Pavlov’s – Bacterium?
Ding! Ding! Ding! More good stuff on theories of mind and classical conditioning. From MIT Technology Review:
A century after Pavlov’s dog first salivated at the sound of a bell, researchers are saying that single-celled organisms such as bacteria can be “trained” to react in a similar way. Rather than use complex networks of nerve cells, or neurons, bacteria can “learn” to associate one stimulus with another by employing molecular circuits, according to a multidisciplinary team from Germany, Holland, and the United Kingdom.
What? Bacteria LEARNING? Well – according to Daniel Dennett, even an object as simple as a thermostat can, in a way, be said to have certain types of ‘beliefs’ about the world. Dennett doesn’t believe that a thermostat is conscious. But he does think that, in certain ways, its behavior can’t be distinguished from beliefs. When considered purely with respect to its specific function, a thermostat could be said to “believe” a room is too cold, and then react by turning the boiler on. Of course, that thermostat doesn’t have a mind, but it does have a purpose, function and design that allow it to react predictably within a specific set of circumstances and conditions. Kind-of like a bacterium.
We humans often attribute feelings or intentions metaphorically to non-human objects. It is convenient for us to adopt the intentional stance when trying to understand human beings, so we try to conserve energy by using it to understand non-human ones too. But – when we try to understand thermometers – or bacteria – at the level of the intentional stance, giving them beliefs about basic aspects of their worlds and ascribing them the ‘desire’ to perform specific functions, do we assume an increased risk of error by extrapolating too broadly?
The mechanisms responsible for controlling behavior vary widely across species – and many actions that are indistinguishable from conscious behavior are observed in the animal and electronic worlds. A mind allows us to perceive environmental stimuli and react adaptively to them by anticipating predictable events. A bimetallic strip allows a thermostat to perceive temperature changes and react to them as well. And in much the same way, biochemical reactions can act like a bacterium’s brain.
Dennett proposes three levels of abstraction (or stances) with respect to behavior. These are the physical, design and intentional stances. The physical stance is the level at which we are concerned with properties like mass, energy, velocity, and chemical composition. The design stance focuses on purpose, function and design. The intentional stance is the level where we consider things in terms of beliefs, thinking and (of course) intent.
As with Pavlov’s dog and all other examples of associative learning, the bacteria in the model learn to build stronger associations between the two stimuli the more they occur together. The Canadian neuropsychologist Donald Hebb established an underlying explanation back in 1945. Now called Hebbian learning, it’s often expressed as a situation in which “neurons that fire together wire together.” In the hungry dog’s case, nerve cells triggered by the smell of food started to make physical links with the nerve cells simultaneously triggered by the sound of a bell. According to Hebb’s theory, the more often the two stimuli are applied at the same time, the greater the link or “synaptic weight” between them.
Bacteria, of course, don’t have synapses or nerve cells. Nonetheless, there are indications that single-celled organisms can learn. In the 1970s, Todd Hennessey claimed that paramecia, the single-celled pond dweller, could be conditioned in the lab. He electrocuted them and associated this with a buzzer. Following the simultaneous exposure to the buzzer and to electric currents, he claimed that the paramecia swam away from the buzzer when they had not done so before. The finding was never properly reproduced, but it raised the intriguing possibility that some sort of associated learning was possible for single-cell life forms.
Eva Jablonka, a theoretical biologist at Tel-Aviv University and a leading researcher in the field, agrees. “This is conceptually a bit difficult,” she says, “but if you look at the definition of learning–because of something happening, you have some kind of physical traces, and this changes the threshold of the response in the future–then this is what you have here.” She adds, “I think that it is a good and potentially very useful paper, and I think they do demonstrate associative learning.”
Think that’s cool? There’s even more. If you or your dog learn something – or even if you develop a conditioned association to a stimulus, that specific bit of “knowledge” is yours and yours alone. But… it seems that bacteria may be capable of passing their associative learning on to their offspring (probably because they reproduce by binary fission instead of that messy sexual reproduction).
Significantly, Fernando estimates that the changes induced in the bacteria could easily persist for the 30-minute life cycle of an E. coli bacterium. This would make the changes, or “learning,” heritable. This is an especially important point when it comes to medical applications for trained bacterium. “After all, diseases or drug doses are going to last longer than 30 minutes,” notes Jablonka.
The trick would be to train bacteria to recognize chemical processes in the body that are associated with danger. This might be an adverse and dangerous reaction to a drug, or to the presence of tumor cells, indicating that a medicine in the system needs to be activated in certain tissues.
Smart bacteria and inheritable learning. Do you suppose we can develop a bacterium that will inoculate us to forgetfulness?