Abstract
The brain is often taken to be a paradigmatic example of a signaling system with semantic and representational properties, in which neurons are senders and receivers of information carried in action potentials. A closer look at this picture shows that it is not as appealing as it might initially seem in explaining the function of the brain. Working from several sender-receiver models within the teleosemantic framework, I will first argue that two requirements must be met for a system to support genuine semantic information: 1. The receiver must be competent—that is, it must be able to extract rewards from its environment on the basis of the signals that it receives. 2. The receiver must have some flexibility of response relative to the signal received. In the second part of the paper, this initial framework will be applied to neural processes, pointing to the surprising conclusion that signaling at the single-neuron level is only weakly semantic at best. Contrary to received views, neurons will have little or no access to semantic information (though their patterns of activity may carry plenty of quantitative, correlational information) about the world outside the organism. Genuine representation of the world requires an organism-level receiver of semantic information, to which any particular set of neurons makes only a small contribution.
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Notes
Adapted from Shannon (1948, p. 379). This simplified version leaves out the noise source and transducer box (which translates the signal back into a form appropriate for its “destination,” which might be either human or machine).
See e.g. Dennett (1986, p. 54).
This is consistent with Jablonka’s (2002) definition of biological information, which “gives a central role to the interpretative system of the receiver of information.” Shea (2007a, b) makes a similar point in the context of genetic information, emphasizing the role of the consumer. Content is determined by its (historical) effect on the (historical) success of the consumer, and there need not be any well-individuated sender with the biological function of producing a correlation between world and signal; correlation between a state of the world and the signal can come about in other ways (e.g. natural correspondence). While receivers can and have evolved without senders, senders cannot evolve without receivers. This means that while a sender is not necessary, the existence of a sender (stabilized by evolution to produce a particular signal) might be a sufficient condition for the signal to carry semantic information—by guaranteeing (often enough) potential receivers for that signal, even if any particular signal happens to go unreceived. What if all the receivers of some signal go extinct while the senders continue to send? In that case I would suggest that that particular signal has lost its meaning.
See Dennett 1984 for a comprehensive discussion of this issue.
Or perhaps not “as well”—if it turns out that we can make artificial intelligences with legitimate mental states but no biological interests. It’s not clear that anything that is neither alive, nor part of a living system, nor a possessor of mental states would fulfill the requirement for having interests, though arguments could perhaps be made for derivative interests, in the case of artifacts designed by humans for some particular end.
The term often used here is “conventional”, but that implies signaling between (at least) two agents. What I mean by arbitrary is essentially the same thing as conventional, but equally applicable to the system with no well-defined sender.
Stegmann (2004) argues that arbitrariness is neither necessary nor sufficient for attributing semantic information to DNA, but I would argue that to the extent that the structure of DNA and RNA is not arbitrary, it rules out the kind of flexibility needed for the information ‘encoded’ to be semantic.
See Dretske (1981, p. 41): knowing you have a one gallon bucket doesn’t tell you that there’s lemonade inside. But it does tell you don’t have two gallons of lemonade (or anything else).
As the reviewer for this paper pointed out. In some sense, the form of many G-protein coupled receptors is arbitrary, in the sense that different subunits can be (and are) mix-and-matched to give different ligand-sensitivities, different dynamics, and different downstream coupling. However, the interaction between receptor and ligand is not at all flexible. Stegmann (2004) also gives an example of allosteric interactions between oxygen molecules binding to hemoglobin—arbitrary but not flexible or semantic.
Although also found elsewhere, e.g. vervet alarm calls, Seyfarth et al. (1980).
Skyrms actually ties the resilience of a generalization to how much it would disrupt our conceptual scheme to give it up, but I will follow Mitchell in taking that disruption to be diagnostic of the objective “strength” of the generalization, rather than a statement about our tendencies to become attached to particular (kinds of) theories.
Computer flexibility is no accident, but a consequence of how we’ve designed to do what we use them for. Nonetheless, they get left out in this sender-receiver picture, because flexibility without interests is not enough to qualify as a bona fide receiver. If we draw the receiver boundary around the computer and its human user (as a single unit), then suddenly the signals coming to that newly defined receiver explode in semantic richness.
Those effects should probably also be characterized in semantic terms, if internal, and teleological terms, if external.
Senders are back in the picture now not because they are required in general for signals to have semantic content, but rather because every receiver in the brain is a competent one primarily in virtue of its ability to be a sender of signals. Sending is the primary flexible action that a receiver in the brain can take. But it is still the receiver that determines whether and what the semantic content of associated signals may be.
See Faisal et al. (2008) for a summary of interesting (and functional) ways in which noise can enter into the system at various levels.
Dretske makes a similar point about what he calls “digitization”. One might argue that there is nothing preventing a sophisticated downstream receiver (DR) from extracting likely information about the world given the states of the previous receiver upstream (UR), but then DR would need some kind of primitive “theory of mind” or “theory of function” about how the UR is likely to respond to various states of the world. Given the impoverished resources of single cells in the brain, this seems like a more likely story at the personal or organismal level than at the cellular level, although again someone might argue that strongly shared interests and lack of deception might make the endeavor more successful at a sub-organismal level. (Of course, even at the personal level, hearsay is not considered as strong evidence as that seen with one’s own eyes). One could also try to make the case that at the cellular level, this kind of “re-interpretation” of previous events is built in, and that that is just what it means to be part of a system. But this seems to me like an overly generous third-party interpretation, since the DR clearly doesn’t need to know what’s going on in the outside world as something going on in the outside world—rather than merely as something going on with UR—in order to do its job properly. It might turn out that the activity of some groups of DRs correlate well with external features of the world, but there still remains the limitation that what the DRs act on will be other cells further downstream in the brain, rather than the state of the world that the UR was responding to.
See Sherman and Guillery (2009).
Other candidate characteristics used to pick out higher-level evolutionary individuals include shared fate/common interests of their constituents, morphological specialization, absence of within-collective conflict, etc. (eg. Queller and Strassman 2009). These are also potentially helpful criteria for picking out those collectives that can be thought of as agent-like in a more general sense.
Dopamine receptors are members of a large class of G-protein coupled receptors (other members of which are sensitive to many other neurotransmitters) that are capable of triggering highly diverse sets responses within single cells (excitation and inhibition, but also complex second-messenger cascades leading to regulation of protein levels and further non-electrical signaling).
Dopamine is closely correlated with organism-level reward—it is released in correspondence with the appearance or expectation of environmental rewards. See Schultz (2006) for a review.
For a review on PSD plasticity, see Voglis and Tavernarakis (2006).
For a review of the role of retrograde messengers in synaptic regulation, see Regehr et al. (2009).
Here as elsewhere in the paper, “representing” is shorthand for “playing receiver to a signal which consequently carries the semantic information that”.
From Lewontin ( p. xi, 200).
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Acknowledgments
I am grateful to Daniel Dennett, Peter Godfrey-Smith, and Nicholas Shea, the discussion group at Tufts, and a reviewer for helpful comments. This work was supported in part by an NSF fellowship.
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Cao, R. A teleosemantic approach to information in the brain. Biol Philos 27, 49–71 (2012). https://doi.org/10.1007/s10539-011-9292-0
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DOI: https://doi.org/10.1007/s10539-011-9292-0