What happens to your cognitive processes when your arousal levels increase? Quite often, the effects are striking, but mixed. For instance, seeing an arousing object such as a gun tends to enhance memory for that object but impair memory for everything around it. Seeing or hearing something arousing also usually impairs memory for what just happened beforehand, but sometimes enhances it. How can we make sense of these opposing effects? Through a series of behavioral and MRI studies, we found that whether arousal will enhance or impair information processing depends on a simple factor: the priority of the information. When a stimulus stands out perceptually or is important to you, your attention to and memory for that stimulus will be enhanced by a surge of arousal. In contrast, your processing of other, less noticeable or important stimuli tends to be impaired by arousal. For instance, consider the array of letters in the below figure. If we play a neutral sound before flashing this array, participants tend to be able to report more of the dark grey letters than the light grey letters, because the dark grey letters are more salient and attract more attention. But if we play an arousing sound (such as a baby crying), this effect of salience increases — participants report even more of the salient letters and fewer of the non-salient letters.

That the effects of arousal depend on priority helps make sense of contradictory findings in the literature and provides a rubric to predict whether arousal will help or hurt your thinking processes. But it also raises another question: How can this work in the brain? How can the neurochemicals released during arousal “know” which neural networks to amp up and which ones to tamp down?

These questions brought us to the locus coeruleus, a small brainstem nucleus that plays a key role in fluctuations in arousal. Neurons in the locus coeruleus have long axons that project throughout most of the brain, allowing for broad release of noradrenaline from varicosities distributed along those axons whenever arousal levels increase — for instance when simply awake or active, or when thinking hard or hearing a sudden loud noise. Thus, activation of the locus coeruleus under arousal seems likely to have a broad diffuse effect. But a broad effect does not fit with our findings. For instance, arousal can simultaneously enhance representations of salient letters and impair representations of non-salient letters.

To address this puzzle, we proposed the ‘Glutamate Amplifies Noradrenergic Effects’ (GANE) model. In this model, locus coeruleus neurons release low-to-moderate levels of noradrenaline throughout the brain when activated. These levels of noradrenaline are sufficient to activate noradrenergic receptors that have relatively low thresholds for activation and often have inhibitory effects. Thus, under arousal, throughout most of the brain, neural activity quiets down. However, where there are already high levels of activity, glutamate spills over from synaptic activity and activates receptors on nearby locus coeruleus axons. This stimulates more noradrenaline release, which stimulates more glutamate release via high threshold receptors, leading to a local hot spot of elevated activity. Thus, by amplifying excitatory activity while inhibiting most other activity, arousal helps focus attention on what really matters.

Learn more about Mara Mather’s article, Norepinephrine ignites local hot spots of neuronal excitation: How arousal amplifies selectivity in perception and memory, published in Behavioral and Brain Sciences. Free access will be available until October 31, 2016.