Recent neuroscience has shown that a region of the brain called the prefrontal cortex (PFC) shifts between two extremes of “hardware configuration”, which are suitable for doing different kinds of things. One mode has been known about for ages. In it, the PFC has a low level of background excitation and localized areas of high excitation. This is good for tasks involving focussed attention, and in older papers it is described as having a good “signal to noise ratio”. An interesting aspect of focussed attention is that when we are using it, we can always say what the next step is, before we perform it.

The other mode is a more recent discovery, perhaps because the tasks that it is good for are harder to test using controlled laboratory experiments. The PFC has a medium level of excitation all over, and the tasks it is good for involve “cognitive flexibility” and working memory. For example, subjects are given three words which can all be the second half of compound words, and the subjects have to guess the common first half of the compound words. So given “shoe”, “flake” and “drop”, the subject might guess “snow”. People are much better at doing this when fMRI machines show that their PFCs are in the second mode. Notice that with this kind of task there is no “working out” involved. We can’t state any steps before we perform them - the word “snow” either pops into the subjects’ minds or it doesn’t.

(I took the fMRI image from Functional organization of the prefrontal cortex in humans at the Institut des Sciences Cognitives.)

What’s really interesting about this is how the brain shifts from one mode to the other. It’s stress that does it. When we (or other animals) become stressed, we release the chemicals norepinephrine and dopamine, and these chemicals bias the cells in the PFC towards the focussed attention mode. This makes sense. The animal is wandering around on the lookout for food or predators, but as soon as it sees a predator it needs to focus in on it, see what it is doing and avoid distraction. So this mode switching would obviously be useful - a bit like “zooming in” on something of interest.If you are interested in looking into this, I suggest starting with the following papers (but you don’t have to read them - I’ll summarize the important bits here):

Noradrenergic Suppression of Synaptic Transmission May Influence Cortical Signal-to-Noise Ratio by Hasselmo et. al. describes low level studies on slices of rats’ brains, and a mathematical model that reproduces what they found. When they dosed the brain slices with norepinephrine they found that “background” signals between cells were strongly suppressed, but signals coming into the slices (from the senses in whole rats) were less suppressed. Most importantly, they say:

This can be interpreted as an increase in signal-to-noise ratio, but the term noise does not accurately characterize activity dependent on the intrinsic spread of excitation, which would more accurately be described as interpretation or retrieval.

Beta-adrenergic Modulation of Cognitive Flexibility during Stress by Alexander et. al. references the Hasselmo paper. They gave human subjects word searching tests as described above, with and without social stress, with and without propranolol (a drug that blocks the effects of norepinephrine). I’ve spent a few years now digging through neuroscience papers, looking for effects which I was sure must exist, but without being a professional in the field myself. The work of this group, led by Prof. David Beversdorf, is outstanding for clarity, specificity and rigour. In this paper they say:

Although the enhanced “signal” aspect of the ratio would be related to superior performance on tasks of attention, the behavioral role of “noise” is less understood. Based on a computational model, however, it has been suggested that “noise” is a representation of intrinsic associative activity, not simply an absence of coherent input to cortical neurons… Related to this model, we propose that the increased intrinsic associative activity of “noise” occurring with decreased [norepinephrine] relates to improved performance on cognitive flexibility tasks involving flexibility of access to the lexical–semantic and associative network.

In the same paper, they say:

Here we show that psychosocial stress in healthy individuals without any history of stress-induced dysfunction impairs performance on tasks that require flexible thinking… The observed specificity of stress and propranolol to affect only cognitive flexibility suggests a role of the noradrenergic system in modulating the neural circuitry that may play a role in underlying such modalities as creativity and insight.

The Effect of Auditory Stressors on Cognitive Flexibility also by the Beversdorf group, describes the use of noise instead of social stress, but finds the same effect. Tellingly, they say:

… cognitive flexibility, for the purposes of this research, encompasses the ability to inhibit strong response preferences in order to explore alternative solutions in problem-solving through searching the ‘possible solutions network’, and are typically solved in an all-or-none insight manner.

Effect of the Cold Pressor Test on Memory and Cognitive Flexibility again by the Beversdorf group, stresses the subjects with physical discomfort (a cold hand) and shows that working memory is also negatively affected by stress.

The Selective Dopamine D4 Receptor Antagonist, PNU-101387G, Prevents Stress-Induced Cognitive Deficits in Monkeys by Arnsten et. al. shows that stressed monkeys suffer similar effects on working memory, this time via a dopamine pathway. Like norepinephrine, dopamine is known to be raised by stress.

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