Snail and Slug Consciousness and Semi-Unlimited (?) Associative Learning
I've just finished reading Simona Ginsburg's and Eva Jablonka's tome on consciousness in non-human animals, The Evolution of the Sensitive Soul. It is an impressively wide-ranging work, covering huge swaths of philosophy, biology, and psychology for many different species. (For an article-length version of their view, see here.)
Ginsburg's and Jablonka's central idea is that consciousness (i.e., phenomenal consciousness, subjective experience, being an entity that there's "something it's like" to be) requires something they call Unlimited Associative Learning. They argue that we see consciousness and Unlimited Associative Learning in vertebrates, at least some arthropods (especially insects), and in some mollusks (especially cephalopods) but not other mollusks (e.g., sea hares), and not in most other animal phyla (e.g., annelida such as earthworms or cnidaria such as jellyfish). If you wonder -- as I do -- where we should draw the line between animal species with consciousness and those without consciousness, theirs is one of the most interesting and well-defended proposals.
I'm not convinced for two broad reasons I discuss here and here. I think all general theories of consciousness suffer from at least the following two epistemic shortcomings. First, all such theories beg the question, right from the start, against plausible views endorsed by leading researchers who see consciousness as either much more abundant in the universe or much less abundant in the universe (e.g., panpsychism and Integrated Information Theory on the abundant side, theories that require sophisticated self-representation on the other side). Second, all such theories are ineliminably grounded in human introspection and verbal report, creating too narrow an evidence base for confident extrapolation to very different species.
But today I don't want to focus on those broad reasons. As regular readers of this blog know, I love snails. So I was interested to note that Ginsburg and Jablonka specifically highlight two genera of terrestrial gastropod (the Limax slug and the Helix snail) as potentially in the "gray area" between the conscious and nonconscious species (p. 395). And I think if you pull a bit on the thread they leave open here, it exposes some troubles that are specific to their theory.
Ginsburg's and Jablonka's view depends essentially on a distinction between Limited Associative Learning and Unlimited Associative Learning. Associative learning, as you might remember from psychology class, is the usual sort of classical and operant conditioning we see when a dog learns to salivate upon hearing a bell associated with receiving food or when a rat learns to press on a lever for a reward. Unlimited Associative Learning, as Ginsburg and Jablonka define it, "refers to an animal's ability to ascribe motivational value to a compound stimulus or action pattern and to use it as the basis for future learning" (p. 35, italics added). Unlimited Associative Learning allows "open-ended behavioral adjustments" (p. 225) and "has, by definition, enormous generativity. The number of associations among stimuli and the number of possible reinforced actions that can be generated are practically limitless" (p. 347). An animal with Limited Associative Learning, in contrast, can only associate "simple ('elemental') stimuli and stereotypical actions" (p. 225).
Immediately, one might notice the huge gap between Limited Associative Learning (no learning of compound stimuli, no stringing together of compound actions) and truly open-ended, truly "unlimited" Unlimited Associative Learning with full generativity and "practically limitless" possibilities for learning. Mightn't there be some species in the middle, with some ability to learn compound stimuli, and some ability to string together compound actions, but only a very limited ability to do so, far, far short of full combinatorial generativity? For example... the garden snail?
Terrestrial snails and slugs are not the geniuses of the animal world. With only about 60,000 neurons in their central nervous system, you wouldn't expect them to be. They don't have the amazing behavioral flexibility and complex learning abilities of monkeys or pigeons. There's not a whole lot they can do. I'd be very surprised, for example, if you could train them to always choose a stimulus of intermediate size between two other stimuli, or if you could train them to engage in long strings of novel behavior. (Certainly, I have heard no reports of this.) But it does seem like they can be trained with some compound stimuli -- not simply "elemental" stimuli. For example, Limax slugs can apparently be trained to avoid the combined scent of A and B, while they remain attracted to A and B separately (Hopfield and Gelperin 1989) -- compound stimulus learning. Terrestrial gastopods also tend to have preferred home locations and home ranges, rather than always moving toward attractive stimuli and away from unattractive stimuli in an unstructured way, and it is likely (but not yet proven) that their homing behavior requires some memory of temporally or spatially compound olfactory and possibly other stimuli (Tomiyama 1992; Stringer et al. 2018).
Nor is it clear that even rat learning is fully generative and compoundable. As Ginsburg and Jablonka acknowledge (p. 303), in the 1960s John Garcia and Robert A. Koelling famously found that although rats could readily be trained to associate audiovisual stimuli with electric shock and gustatory stimuli with vomiting, the reverse associations (audiovisual with vomiting and gustatory with shock) are much more difficult to establish.
Between, on the one hand, "Limited Associative Learning" which is noncompound and reflex and, on the other hand, fully compoundable, fully generative "Unlimited Associative Learning" stands a huge range of potential associative abilities, which with intentional oxymoronity we might call Semi-Unlimited Associative Learning. Ginsburg's and Jablonka's system does not leave theoretical space for this possibility. Terrestrial gastropods might well fall smack into the middle of this space, thus suggesting (once again!) that they are the coolest of animals if you are interested in messing up philosophers' and psychologists' neat theories of consciousness.
[image source Platymma tweediei]