The interplay of spontaneous pupil-size fluctuations and EEG power in near-threshold detection

Abstract

Detection of near-threshold stimuli depends on the properties of the stimulus and the state of the observer. In visual detection tasks, improved sensitivity is associated with larger pre-stimulus pupil size. However, it is still unclear whether this association is due to optical effects (more light entering the eye), correlations with arousal, correlations with cortical excitability (as reflected in alpha power), or a mix of these. To better understand this, we investigated the relative contributions of pupil size and power in the alpha, beta, and theta frequency bands on near-threshold detection. We found that larger pre-stimulus pupil size is associated with improved sensitivity and a more liberal criterion (the likelihood of reporting that a stimulus was detected). Importantly, the relationship between pupil size and sensitivity was not mediated by any of the measured neural variables; the relationship with criterion however was mediated by power in the beta band (12- 30 Hz). Pupil size was positively correlated with power in the alpha and beta bands. Additionally, we show that pre-stimulus theta suppression (4 - 8 Hz) is associated with improved sensitivity and a more liberal criterion. Taken together, our results show an independent effect of pupil size on detection sensitivity that is not driven by cortical excitability, but may be driven by optical effects, arousal, or a mix of both.

Pre-stimulus beta power mediates explicit and implicit perceptual biases in distinct cortical areas

“…This finding is consistent with the results from a study by Nickel et al 43 who showed a decrease in alpha and beta power in the pre-stimulus time window in trials with a high expectation for a painful stimulus in somatosensory channels. In the visual domain, a similar finding was recently obtained by Ruuskanen et al 44 (2024, preprint) showing a mediating effect of pre-stimulus beta power on participants’ decision criterion. Weisz et al 45 identified a pre-stimulus alpha/beta modulation in the contralateral secondary somatosensory cortex during a near-threshold somatosensory detection task.…”

Spontaneous Fluctuations in Pupil Size Shape Retinal Responses to Visual Stimuli

“…The absence of an effect of pupil size on the strength of early retinal responses seems at odds with the well-established finding that larger pupils are associated with increased visual sensitivity (Eberhardt et al, 2022; Mathôt & Ivanov, 2019). For example, Ruuskanen et al (2024) recently found that larger pupils, as in the current study resulting from natural fluctuations, are associated with enhanced visual sensitivity in a near-threshold visual detection task; importantly, a mediation analysis indicated that this association reflects, at least in part, a direct effect of pupil size on sensitivity (as opposed to an indirect effect mediated by various EEG markers of arousal and cortical excitability that were considered in this study). But if pupil size does not affect early retinal responses, then how can pupil size affect visual sensitivity at a behavioral level?…”

“…Based on these results (see Discussion for details), we propose that fast-acting adaptation processes (Baccus & Meister, 2002; Poot et al, 1997) may normalize retinal responses, such that their amplitude is not, or hardly, affected by changes in light influx that result from spontaneous changes in pupil size: an initial form of brightness constancy with respect to pupil size that is implemented at the level of the retina. The finding that larger pupils do not lead to stronger early ERG responses seems at odds with the finding that larger pupils are associated with enhanced visual sensitivity (Ruuskanen et al, 2024). To reconcile these findings, we put forward as a testable hypothesis that increased pupil size may enhance visual sensitivity by reducing the variability, rather than modulating the strength, of early retinal responses; this would explain why, within the range of naturally occurring pupil-size fluctuations, larger pupils allow us to more easily detect faint stimuli (Ruuskanen et al, 2024) without causing us to perceive stimuli as being brighter (Wardhani et al, 2022).…”

“…The finding that larger pupils do not lead to stronger early ERG responses seems at odds with the finding that larger pupils are associated with enhanced visual sensitivity (Ruuskanen et al, 2024). To reconcile these findings, we put forward as a testable hypothesis that increased pupil size may enhance visual sensitivity by reducing the variability, rather than modulating the strength, of early retinal responses; this would explain why, within the range of naturally occurring pupil-size fluctuations, larger pupils allow us to more easily detect faint stimuli (Ruuskanen et al, 2024) without causing us to perceive stimuli as being brighter (Wardhani et al, 2022).…”