Pupil-associated states modulate excitability but not stimulus selectivity in primary auditory cortex

Abstract

Recent research in mice indicates that luminance-independent fluctuations in pupil size predict variability in spontaneous and evoked activity of single neurons in auditory and visual cortex. These findings suggest that pupil is an indicator of large-scale changes in arousal state that affect sensory processing. However, it is not known whether pupil-related state also influences the selectivity of auditory neurons. We recorded pupil size and single-unit spiking activity in the primary auditory cortex (A1) of nonanesthetized male and female ferrets during presentation of natural vocalizations and tone stimuli that allow measurement of frequency and level tuning. Neurons showed a systematic increase in both spontaneous and sound-evoked activity when pupil was large, as well as desynchronization and a decrease in trial-to-trial variability. Relationships between pupil size and firing rate were nonmonotonic in some cells. In most neurons, several measurements of tuning, including acoustic threshold, spectral bandwidth, and best frequency, remained stable across large changes in pupil size. Across the population, however, there was a small but significant decrease in acoustic threshold when pupil was dilated. In some recordings, we observed rapid, saccade-like eye movements during sustained pupil constriction, which may indicate sleep. Including the presence of this state as a separate variable in a regression model of neural variability accounted for some, but not all, of the variability and nonmonotonicity associated with changes in pupil size. NEW & NOTEWORTHY Cortical neurons vary in their response to repeated stimuli, and some portion of the variability is due to fluctuations in network state. By simultaneously recording pupil and single-neuron activity in auditory cortex of ferrets, we provide new evidence that network state affects the excitability of auditory neurons, but not sensory selectivity. In addition, we report the occurrence of possible sleep states, adding to evidence that pupil provides an index of both sleep and physiological arousal.

Short-term Effects of Vagus Nerve Stimulation on Learning and Evoked Activity in Auditory Cortex

“…Since A1 excitability is correlated with the pupil-indexed arousal state (Schwartz et al, 2019) and VNS can increase pupil size by itself (Fig. 3), we considered two possible pathways by which VNS might mediate changes in A1 responses.…”

“…Pupil size was measured using custom written code in MATLAB or Python. The details of pupil diameter measurement in early recordings are similar to Schwartz et al (2019). For the later recordings using Python analysis, we trained a machine learning algorithm to locate the pupil in each video frame and fit an ellipse to the pupil boundary.…”

“…For the later recordings using Python analysis, we trained a machine learning algorithm to locate the pupil in each video frame and fit an ellipse to the pupil boundary. The algorithm was trained on video frames that were labeled using the methods in Schwartz et al (2019). After training, the model performed well on novel video frames from new animals.…”

“…Given our observation that pupil dilation is observed following VNS and changes in pupil correlate with changes in neural excitability in A1 (Schwartz et al, 2019), we sought to dissociate effects of VNS that could and could not be explained by changes in pupil. To accomplish this, we fitted a linear regression model using 40-fold validation to predict response due to pupil effects alone.…”

“…1). First, short-term, reversible changes in excitability should correlate with pupil diameter (Schwartz et al, 2019). Second, long-term, persistent changes in neural activity should reflect synaptic plasticity and learning, and should last longer than changes in pupil.…”