Thursday, December 28, 2017

The cog-neuro of plants

There are many (e.g. Erich Jarvis) who think that the basic hierarchical properties of language are direct reflections of its vocalic expression. This is what makes ASL and other signed languages so useful. The fact that they exist and that modulo their manual articulation they look, so far as we can tell, just like any other language (see here for discussion) puts paid (and paid in full!) to any simple minded idea that linguistic structure is “just” a reflection of their oral articulation.

Why do I mention this? Because I have just been reading some popular pieces about a potentially analogous case in neuroscience. Let me explain.

Several years ago I read a piece on plant “neurobiology” in the New Yorker penned by Michael Pollan (MP) (here). The source of the quotes around ‘neurobiology’ is a central concern of the article in that it explores whether or not it is appropriate  to allow that plants may have a cognitive life (‘bullshit’ is one of the terms tossed around) or whether this is just another case of metaphors run amok. One large influential group of critics thought the idea obscurantist bordering on the incoherent (see here). Here’s a quote from the MP piece quoting one of the critics Lincoln Taiz:

Taiz says that the writings of plant neurobiologists suffer from “over-interpretation of the data, teleology, anthropomorphizing, philosophizing and wild speculations. (6)

Wow, this is really bad, as you can tell when the charge is “philosophizing”!!! Can’t have any of that. At any rate, the main reason for Taiz’s conclusion, it appears, is that plants do not have brains (an uncontroversial point) or neurons (ditto). And if one further assumes that brains with neurons are required for cognition of any kind then the idea that plants might have memory, might use representations, learn and make context sensitive judgments is simply a category mistake. Hence the heat in the above quote.

In a recent Aeon essay, Laura Ruggles (LR) reprises the issues surrounding thinking plants (here). It appears that things are still contentious. This does not really surprise me. After all, the idea that plants cognize really is a weird and wonderful suggestion. So, that it could be false or, at the least, ill supported, strikes me as very plausible. However, as the quote above indicates, this is not the nature of the criticism. The objection is not that the evidence is weak but that the very idea is incoherent. It is not false. It is BS. It is not even high class BS, but a simple category mistake due to bad anthropomorphic philosophical speculation generated by teleologically addled minds. Needless to say, I got interested.

Why the heat? Because it directly challenges the reductive neurocentric conception of cognition that animates most of contemporary cog-neuro (just as ASL challenges the vocalic conception of grammar). And it does so in two ways: (i) It reflects the strong commitment of the methodology of “neuron doctrine” in cog-neuro and (ii) It reflects the strong commitment to the idea that biological memory and computation supervenes on a connectionist architecture (i.e. the relevant computations are inter-neural rather than intra-neural). Let me say a word or two about each point.

The “neuron doctrine” is the idea “that cognitive activity can be accounted for exclusively by basic neuroscience. Neuronal structure and function, as identified by neurophysioplogy, neuroanatomy and neurochemistry, furnish us with all we need to appraise the animal mind/brain complex” (see here: 208). This idea should sound familiar because it is the same one that we discussed in the previous post (here). The position endorses reductionist methodology to the study of the brain (more accurately, a methodological monism), one that sees no fruitful contribution from the mental sciences to cog-neuro. KGG-MMP rehearses the arguments against this rather silly view, but it clearly has staying power. Marr fought it in the 1980s and his proposal that problems in cog-neuro need to be tackled at (at least) three different levels (computational, algorithmic/representational, implementational), which interact but are distinct and provide different kinds of explanatory power is a response to precisely this view. This view, to repeat, was prevalent in his day (well over 30 years ago!) and is still going strong, as witnessed by the fact that KGG-MMP felt the need to reiterate his basic arguments yet again.

The plant “neuroscience” debate is a manifestation of the same methodological dogmatism, the position that takes it for granted that we once we understand neurons, we will understand thought. But it is even more of a challenge to this neurocentric view. If plants can be said to cognize (have representations, memories, process information, learn) then not only is the methodological thesis inappropriate, but the idea that cognition reduces to (exclusively lives on) neuronal structure, is wrong as well (again, think ASL and vocalization wrt grammar). If the plant people are onto something then the having memories is independent of having brains, as is learning and representation and who knows what else. So if the plant people are right, then not only is the neuron doctrine bad methodology, it is also ontologically inadequate.

None of this should be surprising if you have any functionalist sympathies. As Marr noted, the materials that make up a chess board/pieces can vary arbitrarily (wood, marble, bread, papier mache) and the game remains the same (the rules/game of chess is readically independent of the physical make-up of the board and pieces). Whether the relation of cognition to brains is more like the chess case or less is an open question. One view (Searle comes to mind) is that no brains, no cognition. On this view, the connection between brain structure and cognition is particularly tight in that the former is a necessary feature of the latter (thinking can only live in brains (though, IMO, there is more than a touch of mystical vitalism in Searle’s position)). If the plant cognitivists are right, then this is simply incorrect.[1]

In sum, though metaphysical reduction does not lend credence to methodological reduction, if even the former is untenable, then it is quite implausible that the former can stand. Why import neuronal methodological dicta in the study of cognition if cognitive machinery need not live in neuronal wetware?

The neuron doctrine has a more specific expression in todays cog-neuro. It’s the claim that the brain is basically a complex neural net and that memory, learning, cognition are products of such neural nets. In other words, the prevalent view in contemporary cog-neuro is that cognition is an inter-neuronal phenomenon not an intra-neuronal one. Brains are the locus of cognition because it brains have inter-neuronal connections. Memories, for example, are expressed in connection weights and learning amounts to adjusting these inter-neuronal weights. The plant stuff challenges this view as well. How so? Because if plants to cognize they seem to do it without anything analogous to a brain (more exactly, this assumption is common ground in the discussion). So, if plants have memories then it looks like they encode these within cells. LR mentions epigenetic memory as a possible memory substrate. These involve “chromatin marks,” which “are proteins and small chemical groups that attach to DNA within cells and influence gene activity” (LR: 3). This mechanism within cells suffices to physically implement “memory.” And if this is so, then it would provide evidence for the Gallistel-King conjecture that memories can be stored biochemically within cells. Or to state this more carefully: if plants can code memories in this way, why not us too and maybe neuronal connectionism is just a wrong-headed assumption, as Gallistel has been arguing for a while. Here is MP making this point:

How plants do without a brain…raises questions about how our brains do what they do. When I asked Mancuso about the function and location of memory in plants, he…reminded me that mystery still surrounds where and how our memories are stored: “it could be the same kind of machinery, and figuring it out in plants may help us figure it out in humans.” (MP:19)


Ok, there is lots of fun stuff in these essays. It is fun to see how plant people go about arguing for mental capacities in plants. There are nice discussions of experiments that appear to show that plants can “habituate” to stimuli (they pretend-drop plants and see how they react), can “learn” new associations (use wind as conditioned stimulus for light) among stimuli, and can to anticipate what will happen (where sun will be tomorrow) in the absence of the thing being anticipated (in the absence of input from the sun), which suggest that plants can represent the trajectory of the sun. Is this “true” and do plants cognize? I have no idea. But an a priori denial that it is possible is based on conceptions of what proper cog-neuro is that we have every reason to reject.

[1] So too if machines can cognize (Searle’s target), something that seems less challenging for some reason than that plants do. There is some nice speculation in the MP article as to why this might be the case.

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