Pupillary Responses Linked to Neuronal Activity in the Insula

Post by Lincoln Tracy

What's the science?

Much of our knowledge about activity in the brain has been obtained from studies of “task-evoked activations”—or looking at how groups of neurons respond to external events (such as lights or sounds) or experimental conditions. These types of studies have predominantly used functional neuroimaging to examine activity between different areas of the brain. However, due to the limitations surrounding the temporal resolution and signal-to-noise ratio of neuroimaging data, it is challenging to draw conclusions about the profiles and relevance of neuronal activity at the population level. This week in The Journal of Neuroscience, Kucyi and Parvizi developed a combined intracranial electroencephalography (iEEG)-pupillometry paradigm that reliably characterizes the functional significance of spontaneous neuronal activity.

How did they do it?

The authors recruited three patients with refractory focal epilepsy who had electrodes surgically implanted in their brain (specifically, the dorsal anterior insula) to monitor epileptic activity. After surgery, patients completed two tasks: a gradual-onset continuous performance task (where images of cities and mountains were displayed and transitioned from one to the other), and a resting-state task (where patients focused on a white cross displayed over a dark background for five to seven minutes). During the tasks, the authors recorded neuronal activity via the iEEG and changes in pupil diameter via an eye-tracking device.

What did they find?

Insular activity and pupil size both increased following the presentation of the target stimulus during the continuous performance task. The authors examined the specific timing of the increases in insular activity and pupil size to determine whether the task-evoked insular activity was associated with the increase in pupil dilation. They found that the pupil dilation consistently occurred after the task-evoked insular activity. Furthermore, the magnitude of insular activity was positively correlated with how much the pupil increased in size (i.e., more insular activation was associated with a larger increase in pupil size).

The authors also found that there was less frequent and more spontaneous neuronal activity in the insula during the resting-state task compared to the continuous performance task. Although there was less activity during rest, the electrophysiological properties of the activity were similar to the properties of activity during the active task. Finally, the authors further examined the relationship between neuronal activity and pupillary responses at rest. They found that the pupil response was time-locked to the insular activity, regardless of the task.

What's the impact?

This study found that spontaneous or task-induced activity in the insula was linked with brief dilations of the pupil. Such an association may arise from coordination between the insula and a network of cortical and subcortical areas that integrate with autonomic and neuromodulatory systems. While these findings shine the spotlight on a novel link between neural activation and autonomic function, additional work is required to determine whether the insula has a causal role in pupil control.

Kucyi & Parvizi. Pupillary dynamics link spontaneous and task-evoked activations recorded directly from human insula. Journal of Neuroscience (2020). Access the original scientific publication here.