In recent years, there’s been a resurgence in the old romantic sentiment that maybe plants are conscious. I hadn’t realized that an entire sub-field had formed called Plant Neurobiology, the name itself incorporating a dubious claim that plants have neurons. Although later renamed to the more cautious Plant Signalling and Behavior, it’s reportedly still popularly known by the original provocative title.
Apparently a number of biologists have had enough and published a paper in the journal Trends in Plant Science making the case that plants neither possess nor require consciousness. From the abstract:
- Although ‘plant neurobiologists’ have claimed that plants possess many of the same mental features as animals, such as consciousness, cognition, intentionality, emotions, and the ability to feel pain, the evidence for these abilities in plants is highly problematical.
- Proponents of plant consciousness have consistently glossed over the unique and remarkable degree of structural, organizational, and functional complexity that the animal brain had to evolve before consciousness could emerge.
- Recent results of neuroscientist Todd E. Feinberg and evolutionary biologist Jon M. Mallatt on the minimum brain structures and functions required for consciousness in animals have implications for plants.
- Their findings make it extremely unlikely that plants, lacking any anatomical structures remotely comparable to the complexity of the threshold brain, possess consciousness.
The paper is not technical and is fairly easy and interesting reading, although there are numerous summaries in the news. It makes some good points about the metabolic expenses of consciousness, and that plants are simply not in an ecological position where paying that energy price is adaptive for them.
Those of you who’ve followed me for a while might recognize the names Todd Feinberg and Jon Mallatt, as I’ve highlighted their work several times. Their books have been a major influence on my views, so it makes a lot of sense to me that their work would be discussed in this context.
But while they are a major influence, I don’t buy all of their propositions. In particular, I’m not comfortable with their neurobiological essentialism, or this paper citing it as the major driving force for rejecting plant consciousness. Feinberg and Mallatt understandably hold this position because the only place consciousness has been conclusively observed is in such systems.
But evolution has repeatedly shown itself capable of finding alternate solutions to problems. In the case of nervous systems, their low level functionality is actually a re-implementation of functionality that already existed in unicellular organisms. So it seems to me that we should be open to alternate implementations of the high level functionality.
And it’s worth noting that Feinferg and Mallatt’s “structural, organizational, and functional complexity” criteria were developed by looking at vertebrate nervous systems (fish, reptiles, mammals, birds). The invertebrates who they admit into club-consciousness: arthropods (insects, crabs) and cephalopods (octopuses, squids), make it in based on their sense organs and behavior, that is, their observed capabilities.
I think that’s how we should assess the proposition of plant consciousness, cognition, intelligence, etc, by their observed capabilities. Doing so leaves us open to alternative possibilities, while also avoiding animal chauvinism.
Here I’ll pull out my own mental crutch: the hierarchy of consciousness. Each layer builds on the previous one.
- Reflexes: automatic reactions to simuli, fixed action patterns determined by genetics (or programming) although modifiable by local classical conditioning. Some will insist that these action patterns be biologically adaptive; if so, then this and the layers above are inherently biological.
- Perception: predictive models of the environment build on information from distance senses (sight, hearing, smell), expanding the scope of what the reflexes can react to, enabling reaction prior to a direct somatic or chemorecptive encounter
- Attention: prioritization of which perceptions the reflexes are reacting to. Attention can be bottom-up, essentially reflexes about reflexes (meta-reflexes), or top-down, driven by layers 4 and 5.
- Imagination & Sentience: simulations and assessment of possible action scenarios. Based on the results of the simulations, individual reflexes are either allowed or inhibited, decoupling the reflexes, turning them into motivational states (affects) rather than automatic reactions.
- Metacognition: A feedback mechanism for assessing the system’s own cognitive states, particularly the reliability of beliefs. An advanced recursive form of this enables symbolic thought including language, art, mathematics, etc.
Layers 1-4 make up what is commonly referred to as primary consciousness. Based on all the evidence I’ve read, plants are mostly in layer 1, with perhaps some limited layer 2 abilities. I haven’t seen anything implying layers 3 or higher.
Of course, many in the plant neurobiology community might insist that layers 1 and 2 are sufficient for the label “conscious”. But if so, then we’d have to make careful distinctions between animal consciousness vs plant consciousness, and be clear that we don’t mean plants have imagination or self reflection.
Consciousness is in the eye of the beholder, but plant consciousness doesn’t strike me as a productive proposition. If we accept it, what then? Are we required to take their sensibilities into account? Are we being cruel when we mow the yard or trim the hedges? Are vegetarians as much killers as carnivores?
All in all, it would be a lot of trouble for a sketchy proposition, an extraordinary proposition that we should require extraordinary evidence for before accepting.
Unless of course I’m missing something?