Pupil size reflects activation of subcortical ascending arousal system nuclei during rest

  1. Beth Lloyd
  2. Lycia D de Voogd
  3. Verónica Mäki-Marttunen
  4. Sander Nieuwenhuis

  • Curated by eLife

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    eLife assessment

    These are important findings that show that pupil size is not only governed by the locus coeruleus but also by other neuromodulatory subcortical systems. Furthermore, the authors demonstrate that using a standard hemodynamic response kernel is not appropriate for capturing the activity of these systems, at least at rest. Thus, this paper presents compelling evidence against two prevalent working assumptions among researchers in the field.

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Abstract

Neuromodulatory nuclei that are part of the ascending arousal system (AAS) play a crucial role in regulating cortical state and optimizing task performance. Pupil diameter, under constant luminance conditions, is increasingly used as an index of activity of these AAS nuclei. Indeed, task-based functional imaging studies in humans have begun to provide evidence of stimulus-driven pupil-AAS coupling. However, whether there is such a tight pupil-AAS coupling during rest is not clear. To address this question, we examined simultaneously acquired resting-state fMRI and pupil-size data from 74 participants, focusing on six AAS nuclei: the locus coeruleus, ventral tegmental area, substantia nigra, dorsal and median raphe nuclei, and cholinergic basal forebrain. Activation in all six AAS nuclei was optimally correlated with pupil size at 0–2 s lags, suggesting that spontaneous pupil changes were almost immediately followed by corresponding BOLD-signal changes in the AAS. These results suggest that spontaneous changes in pupil size that occur during states of rest can be used as a noninvasive general index of activity in AAS nuclei. Importantly, the nature of pupil-AAS coupling during rest appears to be vastly different from the relatively slow canonical hemodynamic response function that has been used to characterize task-related pupil-AAS coupling.

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  1. Version published to 10.7554/elife.84822 on eLife
  2. eLife

    Author Response

    Reviewer #3 (Public Review):

    The authors took a comprehensive set of analyses to examine the relationship between pupil diameter / derivative and BOLD-signal during rest in the ascending arousal system nuclei in 72 young participants. Focus is on the locus coeruleus, ventral tegmental area, substantia nigra, dorsal and median raphe nuclei and the basal forebrain. Analyses were performed using various processing pipelines: canonical versus custom hemodynamic response functions, with/without smoothing, time to peak analyses and cross spectral power density analyses to define the time lag between both measurements. The authors could not replicate previous correlations between locus coeruleus BOLD and pupil measurements using standard analytic approaches, and also found no relationship between locus coeruleus BOLD and pupil measurements when using custom hemodynamic response functions. When using time to peak and cross-correlation analyses, the authors found that coupling between pupil size and AAS BOLD patterns increases with decreasing time to peak, when the two signals were close in time. The authors conclude that these findings suggest that pupil size could be used as a noninvasive readout of AAS activity under passive conditions.

    These authors did a thorough assessment, and described the methods and results well and in a balanced manner.

    Outstanding questions:

    • the reliability of these observations? would we see the same findings in a different cohort or using a different sequence/field strength?
    • What is the independent association of each assessed nucleus with pupil dilation? That could be informative to understand their shared or unique role.

    We are grateful to the reviewer for their expert advice in helping us strengthen our manuscript. We agree with the reviewer that these two outstanding questions are important and we have done our best to answer these questions below. We believe that our manuscript has greatly improved, thanks to the reviewer’s suggestions for running these additional analyses.

  3. eLife

    eLife assessment

    These are important findings that show that pupil size is not only governed by the locus coeruleus but also by other neuromodulatory subcortical systems. Furthermore, the authors demonstrate that using a standard hemodynamic response kernel is not appropriate for capturing the activity of these systems, at least at rest. Thus, this paper presents compelling evidence against two prevalent working assumptions among researchers in the field.

  4. eLife

    Reviewer #1 (Public Review):

    This paper presents a systematic and novel examination of how pupil size relates to BOLD fMRI signal in a set of subcortical nuclei. It provides some important novel findings that should help advance understanding of how pupil size relates to activity in subcortical nuclei as well as providing important advances in how to measure these relationships.

    The authors first tried replicating prior findings of a relationship between pupil size and BOLD signal using the prior methods. They could not (despite replicating pupil-cortical region relationships), and so tested whether the delay in the hemodynamic response function might differ in subcortical and cortical regions. They found that BOLD signal in the subcortical nuclei showed associations with pupil size at short delays. This is a critical finding as typical fMRI analyses assume a longer delay and so likely obscure the ability to see effects in these subcortical regions. The authors provide a number of helpful 'control' analyses that help strengthen confidence in their findings. For instance, it buttresses their findings that the pons control region did not show any significant effect to time-to-peak on correlations with pupil size or derivative measures. It also is helpful to know that pupil size fluctuations were associated with cortical activity in the regions expected from prior studies. The rigor of the study is also supported by the fact that there was a preregistration and that data are publicly shared.

  5. eLife

    Reviewer #2 (Public Review):

    Lloyd et al examine the relationship between pupil size and fMRI signals in six brain nuclei responsible for providing the four major neuromodulators in the brain: norepinephrine from the locus coeruleus (LC), dopamine from ventral tegmental area (VTA) and substantia nigra, serotonin from the dorsal and median raphe nuclei, and acetylcholine from the cholinergic basal forebrain. Importantly, the authors focus on the relationship between these nuclei in the ascending arousal system (AAS) and the pupil at rest, outside of the context of any task, to determine the extent that small changes in pupil size are predictive of AAS activity.

    Very few previous studies have examined this relationship at rest, perhaps in part because of the increased sensitivity required in the absence of event-based averaging. These nuclei are small (especially the LC), and thus are difficult to measure with standard fMRI.

    The authors use a number of data collection and processing techniques to increase the sensitivity and precision of their recordings targeted to small ROIs. They find robust correlations between multiple AAS nuclei and pupil size with a time course that is not well captured by a standard hemodynamic response function (HRF).

    The latter methodological finding is likely to be useful to the field for future studies focused on extracting useful signals from these nuclei, and the observed relationship between multiple AAS nuclei and the pupil support an emerging consensus from animal research that pupil fluctuations are correlated with neuromodulators besides norepinephrine.

  6. eLife

    Reviewer #3 (Public Review):

    The authors took a comprehensive set of analyses to examine the relationship between pupil diameter / derivative and BOLD-signal during rest in the ascending arousal system nuclei in 72 young participants. Focus is on the locus coeruleus, ventral tegmental area, substantia nigra, dorsal and median raphe nuclei and the basal forebrain. Analyses were performed using various processing pipelines: canonical versus custom hemodynamic response functions, with/without smoothing, time to peak analyses and cross spectral power density analyses to define the time lag between both measurements. The authors could not replicate previous correlations between locus coeruleus BOLD and pupil measurements using standard analytic approaches, and also found no relationship between locus coeruleus BOLD and pupil measurements when using custom hemodynamic response functions. When using time to peak and cross-correlation analyses, the authors found that coupling between pupil size and AAS BOLD patterns increases with decreasing time to peak, when the two signals were close in time. The authors conclude that these findings suggest that pupil size could be used as a noninvasive readout of AAS activity under passive conditions.

    These authors did a thorough assessment, and described the methods and results well and in a balanced manner.
    Outstanding questions:
    - the reliability of these observations? would we see the same findings in a different cohort or using a different sequence/field strength?
    - What is the independent association of each assessed nucleus with pupil dilation? That could be informative to understand their shared or unique role.

  7. Version published to 10.1101/2022.11.04.514984v3 on bioRxiv
  8. Version published to 10.1101/2022.11.04.514984v2 on bioRxiv
  9. Version published to 10.1101/2022.11.04.514984v1 on bioRxiv