporting only random-match operations. This is not to say that it couldn’t play a part in more complex functions, though; the claim is simply that on its own, a random-matching algorithm cannot plausibly support non-random functions. Thus, if the neural mechanism supporting a random-matching mechanism, psy1, were recruited to support a non-random psychological mechanism, psy2, this situation would undermine the presently proposed explanation of neural recruitment. In general, it isn’t plausible that psy1 is replacing psy2 to support certain cognitive tasks if runs of the algorithm characterizing psy1

match observed input to observed output with any probability at chance or below. Indeed, this is precisely how all hypothesized

24 See, for example, Rossion (2006)

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psychological mechanisms are evaluated, and we should demand the same when determining whether one psychological mechanism is replacing another or if a neural mechanism has been recruited. It is premature, then, to claim that neural recruitment reveals only the versatility of the psychological mechanism supervening on the recruited neural mechanism.

The mechanism(s) “downstream” from face recognition “learn(s)” to compensate25

Face recognition tasks require subjects to identify individuals based on differences in their facial features. In the usual case, the psychological mechanisms specialized for processing information relevant to facial features plays a crucial role in executing these tasks. It does not follow, however, that these tasks cannot be executed without the activation of this specialized mechanism. The behavioral output observed in face recognition tasks result not only from the operations of the face-recognition mechanism, but also from the performances of the visual system as a whole and, indeed, the mind as a whole. Success in face recognition tasks, then, may reflect an operational face-recognition mechanism, or they may reflect a visual system that can identify individuals without much information about facial features, or it may reflect the clever guessing strategies of an agent with subtle rational capacities, and there are many other possibilities.

There are two points to note here. First, behavioral output results from the interactions of all the mind’s systems and subsystems. Our overall picture of it conjoined with batteries of behavioral and neurological experimental results allow us to make claims about isolated systems such as the face- and object-recognition mechanisms; in general, no single test suffices to establish the presence or absence of a particular psychological mechanism. It is necessary to deny this sound methodological consideration, however, in order to conclude that prosopagnosic face recognition undermines psycho-neural supervenience. It is much more savory, then, to suppose that success in face recognition tasks results from the operations of systems “downstream” from the face recognition mechanism taken in aggregate.

Second, there are reasons to think that redundancy is widespread among the mind’s mechanisms. To name one: it is widely held that complex systems produced by evolutionary pressures are “constructed…in such a way that the functionality of the whole should be buffered, to some extent, from damage to the parts.”26 Supposing that our mental system are complex products of evolution, then for

25 Peter Carruthers suggested this response in direct communication.26 Carruthers (2007: 13)

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any mental subsystem, if its operations contribute something crucial to the functionality of the whole mind, the overall mental system should be “buffered” from damage to this subsystem. One way to buffer the overall system from damage is to make crucial subsystems redundant. That is, let it be the case either that two or more subsystems support the crucial operation or that the interactions of multiple systems duplicate the operations of a crucial system. In this case, damage to one of the systems supporting the crucial operation won’t eventuate in a major disruption. It is debatable, of course, whether face recognition is crucial to mental functioning and thus likely to be multiply supported, but pima facie it is eligible for such insurance. And so we can plausibly take prosopagnosic face recognition as evidence that face recognition is a crucial operation, or, at the very least, it is evidence that face recognition operations are “buffered” from damage. Hence, while it may not be the case that the so-called non-face object recognition mechanism has the power to support success in face-recognition tasks, it shouldn’t be surprising that some mechanism or collection of mechanisms besides the one supported by the fusiform face area does.

If face recognition tasks can indeed be executed by manifold systems and/or their interactions, then, again, things are no different than we previously thought. The presently known psychological mechanism for face recognition supervenes on the fusiform face area, and the so-called non-face object recognition mechanism supervenes on the object areas including the lateral occipital complex. What we have learned, however, is that a) face recognition is plausibly a psychological process supported by multiple systems, and, if the inference from being buffered to being crucial is justified, then b) there is at least one reason to think that face recognition is crucial to human fitness. Psycho-neural supervenience is untouched.

Notice that these considerations are general, rather than specific to the case of recruitment given above. Whereas the plausibility of the “non-face object mechanism…” response turned on the intuitive similarity between the processing demands of the two systems, this line of reply does not. Rather, it is plausible by dint of general considerations about the organization of the human mind and the production of behavior. Thus, it serves as a reply not only to the alleged counterexample to supervenience regarding the lateral occipital complex, but also to the general argument given in the conclusion.

For any psycho-neural supervenience claim between psy and nu:(v) nu can be recruited to underwrite ~psy(vi) recruited nu is nonetheless neurally indistinguishable

from nu

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(vii) yet, an entity with nu is psychologically distinguishable from an entity with recruited nu: one has psy and the other has ~psy

hence:(viii) psy does not supervene on nu

The argument’s flaw is to conflate instantiations of psy with success at tasks executed by psy in the normal case. Obviously, it does not follow from the fact that a mechanism typically supports certain cognitive tasks in humans that the mechanism supporting these tasks in any particular human is the typical one. Indeed, given that the individual in question has suffered damage to the (usual) neural substrate of the typical mechanism, it is most reasonable to suppose that other mechanisms are supporting the execution of the relevant tasks in this case.

Reply: This line of response and the one just before are both committed to an unattractive claim, namely, that cases in which a cognitive capacity is completely lost result not from damage to localized neural mechanisms, but from the incapacitation of both a localized mechanism and the buffer mechanisms. This is simply inconsistent with the available data.

First, we can make this clear with some intuitive considerations. Let us call a complete loss of a cognitive capacity by its common scientific name plus the modifier “complete”. “Complete prosopagnosia” is thus an exceptionless incapacity to visually recognize faces. “Complete object agnosia” is a complete inability to visually recognize objects; “complete object anomia” is a complete inability to name objects; and so on. Now, these “complete…” impairments are known to be dissociated, as we mentioned eariler. One can have complete object agnosia without also having either complete prosopagnosia or complete object anomia. As we mentioned earlier, one of the more compelling reasons for saying that these psychological capacities are distinct is that that can be dissociated. This was the case, for example, when prosopagnosia was first named. In 1947, Joachim Bodamer described a patient who had suffered a gunshot wound to the head and subsequently lost his ability to recognize faces. This was surely complete prosopagnosia as the patient was unable to recognize his family, his friends, and even himself. Yet, he could recognize his friends and family by other visual information, such as their gaits while walking. Similar cases of isolated object agnosia and object anomia have also been documented.

If either of the accounts of neural recruitment given in the above responses is true, though, then complete impairment of face recognition should appear only when there is at least one other

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deficiency, if not a host of them. The first response predicts that impaired object recognition should accompany complete prosopagnosia, but this is not always the case. Indeed, several documented cases counter this prediction. The second response predicts that complete prosopagnosia will be accompanied by damage to mechanisms “downstream” from the face recognition mechanism; but this too is inconsistent with the available evidence. Rather, prosopagnosia is highly correlated with damage to the fusiform face area and the “occipital face area”, but no others. Accounts of neural recruitment that claim otherwise are simply false.

2. The Advance of Science Objection

Objection: Too much emphasis is being placed on these neural and psychological mechanisms. They are relics of nascent sciences; surely, as cognitive psychology and neuroscience mature, new properties will better explain mental and cerebral phenomena, and our concern should be how these properties relate. Very little of import is revealed by showing that properties in present sciences of the mind do not supervene on properties in the present sciences of the brain.

Reply: As was noted above, I don’t think it is so easy to dismiss this result as confined to present science. For the psychological and neural mechanisms mentioned above are only illustrations of a general point about neural recruitment. Neural recruitment is at the heart of this problem for supervenience, and recruitment isn’t an artifact of present theory. Rather, it is a robust phenomenon uncovered by advances in and increased use of neuronal imaging. As long as neural recruitment looks the way it does today—i.e. as the application of a single neural mechanism for multiple psychological processes—it undermines psycho-neural supervenience. Perhaps it is true that two of the suppositions crucial to understanding recruitment the way we do, the localization of cognitively relevant mechanisms and even the neuron doctrine, are elements of present theory rather than its explananda, but it does not follow from this that recruitment itself is proprietary to the currently fashionable explanans. No matter what future neuroscience looks like, the fact shall remain that brain damage can incite mind-brain mapping to be reconfigured. If this fact remains, so does the failure of psycho-neural supervenience.

3. The Nature of Brain and Behavioral Research Objection

Objection: The supposition that findings in psychology and neuroscience undermine psycho-neural supervenience overlooks a

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crucial feature of mind-brain research, namely, its methodological commitment to psycho-neural supervenience. Given this commitment, it simply isn’t possible that psychological properties could fail to co-vary with neural differences in any sciences resembling those we have now. If psycho-neural supervenience fails at present, this could only be tentative: it is certain that there are research projects determined to explain a) the neural difference between the recruited and un-recruited LOC, b) the neural similarity between the fusiform face area and the recruited LOC; and, furthermore, there will be similar explanations for all cases of neural recruitment. These results will then show how brain damage to, say, the fusiform face area results in changes in the LOC that make it into a “regenerated” fusiform face area, and not a “recruited” LOC.

Reply: I object to the claim that psychological and neuroscientific methodologies are committed to psycho-neural supervenience. As evidence, let us consider the definition of “invariance”, a term applied widely in neuroscientific research, and compare it to supervenience. “Invariance” applies to neural representations and suggests exactly what it sounds like, the consistency in a neural unit’s representations. If a neural unit supports many representations, it is variant; if it supports only one representation, it is maximally invariant. Quiroga, et al (2005), investigating the response activities of single neuronal units when subjects are presented with pictures of faces, give a characteristic gloss:

By invariance we mean that a given unit is activated mainly, albeit not necessarily uniquely, by different pictures of a given individual, landmark or object. (Quiroga, et al (2005: 1102, emphasis added)

Notice that this gloss allows brain units of psychological relevance to co-vary with multiple stimuli, and note the usual methodological assumption in the visual sciences that correlation with a stimulus suggests a correlation with a representation of that stimulus. These authors even reject the suggestion that the units they studied were subserving representation of features common to the stimuli.27

Indeed, for one subject, they found a single neuronal unit that responded differentially to pictures of Jennifer Aniston, but not to pictures of Jennifer Aniston and Brad Pitt, and not to pictures of a woman with many similar facial features, Julia Roberts. In another subject, they found a unit that responded to pictures of Halle Berry, to the letter string HALLE BERRY, and to pictures of Halle Berry dressed

27 Ibid 1104

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as Catwoman, but not to pictures of other people dressed as Catwoman. They propose, quite cautiously, that perhaps they found in the subject a “Halle Berry neuron”. They are careful not to suggest, however, that this neural unit subserves only representations of Jennifer Aniston. They explicitly distance themselves from that view: “We do not mean to imply the existence of single neurons coding uniquely for discrete percepts for several reasons…” (1106) Rather, contrary to psycho-neural supervenience, they allow that this selfsame neuron could, in some cases, underwrite other psychological properties. Indeed, they think it probable:

Given the limited duration of our recording sessions, we can only explore a tiny portion of stimulus space…the fact that we can discover in this short time some images—such as photographs of Jennifer Aniston—that drive the cells suggests that each cell might represent more than one class of images. (1106, emphasis added)

Contrary to the claim that the sciences of the mind and brain presuppose psycho-neural supervenience, then, I marshal these quotations. They suggest that one concept directly related to the prospects for psycho-neural supervenience allows for results inconsistent with it. Furthermore, at least a few scientists working on the mind-brain relation in the lab think that situations inconsistent with psycho-neural supervenience are probable. If it is the case that cognitive psychology and neuroscience presuppose psycho-neural supervenience, it must be shown by reference to the actual practices of those scientists working in the fields, and the presumption should now, given the foregoing demonstration, should now be against this being the case.

At this point one might want to propose that however the psychology and the neurosciences carry on, they ought to respect and work toward psycho-neural supervenience. This may be the case, but it depends on the arguments alluded to above. I propose, however, a modus tollens to their modus ponens. That is, I suggest that we re-evaluate those arguments in light of the fact that the going sciences defy supervenience, rather than a re-evaluation of the sciences.

ConclusionIt is an interesting historical question how the debate over the necessity or contingency of mind-brain intimacy became a debate over mind-brain supervenience—a clearly stronger relation—without vigilance to the experimental evidence. The ambition for a more specific relation than “intimacy” is surely admirable, but the proposed

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relation seems to have been accepted without proper testing. This paper does not dispute that minds and brains bear an intimate relation; rather, it has argued only that mind-brain supervenience does not hold in our very own actuality for the properties of cognitive psychology and neuroscience. In brief, the phenomenon of neural recruitment is a counterexample. The argument has been this. It is inconsistent with all varieties of mind-brain supervenience that two entities with identical neural properties in the actual world have non-identical psychological properties here. This is precisely what occurs, however, in the well-known phenomenon of “neural recruitment”. Prosopagnosic face recognition is submitted as an example of recruitment. Substantial experimental evidence suggests that recognizing faces and recognizing non-face objects are distinct psychological processes typically underwritten by distinct neural mechanisms. Prosopagnosia is a selective impairment of the ability to visually recognize faces by dint of damage to the fusiform face area. In some cases, though, a prosopagnosic has limited success with face recognition tasks despite her damaged fusiform face area. Evidence from fMRI studies suggests that in these cases, the neural mechanism typically reserved for visually recognizing non-face objects is pressed into subserving the recognition of faces as well. Prosopagnosic face recognition is thus a case in which two identical neural properties are correlated with non-identical psychological properties. Advocates of psycho-neural supervenience must explain how neural recruitment could be consistent with their view.

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