I recently read a very interesting opinion in Nature by Tu & Knight titled “Metabolic cycles as an underlying basis for biological oscillations” (2006). The main idea of the paper is that many periodic processes in organisms, specifically, the circadian cycle, hibernation cycle, and sleep-wake cycle, can be partially explained in terms of metabolic ones.
All organisms require food and nutrients to feed metabolic systems in order to survive, grow and reproduce. Key to Tu & Knight’s provocative suggestion is a very interesting distinction between two ways that evolution deals with carrying out metabolic processes that, in some way, conflict. For example, conflict might occur because they are incompatible, or result in the production of toxins. In fact, given the vast number of metabolic reactions necessary for maintaining even simple forms of life these sorts of conflicts are the norm. This raises the question “How does an organism perform the many metabolic reactions that are demanded of it without dire or futile consequences?” (Tu & Knight, p.696).
The most familiar answer to this question is spatial compartmentalization: at a low level, single cells are divided into organelles that carry out different metabolic processes in contained and distinct environments. Higher up, of course, the bodies of more complex organisms are divided into organs, like the heart, stomach, and liver. Though using a different term, Tu & Knight are describing something very close to the familiar notion (to cognitive scientists!) of modularity (I emphasis similar, because here the focus is on the adaptivity of isolating incompatible processes, versus putting things in modules for other reasons that might be adaptive, e.g., processing speed). However, a different, and underappreciated answer to the above question is temporal compartmentalization: the exploitation of the dimension of time such that different metabolic reactions are carried out during different temporal periods (rather than spatial locations). Also, with temporal compartmentalization comes regular fluctuation of metabolites in a cyclical manner.
I think Tu & Knight make a good case that metabolic cycles are the basis for the sorts of cycles they discuss. For example, in the case of the sleep-wake cycle, the recent work on orexin suggests one link between sleep and energy modulation. However, what I find interesting about this piece more generally is the notion of temporal compartmentalization, which seems equally, if not to a much greater degree, underappreciated when it comes to how we think of the mind. Time is another dimension in which we can localize functional processes, and cycles is a key part of this. As one example of how such “temporal” thinking might be fruitful, I will briefly make some halfbaked suggestions about declarative memory and consciousness. I am going to focus on the sleep-wake cycle.
Neither sleep nor wakefulness are monolithic entities; as is now familiar, sleep divides into several stages like REM, non-REM, and non-REM slow-wave sleep (SWS). One correlate of wakefulness is of course, phenomenal consciousness, which does not occur in most sleep stages with the exception of REM sleep. Also, there is very good evidence that different sleep stages (mostly REM and SWS) play important roles in the encoding, consolidaton, and reconsolidation, of different kinds of memory, like declarative memory and procedural memory. So, both consciousness and memory seem to be tied, in someway, to the sleep-wake cycle. Yet, key declarative memory processes only occur during sleep, and only during stages of sleep in which we lack conscious experience (like SWS). A natural question is “why?”.
Temporal compartmentalization suggests one way of thinking about the answer. It is generally thought that memories are stored where they are “used”, which means in cortex. Also, conscious experience correlates with activation in sensory cortex. One explanation for why we are never conscious during certain stages of sleep is that certain processes that are carried out during the non-conscious parts of the sleep wake cycle are incompatible with the sort of activation that correlates with consciousness. A candidate for these processes are ones that are involved in the storage of declarative memory. So certain memory processes are incompatible with us having conscious experience. By the logic of temporal compartmentalization, this would suggest that consciousness, and declarative memory storage, are functions of the same parts of the brain. Just as we might temporally compartmentalize different metabolic processes using the same spatial compartment, the same might be true for memory storage and consciousness.
This, I think, is a interesting idea. If correct, then it might provide an excellent way of trying to triangulate the location AND TIME of neural activation that correlates with conscious experience. For materialists about consciousness, it suggests a way to figure out which neural states just are conscious states. Of course, there are other reasons for why two neural processes cannot occur at the same time; it need not be because they are incompatible and occur at the same place. Still, this is just one example of how one might apply the notion of temporal compartmentalization in how we think of the mind.