Re: Is it possible to have two thoughts at once? (resubmission)

Hi Willem,

     Interesting question. If I understand it correctly, your main question is "Can two separate thoughts (or 'lines of thought') be carried out by the brain simultaneously?" And the secondary question is one of meta-cognition, in which one line of thought is ruminating on the other. From my understanding, the answer would be no to both of these (at least for the average person, and therein may lay the crux of the issue).

     The word 'thought' is a bit tough to work with it, as we don't have a neurobiological signature of a thought - mainly because we don't really know what a 'thought' is. That is to say, if we are going to determine whether two thoughts are possible, we need to know how to define a thought. Is it a sensory perception or when you come up with an idea to solve a problem? Or is it the awareness of that sensory perception or potential solution - the internal dialogue that says, "this painting is nice," or "I can fix the faucet this way!" I think people refer to the latter as consciousness or a conscious event (another poorly defined term). Consciousness is our only 'report' of what's going on in our head. But I guess you're suggesting that a person might have a split consciousness, or that someone could be conscious of two separate events (either event could be external or in one's mind).
     It seems like your ultimate question would be split consciousness of two internal events - "the possibility that we can reflect on (consciously think about) a thought while we are thinking that thought." This opposed to being aware of a cognitive memory, or as you put it, "think[ing] about a thought retrospectively." I can only speak for myself in saying that I have never had an experience where I had split consciousness. But is it possible?

     Ultimately, I think the normal brain is optimized to handle one task at a time, either an external perception or internal monologue. This is borne out in many studies of attention (studied at the perceptual level in animals and at the cognitive level in humans). I use the word attention somewhat loosely to mean 'whatever it is you're focussing on,' whether it be the road when you're driving, or your thoughts of the day when you're lying in bed at night. It also seems like several brain structures (ones that are involved in attention, and thus eventually consciousness) are organized in a way that would select only one sensory perception/internal representation. I must admit, though, that I'm partial to Eric Knudsen's model (that Lane Niles referred you to); I'm working in his lab these days. But we have some anatomy and physiology to back up our ideas.

     The example you bring up, though, is of a fellow who has psychosis. So in this case, all bets are off! He may be able to have two thoughts at once (assuming the caviat below), but it (and other associated maladies he may have) is apparently not necessarily advantageous. That is to say, I don't know if I've encountered the term 'psychosis' with a connotation of the condition being a beneficial one.

     The caveat, as Lane Niles pointed out, is with the possibilities of rapid serial thinking versus parallel thinking; it's hard to prove which is the case. The reason it's hard to know lies in the fact that humans don't really act in 'real time.' We parcellate the world in roughly 100 millisecond (ms) chunks, though they seem seamless. Thus if the a rapid switching paradigm solved the two equations within that 100 ms window, it could be perceived to be simultaneous (as suggested by the fellow you met). And as you accurately point out, EEGs and fMRI are complimentary, but don't really provide you both the temporal and spatial precision you need. So with our current technologies, we can't answer properly address the question. Hopefully better methods will let us test this exact question.

     I think the closest thing that resembles double-thought is the feeling of deja vu. It's often associated with a feeling of 'having been there before,' but I think it's also associated with being in the same 'brain state' in that particular location. (e.g. you can have deja vu in your kitchen - you've been to your kitchen a zillion times, but it's this particular moment, with your thoughts about x,y, or z that is striking). More info on neural theories can be found here. However, this is a rare experience, and one, were it permanent, might be annoying or somehow leading to dysfunction in daily life. Though, perhaps if it were constantly there, it wouldn't grab your attention so much.

     I hope that lends a smidge of clarity as to what folks in the neuroscience community (if I can boldy speak for them) would think.

     cheers,
     Alex